Attaching a ntc epoxy thermistor toa to-220 to monitor temperature - Page 1
Forum section: electronics / beginners

author: george graves
author_rank: Super Contributor
timestamp: February 21, 2012, 07:04:05 am
content: I want to use a ntc epoxy thermistor (that I already have a lot of) to monitor the temperature of a heat sunk'ed to-220. Would there be anything wrong with using some epoxy and gluing it to the plastic case of the to-220?Is there a better or more standard method?

author: Simon
author_rank: Unknown
timestamp: February 21, 2012, 07:05:58 am
content: you need to look at how good your chosen glue is at transmitting heat

author: cybergibbons
author_rank: Frequent Contributor
timestamp: February 21, 2012, 07:13:59 am
content: You'd be better attaching it to the heatsink very close to the package, and then taking into account it will be cooler than the surroundings. The plastic case isn't really designed to dissipate any heat, so temperature readings from it might be misleading.

author: Simon
author_rank: Unknown
timestamp: February 21, 2012, 07:41:35 am
content: using a clip to hold the sensor on and some sort of heat grease like is used between CPU's and a heat-sink is probably your best bet

author: Rerouter
author_rank: Super Contributor
timestamp: February 21, 2012, 07:49:21 am
content: or just buy some $5 syringe of thermal epoxy,. alternativly being a TO-220 you could use a washer to vice the sensor against the packages tab

author: george graves
author_rank: Super Contributor
timestamp: February 21, 2012, 07:56:12 am
content: Hmmmm. I think I might use hardware store epoxy, attach it to the plastic of the t0-220, but I'll "calibrate" it (find the difference) between the thermistor and the metal tab using my fluke 87-5. How does that sound as a plan?

author: Simon
author_rank: Unknown
timestamp: February 21, 2012, 07:57:29 am
content: what sort of accuracy are you looking for ?

author: george graves
author_rank: Super Contributor
timestamp: February 21, 2012, 08:12:56 am
content: Nothing much. It's just for a power supply. I thinking of using a older pentium 4(?) hear sink w/fan, and adjust the fan speed based on the heat the voltage regulator is putting out. That and make sure I'm not blowing anything up.I saw Dave take temp measurements of the dummy load mosfet with using his fluke's temp probe, so I assume that's somewhat accurate way of measuring the tab temperature.

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: February 23, 2012, 09:22:56 pm
content: You want to use something like this:http://www.arcticsilver.com/arctic_silver_thermal_adhesive.htm. In the states, this stuff is available at most stores that cater to the PC modder crowd.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 23, 2012, 10:29:49 pm
content: I was thinking JB Weld would work well but after a little bit of Googling it seems that it has terrible thermal conductivity.Oh well, I guess it can't do it all. My brother-in-lawdidreattach part of his engine block with JB Weld, however. He had only water in the radiator and no antifreeze so part of the engine block quite literally broke off one cold winter. The JB Weld actually worked.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: February 23, 2012, 11:44:34 pm
content: Thermal conductivity is not very important for this attachment.Thermal modelling works similar to Ohms law, T=P*RthThe amount of heat dissipated by the NTC is minimal, so there's very little heat flow. Therefore the temperature drop will be relative small, even if the glue has fairly high thermal resistance.This is analog to probing a voltage using a thick copper wire, or via a resistor. The measurement will stay practically the same, as long as the probing resistance stays much lower than the (high) impedance of the voltmeter.

author: Unknown
author_rank: Unknown
timestamp: February 24, 2012, 12:20:40 am
content: The NTC to heat sink thermal resistance should be much smaller than the NTC to air thermal resistance for an accurate temperature measurement. In this case, accuracy probably doesn't really matter, since it would be a closed loop system. You just want to adjust the fan speed based on the NTC resistance. Figure out a resistance that gives you the highest acceptable heat sink temperature and use that as set point. The resistance to the heat sink shouldn't be much higher than the resistance to air, however, since that would reduce the sensitivity.

author: SeanB
author_rank: Super Contributor
timestamp: February 25, 2012, 07:15:30 am
content: Just use a 2 part epoxy ( Pratley Steel or equivalent) and stick it to the heatsink next to the transistor. Will work well as a sensor for the fan. I have used the quickset steel as a way to stick heatsinks onto processors before, it works well used as a thin film on clean and flat ( made flat on a surface block with 1000 grit waterpaper and a lot of water) heatsink. Handles a old pentium processor with less than a 5C difference between the heatsink and the on board diode.