[mikerz]

Im going to ask if zener diode BZX55C24 can be replaced by IN4749 ?

Both of them are 24 Volts but I don't really know if they are the same because they have same volts.

I just buy this diode here at my local electronics store, I did not put any information only BZX55C24 Zener Diode and they confuse what is the number of that Diode and they said to me confusingly it was 24 Volts? And they gave me IN4749 Zener Diode.

That BZX55C24 Diode was short-circuit and was connected to the gate of WFF3N80 which also have open-circuit gate, these are found when I troubleshoot my Power Supply Unit of computer.

Help...

[KISS]

The package is different and it's rated for 1 Watt rather than 500 mW. It should work. The data sheet for the substitute is here: IN4749A pdf, IN4749A description, IN4749A datasheets, IN4749A view ::: ALLDATASHEET :::

The original part is surface mount, has 3 leads, but one isn't used.

After you get the PS to work, you can then replace with original parts.

[DanielF]

Firstly, I'm not sure what part KISS is referring to as surface-mount, but the BZX55C24 is 500mW and in a DO-35 standard through-hole case, while the IN4749 is 1W and in a DO-41 standard through-hole case. You will probably have trouble fitting the IN4749's thicker leads through the holes for the original diode. Before thinking of drilling-out the holes, make sure the PCB is NOT multi-layer, otherwise you could drill-out an internal connection!

Secondly, what makes you think the WFF3N80's gate is open? This is a power MOSFET with an insulated gate, so you won't measure any resistance with a standard multimeter, though you CAN check if the MOSFET gate is working with a multimeter...

Assuming you have a digital multimeter whose red lead carries a positive (+) voltage (of at least 2V) when in the ohms mode, do the following sequence of tests to check the MOSFET, which must be completely removed from the circuit:

* Short the gate to the source and measure resistance from source (-) to drain (+). It should be open-circuit.
* Remove the gate short, and measure resistance from the source (-) to the gate (+). It should also be open-circuit. Take care not to touch the wires with your fingers - operate on an insulating benchtop by touching the meter prods to the MOSFET wires, without touching them yourself. Don't handle or touch the MOSFET wires between this test and the next one.
* Now re-measure resistance from source (-) to drain (+). It should now measure a low resistance (from ohms to hundreds of ohms).

This works because the gate resistance measurement charges up the gate (from the multimeter's output voltage), and it stays charged (for minutes) because the gate is well insulated. So the MOSFET is now ON, and thus measures low resistance source-to-drain.

If any of these tests fail, the MOSFET is faulty.

Regards,
Daniel

[KISS]

Mmmm! Look here:

http://www.datasheetcatalog.org/data.../BZX55C3V6.pdf

[DanielF]

Yes, but that's not a 'mainstream' manufacturer! Here is the proper datasheet (the BZX55 series originated with Philips, now handled by Vishay):
http://www.fairchildsemi.com/ds/BZ%2FBZX55C24.pdf
http://www.vishay.com/docs/85604/bzx55.pdf

I have the Philips S1 09-82 "Diodes" databook (from 1982), and the BZX55C series is clearly shown as DO-35 case.

Sorry to be so contradictory!

Daniel