Transistor tester from Aliexpress

[ashok]

Some body asked about this device earlier . However this unit can also measure break down voltage of the transistor apart from HFe.
Can we get away by using a small charged cap across the transistor and measure the final voltage across it ?
That is , charge the small value capacitor ( depending on the wattage rating of the transistor) then disconnect from the supply and connect the CE of the transistor to it and check the voltage that it drops to . The transistor should not get destroyed due to the very low power spike it will see.

Will that permanently affect the Hfe of the device or it's noise performance ?
Thanks.

http://www.aliexpress.com/item/1pc-...32263301358.html?spm=2114.40010608.4.1.QxsHE0

 

[Mooly]

I definitely wouldn't use a cap to investigate breakdown voltages. A current limited supply that rises from zero upwards, i.e. a voltage supply available via say a 1 or 10 meg might be one method. That used in conjunction with a series resistor in the earthy end to derive a voltage related to current flowing would probably work.

 

[ashok]

It isn't a plain cap. There is a large value series resistor also to limit peak current. I've done this long ago and didn't destroy the transistor. However at that time I didn't check hfe or noise performance. Just wondered if that would be affected after the test. My memory was it discharged and stayed at the breakdown voltage of the device.
I'll give it a go again as soon as my 'work desk' is ready . I bought one of those testers last year but it was defective and returned it. They couldn't get me a replacement at that time.
Thanks for your post.

 

[Mooly]

Even with a resistor I would still prefer to see it done the other way around, that is to bring the voltage up slowly rather than apply to much suddenly, even if it is limited.

I wouldn't like to say whether the device under test would be affected or not, and I guess a lot depends on the device anyway. Testing a power device this way is probably not going to noticeably affect it, a tiny rf type low noise device could be different.

 

[ashok]

That could be solved by another capacitor across the transistor. It would rise from zero V to breakdown V. I want to do this now but will have to unpack my scope etc to do that. Maybe I should...!
The reason I want to do this is because sometimes it's hard to say if the device is genuine or not . Hfe is lower than expected but will it tolerate the high supply voltage ? So if I can find that out at least I will know it will not break down in the circuit due to the supply voltage.

 

[Mooly]

If you have doubts over devices being genuine then all bets are off You could be working with anything.

 

[ashok]

Yeah, that's true of course . Could still be used in non critical places and in high voltage circuits it would be useful to know how much it can take. Maybe the 'tester' isn't really worth it if one can get parts from genuine sources!

 

[ericj]

I don't know that specific device, but, thought I'd drop this forum thread i found on the general subject of cheap device testers:

http://www.eevblog.com/forum/testgear/$20-lcr-esr-transistor-checker-project/

There's open source software for a lot of them.

 

[cliffforrest]

It may be worth pointing out that all production semiconductor testers (in my now out of date experience!) do the following:

All simple breakdown voltage tests are done at 10uA using a constant current circuit with a defined pulse width.

Secondary breakdown and sustaining breakdown (BVceo etc) are potentially destructive and the SOA curves have to be respected, particularly pulse widths - often 300uS or less - and duty cycle.

Test circuits to do this are complex.