thermistors in parallel...

[John Larkin]

Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Or add a secondary ADC mux, I guess.

 

[Lasse Langwadt Christensen]

lørdag den 6. maj 2023 kl. 04.10.39 UTC+2 skrev John Larkin:

Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Or add a secondary ADC mux, I guess.

if you have enough pins on an FPGA, make 8 ADCs?

or use a cheap 8 channels SPI ADC

 

[whit3rd]

On Friday, May 5, 2023 at 7:10:39 PM UTC-7, John Larkin wrote:

Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Why bother with an ADC? If you know the threshold for speedup, just a comparator
is sufficient; dual transistor per thermistor, and a single DAC output can do the
adjustment, while collectors-connected-together is your OR-invert function.

Extra functionality comes if you dither the threshold for the various sections,
or separate the multiple logic inputs instead of just taking the OR combination.

 

[Anthony William Sloman]

On Saturday, May 6, 2023 at 12:10:39 PM UTC+10, John Larkin wrote:

Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear.

But piece-wise linear.

https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation

> If I parallel four thermistors into each ADC channel, the hottest of the four will dominate. That should work well enough.

The hottest thermistor will have more influence than the cooler parts - if they are interchangeable thermistors - but \"dominate\" isn\'t really the right word.

Some sort of diode thing might select the hottest thermistor, but paralleling is easy.

Or add a secondary ADC mux, I guess.

Better.

--
Bill Sloman, Sydney

 

[Phil Allison]

Anthony William Sloman wrote:
----------------------------------------------------
John Larkin wrote:

If I parallel four thermistors into each ADC channel, the hottest of the four will dominate.
That should work well enough.

** For mugs like JL maybe.

The hottest thermistor will have more influence than the cooler parts -
if they are interchangeable thermistors - but \"dominate\" isn\'t really the right word.

** It a poor solution, general temp rise up in the box would trigger the fan control unnecessarily while no individual NTC Th is actually hot.

Some sort of diode thing might select the hottest thermistor, but paralleling is easy.

Or add a secondary ADC mux, I guess.

** I believe strongly in the KISS principle = fewest components, fail safe design and no non-essential high tech.

IMO A simple NO temp switch, one on each module, wired in parallel would do that.

Shame that gives nothing for JL to boast or bullshit about.


..... Phil

 

[John Larkin]

On Fri, 5 May 2023 20:54:14 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 04.10.39 UTC+2 skrev John Larkin:
Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Or add a secondary ADC mux, I guess.

if you have enough pins on an FPGA, make 8 ADCs?

or use a cheap 8 channels SPI ADC

The FPGA LVDS inputs make decent RRI comparators, at the cost of two
pins each.

It\'s nice to be able to read real temperatures, but the the fan
control algorithm is just a bang-bang thing and the only thing it
needs to know is one bit, is this board too hot? So 8 thermistors into
LVDS or even single-ended FPGA gpio\'s would actually work. Or 8
comparators.

Paralleling thermistors does an interesting nonlinear (aka complex)
weighted average of my 4 or maybe 8 temperatures.

25c 15k
50c 5k
100c 1k

is pretty radical.

We have an 8-channel BIST mux and ADC on every board in this family;
they share a common \"template\" starting-point schematic and layout. I
guess I could add another analog mux ahead of one of the unused BIST
inputs and have the FPGA guy work that into his scan logic. The FPGA
does the 8-channel analog scan and stashes the results in registers;
he\'d just have to add 8 more sub-scan thermistor registers. Share the
pullup resistor. Easy for me to say.

It is appealing to know all 8 temperatures. Each is a cluster of
wirewound resistors on a section of a PCB. Without enough air flow,
they will eventually toast the FR4.

 

[John Larkin]

On Fri, 5 May 2023 21:53:29 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Friday, May 5, 2023 at 7:10:39?PM UTC-7, John Larkin wrote:
Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Why bother with an ADC?

The board already has a multiplexed BIST ADC with three available
inputs.

If you know the threshold for speedup, just a comparator
is sufficient; dual transistor per thermistor, and a single DAC output can do the
adjustment, while collectors-connected-together is your OR-invert function.

Yes, but it would be nice if, in development, we could know the actual
numbers, if that was basically free.

Extra functionality comes if you dither the threshold for the various sections,
or separate the multiple logic inputs instead of just taking the OR combination.

Eight separate measurements is appealing. Paralleling thermistors is
interesting but maybe too tricky. The idea might be interesting in
some other situation.

 

[John Larkin]

On Sat, 6 May 2023 02:01:17 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

Anthony William Sloman wrote:
----------------------------------------------------
John Larkin wrote:

If I parallel four thermistors into each ADC channel, the hottest of the four will dominate.
That should work well enough.

** For mugs like JL maybe.

The wirewpund resistors can run at 250c. The hazard is melting solder
joints and, longterm, toasting the FR4. I\'m sure you have seen
sections of PCBs under resistors that have turned brown and got
crispy. That\'s what I need to avoid, and there is no precise criterion
for the time-temperature profile that\'s safe.

These will be clusters of the stand-up rectangular ceramic 5-watt
wirewound resistors. I want to run them at 7 watts, which should be
fine with lots of air flow.

The hottest thermistor will have more influence than the cooler parts -
if they are interchangeable thermistors - but \"dominate\" isn\'t really the right word.

** It a poor solution, general temp rise up in the box would trigger the fan control unnecessarily while no individual NTC Th is actually hot.

The goal of the fan control is to limit the temp of the hottest part
in the box, at minimum fan noise when nothing is too hot. What\'s wrong
with that?

Some sort of diode thing might select the hottest thermistor, but paralleling is easy.

Or add a secondary ADC mux, I guess.

** I believe strongly in the KISS principle = fewest components, fail safe design and no non-essential high tech.

Absolutely. Sometimes it takes a lot of engineering to make things
simple.

IMO A simple NO temp switch, one on each module, wired in parallel would do that.

I\'d love to have a surface-mount 0603-size temperature switch. I could
use it in lots of places. A thermistor comes close.

Shame that gives nothing for JL to boast or bullshit about.

I find it helpful to discuss designs; I get ideas and it helps me
think. And this discussion group is sci.electronics.design.

Try designing electronics. It\'s fun.

.... Phil

 

[Anthony William Sloman]

On Saturday, May 6, 2023 at 11:55:16 PM UTC+10, John Larkin wrote:

On Fri, 5 May 2023 20:54:14 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 04.10.39 UTC+2 skrev John Larkin:
Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Or add a secondary ADC mux, I guess.

if you have enough pins on an FPGA, make 8 ADCs?

or use a cheap 8 channels SPI ADC

The FPGA LVDS inputs make decent RRI comparators, at the cost of two
pins each.

It\'s nice to be able to read real temperatures, but the the fan
control algorithm is just a bang-bang thing and the only thing it
needs to know is one bit, is this board too hot? So 8 thermistors into
LVDS or even single-ended FPGA gpio\'s would actually work. Or 8
comparators.

For a pretty simple minded idea of \"working\".

Paralleling thermistors does an interesting nonlinear (aka complex)
weighted average of my 4 or maybe 8 temperatures.

25c 15k
50c 5k
100c 1k

is pretty radical.

https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation

is a perfectly explicit relationship. If you are using thermistors, you ought to know about it, and if you are telling other people how to use them, you ought to be telling them about it. \"Pretty radical\" is a lazy cop-out.

We have an 8-channel BIST mux and ADC on every board in this family;
they share a common \"template\" starting-point schematic and layout. I
guess I could add another analog mux ahead of one of the unused BIST
inputs and have the FPGA guy work that into his scan logic. The FPGA
does the 8-channel analog scan and stashes the results in registers;
he\'d just have to add 8 more sub-scan thermistor registers. Share the
pullup resistor. Easy for me to say.

It is appealing to know all 8 temperatures.

It even makes sense. A single over hot resistor can cook it\'s segment of FR4 and that segment stays cooked forever.

> Each is a cluster of wirewound resistors on a section of a PCB. Without enough air flow, they will eventually toast the FR4.

Fairly obviously.

--
Bill Sloman, Sydney

 

[chuck]

On 06/05/2023 5:01 a.m., Phil Allison wrote:

Anthony William Sloman wrote:
----------------------------------------------------
John Larkin wrote:

If I parallel four thermistors into each ADC channel, the hottest of the four will dominate.
That should work well enough.

** For mugs like JL maybe.

The hottest thermistor will have more influence than the cooler parts -
if they are interchangeable thermistors - but \"dominate\" isn\'t really the right word.

** It a poor solution, general temp rise up in the box would trigger the fan control unnecessarily while no individual NTC Th is actually hot.

Some sort of diode thing might select the hottest thermistor, but paralleling is easy.

Or add a secondary ADC mux, I guess.

** I believe strongly in the KISS principle = fewest components, fail safe design and no non-essential high tech.

IMO A simple NO temp switch, one on each module, wired in parallel would do that.

Shame that gives nothing for JL to boast or bullshit about.


.... Phil

Forced Air Cooling power is a trade-off with dust collection and
nuisance noise. Proportional Control is ideal but accurate hotspot
sensing is required. Active cooling is critical to air velocity over the
surface and not volumetric air flow. Thus turbulent air flow enhances
cooling rate more than laminar flow well above the hotspot.

Thus to solved this design problem, you need a thermal sensor for each
hotspot, which could be an LM35 or a diode or a thermistor epoxied to
each hotpot and routed to a shared single point with an LDO controlled
fan speed by shunting the ADJ voltage-controlled. Diode OR linear
control is one method or current pumped by thermal sensor.
Fans are Vdc speed control using Hall sensor commutation so PWM causes
problems but LDO control works fine.

I discovered using smoke & thermocouple tests how using air velocity
design optimized cooling. So I made an insulating material plenum over
the PCB to reduce the aperture to increases the air velocity for all
flow and create eddy current effects over the hotspots to significantly
lower case temperature. I used this on a 180W 1U high 19\" rack design I
did for Lucent/Avaya.

Tony Stewart EE since 1975.

 

[Anthony William Sloman]

On Sunday, May 7, 2023 at 12:14:48 AM UTC+10, John Larkin wrote:

On Sat, 6 May 2023 02:01:17 -0700 (PDT), Phil Allison <palli...@gmail.com> wrote:
Anthony William Sloman wrote:
John Larkin wrote:

Shame that gives nothing for JL to boast or bullshit about.

I find it helpful to discuss designs; I get ideas and it helps me think. And this discussion group is sci.electronics.design.

But there is an obligation to do at least some homework before you expose your rather low level thinking here.

Not knowing about the Steinhart-Hart relationship suggests that you thinking isn\'t up to much.

Getting that kind of idea might be helpful.

> Try designing electronics. It\'s fun.

John Larkin\'s favourite put-down. The joke is that he seems to evolve his electronics, rather than design his circuits.

--
Bill Sloman, Sydeny

 

[Liz Tuddenham]

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:

Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Use 1N4148 (or equivalent) diodes in parallel and a constant feed of
1mA; the hottest one is the only one you are interested in and that one
will determine the voltage drop across the whole circuit. I did exactly
this with the thermal protection of a large amplifier where each output
transistor had its own heatsink.


--
~ Liz Tuddenham ~
(Remove the \".invalid\"s and add \".co.uk\" to reply)
www.poppyrecords.co.uk

 

[John Larkin]

On Sat, 6 May 2023 10:29:25 -0400, chuck <donnyduck@gmail.com> wrote:

On 06/05/2023 5:01 a.m., Phil Allison wrote:
Anthony William Sloman wrote:
----------------------------------------------------
John Larkin wrote:

If I parallel four thermistors into each ADC channel, the hottest of the four will dominate.
That should work well enough.

** For mugs like JL maybe.

The hottest thermistor will have more influence than the cooler parts -
if they are interchangeable thermistors - but \"dominate\" isn\'t really the right word.

** It a poor solution, general temp rise up in the box would trigger the fan control unnecessarily while no individual NTC Th is actually hot.

Some sort of diode thing might select the hottest thermistor, but paralleling is easy.

Or add a secondary ADC mux, I guess.

** I believe strongly in the KISS principle = fewest components, fail safe design and no non-essential high tech.

IMO A simple NO temp switch, one on each module, wired in parallel would do that.

Shame that gives nothing for JL to boast or bullshit about.


.... Phil

Forced Air Cooling power is a trade-off with dust collection and
nuisance noise.

Our boxes are mostly used in aerospace test, in clean control rooms.

A bunch of our gear runs in the world\'s biggest clean room. We don\'t
use air filters and RMAs are not dusty. We\'re lucky in that respect.

Proportional Control is ideal but accurate hotspot
sensing is required. Active cooling is critical to air velocity over the
surface and not volumetric air flow. Thus turbulent air flow enhances
cooling rate more than laminar flow well above the hotspot.

Thus to solved this design problem, you need a thermal sensor for each
hotspot, which could be an LM35 or a diode or a thermistor epoxied to
each hotpot and routed to a shared single point with an LDO controlled
fan speed by shunting the ADJ voltage-controlled. Diode OR linear
control is one method or current pumped by thermal sensor.
Fans are Vdc speed control using Hall sensor commutation so PWM causes
problems but LDO control works fine.

My intent is precisely that, namely to sense the temperature of eight
hot spots on this particular board and adjust the enclosure fans to
limit the worst temperature. The thermistors will be 0603
surface-mount parts.

There was some debate about the fan control algorithm, given the
variety of plugin boards. Some of my guys wanted to do classic PID
control which, predictably, turned out to be a nightmare. We\'re back
to the simple up-down counter approach.

The two 48-volt fans each have a PWM input pin for speed control. At
100%, they are real screamers.

I discovered using smoke & thermocouple tests how using air velocity
design optimized cooling. So I made an insulating material plenum over
the PCB to reduce the aperture to increases the air velocity for all
flow and create eddy current effects over the hotspots to significantly
lower case temperature. I used this on a 180W 1U high 19\" rack design I
did for Lucent/Avaya.

We will have two fans on the front panel. One problem is that that [1]
makes two fire hoses of air onto some boards and leaves some with no
air. So we\'ll have a flat perforated baffle to redistribute the air to
the eight modules.

https://www.dropbox.com/s/gr57bhafemypi63/P940_box_9.jpg?raw=1

Tony Stewart EE since 1975.

[1] It is possible to construct a sentence in which the word \'that\'
appears six times in sucession. Most spell checkers flag just two.

 

[Lasse Langwadt Christensen]

lørdag den 6. maj 2023 kl. 15.55.16 UTC+2 skrev John Larkin:

On Fri, 5 May 2023 20:54:14 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 04.10.39 UTC+2 skrev John Larkin:
Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Or add a secondary ADC mux, I guess.

if you have enough pins on an FPGA, make 8 ADCs?

or use a cheap 8 channels SPI ADC

The FPGA LVDS inputs make decent RRI comparators, at the cost of two
pins each.

It\'s nice to be able to read real temperatures, but the the fan
control algorithm is just a bang-bang thing and the only thing it
needs to know is one bit, is this board too hot? So 8 thermistors into
LVDS or even single-ended FPGA gpio\'s would actually work. Or 8
comparators.

a ninth output to make a ramp
or use IOs and capacitors, measure the recharge time
that\'s basically how the original PC joystick port measured ~100k potmeters (with a 558)
https://www.gamoover.net/redacteur/Iro/Articles/InterfacesPotards/HTML/pc_joystick_fichiers/FIG4.GIF

 

[John Larkin]

On Sat, 6 May 2023 16:25:19 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:

Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Use 1N4148 (or equivalent) diodes in parallel and a constant feed of
1mA; the hottest one is the only one you are interested in and that one
will determine the voltage drop across the whole circuit. I did exactly
this with the thermal protection of a large amplifier where each output
transistor had its own heatsink.

That would behave similar to the parallel thermistors. The diode drops
would be about -2 mV per degree c and the current per diode would be
about 60 mV per decade. The result is a weighted temperature average.

Thanks for the suggestion. We have lots of single and dual
surface-mount diodes. Dual would double the signal.

 

[John Larkin]

On Sat, 6 May 2023 08:53:17 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 15.55.16 UTC+2 skrev John Larkin:
On Fri, 5 May 2023 20:54:14 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 04.10.39 UTC+2 skrev John Larkin:
Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Or add a secondary ADC mux, I guess.

if you have enough pins on an FPGA, make 8 ADCs?

or use a cheap 8 channels SPI ADC

The FPGA LVDS inputs make decent RRI comparators, at the cost of two
pins each.

It\'s nice to be able to read real temperatures, but the the fan
control algorithm is just a bang-bang thing and the only thing it
needs to know is one bit, is this board too hot? So 8 thermistors into
LVDS or even single-ended FPGA gpio\'s would actually work. Or 8
comparators.


a ninth output to make a ramp
or use IOs and capacitors, measure the recharge time
that\'s basically how the original PC joystick port measured ~100k potmeters (with a 558)
https://www.gamoover.net/redacteur/Iro/Articles/InterfacesPotards/HTML/pc_joystick_fichiers/FIG4.GIF

That would work. I designed some electric meters that used
single-slope a/d conversion. Single slope has perfect differential
linearity so is ideal for dithering and signal averaging, which an
electric meter does a lot of. The nuclear spectroscopy folks used to
do that, maybe still do.

But I do have a couple of spare ADC channels.

 

[Fred Bloggs]

On Friday, May 5, 2023 at 10:10:39 PM UTC-4, John Larkin wrote:

Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

I don\'t see how you\'re getting any airflow over the middle boards.

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Or add a secondary ADC mux, I guess.

 

[Lasse Langwadt Christensen]

lørdag den 6. maj 2023 kl. 17.56.04 UTC+2 skrev John Larkin:

On Sat, 6 May 2023 16:25:19 +0100, l...@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

John Larkin <jla...@highlandSNIPMEtechnology.com> wrote:

Imagine a chassis with up to eight plugin boards. The chassis has a
pair of PWM controlled fans.

https://www.dropbox.com/s/8ubv5if7cbnsjzn/P940-8_front.jpg?raw=1

Each board will have a thermistor to sense board temperature. If any
board thinks it\'s too hot, it asks the fan controller to jog the fan
speeds up a notch. If nobody requests more air, the controller jogs
them down. That will work.

But I have a board with eight channels that can each get hot. There\'s
no place on the board that represents the worst-channel temperature. I
only have two available ADC channels so I can\'t use eight thermistors.

But thermistors are radically nonlinear. If I parallel four
thermistors into each ADC channel, the hottest of the four will
dominate. That should work well enough.

Some sort of diode thing might select the hottest thermistor, but
paralleling is easy.

Use 1N4148 (or equivalent) diodes in parallel and a constant feed of
1mA; the hottest one is the only one you are interested in and that one
will determine the voltage drop across the whole circuit. I did exactly
this with the thermal protection of a large amplifier where each output
transistor had its own heatsink.
That would behave similar to the parallel thermistors. The diode drops
would be about -2 mV per degree c and the current per diode would be
about 60 mV per decade. The result is a weighted temperature average.

Thanks for the suggestion. We have lots of single and dual
surface-mount diodes. Dual would double the signal.

maybe scan by grounding each diode in turn with an FPGA output?

Version 4
SHEET 1 920 820
WIRE 192 16 32 16
WIRE 464 16 272 16
WIRE 640 16 464 16
WIRE 848 16 640 16
WIRE 32 32 32 16
WIRE 272 48 272 16
WIRE 464 48 464 16
WIRE 640 48 640 16
WIRE 272 192 272 112
WIRE 464 192 464 112
WIRE 640 192 640 112
WIRE 272 304 272 272
WIRE 464 304 464 272
WIRE 464 304 272 304
WIRE 640 304 640 272
WIRE 640 304 464 304
FLAG 32 112 0
FLAG 272 304 0
FLAG 848 16 Vout
SYMBOL diode 256 48 R0
WINDOW 123 22 83 Left 2
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMATTR Value2 temp={T}
SYMBOL res 176 32 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R1
SYMATTR Value 50k
SYMBOL voltage 32 16 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3
SYMBOL diode 448 48 R0
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL voltage 272 176 R0
WINDOW 3 -5 161 VRight 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(3 0 0 1u 1u 1m 3m)
SYMBOL diode 624 48 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL voltage 464 176 R0
WINDOW 3 -5 161 VRight 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value PULSE(3 0 1m 1u 1u 1m 3m)
SYMBOL voltage 640 176 R0
WINDOW 3 -5 161 VRight 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value PULSE(3 0 2m 1u 1u 1m 3m)
TEXT -2 296 Left 2 !.tran 0 10m 1u
TEXT 208 -200 Left 2 !.step param T 25 100 5

 

[whit3rd]

On Saturday, May 6, 2023 at 7:14:48 AM UTC-7, John Larkin wrote:

The wirewpund resistors can run at 250c. The hazard is melting solder
joints and, longterm, toasting the FR4. I\'m sure you have seen
sections of PCBs under resistors that have turned brown and got
crispy. That\'s what I need to avoid, and there is no precise criterion
for the time-temperature profile that\'s safe.

These will be clusters of the stand-up rectangular ceramic 5-watt
wirewound resistors. I want to run them at 7 watts, which should be
fine with lots of air flow.

The old-school approach was standoff posts to mount hot parts; if you want
it to be printed-wiring, though, and are getting to softening of the solder joints,
maybe perforating the printed wiring board, and having airflow THROUGH the board
rather than parallel to it, is the answer.

Cool air entering from the wiring side, hot parts on the component side, will let
the solder joints chill while the resistors do the work.

 

[John Walliker]

On Saturday, 6 May 2023 at 19:00:53 UTC+1, whit3rd wrote:

On Saturday, May 6, 2023 at 7:14:48 AM UTC-7, John Larkin wrote:

The wirewpund resistors can run at 250c. The hazard is melting solder
joints and, longterm, toasting the FR4. I\'m sure you have seen
sections of PCBs under resistors that have turned brown and got
crispy. That\'s what I need to avoid, and there is no precise criterion
for the time-temperature profile that\'s safe.

These will be clusters of the stand-up rectangular ceramic 5-watt
wirewound resistors. I want to run them at 7 watts, which should be
fine with lots of air flow.
The old-school approach was standoff posts to mount hot parts; if you want
it to be printed-wiring, though, and are getting to softening of the solder joints,
maybe perforating the printed wiring board, and having airflow THROUGH the board
rather than parallel to it, is the answer.

Cool air entering from the wiring side, hot parts on the component side, will let
the solder join

Lead-free solder will help as well.
John