Mosfet Tester - Good Design?

[Craigt81]

Hello everyone,

I am fairly new to the electronics repair field and brand new to bad caps forum. I have only been tinkering with electronics for about the past 2 years in my spare time. So please bare with me.

Recently I acquired a Panasonic Plasma that is dead and blows fuses on plug in and I am trying to test the Power Mosfet N Transistor (RJK5020). I have been searching the web for a decent simple circuit diagram and design to build a test unit. I have built the following and tested the transistor on it. But my findings are not conclusive to what the poster of the design says they should be.

According to the website I got this design from this is what should happen.

Test switch OFF , LED OFF = Good
Test Switch ON , LED ON = Good
Test Switch OFF , LED ON = BAD
Test Switch ON , LED OFF = BAD

But I am getting in both positions my LED on. Any input would be greatly appreciative.

Thank You,
Craig T.

[tom66]

Re: Mosfet Tester - Good Design?

Your MOSFET is bad then, it is short circuit. Assuming the circuit is wired up correctly, of course - try with another similar MOSFET.

[Craigt81]

Re: Mosfet Tester - Good Design?

Thank you very much for the input. I will locate another mosfet from a power supply i have laying around and try it.

[budm]

Re: Mosfet Tester - Good Design?

That circuit is made for testing N-Channel MOSFET only.

[Craigt81]

Re: Mosfet Tester - Good Design?

That is correct I am testing a N-Channel (RJK5020).

[budm]

Re: Mosfet Tester - Good Design?

If you put a jumper between Gate and Source, but do not hit the switch, if the LED is still on, then you have bad MOSFET.

[Tube_Dude]

Re: Mosfet Tester - Good Design?

That circuit is made for testing N-Channel MOSFET only.

Invert the battery polarity, and bingo. Now you have a P-Channel MOSFET Tester.

The best is to use a switch for battery polarity inversion, and you have a both channels Mosfet Tester...

[budm]

Re: Mosfet Tester - Good Design?

Not that simple, you need to reverse the LED also, or he has to use BI-COLOR LED or two LEDs.

[Tube_Dude]

Re: Mosfet Tester - Good Design?

Not that simple, you need to reverse the LED also, or he has to use BI-COLOR LED or two LEDs.

Another LED in parallel and in opposite direction will do the trick... One for P other for N.

[tom66]

Re: Mosfet Tester - Good Design?

Or a filament bulb!

Reminds me of my ELEC1130 lab, some students were confused why dimming a bulb using the function generator didn't work, always at 100% brightness. It was set to AC -- the bulb was receiving essentially the same RMS voltage no matter what the duty cycle was set to.

[Tube_Dude]

Re: Mosfet Tester - Good Design?

Or a filament bulb!

Yes! Maybe the best solution.

[budm]

Re: Mosfet Tester - Good Design?

Agree.

[MDOC]

Re: Mosfet Tester - Good Design?

A filament will work, but I prefer two opposite parallel LEDs and a switch to reverse the battery. You may run across an unknown MOSFET. With LEDs, you'll know the channel type. So, not just a checker, but also a type identifier.

[budm]

Re: Mosfet Tester - Good Design?

One 2-leg BI-COLOR LED (RED/GREEN LED connected in opposite parallel inside). Uses lees power than filament lamp.
One bad thing about the circuit is that if the MOSFET has built-in protection Diode between D and S, one of the LED will be on if the batteries switch is set to the wrong polarity.

[Tube_Dude]

Re: Mosfet Tester - Good Design?

One bad thing about the circuit is that if the MOSFET has built-in protection Diode between D and S, one of the LED will be on if the batteries switch is set to the wrong polarity.

This is not a built-in protection diode between D and S, but a "parasitic" diode inherent to the structure and present in all Mosfets.

[MDOC]

Re: Mosfet Tester - Good Design?

My mistake. Reverse voltage may damage the MOSFET. The MOSFET channel type must be known before it is tested.

[budm]

Re: Mosfet Tester - Good Design?

@TUBE DIODE, I guess you have never design products with MOSFET that has the Diode built-in:
https://cdn.badcaps-static.com/pdfs/...5c85166995.pdf

Most MOSFET had D-S reverse breakdown voltage of 30V or more, so 9V is not going to do any damage.

[Tube_Dude]

Re: Mosfet Tester - Good Design?

@TUBE DIODE, I guess you have never design products with MOSFET that has the Diode built-in:
https://cdn.badcaps-static.com/pdfs/...5c85166995.pdf

Yes I know it! Sometimes is used a faster diode and with lower voltage drop, in parallel with intrinsic diode of the Mosfet NP structure, to speed up things in certain circuits. But in any case, all Mosfet have the intrinsic diode, and that is the important issue in discussion about the use of your Mosfet tester.

Ahh, and my nick name is TUBE DUDE and not TUBE DIODE or TRIODE...

[budm]

Re: Mosfet Tester - Good Design?

Not my MOSFET tester, its CRAIGT81.

[Tube_Dude]

Re: Mosfet Tester - Good Design?

Not my MOSFET tester, its CRAIGT81.

Sorry, about that.