Favourite parts with off-label uses?

On Mon, 6 Apr 2020 11:13:48 +1000, Clifford Heath <no.spam@please.net>
wrote:

On 6/4/20 10:12 am, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 12:43:27 -0700 (PDT), plastcontrol.ru@gmail.com
wrote:

NXP 74aup2g07 - analog switch
https://assets.nexperia.com/documents/data-sheet/74AUP2G07.pdf

Those are great. I use them to discharge the cap in precision
linear-ramp timing circuits.

I know you're concerned about the C-V curve with your fast ramps. How
does this device stack up (against your fave pHEMTs for example)

Clifford Heath

A timing ramp doesn't need to be especially linear. Polynomial
calibration is easy.

It does need low jitter and low tempco and not ring much.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

On Sun, 5 Apr 2020 17:54:54 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Sunday, April 5, 2020 at 5:12:12 PM UTC-7, jla...@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 12:43:27 -0700 (PDT), plastcontrol.ru@gmail.com
wrote:

NXP 74aup2g07 - analog switch
https://assets.nexperia.com/documents/data-sheet/74AUP2G07.pdf

Those are great. I use them to discharge the cap in precision
linear-ramp timing circuits.

The traditional way to discharge the cap would be a controlled current source, and
you take the zero-crossing time to indicate the state of charge instead of using
a fast ADC. That way, all cycles discharge the capacitor to the exact same level.

I don't design from tradition.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote:

Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable.

74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit.

Your faves?

Cheers

Phil Hobbs
(*) who may be bulegoge's good twin, given the similarity of their emails

o CMOS gates for power supplies, precision voltage switching,
class-C r.f. power amps.

o Various semiconductor junctions as varicaps.

o I'm still searching for a way to use crappy ceramic caps as
varactors. Tuning a WWVB loop antenna might finally be that chance.

o LEDs as detectors.

Cheers,
James Arthur

 

[Phil Hobbs]

On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote:

On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 00:09, jlarkin@highlandsniptechnology.com wrote:
On Sat, 4 Apr 2020 17:49:18 -0700 (PDT), pcdhobbs@gmail.com wrote:

Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few
examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is
around 12 GHz, but they're amazingly stable.

They make excellent switches too. Rds-on is never specified for RF
parts, but it's about 2 ohms for the 551.


74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is
almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO
regulators with ceramic output caps. (It's good to be able to
disconnect the supplies during bring-up, and putting the jumper
between the reg and the output cap has this additional benefit.

Your faves?

Some digital bus and USB switches are cheap and are excellent,
super-fast analog multiplexers.

Some chips are useful just for their ESD diodes.

LVDS line receivers are super-fast, dirt cheap RRIO comparators.

I've used them like that at your suggestion. Snazzy if you don't need
super low offset voltage. (And if you do, every comparator slows way down.)


An LED can be used as the voltage reference for an NPN current
source. The tempcos can be made to almost cancel.

High value AlN resistors can be used as thermal bridges.

That's interesting. I recall discussing some very expensive parts sold
specifically for DC isolating thermal pours.


Surface-mount platinum RTDs can be used in experiments, as both
heater and sensor, to quantify resistor heat sinking and thermal
transients.

Surface-mount resistors can be last-resort fuses.

Low-barrier schottky diodes can be used in reverse as
constant-current things. Tempcos are not great. The same diode can
charge and discharge a capacitor.

Interesting. Are they reasonably repeatable unit-to-unit?

For modest values of reasonable. It's Is, which is huge for
schottkies, hundred nA sorts of numbers. I've posted my RF detector
which is a diode and a capacitor. It's in production.



Some self-protecting SSRs can be used as electronic fuses. As can a
3t regulator with the adj pin open.

Depletion fets are nice capacitor bleeders. Ditto 3t reg as a
current sink.



A bit of open-load transmission line can be a high-frequency peaker.
I have that option in my GHz o/e layout.

Interesting. How does that work? Normally I think of O/C tlines as a
series resonance to ground.

Stick a drooling-rise step into one end of a transmission line and it
will overshoot and snap up the waveform at the other end. Adjust the
source impedance, or terminate a little, to trim the step response.
It's sharper than RC peaking, so compensates things like Ft rolloff or
skin effect.

Okay, so not really open-circuited. I'll try it out. That could
potentially have helped that single-diode sampler gizmo--its speed was
limited by the rise time of the line receiver driving the pHEMT switch.
That one used a capacitor plus a short, mismatched shunt stub to make
the sampling pulse from a falling edge.

The soft peaking helps when a step has a soft corner on the rise,
which is very common. That's why people who do fast stuff cheat and
measure risetime 20/80.



I had a profound, life-changing revelation recently. If you don't poke
a fast rise into a passive transmission line, you won't get a fast
reflection. I wasted all those years designing absorptive lowpass
filters.

Gasp!





Someone said never use an opamp as a comparator. They were wrong.

Gotta watch for the antiparallel diodes on the inputs, though. For slow
stuff, LM358s work great as comparators--the inputs survive going way
above the supply.


An rro opamp can also be used as a current limiter/fuse.

You can do all sorts of things with ribbon cable.

Some HV diodes and the c-b junctions of some transistors make
awesome drift step-recovery (Grehkov) diodes.

BFT25s can be used as fA-leakage diodes.

Dual-winding inductors, like DRQ127, can do all kinds of tricks.

Annoying that they don't tell you k in the datasheet, and you can't even
back it out from the series-connected inductance, which is quoted as
exactly four times the parallel-connected value.

The k's are really high. Easy to measure.

That's unusual for a 'coupled inductor'. I normally expect it to be
around 0.85.

The DRQs are bifilar. Two or three 9's.

And a lot of capacitance. They make nice autotransformer flybacks.
Does that have a better name?



A toroidal inductor can be a liquid level sensor.

That one I haven't heard about. Are you looking for the NMR signal?

A conductive liquid is a shorted turn.

At sufficiently low frequency, anyway.

We've started putting little Sensirion T/H sensors in a lot of things.
When using TECs, it's awfully nice to be able to compute the dew point,
for instance, and in outdoor applications (e.g. our fire sensors for
harvesters) it's good to be able to predict when the window is liable to
fog up on the inside.

We use IP67+ enclosures with bags of 5A molecular sieve inside, which is
super cheap and will absorb 50% of its own mass in water. Simon has had
to learn a whole lot about enclosures and mechanical design generally.
Turns out that you have to put an air vent on the enclosure to prevent
pumping water inside due to atmospheric pressure differences. That
leads to working of the O-ring seals, which wears them out.

We considered using a bellows, but atmospheric pressure varies +-7% or
so, which makes for a pretty big, floppy bellows. Hermetic construction
is possible but very expensive, and relies on glass or ceramic insulated
connectors. The glass would have had to be brazed or indium-soldered to
the lid, which leads to CTE mismatch problems.

A balloon inside?

Doesn't help--you need something to reduce the pressure drop across the
seals. An aluminized rubber diaphragm inside the box, with one side
vented, might work.

I made a calculation that 50 grams of 5A molecular sieve would keep the
inside dry for about 10 years.

The board has a T/H sensor on it, so it can complain if the sieve gets
saturated.

We're conformal-coating the boards, and the sensors are running at zero
bias. Doing the latter is unusual for me, but the bandwidth only needs
to be 200 Hz or so.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[mpm]

While not a component...

Early in my career, it was demonstrated to me that you could make a reasonably accurate RF millivoltmeter out of an analog Simpson 260 voltmeter.

(Without modifying it, of course.)

 

[Clifford Heath]

On 6/4/20 5:04 am, George Herold wrote:

On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote:
Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable.

74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit.

Your faves?
Wow nice list!
I'd like to take a 'part' off the alt. use list.
I use to tout the use of 20 zeners rev biased at ~10-100 uA
as audio noise sources. When I went to replace the pack I had
it turned out I had a 'golden string'. I had to order a bunch
from different suppliers to find noisy batch. (And that still wasn't as
good(noisy) as the original... but it's obvious that almost no one wants
a noisy zener.)

George, I'm trying to understand this. What do you need 20 zeners for?

FWIW, I'm currently making a broadband noise source for testing filters
up to 1.5GHz. Still scratching around for the best source to put before
a string of ERA-3 MMIC amplifiers.

The BFR93A data sheet says abs max Vbe is 2V, but it doesn't zener at
5V. Although avalanche zeners produce much more noise, I'd rather not
boost my 5V supply (though I might need to). I wonder how much reverse
current a microwave Shottky diodes (say HSMS-286) would survive. Abs max
peak reverse voltage is 4V, so it might withstand 5V anyway...

Any better suggestions for a device I might have in the drawer already?

Clifford Heath.

 

On Mon, 6 Apr 2020 14:50:46 -0700 (PDT), mpm <mpmillard@aol.com>
wrote:

While not a component...

Early in my career, it was demonstrated to me that you could make a reasonably accurate RF millivoltmeter out of an analog Simpson 260 voltmeter.

(Without modifying it, of course.)

Mo found this at some neighborhood junk sale and bought it for me. $3
or something.

https://www.dropbox.com/s/tz0panr4f1nlqer/RF_Ammeter.JPG?raw=1

It's probably a thermocouple.

How did that Simpson thing work?

I have some Spice models of an RF detector using an SMS7621
low-barrier schottly. It works around 50 mV RMS.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

On Mon, 6 Apr 2020 17:43:46 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 00:09, jlarkin@highlandsniptechnology.com wrote:
On Sat, 4 Apr 2020 17:49:18 -0700 (PDT), pcdhobbs@gmail.com wrote:

Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few
examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is
around 12 GHz, but they're amazingly stable.

They make excellent switches too. Rds-on is never specified for RF
parts, but it's about 2 ohms for the 551.


74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is
almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO
regulators with ceramic output caps. (It's good to be able to
disconnect the supplies during bring-up, and putting the jumper
between the reg and the output cap has this additional benefit.

Your faves?

Some digital bus and USB switches are cheap and are excellent,
super-fast analog multiplexers.

Some chips are useful just for their ESD diodes.

LVDS line receivers are super-fast, dirt cheap RRIO comparators.

I've used them like that at your suggestion. Snazzy if you don't need
super low offset voltage. (And if you do, every comparator slows way down.)


An LED can be used as the voltage reference for an NPN current
source. The tempcos can be made to almost cancel.

High value AlN resistors can be used as thermal bridges.

That's interesting. I recall discussing some very expensive parts sold
specifically for DC isolating thermal pours.


Surface-mount platinum RTDs can be used in experiments, as both
heater and sensor, to quantify resistor heat sinking and thermal
transients.

Surface-mount resistors can be last-resort fuses.

Low-barrier schottky diodes can be used in reverse as
constant-current things. Tempcos are not great. The same diode can
charge and discharge a capacitor.

Interesting. Are they reasonably repeatable unit-to-unit?

For modest values of reasonable. It's Is, which is huge for
schottkies, hundred nA sorts of numbers. I've posted my RF detector
which is a diode and a capacitor. It's in production.



Some self-protecting SSRs can be used as electronic fuses. As can a
3t regulator with the adj pin open.

Depletion fets are nice capacitor bleeders. Ditto 3t reg as a
current sink.



A bit of open-load transmission line can be a high-frequency peaker.
I have that option in my GHz o/e layout.

Interesting. How does that work? Normally I think of O/C tlines as a
series resonance to ground.

Stick a drooling-rise step into one end of a transmission line and it
will overshoot and snap up the waveform at the other end. Adjust the
source impedance, or terminate a little, to trim the step response.
It's sharper than RC peaking, so compensates things like Ft rolloff or
skin effect.

Okay, so not really open-circuited. I'll try it out. That could
potentially have helped that single-diode sampler gizmo--its speed was
limited by the rise time of the line receiver driving the pHEMT switch.
That one used a capacitor plus a short, mismatched shunt stub to make
the sampling pulse from a falling edge.

The soft peaking helps when a step has a soft corner on the rise,
which is very common. That's why people who do fast stuff cheat and
measure risetime 20/80.



I had a profound, life-changing revelation recently. If you don't poke
a fast rise into a passive transmission line, you won't get a fast
reflection. I wasted all those years designing absorptive lowpass
filters.

Gasp!





Someone said never use an opamp as a comparator. They were wrong.

Gotta watch for the antiparallel diodes on the inputs, though. For slow
stuff, LM358s work great as comparators--the inputs survive going way
above the supply.


An rro opamp can also be used as a current limiter/fuse.

You can do all sorts of things with ribbon cable.

Some HV diodes and the c-b junctions of some transistors make
awesome drift step-recovery (Grehkov) diodes.

BFT25s can be used as fA-leakage diodes.

Dual-winding inductors, like DRQ127, can do all kinds of tricks.

Annoying that they don't tell you k in the datasheet, and you can't even
back it out from the series-connected inductance, which is quoted as
exactly four times the parallel-connected value.

The k's are really high. Easy to measure.

That's unusual for a 'coupled inductor'. I normally expect it to be
around 0.85.

The DRQs are bifilar. Two or three 9's.

And a lot of capacitance. They make nice autotransformer flybacks.
Does that have a better name?



A toroidal inductor can be a liquid level sensor.

That one I haven't heard about. Are you looking for the NMR signal?

A conductive liquid is a shorted turn.

At sufficiently low frequency, anyway.

We've started putting little Sensirion T/H sensors in a lot of things.
When using TECs, it's awfully nice to be able to compute the dew point,
for instance, and in outdoor applications (e.g. our fire sensors for
harvesters) it's good to be able to predict when the window is liable to
fog up on the inside.

We use IP67+ enclosures with bags of 5A molecular sieve inside, which is
super cheap and will absorb 50% of its own mass in water. Simon has had
to learn a whole lot about enclosures and mechanical design generally.
Turns out that you have to put an air vent on the enclosure to prevent
pumping water inside due to atmospheric pressure differences. That
leads to working of the O-ring seals, which wears them out.

We considered using a bellows, but atmospheric pressure varies +-7% or
so, which makes for a pretty big, floppy bellows. Hermetic construction
is possible but very expensive, and relies on glass or ceramic insulated
connectors. The glass would have had to be brazed or indium-soldered to
the lid, which leads to CTE mismatch problems.

A balloon inside?

Doesn't help--you need something to reduce the pressure drop across the
seals. An aluminized rubber diaphragm inside the box, with one side
vented, might work.

I meant that it would be vented to the outside. Little plastic tube or
something.

I made a calculation that 50 grams of 5A molecular sieve would keep the
inside dry for about 10 years.

The board has a T/H sensor on it, so it can complain if the sieve gets
saturated.

We're conformal-coating the boards, and the sensors are running at zero
bias. Doing the latter is unusual for me, but the bandwidth only needs
to be 200 Hz or so.

I guess the seive would soak up humidity and reduce the pressure
inside, so a tiny flow through the seals would introduce a little more
humidity. Wouldn't that eventually get to zero humidity and zero
pressure differential?

I guess atmospheric changes would still pump the system slightly.

Mothballs?

Cheers

Phil Hobbs

--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Phil Hobbs]

On 2020-04-06 21:35, jlarkin@highlandsniptechnology.com wrote:

On Mon, 6 Apr 2020 17:43:46 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 00:09, jlarkin@highlandsniptechnology.com wrote:
On Sat, 4 Apr 2020 17:49:18 -0700 (PDT), pcdhobbs@gmail.com wrote:

Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few
examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is
around 12 GHz, but they're amazingly stable.

They make excellent switches too. Rds-on is never specified for RF
parts, but it's about 2 ohms for the 551.


74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is
almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO
regulators with ceramic output caps. (It's good to be able to
disconnect the supplies during bring-up, and putting the jumper
between the reg and the output cap has this additional benefit.

Your faves?

Some digital bus and USB switches are cheap and are excellent,
super-fast analog multiplexers.

Some chips are useful just for their ESD diodes.

LVDS line receivers are super-fast, dirt cheap RRIO comparators.

I've used them like that at your suggestion. Snazzy if you don't need
super low offset voltage. (And if you do, every comparator slows way down.)


An LED can be used as the voltage reference for an NPN current
source. The tempcos can be made to almost cancel.

High value AlN resistors can be used as thermal bridges.

That's interesting. I recall discussing some very expensive parts sold
specifically for DC isolating thermal pours.


Surface-mount platinum RTDs can be used in experiments, as both
heater and sensor, to quantify resistor heat sinking and thermal
transients.

Surface-mount resistors can be last-resort fuses.

Low-barrier schottky diodes can be used in reverse as
constant-current things. Tempcos are not great. The same diode can
charge and discharge a capacitor.

Interesting. Are they reasonably repeatable unit-to-unit?

For modest values of reasonable. It's Is, which is huge for
schottkies, hundred nA sorts of numbers. I've posted my RF detector
which is a diode and a capacitor. It's in production.



Some self-protecting SSRs can be used as electronic fuses. As can a
3t regulator with the adj pin open.

Depletion fets are nice capacitor bleeders. Ditto 3t reg as a
current sink.



A bit of open-load transmission line can be a high-frequency peaker.
I have that option in my GHz o/e layout.

Interesting. How does that work? Normally I think of O/C tlines as a
series resonance to ground.

Stick a drooling-rise step into one end of a transmission line and it
will overshoot and snap up the waveform at the other end. Adjust the
source impedance, or terminate a little, to trim the step response.
It's sharper than RC peaking, so compensates things like Ft rolloff or
skin effect.

Okay, so not really open-circuited. I'll try it out. That could
potentially have helped that single-diode sampler gizmo--its speed was
limited by the rise time of the line receiver driving the pHEMT switch.
That one used a capacitor plus a short, mismatched shunt stub to make
the sampling pulse from a falling edge.

The soft peaking helps when a step has a soft corner on the rise,
which is very common. That's why people who do fast stuff cheat and
measure risetime 20/80.



I had a profound, life-changing revelation recently. If you don't poke
a fast rise into a passive transmission line, you won't get a fast
reflection. I wasted all those years designing absorptive lowpass
filters.

Gasp!





Someone said never use an opamp as a comparator. They were wrong.

Gotta watch for the antiparallel diodes on the inputs, though. For slow
stuff, LM358s work great as comparators--the inputs survive going way
above the supply.


An rro opamp can also be used as a current limiter/fuse.

You can do all sorts of things with ribbon cable.

Some HV diodes and the c-b junctions of some transistors make
awesome drift step-recovery (Grehkov) diodes.

BFT25s can be used as fA-leakage diodes.

Dual-winding inductors, like DRQ127, can do all kinds of tricks.

Annoying that they don't tell you k in the datasheet, and you can't even
back it out from the series-connected inductance, which is quoted as
exactly four times the parallel-connected value.

The k's are really high. Easy to measure.

That's unusual for a 'coupled inductor'. I normally expect it to be
around 0.85.

The DRQs are bifilar. Two or three 9's.

And a lot of capacitance. They make nice autotransformer flybacks.
Does that have a better name?



A toroidal inductor can be a liquid level sensor.

That one I haven't heard about. Are you looking for the NMR signal?

A conductive liquid is a shorted turn.

At sufficiently low frequency, anyway.

We've started putting little Sensirion T/H sensors in a lot of things.
When using TECs, it's awfully nice to be able to compute the dew point,
for instance, and in outdoor applications (e.g. our fire sensors for
harvesters) it's good to be able to predict when the window is liable to
fog up on the inside.

We use IP67+ enclosures with bags of 5A molecular sieve inside, which is
super cheap and will absorb 50% of its own mass in water. Simon has had
to learn a whole lot about enclosures and mechanical design generally.
Turns out that you have to put an air vent on the enclosure to prevent
pumping water inside due to atmospheric pressure differences. That
leads to working of the O-ring seals, which wears them out.

We considered using a bellows, but atmospheric pressure varies +-7% or
so, which makes for a pretty big, floppy bellows. Hermetic construction
is possible but very expensive, and relies on glass or ceramic insulated
connectors. The glass would have had to be brazed or indium-soldered to
the lid, which leads to CTE mismatch problems.

A balloon inside?

Doesn't help--you need something to reduce the pressure drop across the
seals. An aluminized rubber diaphragm inside the box, with one side
vented, might work.

I meant that it would be vented to the outside. Little plastic tube or
something.



I made a calculation that 50 grams of 5A molecular sieve would keep the
inside dry for about 10 years.

The board has a T/H sensor on it, so it can complain if the sieve gets
saturated.

We're conformal-coating the boards, and the sensors are running at zero
bias. Doing the latter is unusual for me, but the bandwidth only needs
to be 200 Hz or so.



I guess the seive would soak up humidity and reduce the pressure
inside, so a tiny flow through the seals would introduce a little more
humidity. Wouldn't that eventually get to zero humidity and zero
pressure differential?

I guess atmospheric changes would still pump the system slightly.

If we start with a dry atmosphere inside, humidity by itself would not
be a first-order forcing function.

Air pressure varies +- 7% or thereabouts on time scales of a few days.
If you assume that the variations are the same inside the box, you can
calculate how much water will get in.

Our stuff has to work in medium-extreme conditions. Harvesters are
still working in January in some places--I've spent a little January
time in West Texas installing and verifying installations, at
temperatures well below freezing and with high winds. Simon has done
about three times more of that stuff. It's difficult.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Michael Terrell]

On Monday, April 6, 2020 at 9:43:33 PM UTC-4, jla...@highlandsniptechnology..com wrote:

On Mon, 6 Apr 2020 14:50:46 -0700 (PDT), mpm wrote:

While not a component...

Early in my career, it was demonstrated to me that you could make a reasonably accurate RF millivoltmeter out of an analog Simpson 260 voltmeter.

(Without modifying it, of course.)

Mo found this at some neighborhood junk sale and bought it for me. $3
or something.

https://www.dropbox.com/s/tz0panr4f1nlqer/RF_Ammeter.JPG?raw=1


It's probably a Thermocouple.

Yes, they are Thermocouple meters. Those were used to measure the current to an antenna.

Larger ones were used in Broadcasting to log transmitter output in the station's daily engineering logs for the FCC.

How did that Simpson thing work?

I have some Spice models of an RF detector using an SMS7621
low-barrier Schottly. It works around 50 mV RMS.

Simple RF probes. There are a lot of minor variations. I have a Fluke85RF. The link is for a two page manual, and schematic. It used a Germanium diode, but modern diodes are usable. and have a better frequency response. The ARRL handbook has had a probe design in every issue that I've ever seen. The early ones used a dual diode tube like a 6AL5.

If you need better accuarcy at low levels, an old Boonton 92 series is hard to beat. Its sucessor, the 9200 is even better. It is digital and it has IEEE-488 interface as an option. Probes often cost more than the meters, and the cables are as well, but you can find them on Ebay. I've ought 'untested' 92 meters with a probe and cable cheap, because the seller is clueless. I will be building clones of some of their probes, now that I have a lathe. and suitable shielded cable. Wirepro still makes the origin Amphenol two pin microphone connectors, and I have a new, 250 foot spool of the cable.

<https://www.schematicsunlimited.com/f/fluke/fluke-85rf>

 

[Phil Hobbs]

On 2020-04-06 21:35, jlarkin@highlandsniptechnology.com wrote:

On Mon, 6 Apr 2020 17:43:46 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 00:09, jlarkin@highlandsniptechnology.com wrote:
On Sat, 4 Apr 2020 17:49:18 -0700 (PDT), pcdhobbs@gmail.com wrote:

Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few
examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is
around 12 GHz, but they're amazingly stable.

They make excellent switches too. Rds-on is never specified for RF
parts, but it's about 2 ohms for the 551.


74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is
almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO
regulators with ceramic output caps. (It's good to be able to
disconnect the supplies during bring-up, and putting the jumper
between the reg and the output cap has this additional benefit.

Your faves?

Some digital bus and USB switches are cheap and are excellent,
super-fast analog multiplexers.

Some chips are useful just for their ESD diodes.

LVDS line receivers are super-fast, dirt cheap RRIO comparators.

I've used them like that at your suggestion. Snazzy if you don't need
super low offset voltage. (And if you do, every comparator slows way down.)


An LED can be used as the voltage reference for an NPN current
source. The tempcos can be made to almost cancel.

High value AlN resistors can be used as thermal bridges.

That's interesting. I recall discussing some very expensive parts sold
specifically for DC isolating thermal pours.


Surface-mount platinum RTDs can be used in experiments, as both
heater and sensor, to quantify resistor heat sinking and thermal
transients.

Surface-mount resistors can be last-resort fuses.

Low-barrier schottky diodes can be used in reverse as
constant-current things. Tempcos are not great. The same diode can
charge and discharge a capacitor.

Interesting. Are they reasonably repeatable unit-to-unit?

For modest values of reasonable. It's Is, which is huge for
schottkies, hundred nA sorts of numbers. I've posted my RF detector
which is a diode and a capacitor. It's in production.



Some self-protecting SSRs can be used as electronic fuses. As can a
3t regulator with the adj pin open.

Depletion fets are nice capacitor bleeders. Ditto 3t reg as a
current sink.



A bit of open-load transmission line can be a high-frequency peaker.
I have that option in my GHz o/e layout.

Interesting. How does that work? Normally I think of O/C tlines as a
series resonance to ground.

Stick a drooling-rise step into one end of a transmission line and it
will overshoot and snap up the waveform at the other end. Adjust the
source impedance, or terminate a little, to trim the step response.
It's sharper than RC peaking, so compensates things like Ft rolloff or
skin effect.

Okay, so not really open-circuited. I'll try it out. That could
potentially have helped that single-diode sampler gizmo--its speed was
limited by the rise time of the line receiver driving the pHEMT switch.
That one used a capacitor plus a short, mismatched shunt stub to make
the sampling pulse from a falling edge.

The soft peaking helps when a step has a soft corner on the rise,
which is very common. That's why people who do fast stuff cheat and
measure risetime 20/80.



I had a profound, life-changing revelation recently. If you don't poke
a fast rise into a passive transmission line, you won't get a fast
reflection. I wasted all those years designing absorptive lowpass
filters.

Gasp!





Someone said never use an opamp as a comparator. They were wrong.

Gotta watch for the antiparallel diodes on the inputs, though. For slow
stuff, LM358s work great as comparators--the inputs survive going way
above the supply.


An rro opamp can also be used as a current limiter/fuse.

You can do all sorts of things with ribbon cable.

Some HV diodes and the c-b junctions of some transistors make
awesome drift step-recovery (Grehkov) diodes.

BFT25s can be used as fA-leakage diodes.

Dual-winding inductors, like DRQ127, can do all kinds of tricks.

Annoying that they don't tell you k in the datasheet, and you can't even
back it out from the series-connected inductance, which is quoted as
exactly four times the parallel-connected value.

The k's are really high. Easy to measure.

That's unusual for a 'coupled inductor'. I normally expect it to be
around 0.85.

The DRQs are bifilar. Two or three 9's.

And a lot of capacitance. They make nice autotransformer flybacks.
Does that have a better name?



A toroidal inductor can be a liquid level sensor.

That one I haven't heard about. Are you looking for the NMR signal?

A conductive liquid is a shorted turn.

At sufficiently low frequency, anyway.

We've started putting little Sensirion T/H sensors in a lot of things.
When using TECs, it's awfully nice to be able to compute the dew point,
for instance, and in outdoor applications (e.g. our fire sensors for
harvesters) it's good to be able to predict when the window is liable to
fog up on the inside.

We use IP67+ enclosures with bags of 5A molecular sieve inside, which is
super cheap and will absorb 50% of its own mass in water. Simon has had
to learn a whole lot about enclosures and mechanical design generally.
Turns out that you have to put an air vent on the enclosure to prevent
pumping water inside due to atmospheric pressure differences. That
leads to working of the O-ring seals, which wears them out.

We considered using a bellows, but atmospheric pressure varies +-7% or
so, which makes for a pretty big, floppy bellows. Hermetic construction
is possible but very expensive, and relies on glass or ceramic insulated
connectors. The glass would have had to be brazed or indium-soldered to
the lid, which leads to CTE mismatch problems.

A balloon inside?

Doesn't help--you need something to reduce the pressure drop across the
seals. An aluminized rubber diaphragm inside the box, with one side
vented, might work.

I meant that it would be vented to the outside. Little plastic tube or
something.

Vented might be okay. Trouble is, the gas diffusion rate in elastomers
is many orders of magnitude higher than in metal or glass, so it would
need to be metallized. The metal film would have a lot of cracks, of
course (*), but most of the surface would still be metal.

I made a calculation that 50 grams of 5A molecular sieve would keep the
inside dry for about 10 years.

The board has a T/H sensor on it, so it can complain if the sieve gets
saturated.

We're conformal-coating the boards, and the sensors are running at zero
bias. Doing the latter is unusual for me, but the bandwidth only needs
to be 200 Hz or so.



I guess the seive would soak up humidity and reduce the pressure
inside, so a tiny flow through the seals would introduce a little more
humidity. Wouldn't that eventually get to zero humidity and zero
pressure differential?

I guess atmospheric changes would still pump the system slightly.

Simon's the seal expert at this point. A sufficiently stiff box, with
enough screws holding the lid on, and hermetic connectors, ought to be
able to stay sealed pretty well. The pressure changes are nontrivial
though--our box is about 3 x 5 inches, so a 7% pressure change amounts
to about 15 pounds over the surface of the lid. They're also fairly
slow, so it doesn't take much of a leak rate to equalize the pressure.

We're using a cable gland rather than a hermetic connector, primarily
for cost reasons. I suspect that enough air will flow inside the cable
to manage the vent job, but we'll probably have to measure that to find out.

Mothballs?

If we get critters in there, we'll certainly have other problems first.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Monday, April 6, 2020 at 5:43:54 PM UTC-4, Phil Hobbs wrote:

On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:
big snip

We've started putting little Sensirion T/H sensors in a lot of things.
When using TECs, it's awfully nice to be able to compute the dew point,
for instance, and in outdoor applications (e.g. our fire sensors for
harvesters) it's good to be able to predict when the window is liable to
fog up on the inside.

We use IP67+ enclosures with bags of 5A molecular sieve inside, which is
super cheap and will absorb 50% of its own mass in water. Simon has had
to learn a whole lot about enclosures and mechanical design generally.
Turns out that you have to put an air vent on the enclosure to prevent
pumping water inside due to atmospheric pressure differences. That
leads to working of the O-ring seals, which wears them out.

We considered using a bellows, but atmospheric pressure varies +-7% or
so, which makes for a pretty big, floppy bellows. Hermetic construction
is possible but very expensive, and relies on glass or ceramic insulated
connectors. The glass would have had to be brazed or indium-soldered to
the lid, which leads to CTE mismatch problems.

A balloon inside?

Doesn't help--you need something to reduce the pressure drop across the
seals. An aluminized rubber diaphragm inside the box, with one side
vented, might work.

I made a calculation that 50 grams of 5A molecular sieve would keep the
inside dry for about 10 years.

Phil, I'm not sure I understand the problem. I wonder if you could say
a bit more?
(I do dream of someday maybe using a sealed box...)
You've got a sealed box.
But pressure differences can cause the seals to fail.
So you put a vent in it.
but now that will pump water into your box.
(Because you have molecular sieve in there and it will
reduce the vapor pressure of water in the box.)
So you need a long vent such that the diffusion time for
H2O is long....

Is that close to stating the problem correctly?

Some sort of long thin tube?

George H.

The board has a T/H sensor on it, so it can complain if the sieve gets
saturated.

We're conformal-coating the boards, and the sensors are running at zero
bias. Doing the latter is unusual for me, but the bandwidth only needs
to be 200 Hz or so.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Monday, April 6, 2020 at 7:32:00 PM UTC-4, Clifford Heath wrote:

On 6/4/20 5:04 am, George Herold wrote:
On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote:
Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable.

74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit.

Your faves?
Wow nice list!
I'd like to take a 'part' off the alt. use list.
I use to tout the use of 20 zeners rev biased at ~10-100 uA
as audio noise sources. When I went to replace the pack I had
it turned out I had a 'golden string'. I had to order a bunch
from different suppliers to find noisy batch. (And that still wasn't as
good(noisy) as the original... but it's obvious that almost no one wants
a noisy zener.)

George, I'm trying to understand this. What do you need 20 zeners for?

FWIW, I'm currently making a broadband noise source for testing filters
up to 1.5GHz. Still scratching around for the best source to put before
a string of ERA-3 MMIC amplifiers.

The BFR93A data sheet says abs max Vbe is 2V, but it doesn't zener at
5V. Although avalanche zeners produce much more noise, I'd rather not
boost my 5V supply (though I might need to). I wonder how much reverse
current a microwave Shottky diodes (say HSMS-286) would survive. Abs max
peak reverse voltage is 4V, so it might withstand 5V anyway...

Any better suggestions for a device I might have in the drawer already?

Clifford Heath.

Arghh! typo. sorry a 20V zener! Run near the knee you get these big
avalanche spikes, with ~1us rise/ fall times.

1 GHz noise sounds hard. What about a spectrum analyzer and tracking
generator?


George H.

 

On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote:

Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable.

74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit.

Your faves?

Cheers

Phil Hobbs
(*) who may be bulegoge's good twin, given the similarity of their emails

all the same. I do not try to be anonymous, I just try to make myself anonymous to the first level of google search.

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Monday, April 6, 2020 at 2:09:59 PM UTC-4, dagmarg...@yahoo.com wrote:

On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote:
Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable.

74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit.

Your faves?

Cheers

Phil Hobbs
(*) who may be bulegoge's good twin, given the similarity of their emails

o CMOS gates for power supplies, precision voltage switching,
class-C r.f. power amps.

o Various semiconductor junctions as varicaps.

o I'm still searching for a way to use crappy ceramic caps as
varactors. Tuning a WWVB loop antenna might finally be that chance.

Hi James, I spent part of the weekend trying to think about
crappy ceramic caps as part of a parametric oscillator.
(After re-reading Pippard's "Physics of Vibration"
the oscillator seems like an easier first step.)
But I have no idea how to couple two different signals
into/ through the capacitor. Well maybe a bridge.

Do you have any clever ideas?

George H.

o LEDs as detectors.

Cheers,
James Arthur

 

On Tue, 7 Apr 2020 06:03:15 -0700 (PDT), George Herold
<ggherold@gmail.com> wrote:

On Monday, April 6, 2020 at 7:32:00 PM UTC-4, Clifford Heath wrote:
On 6/4/20 5:04 am, George Herold wrote:
On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote:
Following up on blocher's sterling work,(*)

Many of us use parts off-label, often very successfully. A few examples:

SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable.

74HC4352s make good flying-capacitor diff amp front ends.

TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster.

Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit.

Your faves?
Wow nice list!
I'd like to take a 'part' off the alt. use list.
I use to tout the use of 20 zeners rev biased at ~10-100 uA
as audio noise sources. When I went to replace the pack I had
it turned out I had a 'golden string'. I had to order a bunch
from different suppliers to find noisy batch. (And that still wasn't as
good(noisy) as the original... but it's obvious that almost no one wants
a noisy zener.)

George, I'm trying to understand this. What do you need 20 zeners for?

FWIW, I'm currently making a broadband noise source for testing filters
up to 1.5GHz. Still scratching around for the best source to put before
a string of ERA-3 MMIC amplifiers.

The BFR93A data sheet says abs max Vbe is 2V, but it doesn't zener at
5V. Although avalanche zeners produce much more noise, I'd rather not
boost my 5V supply (though I might need to). I wonder how much reverse
current a microwave Shottky diodes (say HSMS-286) would survive. Abs max
peak reverse voltage is 4V, so it might withstand 5V anyway...

Any better suggestions for a device I might have in the drawer already?

Clifford Heath.

Arghh! typo. sorry a 20V zener! Run near the knee you get these big
avalanche spikes, with ~1us rise/ fall times.

1 GHz noise sounds hard. What about a spectrum analyzer and tracking
generator?


George H.

NoiseCom sells noise zeners that go to 110 GHz.

https://www.noisecom.com/products/components/nc100-200-300-400-series-chips-and-diodes

You get spikes at low zener current. As current increases, it
approaches symmetric gaussian noise.

Here is my extensive scientific research on zener noise:

https://www.dropbox.com/s/z6ozl646hfnbb2q/Zener_Noise.pdf?dl=0

But to characterize a filter, a sweep generator sounds better than
noise. Noise is too, well, noisy.




--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[whit3rd]

On Tuesday, April 7, 2020 at 6:03:27 AM UTC-7, George Herold wrote:

1 GHz noise sounds hard. What about a spectrum analyzer and tracking
generator?

The ones I've seen are waveguides around a gas-filled discharge tube.
Recombination noise just couples to the ports, like holding a conch shell to your ear.

 

[George Herold]

On Tuesday, April 7, 2020 at 2:23:52 PM UTC-4, whit3rd wrote:

On Tuesday, April 7, 2020 at 6:03:27 AM UTC-7, George Herold wrote:

1 GHz noise sounds hard. What about a spectrum analyzer and tracking
generator?

The ones I've seen are waveguides around a gas-filled discharge tube.
Recombination noise just couples to the ports, like holding a conch shell to your ear.

Rigol has a ~1 GHz SA with TG option. ~$3k?
Maybe a used one?
Or a boat anchor.

George H.

 

[Phil Hobbs]

On 2020-04-07 09:15, George Herold wrote:

On Monday, April 6, 2020 at 5:43:54 PM UTC-4, Phil Hobbs wrote:
On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:
big snip

We've started putting little Sensirion T/H sensors in a lot of things.
When using TECs, it's awfully nice to be able to compute the dew point,
for instance, and in outdoor applications (e.g. our fire sensors for
harvesters) it's good to be able to predict when the window is liable to
fog up on the inside.

We use IP67+ enclosures with bags of 5A molecular sieve inside, which is
super cheap and will absorb 50% of its own mass in water. Simon has had
to learn a whole lot about enclosures and mechanical design generally.
Turns out that you have to put an air vent on the enclosure to prevent
pumping water inside due to atmospheric pressure differences. That
leads to working of the O-ring seals, which wears them out.

We considered using a bellows, but atmospheric pressure varies +-7% or
so, which makes for a pretty big, floppy bellows. Hermetic construction
is possible but very expensive, and relies on glass or ceramic insulated
connectors. The glass would have had to be brazed or indium-soldered to
the lid, which leads to CTE mismatch problems.

A balloon inside?

Doesn't help--you need something to reduce the pressure drop across the
seals. An aluminized rubber diaphragm inside the box, with one side
vented, might work.

I made a calculation that 50 grams of 5A molecular sieve would keep the
inside dry for about 10 years.

Phil, I'm not sure I understand the problem. I wonder if you could say
a bit more?
(I do dream of someday maybe using a sealed box...)
You've got a sealed box.
But pressure differences can cause the seals to fail.
So you put a vent in it.
but now that will pump water into your box.
(Because you have molecular sieve in there and it will
reduce the vapor pressure of water in the box.)
So you need a long vent such that the diffusion time for
H2O is long....

Is that close to stating the problem correctly?

Some sort of long thin tube?

Air pressure varies by about +-7% (~1 psi) over time scales of a day or
two. Our enclosures have a volume of about 300 ml, and will be mounted
on top of a harvester. It will be painted a famous dark green colour,
so its albedo will be low. Thus on sunny days, it will experience
temperature fluctuations of as much as 70C over time scales of hours.
That will lead to fluctuations on the order of +-2 psi in a sealed
enclosure.

This isn't a whole lot of force, but there are all sorts of white papers
from Parker and other companies that say that pumping liquid water past
O-rings is a significant problem in outdoor equipment. One reason is
that the pressure variations basically centre on zero, and O-ring seals
really really like a single sign of pressure difference, which causes
the O-rings to stay seated.

In a vented 300 ml enclosure with 50g of 5A molecular sieve, we can get
about 24000 cycles of 20% air exchange at a dew point of 20C before the
sieve saturates, so that's many years.

Liquid getting in is much worse.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Carl]

"Phil Hobbs" wrote in message
news:bc1dc43e-b9a1-7e91-15dc-5f851bc46c7b@electrooptical.net...

On 2020-04-07 09:15, George Herold wrote:
On Monday, April 6, 2020 at 5:43:54 PM UTC-4, Phil Hobbs wrote:
On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote:
On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:
big snip

We've started putting little Sensirion T/H sensors in a lot of things.
When using TECs, it's awfully nice to be able to compute the dew
point,
for instance, and in outdoor applications (e.g. our fire sensors for
harvesters) it's good to be able to predict when the window is liable
to
fog up on the inside.

We use IP67+ enclosures with bags of 5A molecular sieve inside, which
is
super cheap and will absorb 50% of its own mass in water. Simon has
had
to learn a whole lot about enclosures and mechanical design generally.
Turns out that you have to put an air vent on the enclosure to prevent
pumping water inside due to atmospheric pressure differences. That
leads to working of the O-ring seals, which wears them out.

We considered using a bellows, but atmospheric pressure varies +-7% or
so, which makes for a pretty big, floppy bellows. Hermetic
construction
is possible but very expensive, and relies on glass or ceramic
insulated
connectors. The glass would have had to be brazed or indium-soldered
to
the lid, which leads to CTE mismatch problems.

A balloon inside?

Doesn't help--you need something to reduce the pressure drop across the
seals. An aluminized rubber diaphragm inside the box, with one side
vented, might work.

I made a calculation that 50 grams of 5A molecular sieve would keep the
inside dry for about 10 years.

Phil, I'm not sure I understand the problem. I wonder if you could say
a bit more?
(I do dream of someday maybe using a sealed box...)
You've got a sealed box.
But pressure differences can cause the seals to fail.
So you put a vent in it.
but now that will pump water into your box.
(Because you have molecular sieve in there and it will
reduce the vapor pressure of water in the box.)
So you need a long vent such that the diffusion time for
H2O is long....

Is that close to stating the problem correctly?

Some sort of long thin tube?


Air pressure varies by about +-7% (~1 psi) over time scales of a day or
two. Our enclosures have a volume of about 300 ml, and will be mounted on
top of a harvester. It will be painted a famous dark green colour, so its
albedo will be low. Thus on sunny days, it will experience temperature
fluctuations of as much as 70C over time scales of hours. That will lead to
fluctuations on the order of +-2 psi in a sealed enclosure.

This isn't a whole lot of force, but there are all sorts of white papers
from Parker and other companies that say that pumping liquid water past
O-rings is a significant problem in outdoor equipment. One reason is that
the pressure variations basically centre on zero, and O-ring seals really
really like a single sign of pressure difference, which causes the O-rings
to stay seated.

In a vented 300 ml enclosure with 50g of 5A molecular sieve, we can get
about 24000 cycles of 20% air exchange at a dew point of 20C before the
sieve saturates, so that's many years.

Liquid getting in is much worse.

Cheers

Phil Hobbs

Rube Goldberg (or someone bored and tired of staring at the walls ) would
suggest a spring-loaded piston with a solenoid release. Compress spring and
"arm" solenoid release, seal in box with oring seals and hermetic wire
feedthroughs, trigger solenoid so piston releases and raises the pressure in
the box, and voila, happy orings . If you have to open the box for
service, just rearm and fire the piston again once the box is resealed.

--
Regards,
Carl Ijames