Bipolar Junction Transistor Testing Basics

[EchoWars]

Another question: When you measure hFE for a transistor with a multimeter does this value correspond the Maximum DC current gain which is listed on the data sheet?

The gain of the transistor will vary depending on the collector current...sometimes a little, sometimes a lot. Depends on the transistor. Yes, you are measuring DC current gain, but at a low current. I would not venture to say that what you get is the maximum value and thus equal to the datasheet value of Max Gain...it would be a coincidence if you did.

 

[rulerboyz]

Diode test measure what?

From your post I gather that Diode test measures a value in Volts correct?
On my multimeter I am testing a random NPN transistor and I get the following values:

B-E= 1077
B-C= 1063

Hfe test= 221

My question is, does the number 1077 indicate 1.077 volts?

 

[fropiler]

Outstanding!

As many times as people ask me how to test, well, maybe not such a bad idea.

Thatch?

I'd been looking for a transistor refresher lesson, and this is perfect.

Thank you EW.

Ed

(Now you got me itching to get home and see if my Fluke 77 has a diode tester setting)

 

[Yamaha B-2]

How do I find replacements for transistors that I can't seem to match.
How do you find an approximate match when you can't locate what you are looking for?
The service manual for Rotel RTC-850 preamplifier seems impossible to find. I was able to find the owners manual, but that doesn't have a schematic or anything detailed in it.

Do you mean cross-reference? 'Match' in the audio world generally means you are actually matching the performance curves of the individual devices with some sort of curve tracer, etc, within a certain percentage level. Cross-reference means you are trying to find a replacement transistor for something that is (generally) no longer available. In the ideal world you would replace transistors with 'matched' devices, but, depending on the application/circuit, may not be that critical.

 

[rulerboyz]

Yeah, I am willing to settle for cross-referenced replacement in the applications I am tinkering with.

I have another question concerning transistors that are of the T066 type and look like this:

When you buy these they come with two plastic washers for the two screws and plastic film that isolates the body (collector) of the transistor from the heatsink.

The two pins represent the base and the emitter. Is the collector connected to the circuit via the screws?

Am I correct in assuming that if you would put the plastic washers in the holes at the top of the transistor you could prevent the collector from making contact with the circuit?

What happens if you don't put that clear plastic gasket between the transistor and the heatsink? What would happen if you used clear plastic tape instead of the plastic gasket?

 

[EchoWars]

RE: Your meter tests. 1+ volts is very high...something is wrong wth your procedure, or your meter. At the tiny current induced by a meter, 1/2 a volt would be more like it.

The 'gasket' is a mica washer to electrically insulate the body (which is the collector) from the heatsink. The screws generally do not screw into the heatsink, but into the transistor socket, and that is how connection is made to the collector. You do not want to use an insulator on the hold-down screws, but you must use the mica washer, else sparks 'n smoke 'n blown fuses 'n burnt parts. :yes:

 

[rulerboyz]

I think it must be my multimeter because every brand new transistor i am testing is giving me a reading around that value.

Well the heatsink I am screwing onto does not appear to need the mica washers because the screws clear the heatsink metal and go directly into the transistor socket. The only part of the transistor that contacts the heastink would be insulated from the clear plastic gasket. As far as I can remember the original transistors didn't have those washers.

I believe that's what caused the smoking resistor last time I attached a speaker to it. I took those mica washers out and checked the bias again.
Now it is reading really high...around 200mv.

 

[EchoWars]

Bump...

This would be a good sticky, since I get Q's all the time about this.

Hint Hint...

 

[drspiff]

Cautionary note when using hfe to match transistors.

Diode junctions including those in a transistor make excellent thermistors or temperature sensing devices. The heat of your fingers placing the transitor into the hfe slots on your meter will skew the gain of the Device Under Test (DUT).

To match transistors, you need to have repeatable conditions. To do this you need to eliminate as many variables as you can from the process. This includes drafts, the heat you applied while placing the transistor, the "freshness" of the meter batteries, and proximity to heat sources including your body and hands.

Place the transistor in the hfe slots, and WAIT. Wait until the reading stabilizes. This may take a couple of minutes or more. Watch the meter, and wait until the reading has updated several times. Once you are sure that the gain has stabilized, then record the reading for that transistor. Repeat the process for the rest of the transistors you are matching.

 

[EchoWars]

Good advice.

 

[d3imlay]

(note: most transistors fail with a dead-short from the emitter to the collector, especially in the later power stages of an amplifier. Knowing this can allow you to check high-power stages quickly for obvious failures)

I teach classes for troubleshooting Uninterruptible Power Supplies on a regular basis. I stress over and over EW's comment. The higher the power levels, the more truth to his statement.

One other thing: Heat sink compound. A little dab'll do ya. It shouldn't come oozing out under the xistor. Better yet use silpads rather than mica insulators and forget the grease. I never reuse silpads as a precaution. I also have a few thousand of them so it's not necessary.

 

[rulerboyz]

A new transistor will come with a sil-pad in the package. It's a good idea to remember to fasten your transistor to the heatsink first, getting the sil-pad correctly placed, before attempting to solder the new transistor to the PCB. It is the heatsink screw that determines the lead height of the transistor.

 

[macaltec]

Finally got around to trying this out. How do you go about testing ouput transistors while the unit is powered up? I'd prefer not to pull them yet as the problem is somewhat intermittent and the device has not failed completely. I'm just looking to eliminate them as the cause of my problem before moving on. Thanks

mac

 

[d3imlay]

If the ammp is operating, the outputs are probably O.K. The only way to test them is to drive the amp into clipping with an 8 ohm load and read the voltage drop across the emitter resistors. The absolute value isn't important as longs as they are all very close.

 

[macaltec]

If the ammp is operating, the outputs are probably O.K. The only way to test them is to drive the amp into clipping with an 8 ohm load and read the voltage drop across the emitter resistors. The absolute value isn't important as longs as they are all very close.

Thanks. I didn't want to hijack this thread so, I started a new thread on the amp I am woking on. It's in the Pioneer forum "Working On My SX 939, Oops!" I need to go and add some more details of what the original problem is.

Jason

 

[MAXZ28]

Good info, for sure.

Bump for STICKY status, please.

 

[lorne]

Great thread! Has to be a sticky for sure ... so some people can refresh, and others can begin learning how to do this.

 

[240sx4u]

Good read here folks. Very useful.

Evan

 

[xargos]

Another vote for sticky! This thread is quite useful.

 

[lorne]

One other thing: Heat sink compound. A little dab'll do ya. It shouldn't come oozing out under the xistor. Better yet use silpads rather than mica insulators and forget the grease.

Please:

1) What's a silpad — the same thing as the mica insulator but made from silica? Which would be essentially ... er ... glass?

2) Does this imply that the silpad makes the grease redundant? Does this mean that the silpad conforms to surface better than mica wafers?

3) Is this post off-topic? Should I delete or move it?

I found piles of NOS wafers in two sizes. They look like mica and I've been
using fresh ones and changing out grease ....

Heat sink compound. A little dab'll do ya. It shouldn't come oozing out under the xistor.

4) ... and my applications have been oozing to match what I found in the amps. Is oozing just an untidy, unnecessary thing? Or something else?