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Old 28th March 2008, 06:48 PM   #1
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Join Date: Nov 2004
Default Help me identify a thermistor?

I can put up a photo if necessary, but it's a pretty basic little thing.

I'm in the middle of a shop project using a couple of digital LED temperature gauges. They are Cyberdyne brand 12vdc automotive gauges. The 'tech' support at cyberdyne is no help since they are only importers of the gauges and only know how to wire them to the car. They won't tell me who manufactures the gauge.

I cannot use the sending unit they supply because of space restrictions. I took it apart and there is a little disc/wafer thermistor inside. It's approx 1.5mm thick, 3.0mm diameter round disc. Black composite with silver cladding on either side. Measures about one ohm at 25c.

Is there any way to measure this thing more precisely with a common ohm meter? I need to replace it with something having two leads and very compact.

Any advice would be HUGELY appreciated. I'm really stuck.
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Old 28th March 2008, 11:45 PM   #2
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Location: Phoenix, AZ
Hello Bluebeard, A couple of thoughts here. One, posting a picture would help all help you. Remember "a picture is worth a thousand words". Although I will still try to give you the thousand words!
A couple thoughts on testing the thermistor.
1) put it in a bucket of water that you know the temp of, ice cubes help a lot here. Measure the value
2) stick the thermistor in boiling water (212 F, 100 C). Measure the value.
If you have a way of creating temperatures in between these that you can measure some other way, go for it. This will allow you to plot the measured values and create a graph. From this you can get a curve to compare to the datasheet of some known thermistors. This way you can find one with similar specs.

Peace,

Dave
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Old 29th March 2008, 05:45 PM   #3
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dave thanks for that.
I suppose I could set a cooking thermometer in a gallon of boiling water and drop the the thermistor (hooked to my ohm meter) into it?

From there it would just be a matter of patience? I'd just sit there and jot down the corresponding temps/ohms as the water settled back down to room temp. Guess I could start tossing in ice cubes if I really needed to extend the curve. Now that I think of it, it may not matter since this is only going to be used for room temps on up anyway. I understand that one of the disadvantages of thermistors is non-linearity. That's why I'm in this predicament.

The senders that came with these identical gauges had different colored pottings. They were 1/4"npt brass plugs with sensors potted inside. I was a little suspicious of the different colors of epoxy pot and sure enough when I dismantled them, one used a little disc as I'd described (picture the tiniest watch battery available) and the other used an extremely small SMD thermistor nestled across a couple of solder blobs.

I don't understand why an engineer would design around a thermistor for a digital thermometer anyway when the LM34 (F) and LM35 (C) do the work for you. Perhaps it's because a thermistor is so much less expensive per thousands? Perhaps the thermistor is more rugged?

I don't know, but it's too damn bad I can't just use an LM34. But this gauge I have has been designed around whatever curve this thermistor exhibits.

(sorry for the thousand words here. A picture really wouldn't help anyway)

I'll give your suggestions a try. Thanks.
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Old 29th March 2008, 05:53 PM   #4
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Location: Phoenix, AZ
Glad to help. Another suggestion, turn on the tunes, get beer and sit back. It will make the measuring you mentioned a lot easier

Peace,

Dave
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Old 31st March 2008, 12:14 AM   #5
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Funny that's precisely the scenario when I checked back in here.
I got a pretty graph out of it on my laptop.
Just scanned through Digikey's catalog and there are dozens and dozens that appear as if they would suit my needs.
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Old 31st March 2008, 11:17 AM   #6
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This will probably make things worse, but it may keep you from wasting a lot of time.

Thermistor hunting can be tough. I looked at curves and tutorials until I couldn't stand it any more.


I derived the following as the two factors that make the situation so difficult.

There are two aspects to a thermistor.

(1) reacting to external temperature

(2) self-heating.

In some applications a thermistor is used primarily for the self-heating in order to regulate current.

In your case it is the external temperature. The low resistance may be to exclude self-heating from the overall response.

I finally had to give up the search, but I can appreciate why there are so many types of thermistors.

The size of the thermistor matters, not just the resistance. There are different mixes of resistive material.

Good Luck, Mark.
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Old 31st March 2008, 07:57 PM   #7
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Mark,

Thank you. I appreciate the forewarning. That does explain the vast array of thermistors available. In my favor is that this is not part of a process, and not controlling anything too dependant on accuracy.
I'm just using a couple of digital LED displays to monitor in and out temp in a welding water cooling loop.

It'd be nice to know that what is displayed is accurate all along the curve. The gauges are trimable, so I can at least get good accuracy at one point. If the thermistor curve is even a little close to the original design, I'll be ok I guess. The gauges are really just a head's up for me to see if there are any problems developing and to roughly gauge the efficiency of the cooler at any given point.
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Old 31st March 2008, 09:03 PM   #8
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If it is trimmable then you will probably get there.


Let us know how it turns out, Mark
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