diode sense for temperature

[HapticZ]

would it be usefull to use two similar 1n4148 diodes to sense air
tempreature differential?

my application is to use one diode at floor level and another diode at
ceiling level to detect gross temperature
differences and then turn on a fan if the difference is too great.

(high celings are architecturally trendy, but bad for effective heat usage,
might be cheaper to use radiant after all!)

a simple comparator and a single dc/ac isolated 120vac switch (solid state
relay) will be the basic components

a single pot or trimmer would set the range of the tempreature difference
before the switch turns a fan on.

 

[Dave]

I think it sounds like a great idea! Unfortunately, I am a hobbyist, and
not an EE. PLEASE post the schematic, if you come up with anything.

Thanks,

Dave

 

[Eeyore]

would it be usefull to use two similar 1n4148 diodes to sense air
tempreature differential?

Not sure about how well 1N4148s fare in such applications but I've used the Vbe
of transistors very sucessfully in temperature monitoring applications.

Do bear in mind that a couple chosen at random may not match very closely.

Graham

 

[Jamie]

would it be usefull to use two similar 1n4148 diodes to sense air
tempreature differential?

my application is to use one diode at floor level and another diode at
ceiling level to detect gross temperature
differences and then turn on a fan if the difference is too great.

(high celings are architecturally trendy, but bad for effective heat usage,
might be cheaper to use radiant after all!)

a simple comparator and a single dc/ac isolated 120vac switch (solid state
relay) will be the basic components

a single pot or trimmer would set the range of the tempreature difference
before the switch turns a fan on.

http://www.mouser.com/catalog/632/493.pdf>

Look at those, NTC and PTC, perfect for what you want to
do.
you'll need to have a pot on one of them to balance the circuit.

 

[terryS]

http://www.mouser.com/catalog/632/493.pdf>

Look at those, NTC and PTC, perfect for what you want to
do.
you'll need to have a pot on one of them to balance the circuit.

Could you stick them in some sort of 'Wheatstone' bridge circuit.
Balance the bridge while both diodes even if they different are at
same temperature and then calibrate as one thinks you will have to
anyway for actual differences in their respective tempearures? Just an
idea?

 

[HapticZ]

only looking for "gross" differences in temp. im not looking for anything
resembling laboratory accuracy

difference span may be 5-10-20 degrees or more,

repeatabilty is non-critical, within a few degrees is ok.

tolerance can be +- 2-4 deg either way to allow for built in hysteysis
(hysterical)

u know what i mean , oh jeez i'm getting my friend al's hymers disease!

or maybe i'll just buy some warm wool sweaters and toss this project ;-))
..

 

[Bob Masta]

would it be usefull to use two similar 1n4148 diodes to sense air
tempreature differential?

my application is to use one diode at floor level and another diode at
ceiling level to detect gross temperature
differences and then turn on a fan if the difference is too great.

(high celings are architecturally trendy, but bad for effective heat usage,
might be cheaper to use radiant after all!)

a simple comparator and a single dc/ac isolated 120vac switch (solid state
relay) will be the basic components

a single pot or trimmer would set the range of the tempreature difference
before the switch turns a fan on.

Diodes make exquisitely linear temperature sensors,
but note that each unit will have a different calibration
factor. When fed with constant currents, the (negative)
voltage slopes will be different. (You can use an ordinary
ohms range on a DMM to test.) You can manually adjust
for this, or there are ways to compensate using current mirrors with
transistor pairs instead of diodes (look up "band gap reference"), but
if that is too much trouble, consider using temperature
measurement chips that already do that for you.

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

 

[John Larkin]

would it be usefull to use two similar 1n4148 diodes to sense air
tempreature differential?

my application is to use one diode at floor level and another diode at
ceiling level to detect gross temperature
differences and then turn on a fan if the difference is too great.

(high celings are architecturally trendy, but bad for effective heat usage,
might be cheaper to use radiant after all!)

a simple comparator and a single dc/ac isolated 120vac switch (solid state
relay) will be the basic components

a single pot or trimmer would set the range of the tempreature difference
before the switch turns a fan on.

Sure, that will work, but an IC temperature sensor, or a thinfilm RTD,
would be more repeatable. Or a pair of series-wired, back-to-back
thermocouples, but you'd only get about 30-40 microvolts per deg C.

Put ceramic caps right across the diodes so they don't pick up rf from
cell phones and such.

John

 

[gearhead]

only looking for "gross" differences in temp. im not looking for anything
resembling laboratory accuracy

difference span may be 5-10-20 degrees or more,

repeatabilty is non-critical, within a few degrees is ok.

tolerance can be +- 2-4 deg either way to allow for built in hysteysis
(hysterical)

u know what i mean , oh jeez i'm getting my friend al's hymers disease!

or maybe i'll just buy some warm wool sweaters and toss this project ;-))
.

It's not that hard to do, so don't give up yet.
You can use execute your original idea using two diodes and a
comparator. You need some way to trim (calibrate) the circuit; a
single potentiometer will adjust for the diode difference and the
comparator offset, all in one fell swoop.

 

[Ben Jackson]

would it be usefull to use two similar 1n4148 diodes to sense air
tempreature differential?

Maybe if you were going to make 1000 of them and wanted to save a few
bucks. Why not use something like LM34? The datasheet even has
"remote" application examples.

 

[Rich Grise]

would it be usefull to use two similar 1n4148 diodes to sense air
tempreature differential?

my application is to use one diode at floor level and another diode at
ceiling level to detect gross temperature differences and then turn on a
fan if the difference is too great.

(high celings are architecturally trendy, but bad for effective heat
usage, might be cheaper to use radiant after all!)

a simple comparator and a single dc/ac isolated 120vac switch (solid
state relay) will be the basic components

a single pot or trimmer would set the range of the tempreature
difference before the switch turns a fan on.

You'd have to either match the diodes or calibrate them for Vf.

But once you know the offset, they both follow the same curve, so
sure, why not?

FWIW, I did a little experiment not too long ago with just one, and
found that they work quite well.

Good Luck!
Rich

 

[Rich Grise]

Maybe if you were going to make 1000 of them and wanted to save a few
bucks. Why not use something like LM34? The datasheet even has
"remote" application examples.

I'm guessing:
A: he's already got the diodes
B: they don't need a third wire for a power source
C: he's only looking for a differential reading

At least, those are the reasons I'd use diodes.

Cheers!
Rich