Testing a ntc thermistor (icl)

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Old 11-18-19, 09:29 PM
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Testing a ntc thermistor (icl)

Simple question. Can you test resistance on a inrush current limiter with its connected to a circuit board or must it be on its own?
I tested one on the board and got continuity but it did not register a resistance at room temperature

 
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Old 11-19-19, 07:33 AM
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complicated answer. Usually yes, but depends on the circuit. Got a schematic?
 
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Old 11-19-19, 04:49 PM
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Thank you for the reply. Itís a ntc thermistor (icl) on an ecm control module for a furnace blower.
This particular GE module is famous for its thermistors blowing up, mine shows no real visible damage other than a tiny chip and some stress marks. No tell tale burn marks on the board

I am I could try to post a picture of it?

I want to test the thermistor before I desolder it but I am not sure off the rest is accurate that way?

 
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Old 11-19-19, 05:53 PM
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It's actually a varistor.

It should measure a very low resistance when cold....... I'm going to guess well under 5 ohms.
Once heated they are closer to 1 or 0 ohms.
A chunk missing is usually a good sign it's gone.
How-to-insert-pictures.
 
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Old 11-19-19, 06:26 PM
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Hi I will try to post a picture but it is a ntc thermistor also known as a inrush current limiter. Itís used in ecm control modules for variable speed blowers for furnaces

the specific thermistor is a sl22 1R020 inrush current limiter limiter
 
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Old 11-19-19, 06:37 PM
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I see them called thermistors. I've always called them varistors. They're listed as the same part.
Basically a thermally sensitive resistor. The 1R is the resistance...... 1 ohm.
 
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Old 11-19-19, 06:37 PM
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https://www.digikey.ca/product-detail/en/ametherm/SL22-1R020/570-1041-ND/749881

that is the specific thermistor on the board

I tested for continuity while on the board and got continuity but no resistance
 
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Old 11-19-19, 06:46 PM
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A one ohm part is a good candidate for in circuit testing. Any reading over a couple ohms would lead me to believe its bad.
 
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Old 11-19-19, 06:48 PM
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https://youtu.be/e8ye0rwv7tI


attacyed is a YouTube video of the identical module with the thermistor being removed

before I do this I just wanted a way to test it. I had read it should show a resistance and when heated the resistance would drop... I just donít know if I can do that test whiles itís soldered to the circuit board? Given I do t show any resistance is it already a sign itís bad? In theory if I apply heat to the thermistor with the tip of a soldering iron it canít drop resistance because it does not start with any resistance

Theses new modules cost over 500 and a new thermistor costs 5 bucks so i am praying I have a bad thermistor
 
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Old 11-19-19, 06:53 PM
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What if it reads continuity while in circuit?
 
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Old 11-19-19, 07:12 PM
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It should read continuity. It should read about 1 ohm.
1 ohm is basically the same thing as shorting your probes together.
 
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Old 11-19-19, 07:19 PM
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Shoot so it may be good.
Would any readings change if heat is applied to the thermistor?


thank you very much for the information
 
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Old 11-19-19, 07:23 PM
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When cold..... it should be 1 ohm. When hot it should be 0 ohms.
 
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Old 11-19-19, 07:26 PM
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On the 200 setting level on my multimeter it reads 1.4 ohms, initially the number read all over the map but it settles to 1.4

so itís possible the issue is not the thermistor?
 
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Old 11-19-19, 07:57 PM
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What is your shorted lead resistance ?
You would subtract that from 1.4 ohms to get the exact part value.

It sounds like that part is ok.
 
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Old 11-19-19, 08:09 PM
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How do it get the shorted lead resistance?
 
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Old 11-20-19, 11:55 AM
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Normally when measuring resistance or ohms or continuity, at least one of the respective places to which the two meter leads are touched must be unhooked.

This is to prevent confusion with the reading as measured, as pertaining only to the subject item (here, the thermistor) versus pertaining to other paths in other circuitry to which the subject item is wired.

Before disconnecting wires, label each one so the circuitry or wiring can be easily put back the way it was.
 
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Old 11-20-19, 01:49 PM
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Shorted lead resistance is just what it says.

You short the meter leads together to see if there is a zero offset in the meter.
Then you subtract it from any reading you get measuring a component.
 
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Old 11-20-19, 06:29 PM
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Thank you for all the great information and advice

itís nice that people take time out to help other amd itís much appreciated
 
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