Re: emitter followers / Darlington - need help

On Thu, 14 Dec 2006 12:59:59 -0500, John Popelish <jpopelish@xxxxxxxx>
wrote:

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.

He's got the emitter of the first transistor floating on the mosfet
gate, if I'm reading him right. When the BJT turns off, the emitter
just hangs there with little more than leakage to allow the mosfet
gate to fall. So it stays on, I think.

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

Also, it may be a good idea to look for isolation here. I gather this
is to drive some motor ("I'd like to be able to control a small motor
via my parallel port.") I don't know what's providing that power, but
it may be nice to galvanically isolate these two devices; the PC port
and the motor circuit.

Jon
.

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