I have a circuit, here is a snip of it..

the incoming BUS_ON signal is 5volts when on...
the SS_PIN is outgoing to SlaveSelect on SPI port of AVR
so i take the inverted bus_on and send it through a 1k to the base of a PNP trans, emitter to 5v and collector to source of device

anyway... what i need to happen and it does in this circuit is power up a device that needs about 500ma when bus_on is high... like i said it all works fine here..


it all works, until i find out i need to control a 12v device same 500ma
if i connect the emitter to 12 volts, it no longer works, the transistor is always on

why?

and better yet how do i fix?

i now need to drive a 12 volt device that uses about 500ma...

could a P-channel mosfet work here?

attached is the partial schematic

some help please...
maybe some education too

mitch

Attached Images

circuit_389.jpg (40.6 KB, 307 views)

 

Try my PIC tutorial Hardware Extras page at http://www.winpicprog.co.uk/pic_tutorial_extras.htm which shows how to do it, basically you need to switch the bottom end of the load. To switch the top end would require two transistors.

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in my application, i cannot SINK
i have to source...

can you show me the 2 transistor method?


the device has a 12volt turn on lead, which i can use
it will be much much less current...



mitch

 

what is the device..?
i assume that the right half of the circuit is still 5V..?

 

The PNP transistor open when emitter-base voltage higher as 0.6V. If the emitter connected to +12V and the base on +5V, the 7V difference much higher as 0.6V. Apply a zener as level-shifter and a resistor for proper closing.

Attached Images

circuit_389_539.jpg (16.0 KB, 260 views)

 

ok...

so when the logic goes to 5v, the voltage difference of 12-5 = 7
will keep the zener from conducting...
and the transistor will be off

is that correct?

mitch