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DAVEHALL 16th May 2010 12:07 am

Testing transistors - analogue version
Sticky mentions testing transistors with a digital meter which measures junction drop .
However ,in early days ,the test was to measure the junction resistance .
But ,the meter of choice was usually the AVO .
Range 1 - ohms X 100 -
Range 2 - ohms
Range 3 - ohms /100

Now what is correct range ,to avoid damage to transistor .
R1 - battery =9v ,a bit too high for the junction .
R2 - BATTERY =1.5, half FSD = 500 . Current = 3 mA ,fsd =6 mA ,ACCEPTABLE FOR TRANSISTORS .
R3 -BATTERY =1.5V, half FSD = 5, current = BUT TOO HIGH FOR JUNCTION .

Then , base -emitter ( and collector) - one way ,high, other way low resistance .
Emitter/ collector - O/C both ways .

And don't forget that the AF FAMILY had a nasty habit of going collector /screen s/c ( proved by cutting screen) .

Herald1360 16th May 2010 10:21 am

Re: Testing transistors - analogue version
Don't forget the wet finger check for a bit of gain:

Connect AVO on ohms x 100 to collector and emitter (remember lead polarity is reversed on ohms), lick fingers, connect thumb to collector, forefinger to base (or vice versa), look for noticeable increse in meter deflection.

For a quick sanity check, repeat wet finger bit with just the meter and check that deflection is now less than with transistor in circuit.

kalee20 16th May 2010 7:21 pm

Re: Testing transistors - analogue version
Herald1360's method is exactly how I test transistors. (The wet finger directly between collector and emitter leads, with approximately the same area of contact and finger pressure as you used between collector and base, allows you to 'calibrate out' your current saliva conductivity).

You can also use this method to test power mosfets - the wet finger between drain and gate turns them 'on' and the meter deflection should remain when your finger is removed; finger between source and gate should turn them 'off' and now the meter should stay at infinity when your finger is removed. Drift of meter needle indicates leakage and possible electrostatic damage (assuming the transistor is clean!). Always have your finger roll onto the gate lead last, and break contact with gate lead first.

Lee.Wilkerson 7th Sep 2010 12:51 pm

Re: Testing transistors - analogue version
A 1K ohm resistor works great between base and collector. You will get a lower resistance reading (indicating gain) than if you have collector and emitter reversed. 100 ohms works well for high-power transistors (TO-3 cases, etc).

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