AoE x-Chapters - 1x.2 Resistors

On Tuesday, 6 August 2019 19:29:42 UTC+1, Tim Williams wrote:

tabbypurr> wrote in message
news:3a5609d9-916f-4d3e-aea4-af1bbcae445c@googlegroups.com...

If you think that's bad, I've seen many circuits in the past where
adjusting a preset pot to one end of its travel would destroy a
transistor. That was somewhat common at one time.


Y'mean audio amp bias pots? Or the ones without wiper resistors so when
they inevitably go open circuit, the output stage grenades.

I suspect that's far more endemic to equipment in general, than you think.
I've seen far too many things with trimpots of absurd ranges...

It goes the same for simple analog design, as much as digital [hardware]
design, and software design: limit your input domain and output range!
Cover only as much as you need, no more, no less!

The theory of matched ranges, applied to analog, is to say that you have,
say, a cascade of a few stages, including amplifiers and other signal
processing. The output [voltage] range from each one, must be included in
the input range of the next, and so on. And current, at least in terms of
capability (fanout) for voltage-mode circuits. So, even more generally, the
most famous case is simply the (power transfer) impedance matching theorem.

In digital hardware, we're concerned with bit values and patterns, and
devising tests that can explore a predominant fraction of the input space,
and verifying it against the intended output. Failure to do so results in
famous bugs like Pentium FDIV.

In software, we're concerned with functions that take parameters, and what
intended direct effects, and indirect side-effects, they have. Perform
bounds checking. Pass around, say, structs showing the size of your arrays.
Don't do willy-nilly pointer arithmetic! We shouldn't have to put up with
buffer overflows, this isn't 1970... and yet!

...And so for trimpots, use only the range you need, no more, no less.
Trimming out a resistor tolerance? Great, pad that sucker down to the,
whatever, +/-1% range it needs. The circuit should work fundamentally the
same no matter what any trimpot is set to; it should always function safely,
if terribly inaccurately.

Sometimes it's not possible to ensure function under those conditions, or
even safety; in that case, efforts should be redoubled to address those, and
protective measures added to detect and constrain those conditions.

Example: CRT monitors with deflection lockout: beam current is cut off if
deflection (width or height) falls well below the normal adjustable range.
Incidentally, classic TVs usually did this automatically, since horizontal
sweep generated high voltage; wide-sync monitors however had independent HV
supplies, so could dutifully burn a trench straight across the phosphor
screen if deflection were lost and no protection was in place.

Tim

OTOH most stuff is cost cutting now, and less skilled engineers are also cheaper.
Yeah, old audio mainly


NT

 

[George Herold]

On Monday, August 5, 2019 at 9:51:49 PM UTC-4, Phil Hobbs wrote:

On 8/5/19 9:19 PM, George Herold wrote:
On Monday, August 5, 2019 at 5:44:56 PM UTC-4, Winfield Hill wrote:
tabbypurr@gmail.com wrote...

A dry tome? If you don't want that you won't be
buying AoE It's a book of engineer's factual
info, not a junk soap.

Thanks, I think. Actually we're a little of each;
we get tired of the endless spreadsheets of parts
data, bench meas, writing dry-tome stuff, and Paul
breaks out in the Big Lebowski, or whatever.
Gotta jazz up the stories.

And we love name dropping. John sends us his huge
resistor fixture, we explode it, damn right we fess
up. Are we gonna post pictures, claim we made it?

OK, could ... Nah.


--
Thanks,
- Win

AoE has always had a relaxed folksy feel. Transistor man, for instance,
is in the first ed.
I'm happy that is not going away. :^)

George H.

Transistor man vs Ebers-Moll, mumble frotz. Ebers-Moll for me.

Grin, Ebers-Moll is cool now that I'm older, Transistor man
get's you going when you don't know as much.

GH

Cheers

Phil Hobbs

(Relaxed and folksy is key, though.)

 

[Phil Hobbs]

On 8/6/19 9:38 PM, George Herold wrote:

On Monday, August 5, 2019 at 9:51:49 PM UTC-4, Phil Hobbs wrote:
On 8/5/19 9:19 PM, George Herold wrote:
On Monday, August 5, 2019 at 5:44:56 PM UTC-4, Winfield Hill wrote:
tabbypurr@gmail.com wrote...

A dry tome? If you don't want that you won't be
buying AoE It's a book of engineer's factual
info, not a junk soap.

Thanks, I think. Actually we're a little of each;
we get tired of the endless spreadsheets of parts
data, bench meas, writing dry-tome stuff, and Paul
breaks out in the Big Lebowski, or whatever.
Gotta jazz up the stories.

And we love name dropping. John sends us his huge
resistor fixture, we explode it, damn right we fess
up. Are we gonna post pictures, claim we made it?

OK, could ... Nah.


--
Thanks,
- Win

AoE has always had a relaxed folksy feel. Transistor man, for instance,
is in the first ed.
I'm happy that is not going away. :^)

George H.

Transistor man vs Ebers-Moll, mumble frotz. Ebers-Moll for me.
Grin, Ebers-Moll is cool now that I'm older, Transistor man
get's you going when you don't know as much.

I suppose, in a purely elementary treatment. But in a combination
textbook + professional reference, I think it's out of place. ('Tain't
my call, of course.)

If JL ever gets round to writing "Electronics From Scratch", he could
get away with something like TM just fine.

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

 

[Phil Hobbs]

On 8/6/19 10:00 AM, John Larkin wrote:

On Tue, 6 Aug 2019 10:35:55 +0100, David Nadlinger
david@klickverbot.at> wrote:

On 05.08.19 3:58 PM, John Larkin wrote:
Caddock makes a resistor

https://www.dropbox.com/s/lxq4ujmkvo755uy/HP54006_probe.zip?dl=0

that is almost perfect.

Can you still get those anywhere, or something equivalently proven? I
had a look a while ago for some tricky probing, but couldn't find a
source (in small quantities, anyway).

— David

Sure, Caddock sells them. I got 5 samples. They are good for probing
kilovolt pulses, too.

Caddock is great to work with and makes some really interesting parts.

They don't list the MD1248 series now.

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

 

[John Miles, KE5FX]

On Monday, August 5, 2019 at 2:13:43 AM UTC-7, Tom Del Rosso wrote:
> So quit doing this. Just quit it.

Are there drawbacks other than resistor and voltage tolerances? I wouldn't
do that because the small output voltage would inherit a lot of noise/
inaccuracy/drift from the power rails, but that's the only gotcha I can
think of as long as 0.1% resistors are used.

Not a big fan of pulling DC current out of the wiper, as done in some
other implementations.

Not a big fan of trimpots in general for that matter. They are a hassle
in production, a liability in calibration, and impractical to automate.
They are one of those components whose quality can only be expected to
get worse as the volume drops. I've rambled about that before.

Don't use trimpots, use DACs.

-- john

 

[John Larkin]

On Tue, 6 Aug 2019 21:20:56 -0700 (PDT), "John Miles, KE5FX"
<jmiles@gmail.com> wrote:

On Monday, August 5, 2019 at 2:13:43 AM UTC-7, Tom Del Rosso wrote:
So quit doing this. Just quit it.

Are there drawbacks other than resistor and voltage tolerances? I wouldn't
do that because the small output voltage would inherit a lot of noise/
inaccuracy/drift from the power rails, but that's the only gotcha I can
think of as long as 0.1% resistors are used.

Not a big fan of pulling DC current out of the wiper, as done in some
other implementations.

Not a big fan of trimpots in general for that matter. They are a hassle
in production, a liability in calibration, and impractical to automate.
They are one of those components whose quality can only be expected to
get worse as the volume drops. I've rambled about that before.

Don't use trimpots, use DACs.

-- john

What's the fastest MDAC around? Fastest DPOT?


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Jan Panteltje]

On a sunny day (Tue, 6 Aug 2019 21:20:56 -0700 (PDT)) it happened "John Miles,
KE5FX" <jmiles@gmail.com> wrote in
<81b818de-91d5-4482-bed9-5e355ffca4e1@googlegroups.com>:

On Monday, August 5, 2019 at 2:13:43 AM UTC-7, Tom Del Rosso wrote:
So quit doing this. Just quit it.

Are there drawbacks other than resistor and voltage tolerances? I wouldn't
do that because the small output voltage would inherit a lot of noise/
inaccuracy/drift from the power rails, but that's the only gotcha I can
think of as long as 0.1% resistors are used.

Not a big fan of pulling DC current out of the wiper, as done in some
other implementations.

Not a big fan of trimpots in general for that matter. They are a hassle
in production, a liability in calibration, and impractical to automate.
They are one of those components whose quality can only be expected to
get worse as the volume drops. I've rambled about that before.

Don't use trimpots, use DACs.

-- john

If you have ever been through the calibration procedure of the old Tek scopes,
it involved step by step adjusting of many trimpots.
and people still sing the praises of that stuff today.
They did not even use 10 turns.

When I worked in the TV studio in the seventies THOUSANDS of trimpots were one way or the other in signal and sync chain.
Those never caused problems.

I use ten turns a lot.
Some are better than others, Bourns was OK, using cheap Chinese ones now.
But still have to see a problem with any trimpot.

DACs or digital potmeters when you need to dynamically change settings.
Or maybe in strong vibration situations.

 

[John Miles, KE5FX]

On Tuesday, August 6, 2019 at 10:21:45 PM UTC-7, John Larkin wrote:
> What's the fastest MDAC around? Fastest DPOT?

Dunno, good question. If I want a digitally-controlled attenuator for
faster signals, I'll use something labeled for that purpose. Some very
nice CMOS parts are available up to (mumble) GHz from outfits like
Peregrine. They will work down to the audio range but the power has
to be derated as you go below a MHz or so.

I don't think there are any good one-size-fits all trimpot substitutes,
at least nothing affordable. VGAs bridge the gap between RF and DC,
but most are noisier than I'd like. VGA designers don't seem to
understand the whole Friis noise equation thing.

There are all kinds of corner cases where you have to use very different
parts in order to avoid trimpots. I wouldn't try to claim otherwise...
just saying that when I can avoid using a physical pot, I'll try pretty
hard to do so.

-- john, KE5FX

 

On Wednesday, 7 August 2019 06:41:44 UTC+1, Jan Panteltje wrote:

On a sunny day (Tue, 6 Aug 2019 21:20:56 -0700 (PDT)) it happened "John Miles,

Don't use trimpots, use DACs.

If you have ever been through the calibration procedure of the old Tek scopes,
it involved step by step adjusting of many trimpots.
and people still sing the praises of that stuff today.
They did not even use 10 turns.

When I worked in the TV studio in the seventies THOUSANDS of trimpots were one way or the other in signal and sync chain.
Those never caused problems.

I use ten turns a lot.
Some are better than others, Bourns was OK, using cheap Chinese ones now.
But still have to see a problem with any trimpot.

I've seen countless that were bad. They were a major cause of failures.

NT

 

[Jan Panteltje]

On a sunny day (Wed, 7 Aug 2019 02:37:15 -0700 (PDT)) it happened
tabbypurr@gmail.com wrote in
<b8ea32f9-5645-4f12-b29c-eb3e601ab438@googlegroups.com>:

On Wednesday, 7 August 2019 06:41:44 UTC+1, Jan Panteltje wrote:
On a sunny day (Tue, 6 Aug 2019 21:20:56 -0700 (PDT)) it happened "John Miles,

Don't use trimpots, use DACs.

If you have ever been through the calibration procedure of the old Tek scopes,
it involved step by step adjusting of many trimpots.
and people still sing the praises of that stuff today.
They did not even use 10 turns.

When I worked in the TV studio in the seventies THOUSANDS of trimpots were one way or the other in signal and sync chain.
Those never caused problems.

I use ten turns a lot.
Some are better than others, Bourns was OK, using cheap Chinese ones now.
But still have to see a problem with any trimpot.

I've seen countless that were bad. They were a major cause of failures.

NT

It needed precision resistors, but this works OK:
http://panteltje.com/pub/audio_90_degrees_phase_shifter_PCB_top_view_IMG_6962.JPG

 

[Phil Hobbs]

On 8/7/19 1:58 AM, John Miles, KE5FX wrote:

On Tuesday, August 6, 2019 at 10:21:45 PM UTC-7, John Larkin wrote:
What's the fastest MDAC around? Fastest DPOT?

Dunno, good question. If I want a digitally-controlled attenuator for
faster signals, I'll use something labeled for that purpose. Some very
nice CMOS parts are available up to (mumble) GHz from outfits like
Peregrine. They will work down to the audio range but the power has
to be derated as you go below a MHz or so.

I don't think there are any good one-size-fits all trimpot substitutes,
at least nothing affordable. VGAs bridge the gap between RF and DC,
but most are noisier than I'd like. VGA designers don't seem to
understand the whole Friis noise equation thing.

There are all kinds of corner cases where you have to use very different
parts in order to avoid trimpots. I wouldn't try to claim otherwise...
just saying that when I can avoid using a physical pot, I'll try pretty
hard to do so.

-- john, KE5FX

I almost always use the voltage-divider-on-the-wiper approach for DC
adjustments.

I've never seen a current-related problem with a trimpot wiper at < 10
mA. Trimmers that are getting worn out (their lifetime is only 100
cycles or so) will get momentary opens on the wiper, and the voltage
divider on the wiper prevents that from causing havoc, e.g. blowing up
your diode laser.

Dpots and MDACs crap out below 1 MHz, making them essentially useless
for most of what I do. Their bandwidth and isolation are both strong
functions of the code selected, so from an electrical standpoint they're
mostly crap. I do use dpots for current limits, which occasionally
means programming them via a bus pirate and USB isolator because they're
hanging off some higher-voltage rail.

The nice thing about a dpot is that it's hard to casually mis-adjust and
then lie about it.

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 Tuesday, August 6, 2019 at 10:25:34 PM UTC-4, Phil Hobbs wrote:

On 8/6/19 9:38 PM, George Herold wrote:
On Monday, August 5, 2019 at 9:51:49 PM UTC-4, Phil Hobbs wrote:
On 8/5/19 9:19 PM, George Herold wrote:
On Monday, August 5, 2019 at 5:44:56 PM UTC-4, Winfield Hill wrote:
tabbypurr@gmail.com wrote...

A dry tome? If you don't want that you won't be
buying AoE It's a book of engineer's factual
info, not a junk soap.

Thanks, I think. Actually we're a little of each;
we get tired of the endless spreadsheets of parts
data, bench meas, writing dry-tome stuff, and Paul
breaks out in the Big Lebowski, or whatever.
Gotta jazz up the stories.

And we love name dropping. John sends us his huge
resistor fixture, we explode it, damn right we fess
up. Are we gonna post pictures, claim we made it?

OK, could ... Nah.


--
Thanks,
- Win

AoE has always had a relaxed folksy feel. Transistor man, for instance,
is in the first ed.
I'm happy that is not going away. :^)

George H.

Transistor man vs Ebers-Moll, mumble frotz. Ebers-Moll for me.
Grin, Ebers-Moll is cool now that I'm older, Transistor man
get's you going when you don't know as much.

I suppose, in a purely elementary treatment. But in a combination
textbook + professional reference, I think it's out of place. ('Tain't
my call, of course.)

Sure, AoE has changed much with the 3rd ed. I saw the 1st ed as a
book to teach physics students enough to get things done in the lab.

It's much more than that now.

George H.

If JL ever gets round to writing "Electronics From Scratch", he could
get away with something like TM just fine.

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

 

[Phil Hobbs]

On 8/7/19 12:20 AM, John Miles, KE5FX wrote:

On Monday, August 5, 2019 at 2:13:43 AM UTC-7, Tom Del Rosso wrote:
So quit doing this. Just quit it.

Are there drawbacks other than resistor and voltage tolerances? I wouldn't
do that because the small output voltage would inherit a lot of noise/
inaccuracy/drift from the power rails, but that's the only gotcha I can
think of as long as 0.1% resistors are used.

Not a big fan of pulling DC current out of the wiper, as done in some
other implementations.

Not a big fan of trimpots in general for that matter. They are a hassle
in production, a liability in calibration, and impractical to automate.
They are one of those components whose quality can only be expected to
get worse as the volume drops. I've rambled about that before.

Don't use trimpots, use DACs.

-- john

Good luck at 2 GHz. Not every trimpot is used for DC offsets, by a lot.

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

 

[John Larkin]

On Tue, 6 Aug 2019 22:25:28 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 8/6/19 9:38 PM, George Herold wrote:
On Monday, August 5, 2019 at 9:51:49 PM UTC-4, Phil Hobbs wrote:
On 8/5/19 9:19 PM, George Herold wrote:
On Monday, August 5, 2019 at 5:44:56 PM UTC-4, Winfield Hill wrote:
tabbypurr@gmail.com wrote...

A dry tome? If you don't want that you won't be
buying AoE It's a book of engineer's factual
info, not a junk soap.

Thanks, I think. Actually we're a little of each;
we get tired of the endless spreadsheets of parts
data, bench meas, writing dry-tome stuff, and Paul
breaks out in the Big Lebowski, or whatever.
Gotta jazz up the stories.

And we love name dropping. John sends us his huge
resistor fixture, we explode it, damn right we fess
up. Are we gonna post pictures, claim we made it?

OK, could ... Nah.


--
Thanks,
- Win

AoE has always had a relaxed folksy feel. Transistor man, for instance,
is in the first ed.
I'm happy that is not going away. :^)

George H.

Transistor man vs Ebers-Moll, mumble frotz. Ebers-Moll for me.
Grin, Ebers-Moll is cool now that I'm older, Transistor man
get's you going when you don't know as much.

I suppose, in a purely elementary treatment. But in a combination
textbook + professional reference, I think it's out of place. ('Tain't
my call, of course.)

If JL ever gets round to writing "Electronics From Scratch", he could
get away with something like TM just fine.

Cheers

Phil Hobbs

I never met Transistor Man. But I can (and do) explain transistors
pretty well in about a half hour at a whiteboard with just a few
concepts and a few numbers. Ebers-Moll is not the whole story at all.

Last few instruments that I designed, I don't think I've used a single
bipolar transistor. They are going the way of tubes.

I still like relays!


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Wed, 07 Aug 2019 05:41:35 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Tue, 6 Aug 2019 21:20:56 -0700 (PDT)) it happened "John Miles,
KE5FX" <jmiles@gmail.com> wrote in
81b818de-91d5-4482-bed9-5e355ffca4e1@googlegroups.com>:

On Monday, August 5, 2019 at 2:13:43 AM UTC-7, Tom Del Rosso wrote:
So quit doing this. Just quit it.

Are there drawbacks other than resistor and voltage tolerances? I wouldn't
do that because the small output voltage would inherit a lot of noise/
inaccuracy/drift from the power rails, but that's the only gotcha I can
think of as long as 0.1% resistors are used.

Not a big fan of pulling DC current out of the wiper, as done in some
other implementations.

Not a big fan of trimpots in general for that matter. They are a hassle
in production, a liability in calibration, and impractical to automate.
They are one of those components whose quality can only be expected to
get worse as the volume drops. I've rambled about that before.

Don't use trimpots, use DACs.

-- john

If you have ever been through the calibration procedure of the old Tek scopes,
it involved step by step adjusting of many trimpots.
and people still sing the praises of that stuff today.
They did not even use 10 turns.

When I worked in the TV studio in the seventies THOUSANDS of trimpots were one way or the other in signal and sync chain.
Those never caused problems.

I use ten turns a lot.
Some are better than others, Bourns was OK, using cheap Chinese ones now.
But still have to see a problem with any trimpot.

DACs or digital potmeters when you need to dynamically change settings.
Or maybe in strong vibration situations.

Most of our products are closed-box calibrated, with a cal table saved
in flash inside. Testing is automated.

I once designed a PWM DAC circuit that used two trimpots to cal offset
and gain. The interaction was so diabolical that nobody could cal it,
not even the brilliant designer.

But to tweak the gain of a small low-noise DC-x00 MHz amp to 1%, a
trimpot is perfect.

This is all analog, and has one pot:

http://www.highlandtechnology.com/DSS/J730DS.shtml

The analog output is trimmed to 1 volt/mW at 850 nm. The photodiode
gains are all over the place.

I don't know how Phil deals with parts whose specs regularly vary over
a 3:1 range, and jump around every production batch, and are only
vaguely suggested on data sheets.

I made a personal deal with one laser supplier that they would alert
me to any major changes. Of course they didn't. I think they buy their
lots of pds and lasers in dark alleys in Shanghai. Their fiber
connector housings must be made from melted-down beer cans. An
unmate/mate cycle is good for 3 dB gain change.







--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Tue, 6 Aug 2019 22:58:49 -0700 (PDT), "John Miles, KE5FX"
<jmiles@gmail.com> wrote:

On Tuesday, August 6, 2019 at 10:21:45 PM UTC-7, John Larkin wrote:
What's the fastest MDAC around? Fastest DPOT?

Dunno, good question. If I want a digitally-controlled attenuator for
faster signals, I'll use something labeled for that purpose. Some very
nice CMOS parts are available up to (mumble) GHz from outfits like
Peregrine. They will work down to the audio range but the power has
to be derated as you go below a MHz or so.

I don't think there are any good one-size-fits all trimpot substitutes,
at least nothing affordable. VGAs bridge the gap between RF and DC,
but most are noisier than I'd like. VGA designers don't seem to
understand the whole Friis noise equation thing.

There are all kinds of corner cases where you have to use very different
parts in order to avoid trimpots. I wouldn't try to claim otherwise...
just saying that when I can avoid using a physical pot, I'll try pretty
hard to do so.

-- john, KE5FX

A screwdriver is a pretty good human interface.

There are RF digital step attenuators, and some (possibly) work down
to DC, but they work in -shock- steps. And they need a digital input
from somewhere. I've faked a digital attenuator with a half-pitch
dipswitch and a few resistors, at the 4-bit level, but it's only 16
steps.

I don't see anything wrong with using a trimpot to tweak, say, a
photodiode gain in a 300 MHz circuit, from a 2:1 variation down to 1%.

To me, there's nothing to avoid. Pots are cheap, easy, small, and
reliable.




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Miles, KE5FX]

On Wednesday, August 7, 2019 at 11:00:37 AM UTC-7, Phil Hobbs wrote:

Murata PVA2. JL put me on to them long ago, but unfortunately they've
been discontinued. Bourns TC33 look pretty similar.

Thanks, I'll give those a try.

-- john

 

[Phil Hobbs]

On 8/7/19 1:53 PM, John Miles, KE5FX wrote:

On Wednesday, August 7, 2019 at 6:11:19 AM UTC-7, Phil Hobbs wrote:
Good luck at 2 GHz. Not every trimpot is used for DC offsets, by a lot.

What are some good trimpots for use at 2 GHz?

Seriously, I could use some of those for the variable-gain CFB amps I like
to keep around the bench.

-- john, KE5FX

Murata PVA2. JL put me on to them long ago, but unfortunately they've
been discontinued. Bourns TC33 look pretty similar.

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

 

[John Miles, KE5FX]

On Wednesday, August 7, 2019 at 2:37:19 AM UTC-7, tabb...@gmail.com wrote:
> I've seen countless that were bad. They were a major cause of failures.

I've seen plenty of bad ones, too, in older gear. I don't see many in
newer equipment at all, whether good or bad.

I've only used one trimpot in a commercial product. Brand new parts
from Bourns, straight outta Thief River Falls and priced accordingly.
A bunch of them failed after soldering. I took the hint.

-- john, KE5FX

 

[Phil Hobbs]

On 8/7/19 11:09 AM, John Larkin wrote:

On Wed, 07 Aug 2019 05:41:35 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Tue, 6 Aug 2019 21:20:56 -0700 (PDT)) it happened
"John Miles, KE5FX" <jmiles@gmail.com> wrote in
81b818de-91d5-4482-bed9-5e355ffca4e1@googlegroups.com>:

On Monday, August 5, 2019 at 2:13:43 AM UTC-7, Tom Del Rosso
wrote:
So quit doing this. Just quit it.

Are there drawbacks other than resistor and voltage tolerances? I
wouldn't do that because the small output voltage would inherit a
lot of noise/ inaccuracy/drift from the power rails, but that's
the only gotcha I can think of as long as 0.1% resistors are
used.

Not a big fan of pulling DC current out of the wiper, as done in
some other implementations.

Not a big fan of trimpots in general for that matter. They are
a hassle in production, a liability in calibration, and
impractical to automate. They are one of those components whose
quality can only be expected to get worse as the volume drops.
I've rambled about that before.

Don't use trimpots, use DACs.

-- john

If you have ever been through the calibration procedure of the old
Tek scopes, it involved step by step adjusting of many trimpots.
and people still sing the praises of that stuff today. They did
not even use 10 turns.

When I worked in the TV studio in the seventies THOUSANDS of
trimpots were one way or the other in signal and sync chain. Those
never caused problems.

I use ten turns a lot. Some are better than others, Bourns was OK,
using cheap Chinese ones now. But still have to see a problem with
any trimpot.

DACs or digital potmeters when you need to dynamically change
settings. Or maybe in strong vibration situations.

Most of our products are closed-box calibrated, with a cal table
saved in flash inside. Testing is automated.

I once designed a PWM DAC circuit that used two trimpots to cal
offset and gain. The interaction was so diabolical that nobody could
cal it, not even the brilliant designer.

But to tweak the gain of a small low-noise DC-x00 MHz amp to 1%, a
trimpot is perfect.

This is all analog, and has one pot:

http://www.highlandtechnology.com/DSS/J730DS.shtml

The analog output is trimmed to 1 volt/mW at 850 nm. The photodiode
gains are all over the place.

I don't know how Phil deals with parts whose specs regularly vary
over a 3:1 range, and jump around every production batch, and are
only vaguely suggested on data sheets.

Photodiodes are generally pretty good, assuming you control the etalon
fringes. The responsivity of BPW34s varies only a few percent
unit-to-unit unless you're out in the tails of the response.

Diode lasers, now that's another issue. It's not uncommon for the
threshold current to vary +-40%, and the major damage mechanism is facet
overheating from too-high output power. The best way to handle that is
a constant-power feedback loop and a current limit set by a dpot.

If I needed to modulate it rapidly with some known AM depth and/or FM
deviation, I'd probably use a built-up R-2R attenuator and program it
with hot tweezers.

I made a personal deal with one laser supplier that they would alert
me to any major changes. Of course they didn't. I think they buy
their lots of pds and lasers in dark alleys in Shanghai.

Everybody seems to make their lasers in pots and pans, except for the
very highest-volume ones. I'm using a $700 Nichia 488-nm diode today,
which has a slow-axis beam divergence of 7 to 13 degrees, typical 10,
and an aiming tolerance of +-5 degrees! Maintaining a consistent
collimated beam diameter requires either a selection of different
collimating lenses or some sort of varifocal or zoom arrangement. Blech.

Their fiber connector housings must be made from melted-down beer
cans. An unmate/mate cycle is good for 3 dB gain change.

1 dB is more usual, at least with Kyocera ferrule inserts.

As my Dad used to say when I complained about some chore, "If it were
easy, I'd have done it myself."

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