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How to determine if your transistors are "too hot"
How to determine if your transistors are "too hot"
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Old 12th October 2012, 06:57 PM   #1
Fusion916 is offline Fusion916  United States
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Default How to determine if your transistors are "too hot"

Say you have a TO220 with a heatsink, just by empirical methods, how do you determine if it's dissipating too much power? By some kind of temp meter or touching the heatsink? What is the accepted method?
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Old 12th October 2012, 07:12 PM   #2
wg_ski is offline wg_ski  United States
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A thermocouple with the right adapter will plug right into a DMM and read deg. C as millivolts. Where you measure is equally important. It needs to be located as close to the heat source as possible (the transistor die). The best method I've found is into a hole drilled into the heat sink, right under where the die is located in the pacakge. The mounting tab usually measures quite a bit cooler and is inaccurate.
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Old 12th October 2012, 07:22 PM   #3
RJM1 is offline RJM1  United States
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It all depends on what you call too hot. Heat will shorten the life of semiconductors and capacitors around them. If it can run all day, then it isn't too hot.If you want it to last a long time and you ask the question "Is it too hot?" then it probably is.

Last edited by RJM1; 12th October 2012 at 07:33 PM.
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Old 12th October 2012, 07:40 PM   #4
Yooper is offline Yooper  United States
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An industry rule of thumb is that for every 10 degree (C) rise in a semiconductor, the MTBF of the device will be halved.

Best method of temp measurement was described above. Drill a hole through the heatsink and measure the tab of the device. I always use thermal compound for better contact, and remember to power down the system right before taking an instantaneous reading.

Last edited by Yooper; 12th October 2012 at 07:49 PM. Reason: Halved...not doubled.
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Old 12th October 2012, 07:44 PM   #5
Mooly is offline Mooly  United Kingdom
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How to determine if your transistors are "too hot"
With experience comes being able to judge these things empirically. You just know

If its a TO220 then the metal tab of the transistor which has the die mounted on it should be touchable even if there is discomfort and the leads should similarly be touchable and certainly not sizzle water. A single drop of water placed on the metal tab should take around 5 seconds minimum to evaporate. The heatsink should be touchable for at least 5 seconds or so with firm finger pressure.
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Old 12th October 2012, 08:46 PM   #6
Fusion916 is offline Fusion916  United States
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Thanks for the info guys.
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Old 13th October 2012, 04:56 AM   #7
sregor is offline sregor  United States
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I don't recall what they are called, but where I used to work (about 25 years ago) we used small orange adhesive dots which turned white if the transistor it was on got hotter than 80C. Sorry, I don't remember any other details.
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Old 13th October 2012, 05:47 AM   #8
Ian Finch is offline Ian Finch  Australia
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Even back in the 1960s, "Thermopaper" and "Thermodot" were some brands of this stuff for lab. and specific industry applications.
They worked by a melting wax wetting out specific pigments that seemed to appear or disappear when hot. IIRC, a similar principle is
used in all the thermal paper printers for POS receipts, cheap calculators and older facsimilie machines.

But then thermocouple probes became cheap and now IR toys are everywhere.

Edit:Whaddaya know, I remembered! http://paperthermometer.com/default/...tp-series.html
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Last edited by Ian Finch; 13th October 2012 at 05:57 AM. Reason: add link
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Old 13th October 2012, 08:01 AM   #9
Mooly is offline Mooly  United Kingdom
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How to determine if your transistors are "too hot"
Quote:
Originally Posted by sregor View Post
I don't recall what they are called, but where I used to work (about 25 years ago) we used small orange adhesive dots which turned white if the transistor it was on got hotter than 80C. Sorry, I don't remember any other details.
I remember those kind of things. In fact a pack of them were given as a freebie on the cover of Wireless World way back... and I still have them
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Old 13th October 2012, 12:31 PM   #10
AndrewT is offline AndrewT  Scotland
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My Sugden p28 uses To92 for VAS & Drivers.
The VAS and its CCS run too hot to hold.

That bothers me, but that is not what stops me using them. The output goes to rail if one fuse blows. That stops me using them until I modify them to have DC detection to activate the relay they use for speaker muting.
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