Re: emitter followers / Darlington - need help

mrdarrett@xxxxxxxxx wrote:
Hello,

I'm trying to use a low-voltage signal from my PC's parallel port (<
5V) to activate the gate of a mosfet, which will switch a 9-12V supply
and lamp.

Something similar to this, under "NPN Darlington Configuration":
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/emitfol.html

When I built this, though, it seems that the mosfet ALWAYS wants to
stay on. Really weird. At first I thought I fried my transistors
somehow, but there's no short (after testing with my dmm).

I'm using a TIP31A for the first transistor, and an IRF530 as the
mosfet.

Why I'm doing it: I'd like to be able to control a small motor via my
parallel port, basically simulating a PWM. Unfortunately, the feeble
5V from the parallel port's Pin 2 (plus 2000 ohms of resistor) isn't
enough to fully open the gate mf my mosfet. So, I'm trying to amplify
the signal, so to speak.

Am I on the wrong track here? Any pointers?

I think so. I don't know what you are doing wrong that is keeping the mosfet on all the time, but an emitter follower provides no voltage gain, but actually loses a diode drop off the input voltage, just to forward bias the transistor.

If I were you, I would start with a logic level mosfet (one with a low turn on voltage that could be connected directly to the parallel port output). Otherwise, you need a 10 to 12 volt supply and an amplifier with voltage gain, that will increase the voltage swing available from the parallel port to about 10 volts needed to switch a non logic level mosfet.

Such an amplifier can certainly be built with one to three transistors, but you can also get integrated gate drivers that accept the parallel port signals as inputs and provide full supply swing output with a low enough resistance to charge the gate capacitance very quickly, to provide low loss switching.

For instance:
http://ww1.microchip.com/downloads/en/DeviceDoc/21393C.pdf
.

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