HFE and Testing

[killian6pk]

I have an autoranging DMM that has a spot to put transistors labeled hfe and with a hole for: base, collector, emitter and collector and on my selector switch there is an hfe setting. Two questions. Is this sufficient to test these transistors and mosfets? Reading wise if I get a reading that falls within the minimum and maximum ratings on a Data Sheet for that transistor etc. does that mean that these are not shorted and are ok?

[PlainBill]

Re: HFE and Testing

I have an autoranging DMM that has a spot to put transistors labeled hfe and with a hole for: base, collector, emitter and collector and on my selector switch there is an hfe setting. Two questions. Is this sufficient to test these transistors and mosfets? Reading wise if I get a reading that falls within the minimum and maximum ratings on a Data Sheet for that transistor etc. does that mean that these are not shorted and are ok?

Hooo, boy!!! Lots of good questions, let's see if I can come up with some good answers, and tehn elaborate some more.

1. That is one method of testing a bipolar (base, collector, emitter) transistor. Mosfets work on a different principal, and this tester may not give meaningful results.

2. If a transistor is rated at a beta of 400-650 and you get a reading in that range, the transistor is good.

Now for the comments. Using this feature of your dmm is overkill in most cases. The only exception in monitor repair would be if you suspected you had a bad (low hfe) lot of 2SC5707 transistors (used in the inverters in Benq, Dell, and HP monitors).

A simple base-collector, base-emitter, collector-emitter resistance check will find any shorted transistor without the need of pulling it from the circuit board. There are a very few cases where the resistance test will give a false 'bad' reading - chiefly on the horizontal output transistor of CRT monitors. If your DMM has a diode test function, a simple base-emitter, base-collector test will ensure you aren't dealing with an open transistor.

PlainBill

[killian6pk]

Re: HFE and Testing

Thanks Plain Bill you are always johnny on the spot with your answers. I appreciate it and I know everyone you help appreciates it also. I don't know how your read my mind, but the 5707 is what I was thinking about when I asked this question. I have one of the Dell/BenQ 172fpb 17" monitors that I decided to try to work on and that was the part I knew was bad sometimes. So add mind reading to your many other abilities.

[retiredcaps]

Re: HFE and Testing

I have one of the Dell/BenQ 172fpb 17" monitors

These Benq boards have well known documented problems. Check c5707s, fu9024, and the picofuse. Resolder all the inverter transformer pins.

[Mikeley]

Re: HFE and Testing

I had one of these today with one shorted 5705. I always replace the 5707's in pairs due to lot to lot variation in transistors which can cause failure upon replacement. Both transistors in the royer need to be close gain wise. fu9024 and fuse were fine but will be testing it before it goes back together.

While I'm waiting on deliveries, I re-soldered all traffo, caps, trannies, diodes, links inductors etc on the inverter and also replaced 2 1000uF 10V caps(ESR >> than spec). All others had excellent Capacitance and ESR... so brazenly I have left them in! Will be picking it up again after the Ireland v's Wales rugby tomorrow morning! Come on Ireland!