Three blown fet's?

[pentium]

I was anxious to get my new tape drive going and when it came time it betrayed me.
After a few hours of frusterated searching, I discovered what appeared to be three burned and cracked FET's. While I do not know for certain if they really are, I was able to get their numbers:

-IRFD
8AI(symbol like diode)R1P
9014

-IRFD
9NI(symbol like diode)R5X
010

-IRFD
8UI(symbol like diode)R7Z
9014

there is also another one with a discoloured terminal. They have two terminals on one side and a single one on the other.
sorry no pics.

[gonzo0815]

Re: Three blown fet's?

Desolder them, and chek the for short between gate - drain, gate -source and between the two others. Then chek if there is an short in the circuit. Remeber, mosfets are elektrostatic sensitive devices. If those mosfets are powering a coil or dc motor, make shure there is a free wheeling diode and or a snubber network in the circuit.

[pentium]

Re: Three blown fet's?

Wouldn't the crack on each of them scream "replace!"?

anyways, yes they were used for the motor.

[gonzo0815]

Re: Three blown fet's?

Yes, the crake obviousely do that. But if you do not measure the circuit for other defekts, your replacemnt will instantly do the same.

[pentium]

Re: Three blown fet's?

Testing showed that the board they were soldered on is okay however one resistor (located under one fet) was burned out and was caused probably by the severe heat from the fet.
Testing shows nothing every direction for two and a short in the third.

[gonzo0815]

Re: Three blown fet's?

If the burned resistor is conected to the gate of the fet, it is for a saefty measure, to limite current flowing through the gate. The value depends from the needed switching speed and from other factors. Usually a few ohms to a few 100. If there are the same resistors on the other fets, you can probably use the same value.

The mosfets are no problemtic ones, either go with the original parts, or some similar ones. Datasheets are availiable at http://www.datasheetcatalog.net, you just have to type in the part number, e.g. IRFD9014. Those mosfets do probably form a H bridge, wich can shift the polarity of the motor supply voltage, to alter the direction. Usually you need 4 mosfet for a full H bridge, unles the DC motor does have 3 wires.