Testing transistors - analogue version - UK Vintage Radio Repair and Restoration Discussion Forum

DAVEHALL · 16th May 2010, 12:07 am

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

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

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

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).

16th May 2010, 12:07 am	#1
DAVEHALL Retired Dormant Member Join Date: Jan 2010 Location: Coventry, Warwickshire, UK. Posts: 339	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) .

16th May 2010, 10:21 am	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,471	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. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

16th May 2010, 7:21 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 6,934	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.

7th Sep 2010, 12:51 pm	#4
Lee.Wilkerson Retired Dormant Member Join Date: Sep 2010 Location: Lexington, Kentucky, USA Posts: 6	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).