problem with transistor

Britannicus

help me I'm doing something dumb,

I set up a simple transistor circuit using a BC558B PNP transistor - it seemed to work ok (see example in attached diagram (first)

with the collector conected to vdd, and the emitor to vss with an LED in between. When is pulsed P11 between high and low, good result and the LED flashes on when P11 is low.

now for the problem.

As per the crude diagram (second) I replaced the vdd conection to a 6 volt supply, replaced the LED with a 6V 1a motor, replacing the connection from the emitor to Vss with the negative on the battery - I'd expected this to pulse the motor in the same way as the LED flashed (nb the motor runs fine if connected to the battery directly to +ve and -ve terminals) - the damn thing doesn't even hiccup - what am I doing wrong ??

Should I be using a different spec of transistor or is it more fundamentally stupid of me ?

Mike G

Do you have a common ground rail?

Look up transistor switch and make sure you have a diode so when the magnetic field collapses you don't blow something up.
http://www.pykett.org.uk/EvolutionTransistorSwitch.gif

Mike Green

For this type of circuit (common emitter), the emitter (E) of a PNP transistor should be connected to +6V and the collector (C) should be connected through the load (motor) to -6V. For a 1A load (assuming the transistor is designed for this load), the base will need something like 30mA to 40mA to switch on properly, so you'll need a 180 Ohm or 200 Ohm base (B) resistor. When the base resistor is connected to Vss (-6V), the transistor should switch on. You'll need the diode across the motor with the cathode (marked with a band) connected to the collector end of the motor. A 1N4001 diode or equivalent should work fine.

You have the PNP transistor connected in reverse for this sort of use (collector to +6V, emitter to load). It's not meant for this although it will work.

Britannicus

Cheers Gentlemen - once again I'm in your debt and my total-dummy status is exposed shamelessly !!

I'm assuming that the same essential structure would apply just as well with an NPN transistor, but this will work in reverse ( i.e. will function when High) - I've not been using a common ground, so will explore this circuit further.