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Clever ways to use ordinary parts to sense temperature? Answered

While designing a flyback driver circuit I figured the best way to cool the transistors in use would be to use the stock intel CPU cooler. They are small, compact, and effective, and the delta fan is of reasonable quality. I would like to control this PWM (4 pin) fan with the 2 extra comparators I have left over from my LM339.

What common passive (or active) components have a large enough temp-co to allow their use in sensing tempurature? Or will I just need to buy some specialized ICs purpose designed for this task?

Discussions

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-max-iceng

Answer 2 years ago

Nice! This is exactly what I was looking for! I am definitely building this one! What's the point of all the twisted wires? Would a diode work? Also, this appears to be EXACTLY what had failed in my AM220 multimeter. I damaged it by leaving it in the capacitance range and measuring a 170V voltage rail when I was trying to repair my old scope. (and it went out with a BANG!!!!) Since then it was only able to measure voltage and current. Turned out to be a TO92 S8050 that had failed when I took it apart today, and replacing it fixed all the ranges on it! :D Considering it went out with a bang, I was surprised this transistor looked perfectly fine! I don't know what exploded in it! no damage anywhere.

Strangely, This is how a S8050 was configured! I could not work out why it tied the emitter to the base like that, my guess was to get a 0.6v voltage reference (?) seems very unlikely, or maybe a DC offset or some sort of analog voodoo I was not aware of yet. So now I think it must be the way they were sensing temperature of the meter to help compensate the temp-co of all the other critical components. Or maybe a part that is populated for the use of the K type thermocouple in the more expensive unit. (?) Would this be right?

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iceng-max-

Answer 2 years ago

Important -Twisting avoids ac magnetic field induction interference.... Signal to noise is worsened when a changing magnetic field passes through a wire.. If every 1/4" the induced voltage is canceled by the next twist which is a desirable effect while you bathe in the 60 cycle magnetic fields created by wiring running through the walls of your shop.

Never succeeded with a PN diode.

A thermo_couple needs a cold reference and thats an annoying bit more ckts.

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-max-iceng

Answer 2 years ago

I wasn't planning to use the temperature sensor a long distance away, but instead soldered to a custom PCB. Certainly if I was going a long distance I would use scrap twisted pair wire or coax. I was able to cause the forward voltage voltage drop to fall from 0.65V to 0.32V by touching the iron to the transistor heating it up. I achieved almost identical results using a 1N4148. I am surprised how sensitive the junction is to temperature! Using this property, I was able to use the comparator to trigger when the soldering iron was brought near the temperature sensing forward biased diode. I used another diode as a voltage reference for the other comparator input. Both diodes used a 3.3K pullup resistor and I used 12V power supply.

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Jack A Lopez

2 years ago

I think the most hassle-free way to sense temperature, is going to be by using a temperature sensing IC, like LM34, LM35, or similar.

But since you asked specifically for way to do this with ordinary parts, I have upped some pictures from the book came with an old RadioShack N-1 project kit, specifically, Project 21, "Electronic Thermometer"

which uses a single NPN transistor to sense temperature somehow.

I mean it measures collector current, Ic, directly with a little ammeter. I'm not sure how Ic is expected to respond to temperature.

The formula from the paper Iceng just linked to, "AND8494-D.PDF" might be useful here, for to tell us what that relationship is, uh..

logn(Ic/Is) = (q/K)*(Vbe)*(1/T)

I dunno.

If you built the PNP version of this circuit, I think that would flip that 10K resistor upside down, so one side of that resistor was attached to ground, and that would provide a convenient way to measure Ic, as a voltage, Ic*10K, relative to ground.

Links to large upped images:

https://www.instructables.com/file/F0H294RIOHGWBLR/
https://www.instructables.com/file/FAH8VZLHW8T314U/

radioshack-160-in-1--project-21-electronic-thermometer.jpg
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-max-Jack A Lopez

Answer 2 years ago

I built up a circuit using 2 2N4148 transistors and used a 3.3K pullup resistor for both of them, and these were connected to the input of the comparator. Because the bias voltage was very close to the same for both, when I brought my iron near one of the diodes, I was able to change the state of the comparator. The only problem I see now is tolerance issues between different resistors, as the bias current greatly affects the trigger temperature. Also because I am using 2 diodes, I basically sensing a temperature difference since the reference voltage diode is equally sensitive to tempurature as the tempurature sensing diode. I will probably need to use a better more stable voltage reference.

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Josehf Murchison

2 years ago

You only need one comparator and a thermistor to control a fan.

Cooling Fan 2.gif
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-max-Josehf Murchison

Answer 2 years ago

I don't have a thermister. If I did it would be trivial ;)

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Josehf Murchison-max-

Answer 2 years ago

If you have an old power supply with a fan look at the heat sinks.

The heat sink will have bracketed or glued to it sometimes just next to it, a component that looks like a solid, green, red or black capacitor with no writing on it. that is a thermistor.

Some times you can find them on the transformer.

If you connect them to an ohm meter it will give you the resistance of the thermistor at room temp. hold it in your hand and the resistance will change.

Some thermistors look like a 1N4148 diode without any markings.

Here are a couple other schematics you might like.

Cooling Fan 1.gifCooling Fan 3.gif
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-max-Josehf Murchison

Answer 2 years ago

Yeah, I think I have a few MOVs mixed in with my salvaged ceramic capacitors. But I will need aa lot of identical ones. I guess my only option is to buy the parts.

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-max-Josehf Murchison

Answer 2 years ago

I already bought lots of NPN and complementary PNP transistors! ;) Considering I have 2 unused comparators on the kit I hope to sell, I want to make use of them without adding more parts to the bill of materials. Speaking of which, you want to have a look at my current design?

Circuit:

https://easyeda.com/editor#id=BLztPfAkw|vUO60iAua

PCB layout:

https://easyeda.com/editor#id=BLztPfAkw

I think this PCB will be pretty $$$ at the moment as it is approx 9 inches squared. :( I think I will make the PCB much smaller and figure out a different way to mount the MOSFET to my stock intel CPU cooler to make this kit cheaper.

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Josehf Murchison-max-

Answer 2 years ago

That is an interesting sight, is it free or monthly?

I was just showing you mouser they have just about everything you might want datasheets included, and if you shop around the prices are good.

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-max-Josehf Murchison

Answer 2 years ago

What, easy EDA? Its free online PCB design software. It works reasonably well too. I like it. I learned about it when Great Scott (on YouTube) reviewed it.

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-max-Josehf Murchison

Answer 2 years ago

I have lots of random components includes really old carbon film resistors, wire wound 25mohm 15W resistors, all sorts of capacitors, and lots of 1N4004's, SCRs.. can any of these be used? If not I will have to order more parts on digikey.