help with ntc thermistor

jarthel · 1st October 2006, 03:59 PM

I decided to connect a 10ohm ntc thermistor on the "hot" wire of the IEC socket. I then connected the leads of the VOM on both leads of the thermistor.

from what I understand, an ntc thermistor's resistance will decrease. This is also the same in my situation. BUT once the resistance measures near zero, it begins to go up and down.

any ideas?

thank you.

lgreen · 1st October 2006, 04:05 PM

Not sure what a VOM is (voltmeter?) but if the load is not drawing constant current, the thermister may cool off and resistance will rise.

Just to restate the obvious...if you are using a voltmeter, are you measuring voltage or resistance across the thermister? You want to measure the voltage across it, if the power is "on" not resistance.

jarthel · 1st October 2006, 04:08 PM

Quote:

Originally posted by lgreen
Not sure what a VOM is (voltmeter?) but if the load is not drawing constant current, the thermister may cool off and resistance will rise.

Just to restate the obvious...if you are using a voltmeter, are you measuring voltage or resistance across the thermister? You want to measure the voltage across it, if the power is "on" not resistance.

volt-ohm-meter (old school here even though the meter I've used in university 10 years ago are digital)

the current load is constant since they are tube heaters. (B+ supply isn't connected yet. just testing the DC supply).

I thought measuring resistance would have the same effect as measuring voltage?

poobah · 1st October 2006, 04:41 PM

You can't measure resistance of a live circuit... you can even roast your meter.

You can measure the voltage though...

jarthel · 1st October 2006, 04:57 PM

Quote:

Originally posted by poobah
You can't measure resistance of a live circuit... you can even roast your meter.

You can measure the voltage though...

I never knew that.

I did measure the voltage and the thermistor seems to work as intended.

thanks for the help

1st October 2006, 03:59 PM	#1
jarthel diyAudio Member Join Date: Jan 2002 Location: somewhere in Australia	help with ntc thermistor I decided to connect a 10ohm ntc thermistor on the "hot" wire of the IEC socket. I then connected the leads of the VOM on both leads of the thermistor. from what I understand, an ntc thermistor's resistance will decrease. This is also the same in my situation. BUT once the resistance measures near zero, it begins to go up and down. any ideas? thank you.

1st October 2006, 04:05 PM	#2
lgreen diyAudio Member Join Date: May 2003 Location: San Diego, USA	ntc Not sure what a VOM is (voltmeter?) but if the load is not drawing constant current, the thermister may cool off and resistance will rise. Just to restate the obvious...if you are using a voltmeter, are you measuring voltage or resistance across the thermister? You want to measure the voltage across it, if the power is "on" not resistance. __________________ My DIY audio projects- PartTimeProjects.com.

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1st October 2006, 04:41 PM	#4
poobah diyAudio Member Join Date: Nov 2005	You can't measure resistance of a live circuit... you can even roast your meter. You can measure the voltage though...

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