Electronics > Beginners

Matching transistors room temprature

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Jan Audio:
So i am matching transistors again, this is difficult to do.
My room is steady temprature without heating 17,5 degrees.
With heating on to 20 degrees the temprature is not stable enough, the readings vary to much.
Is it bad to measure at cold temprature ?

i readed in a lab with climate control is 21 to 25 degrees ?
Should i wait for a warmer day ?

Some more info how i measure :
cost me 1 breadboard + cables, some flexible styrofoam to place around the row transistors.
The cables are to have my hand away from the transistors, it changes the reading when to close.

So how do hobbyists fix this without climate controlled lab ?

jeremy:
If you are going to run the transistors at 17.5 C, then it is probably best to match them also at 17.5 C. It doesn't really matter at what temperature you match them as long as they are also used as close as possible to that same temperature (or have a well characterised temperature coefficient). It's roughly the same problem with precision mechanical measurements (fancy CNC machines, optics, etc); the absolute temperature is not so important itself, it's more important that the temperature is stable.

Laboratory climate control is not the same in all contexts, but (for example) the US NIST Normal Temperature and Pressure (NTP) is 20 C. This is more out of necessity than anything else; you just have to pick a standard and stick with it, and 20 C is relatively easy to achieve in most parts of the world (and I suppose comfortable enough for humans).

You can find a list of common standards here: https://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

Jan Audio:
Thanks.
Wont the transistors matched at 17,5c be matched at the temprature inside the enclosure ?

bd139:
I just don't do it. If I need a matched pair I usually bodge a CA3046 / LM194/394 array in somehow.

David Hess:
What characteristics are you matching?  I usually settle for simple Vbe measurements to get within a couple millivolts and it does not take long enough for the temperature to change significantly.  For transistors in the same lot, Vbe correlates well with other characteristics.

A more complex method is to make a delta Vbe measurement to get the temperature of the transistor junction while forcing it to a specific temperature and then making the measurement of whatever is being matched.

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