Help... transistors/mosfets confusion

[mitchjs]

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

 

[Nigel Goodwin]

Try my PIC tutorial Hardware Extras page at 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.

 

[mitchjs]

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

 

[williB]

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

 

[Sebi]

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.

 

[mitchjs]

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