Different between analog VOM with DVM on testing transistor or mosfet

[eccerr0r]

Re: Different between analog VOM with DVM on testing transistor or mosfet

Weird... I just measured my analog meter in resistance mode:
Rx1 - measuring 0 ohms, it passes about 120mA (uses D-cell only)
Rx100- measuring 0 ohms, it passes about 1.2mA (also uses D-cell only)
Rx10K - measured around 65 microamperes?!?! (uses D-cell and 4xAA)

Trying to measure a 1W LED, only the Rx10K (expectedly) was able to light the LED. And I'm surprised the probes are actually correct (though resistance doesn't matter, measuring diodes it would be nice if the red probe is the current source and black the sink, when measuring resistance, just to know which is which.

I've only seen one analog meter so far that was hooked up backwards...

[Longbow]

Re: Different between analog VOM with DVM on testing transistor or mosfet

I am still hesitant about this. I have read that some website said that DVM couldn't test transistor accurate because the voltage from DVM probes didn't enough

The "diode test" function which is found on DVM's is designed to forward bias transistor and diode junctions. The meter will not damage the junction because it will limit the current applied to the junction to a safe level. They already thought of that. No reason to worry.

[senz_90]

Re: Different between analog VOM with DVM on testing transistor or mosfet

thank you budm.

[budm]

Re: Different between analog VOM with DVM on testing transistor or mosfet

http://www.littelfuse.com/products/resettable-ptcs.aspx

[senz_90]

Re: Different between analog VOM with DVM on testing transistor or mosfet

Btw, what is resettable fuse? i have read the manual one again and it said this DVM has a resettable fuse. i don't know what it looks like?

[senz_90]

Re: Different between analog VOM with DVM on testing transistor or mosfet

I am see one of norcal715 video, he explained and showed how to test mosfet with his fluke meter, but after read some site and jestine yong article, I am really hesitant about this so maybe one of you had experienced before. My internet quota is almost finished now so i'll try to see it later. thanks.

[Agent24]

Re: Different between analog VOM with DVM on testing transistor or mosfet

Take a look https://www.youtube.com/watch?v=qM7_h8my8eA he also has a video on MOSFET testing

Really, a transistor tester is probably a good idea, if you want to do it 'properly' - but it looks like this method would work most of the time.

[senz_90]

Re: Different between analog VOM with DVM on testing transistor or mosfet

I don't wanna ever try to measure everything high current like 10A with this meter, it would kill myself and burn my house if something bad happened.

I'm not frequently makes an experiment with high voltage, just troubleshoot SMPS, TV, or Monitor. It has caps range that suitable for measure CRT monitor flyback internal caps so this meter is enough for me, maybe I would get an autoranging decent meter (don't consider Fluke because just for hobby) after have some budget maybe on $30-60 range DVM.

[tom66]

Re: Different between analog VOM with DVM on testing transistor or mosfet

The thick wire is the current shunt, it drops about 10mV per amp (0.01 ohm shunt.) Even expensive meters have this. But they are usually fused.

Be very careful with unfused range. If you accidentally put that across the mains, an explosion is possible. That thick shunt can very quickly become an explosion of molten metal, if the building is not correctly protected against such faults. You would not want to be holding onto the meter if this happened.

It's crazy really that they don't bother to fuse the 10A range. I have seen UNI-T recently using BS1363 fuses. This is a good idea IMHO, because the 13 amp ones are rated for high interrupt current and are suitable for mains use up to around 250V. Plus they're cheap, like less than £0.10 (~$0.15) each in quantity, so they don't add much to the meter's cost.

I wouldn't use a BS1363 on a commercial circuit, but I wouldn't feel in danger if I were testing household mains. Luckily my meter uses HRC fuses.. but they cost about £5 each... I have been very careful not to blow any!

[senz_90]

Re: Different between analog VOM with DVM on testing transistor or mosfet

my cheap DVM like this. So the conclusion is 3V enough to test transistor and diode except power mosfet? I don't know that diode test mode have a constant current source, got a new knowledge again thanks . The suck thing from this DVM is 10A range probe didn't have any fuse just a thick wire on it..LOL
on small Ampere probe it just have NTC if i am not wrong reading from manual.

I have try to open it and it looks like small MOV on PC power supply with some number on it, and the thick SUCK wire.

[tom66]

Re: Different between analog VOM with DVM on testing transistor or mosfet

3V will turn on most transistors (with the exception of power MOSFETs which are not logic-level/5V drive capable) and almost all diodes.

Most modern DMM resistance ranges produce 0.2V to 0.3V. The reason for this is it won't turn on most diodes. Also, the voltage is often reverse polarity, (-0.2V to -0.3V) to allow you to test the "normal" way across a diode without turning it on, though it can create confusion.

Because this voltage range is too low to test a diode, there is a dedicated diode test mode on most meters, which on mine can produce about 4V open circuit. This is quite useful because it is high enough to turn on a white LED (Vf=3~3.5V at low currents.) It is actually usually a constant current source (typical circuit is PNP transistor with two resistors) limited to a couple of mA which is ideal for testing diodes.

On most cheap multimeters, there is no extra diode test range, instead it is just integrated into the 2000 or 20k ohm range. This is usually indicated by the diode symbol next to this range.... sometimes it is constant current but not always, so don't depend on it.

[senz_90]

I am still hesitant about this. I have read that some website said that DVM couldn't test transistor accurate because the voltage from DVM probes didn't enough, Jestine yong also said this. I have try to measure my DVM on diode test mode has 3VDC output compare with my analog VOM has 3VDC output too except x10k range(12V DC). on resistance mode (DVM) it has about 0.3VDC as measure. I know most of you use Fluke DVM and analog VOM didn't famous anymore. Is this true ? anyone have experience with this before where is the transistor tested good with DVM but tested bad with analog one?

I am appreciated comment and glad to hear your sharing.