measuring HFE ? Does it take time for the reading to "settle in"?

[rosssurf]

I recently purchased a cheap MM from Harbor Frieght. I bought this because I am building a Ross Comp clone and it has the ability to measure HFE.
When I first put in my 5088 it measured around 525 then after a minute of so it settled down to 520. Is this normal? Does it take a little time for the measurement to settle in?

[Peter Snowberg]

Try holding your fingers on the transistor while it's in the test socket. Now try it with a Ge transistor.

[rosssurf]

after the brief afore mentioned "settle in" time. The reading does stay put. It takes about 30 seconds. is that normal?

[Stephen]

I have a craftsman from SEARS same thing!!!  Normal  good way to find the EBC if you dont know the transistor....

I believe a germanium will never stop!!!!!!!!!!!!!!!!!..................Well sometimes but they leak on design so the reading isnt close

[R.G.]

Welcome to the world of thermal modelling.

Heat flow can be modelled just like electrical R-C circuits. Mass is analogous to capacitance, temperature differences to voltage differences, and heat flow to current.

So a body of mass N can exchange heat with another body of Mass M through the thermal resistance between them. The temperature difference will have exactly the same shape of curve that the voltage between two capacitors connectec by a resistor does - a relaxing exponential. The time to do this relaxation is determined by the thermal time constant, i.e. the product of the thermal capacity of the bodies and the thermal resistance between them.

When you have a transistor in a drawer and the room air has been homogenous for a long time, the transistor comes to room temperature. When you touch it, your 86F skin heats up the transistor. When you put it into a socket, the socket cools the legs, and your fingers continue to heat it. Your body heats the air around you, which may or may not pull room temperature air over the transistor, and the electricity in the device causes a small, but measurable self heating.

The thermal time constant of a transistor into room air may be many seconds.

[rosssurf]

Thanks ! I am learning