Need a particular Transistor or device

[april]

1.5V at the gate. I tried two devices with the same result supply was about 12 V I wasnt precise
I set the voltage a little higher at 13.1 and I get 1.8 V on the gate, 10.7 Volts on the drain,and it still gets hot

EDIT I had a BDX540 instead of an IRF540

 

[MikeMl]

1.5V at the gate.

You must have a screw up in the PC layout. There should be 12V on the gate (with respect to ground or the source of the Fet), which will turn the Fet on as hard as it can be turned on. That should make the drain-to-source voltage less than 150mV, which means the Fet wont even get luke warm when switching an Amp.

 

[april]

Apologies Mike . I had a BDX540 instead of an IRF540 . Mixed up in my box and I thought I had the right ones. I dug a bit deeper and got the right one. It is still turned on fully but the voltages are now a supply of 13V with a drop of 50 millivolts source to drain and about 12.96 volts on the gate.

The light is a LED light When the pulse goes high the light goes off

What I need is when the pulse goes high the light comes on. I can change the software but is it possible to alter it in the circuit?Mike and what is a BDX540 anyone know?

 

[MikeMl]

...
The light is a LED light When the pulse goes high the light goes off

What I need is when the pulse goes high the light comes on. I can change the software but is it possible to alter it in the circuit?Mike and what is a BDX540 anyone know?

I have no idea what a BDX540 is. I googled for it and came up with nothing.

If it is an NFet like a IRF540, then the circuit I posted with the two 2N3904s would cause current to flow through the Load when the port pin is high. If that is not happening, you have mixed up the pins on the 2N3904s or on the FET, or the FET is not what you think it is (namely an NFet).

 

[throbscottle]

I still think BDX540 might be bipolar - have you actually tested one with a meter to see?

 

[april]

Yep I had blown the first transistor-changed that and all worked as it should -Thanks Mike
throbswhatever - I have been thinking they were MOSFETS and not game to put a meter near them.
So what tests do you think I should try -normal diode checks both sides? would that be safe if they are a MOSFET?

 

[throbscottle]

So long as you and all the mosfet's leads and your meter are all at the same potential there should be no problem - so just make sure you touch everything you're using before you start and don't go walking around, mid-test. An enhancement mode mosfet should show up as a diode between source and drain (I think in a depletion mode one you'd see a low resistance there but I don't know for sure), with the gate appearing open circuit. A bipolar transistor should show up as two diodes connected back to back with the base being the anodes (for NPN) or cathodes (for PNP).

I work on an old metal tea-tray. It's not earthed, just at "me" voltage.

 

[april]

Make that a BDX54C NPN Darlington and the two I tested with in circuit still work OK
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/4103.pdf'