thermistors in parallel...

[John Larkin]

On Sat, 6 May 2023 10:58:57 -0700 (PDT), whit3rd <whit3rd@gmail.com>
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;

We have a pick-and-place machine for surface-mount parts and a
selective solder machine for thru-hole; zero hand wiring. We can have
surface mounts on the bottom too, if they are not too close to leaded
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.

Only parallel in our case. Cutting holes in the board reduces heat
spreading in the copper planes anyhow.

The resistors do have a sort of tunnel on the bottom that might help
cool the pads a bit. This style:

https://www.mouser.com/ProductDetail/Ohmite/TWW5J3R9E?qs=EnuEBe%2FYa%2Fo765s1eHMwlQ%3D%3D

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.

I\'ll have air flow, up to 200 LFPM, on both sides of the board. Big
copper pours on the bottom side will help cool the resistor leads.

 

[John Larkin]

On Sat, 6 May 2023 10:22:38 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

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

LT Spice aborts that, \"unknown syntax\" error.

 

[John Larkin]

On Sat, 6 May 2023 09:10:47 -0700 (PDT), Fred Bloggs
<bloggs.fredbloggs.fred@gmail.com> wrote:

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.

A perforated plastic baffle will drop in between the fans and the
boards. It will balance the air flow pretty well.

 

[John Larkin]

On Sat, 6 May 2023 12:41:29 -0700 (PDT), John Walliker
<jrwalliker@gmail.com> wrote:

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

We\'re all lead-free, but we couldn\'t let the PCB get anywhere close to
solder melting temps.

 

[Liz Tuddenham]

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:

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.

You shouldn\'t get an average across the parallel circuit, the voltage
will be that of the hottest (lowest voltage) diode which will take the
current away from all the other diodes. (That\'s why you restrict the
current to around 1mA to prevent self-heating effects.).

I expect they will work out a lot cheaper than thermistors and it is
easy to find alternative types in the event of supply difficulties.
Future availability of equivalent parts is almost guaranteed.

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

You could use even more than that if you wanted a really big signal or
an average of the temperature over the hot area. I made a solar panel
pump controller with 7 diodes per side in a differential comparator
bridge, it could easily respond to a 1degree C temperature difference.


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

 

[Lasse Langwadt Christensen]

lørdag den 6. maj 2023 kl. 21.52.56 UTC+2 skrev John Larkin:

On Sat, 6 May 2023 10:22:38 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

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

LT Spice aborts that, \"unknown syntax\" error.

weird, I just tried it and it works here

https://imgur.com/a/ZKWEVur

 

[John Larkin]

On Sat, 6 May 2023 13:18:18 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 21.52.56 UTC+2 skrev John Larkin:
On Sat, 6 May 2023 10:22:38 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

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

LT Spice aborts that, \"unknown syntax\" error.

weird, I just tried it and it works here

https://imgur.com/a/ZKWEVur

Found it. There were three strange \"blank\" lines at the end of the
..asc that it didn\'t like.

That\'s sneaky, using the diodes as their own mux.

 

[John Larkin]

On Sat, 6 May 2023 20:57:22 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:

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.

You shouldn\'t get an average across the parallel circuit, the voltage
will be that of the hottest (lowest voltage) diode which will take the
current away from all the other diodes. (That\'s why you restrict the
current to around 1mA to prevent self-heating effects.).

Mostly, but the diode conduction curve isn\'t a brick wall. A 10 deg c
difference will be about 20 mv, and that\'s not enough for the hot
diode to steal all the current from the other 3 or 7 diodes.

I expect they will work out a lot cheaper than thermistors and it is
easy to find alternative types in the event of supply difficulties.
Future availability of equivalent parts is almost guaranteed.

I don\'t know that I could trust the diode temp curve. The thermistor
is guaranteed and costs 4 cents.

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

You could use even more than that if you wanted a really big signal or
an average of the temperature over the hot area. I made a solar panel
pump controller with 7 diodes per side in a differential comparator
bridge, it could easily respond to a 1degree C temperature difference.

 

[DJ Delorie]

John Larkin <jlarkin@highlandSNIPMEtechnology.com> writes:

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.

Three cool boards will stop the hottest from getting its fan on at the
same temperature as four hot boards.

Could diodes be used with the thermisters, so that only the hottest one
counts?

 

[Lasse Langwadt Christensen]

lørdag den 6. maj 2023 kl. 22.55.40 UTC+2 skrev John Larkin:

On Sat, 6 May 2023 13:18:18 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 21.52.56 UTC+2 skrev John Larkin:
On Sat, 6 May 2023 10:22:38 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

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

LT Spice aborts that, \"unknown syntax\" error.

weird, I just tried it and it works here

https://imgur.com/a/ZKWEVur
Found it. There were three strange \"blank\" lines at the end of the
.asc that it didn\'t like.

That\'s sneaky, using the diodes as their own mux.

could work with NTCs too with opencollector/threestate outputs

 

[John Larkin]

On Sat, 06 May 2023 17:02:54 -0400, DJ Delorie <dj@delorie.com> wrote:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> writes:
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.

Three cool boards will stop the hottest from getting its fan on at the
same temperature as four hot boards.

Each board will have a bit that tells the controller \"I\'m too hot.\" If
any is too hot, jog the fans up. If none is complaining, jog down.

The case now is to extend that to 8 zones of a single board.

If I analog mux and actually measure the individual temps, the FPGA
can consolidate 8 measurements into that one bit to pass to the fan
controller, basically set the too-hot bit based on the highest temp of
the eight.

The paralleled thermistors would mostly work. The hottest will
dominate the weighted mean temperature.

Could diodes be used with the thermisters, so that only the hottest one
counts?

Something like that would work.

 

[Phil Hobbs]

On 2023-05-06 16:55, John Larkin wrote:

On Sat, 6 May 2023 13:18:18 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 21.52.56 UTC+2 skrev John Larkin:
On Sat, 6 May 2023 10:22:38 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

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

LT Spice aborts that, \"unknown syntax\" error.

weird, I just tried it and it works here

https://imgur.com/a/ZKWEVur


Found it. There were three strange \"blank\" lines at the end of the
.asc that it didn\'t like.

That\'s sneaky, using the diodes as their own mux.

In the early \'90s, I built a diffraction-based sensor as an online
monitor for post-exposure bake in semiconductor lithography that worked
like that. It had seven 1 x 3 inch solar cells arranged in a cone (like
a poker hand, but 360 degrees).

All the cathodes were connected to a TIA together, and the anodes were
grounded one at a time, using a zero-power PAL with tri-state outputs.
(A PALCE22V10Z, iirc.) It had to be 3-state, because a logic high would
have forward-biased the solar cells. The photocurrent was pretty small,
so even a 1990 CMOS output resistance was negligible.

A normal PAL might not have worked, even in 3-state, because their low
output levels weren\'t that well controlled.

Lasse\'s suggestion is a bit different because the selected diode is
forward-biased, so a totem-pole output would be OK as long as it goes to
0V accurately enough.

Dunno about FPGAs--do their outputs really go to ground like that?

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 Sat, 6 May 2023 19:14:37 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-05-06 16:55, John Larkin wrote:
On Sat, 6 May 2023 13:18:18 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 21.52.56 UTC+2 skrev John Larkin:
On Sat, 6 May 2023 10:22:38 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

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

LT Spice aborts that, \"unknown syntax\" error.

weird, I just tried it and it works here

https://imgur.com/a/ZKWEVur


Found it. There were three strange \"blank\" lines at the end of the
.asc that it didn\'t like.

That\'s sneaky, using the diodes as their own mux.


In the early \'90s, I built a diffraction-based sensor as an online
monitor for post-exposure bake in semiconductor lithography that worked
like that. It had seven 1 x 3 inch solar cells arranged in a cone (like
a poker hand, but 360 degrees).

All the cathodes were connected to a TIA together, and the anodes were
grounded one at a time, using a zero-power PAL with tri-state outputs.
(A PALCE22V10Z, iirc.) It had to be 3-state, because a logic high would
have forward-biased the solar cells. The photocurrent was pretty small,
so even a 1990 CMOS output resistance was negligible.

A normal PAL might not have worked, even in 3-state, because their low
output levels weren\'t that well controlled.

Lasse\'s suggestion is a bit different because the selected diode is
forward-biased, so a totem-pole output would be OK as long as it goes to
0V accurately enough.

Dunno about FPGAs--do their outputs really go to ground like that?

That\'s interesting. The Trion is pretty low power so its ground paddle
is probably close to actual ground. But we could measure it.

 

[Martin Brown]

On 06/05/2023 03:10, 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. 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.

You would probably get away with it if you added a trimmer to make the
normal operating conditions equal resistance on each thermistor line.

Otherwise you will end up too sensitive to the channel with the lowest
resistance at ambient temperature.

Diodes dV/dT in series might be more reproducible.

--
Martin Brown

 

[Phil Hobbs]

On 2023-05-07 08:09, Martin Brown wrote:

On 06/05/2023 03:10, 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. 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.

You would probably get away with it if you added a trimmer to make the
normal operating conditions equal resistance on each thermistor line.

Otherwise you will end up too sensitive to the channel with the lowest
resistance at ambient temperature.

Diodes dV/dT in series might be more reproducible.

\'Tis \'t\'other way round.

A good thermistor has a 3-4%/K tempco and ~0.4K interchangeability.
(Some are better than that.)

The diode tempco is about -2 mV/K, so to be competitive, the diode
forward voltage at 1 mA would have to be consistent to better than 0.8
mV unit-to-unit, which is a bit hard to believe.

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 Sun, 7 May 2023 13:09:33 +0100, Martin Brown
<\'\'\'newspam\'\'\'@nonad.co.uk> wrote:

On 06/05/2023 03:10, 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. 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.

You would probably get away with it if you added a trimmer to make the
normal operating conditions equal resistance on each thermistor line.

We very rarely use trimmers; people have to set them. The only case in
the past 15 years is to adjust the gain on a GHz-class o/e converter
that has no microprocessor.

Otherwise you will end up too sensitive to the channel with the lowest
resistance at ambient temperature.

The thermistors are 2% resistance accuracy at 25c.

Diodes dV/dT in series might be more reproducible.

Or LM45s, but they have too many quirks.

I guess I\'ll just brute-force mux 8 thermistors into an ADC and let
code take over.

 

[Lasse Langwadt Christensen]

søndag den 7. maj 2023 kl. 01.16.17 UTC+2 skrev Phil Hobbs:

On 2023-05-06 16:55, John Larkin wrote:
On Sat, 6 May 2023 13:18:18 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

lørdag den 6. maj 2023 kl. 21.52.56 UTC+2 skrev John Larkin:
On Sat, 6 May 2023 10:22:38 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

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

LT Spice aborts that, \"unknown syntax\" error.

weird, I just tried it and it works here

https://imgur.com/a/ZKWEVur


Found it. There were three strange \"blank\" lines at the end of the
.asc that it didn\'t like.

That\'s sneaky, using the diodes as their own mux.

In the early \'90s, I built a diffraction-based sensor as an online
monitor for post-exposure bake in semiconductor lithography that worked
like that. It had seven 1 x 3 inch solar cells arranged in a cone (like
a poker hand, but 360 degrees).

All the cathodes were connected to a TIA together, and the anodes were
grounded one at a time, using a zero-power PAL with tri-state outputs.
(A PALCE22V10Z, iirc.) It had to be 3-state, because a logic high would
have forward-biased the solar cells. The photocurrent was pretty small,
so even a 1990 CMOS output resistance was negligible.

A normal PAL might not have worked, even in 3-state, because their low
output levels weren\'t that well controlled.

Lasse\'s suggestion is a bit different because the selected diode is
forward-biased, so a totem-pole output would be OK as long as it goes to
0V accurately enough.

Dunno about FPGAs--do their outputs really go to ground like that?

here\'s the plot of the Artix7 LVCMOS33 pull down from the ibis file (min/max/typ)

https://imgur.com/a/qFTQwNg

 

[Anthony William Sloman]

On Monday, May 8, 2023 at 12:22:13 AM UTC+10, John Larkin wrote:

On Sun, 7 May 2023 13:09:33 +0100, Martin Brown <\'\'\'newspam\'\'\'@nonad.co.uk> wrote:
On 06/05/2023 03:10, John Larkin wrote:

<snip>

Otherwise you will end up too sensitive to the channel with the lowest resistance at ambient temperature.

The thermistors are 2% resistance accuracy at 25c.

Cheap ones might be. Interchangeable thermistors do rather better.

I liked the Betathherm 0.2C interchangeable parts. That 0.8% accuracy. Yellow Springs thermistors coukld do better, but they cost even more.

Diodes dV/dT in series might be more reproducible.

Or LM45s, but they have too many quirks.

https://www.ti.com/lit/ds/symlink/lm45.pdf?ts=1683472832545&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM45

It\'s a 12 page data sheet (with eight extra pages for the package details). John Larkin seems to get quirked out easily.

Interchangeable thermistors are quite a bit more precise.

> I guess I\'ll just brute-force mux 8 thermistors into an ADC and let code take over.

Let somebody else do the actual design.

--
Bill Sloman, Sydney

 

[Lasse Langwadt Christensen]

søndag den 7. maj 2023 kl. 17.26.17 UTC+2 skrev Anthony William Sloman:

On Monday, May 8, 2023 at 12:22:13 AM UTC+10, John Larkin wrote:
On Sun, 7 May 2023 13:09:33 +0100, Martin Brown <\'\'\'newspam\'\'\'@nonad.co..uk> wrote:
On 06/05/2023 03:10, John Larkin wrote:
snip
Otherwise you will end up too sensitive to the channel with the lowest resistance at ambient temperature.

The thermistors are 2% resistance accuracy at 25c.
Cheap ones might be. Interchangeable thermistors do rather better.

I liked the Betathherm 0.2C interchangeable parts. That 0.8% accuracy. Yellow Springs thermistors coukld do better, but they cost even more.
Diodes dV/dT in series might be more reproducible.

Or LM45s, but they have too many quirks.
https://www.ti.com/lit/ds/symlink/lm45.pdf?ts=1683472832545&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM45

It\'s a 12 page data sheet (with eight extra pages for the package details). John Larkin seems to get quirked out easily.

he\'s just experienced and knows the datasheet rarely tells the whole story, afaict the LM45 is notorious for being prone to latchup, oscillation and generally finicky

 

[Anthony William Sloman]

On Monday, May 8, 2023 at 1:50:12 AM UTC+10, Lasse Langwadt Christensen wrote:

søndag den 7. maj 2023 kl. 17.26.17 UTC+2 skrev Anthony William Sloman:
On Monday, May 8, 2023 at 12:22:13 AM UTC+10, John Larkin wrote:
On Sun, 7 May 2023 13:09:33 +0100, Martin Brown <\'\'\'newspam\'\'\'@nonad.co.uk> wrote:
On 06/05/2023 03:10, John Larkin wrote:
snip
Otherwise you will end up too sensitive to the channel with the lowest resistance at ambient temperature.

The thermistors are 2% resistance accuracy at 25c.
Cheap ones might be. Interchangeable thermistors do rather better.

I liked the Betathherm 0.2C interchangeable parts. That 0.8% accuracy. Yellow Springs thermistors coukld do better, but they cost even more.
Diodes dV/dT in series might be more reproducible.

Or LM45s, but they have too many quirks.

https://www.ti.com/lit/ds/symlink/lm45.pdf?ts=1683472832545&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM45

It\'s a 12 page data sheet (with eight extra pages for the package details). John Larkin seems to get quirked out easily.

he\'s just experienced and knows the datasheet rarely tells the whole story, afaict the LM45 is notorious for being prone to latchup, oscillation and generally finicky.

But he couldn\'t be bothered to spell out how it is quirky.

If you are posting here as an experienced person as bit more detail is expected, though perhaps not from John who is fishing for flattery rather than being helpful.

Figures 16 and 17 in the data sheet make it clear that it is intolerant of capacitative loads. If you didn\'t read the data sheet this might come as a surprise.

Latchup isn\'t mentioned. If you can provoke the device into oscillation I suppose anything is possible. Getting the output above the supply rail can provoke all sort of odd behavior, and getting the output below the negative rail can inject charge carriers into the substrate, which can be even worse.

--
Bil Sloman, Sydney