Multi-Meters

[Rodgre]

I'm still getting familiar with the Radio Shack meter I bought a few years ago right before I stopped building pedals for a while. I started building again in the last few months and found the meter and realized it had an Hfe test on it.

I popped in a tranny from a Rangemaster from Small Bear and lo and behold, it told me pinout, type (npn or pnp) and gain! It came up with the same value that Steve had stated it was.

Now I feel way more confident about selecting trannies. My only question is should I expect some germanium trannies to have a ton of leakage? I ask this, as I was testing a bunch of ECG158s (I bought a bunch for Fuzz Faces back in the day) and their gain kept going up and up and up and up. Should that be read as being leaky, or is there a problem?

Now I can see the difference in gain between 3904s and 5088s!

Roger

[Rob Strand]

It came up with the same value that Steve had stated

I wouldn't read too much into that, transistors gain depends on a number of factors (current and temperature) and all that means is your meter probably measured the gain under conditions close to what Steve used.  This multi-meter testers test the transistors at fairly low currents which is fine for those small transistors.  For larger transistors the gain is usually low.  By all means keep using you meter, just be aware transistor gain isn't something which is a precise single number.

My only question is should I expect some germanium trannies to have a ton of leakage? [code]

They vary a *lot* from unit to unit.  The presence of leakage will throw-out the gain measurement.  The only way to compensate is to set-up a jig like on RG's site.

gain kept going up and up and up and up

That could be leakage (or gain variation with temperature), it's likely you were holding the case of the device an that heats it up and changes the gain.  Try not touching it.   Keep in mind though, with those transistors you will get different readings on cold and hot days.

Now I can see the difference in gain between 3904s and 5088s!

Ballpark gain measurements is where those meters are really useful.

[Bluesgeetar]

Since I just bought mine and I still have the manaul and box I can post what the manual says about what the unit does when it reads trannys.

From Radio Shack Auto-Range DMM owners manual:

Notes:
*  Do not take the hfe reading as an absolute measurement, but rather as an indication that the transistor is operating.  The true gain of a transistor depends on its operating current.  This meter applies 500 to 1000uA to the emitter and collector and measures the base current to calculate base gain.

* You cannot measure the hfe of a transistor that is connected in a circuit.

* You cannot measure the hfe of an FET or other non-bipolar transistor.

* Some power darlington transistors contain internal base-to-emitter resistors.  Because the meter uses two current readings to calculate hfe, any internal transistor resistance causes undependable readings.


Hope that helped.