Driving motors, solenoids, etc. from a parallel port

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I've been trying to rig up some motors (both DC and stepper) and
solenoids to a PC's parallel port. I seem to be running into two
problems:

1) My buffering IC (between the parallel port and the devices) cannot
source the current needed for the devices.
2) The DC-DC power supply run off of the canibalized wall-wart does not
seem to keep up with the current draw I'm taking.

Now, I took a bunch of EE classes in college and work around (but not
directly on) a lot of EE stuff. So, I've got the basics filed away in
my brain somewhere, but I feel like I'm just stumbling around here. So
I guess my questions are, for 1) can I build some sort of circuit using
indivdual transistors for each signal from the parallel port to gain
more current capability? And for 2) can I put down a bunch of caps in
parallel with Vdd and Gnd to buffer up enough current to drive the
devices when I turn them on (I'm only ever turning them on
momentarily)? Or, do I have to go to a more polished voltage
generator?

Thanks for any help.
Todd

 

[Randy Day]

I've been trying to rig up some motors (both DC and stepper) and
solenoids to a PC's parallel port. I seem to be running into two
problems:

1) My buffering IC (between the parallel port and the devices) cannot
source the current needed for the devices.
2) The DC-DC power supply run off of the canibalized wall-wart does not
seem to keep up with the current draw I'm taking.

Now, I took a bunch of EE classes in college and work around (but not
directly on) a lot of EE stuff. So, I've got the basics filed away in
my brain somewhere, but I feel like I'm just stumbling around here. So
I guess my questions are, for 1) can I build some sort of circuit using
indivdual transistors for each signal from the parallel port to gain
more current capability? And for 2) can I put down a bunch of caps in
parallel with Vdd and Gnd to buffer up enough current to drive the
devices when I turn them on (I'm only ever turning them on
momentarily)? Or, do I have to go to a more polished voltage
generator?

http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#p_input

HTH

 

[Bob Masta]

I've been trying to rig up some motors (both DC and stepper) and
solenoids to a PC's parallel port. I seem to be running into two
problems:

1) My buffering IC (between the parallel port and the devices) cannot
source the current needed for the devices.
2) The DC-DC power supply run off of the canibalized wall-wart does not
seem to keep up with the current draw I'm taking.

Now, I took a bunch of EE classes in college and work around (but not
directly on) a lot of EE stuff. So, I've got the basics filed away in
my brain somewhere, but I feel like I'm just stumbling around here. So
I guess my questions are, for 1) can I build some sort of circuit using
indivdual transistors for each signal from the parallel port to gain
more current capability? And for 2) can I put down a bunch of caps in
parallel with Vdd and Gnd to buffer up enough current to drive the
devices when I turn them on (I'm only ever turning them on
momentarily)? Or, do I have to go to a more polished voltage
generator?

Thanks for any help.
Todd

The basic idea is to have an external supply that is
capable of driving the motor or other load. Connect
the negative side of that to the ground pins of the
port. Connect the emitter of an NPN power transistor
to the same ground. Connect the base to the port
output through a current-limiting resistor (say, 1K).
Connect the motor between the collector and the
positive lead of the external supply. Put a rectifier
across the motor, oriented such that no current
flows normally (arrow points to positive supply).
That's just to block inductive kick-back.

Now when the port output goes high, the NPN goes
on and pulls the collector low, turning the motor on.
The success of this simple circuit depends upon how
much current the motor needs versus the gain of the
NPN. You might need a Darlington (arrangement
of 2 NPNs) to get extra gain in some cases.

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!

 

[Mike]

I've been trying to rig up some motors (both DC and stepper) and
solenoids to a PC's parallel port. I seem to be running into two
problems:

1) My buffering IC (between the parallel port and the devices) cannot
source the current needed for the devices.
2) The DC-DC power supply run off of the canibalized wall-wart does not
seem to keep up with the current draw I'm taking.

Now, I took a bunch of EE classes in college and work around (but not
directly on) a lot of EE stuff. So, I've got the basics filed away in
my brain somewhere, but I feel like I'm just stumbling around here. So
I guess my questions are, for 1) can I build some sort of circuit using
indivdual transistors for each signal from the parallel port to gain
more current capability? And for 2) can I put down a bunch of caps in
parallel with Vdd and Gnd to buffer up enough current to drive the
devices when I turn them on (I'm only ever turning them on
momentarily)? Or, do I have to go to a more polished voltage
generator?

Thanks for any help.
Todd

You may have enough time to charge a cap to supply the current you need,
but I sure wouln't mess with that. Just get a bigger wall wart and be
done with it. There are about a zillion surplus power supplies available cheap.
Maybe even an old PC power supply if your motors will run on 5 or 12 volts.

Mike



"The scientist is possessed by the sense of universal
causation...His religious feeling takes the form of
rapturous amazement at the harmony of natural law,
which reveals the intelligence of such superiority
that, compared with it, systematic thinking and acting
of human beings is an utterly insignificant reflection."
Albert Einstein (theoretical physicist)

 

[Chris]

I've been trying to rig up some motors (both DC and stepper) and
solenoids to a PC's parallel port. I seem to be running into two
problems:

1) My buffering IC (between the parallel port and the devices) cannot
source the current needed for the devices.
2) The DC-DC power supply run off of the canibalized wall-wart does not
seem to keep up with the current draw I'm taking.

Now, I took a bunch of EE classes in college and work around (but not
directly on) a lot of EE stuff. So, I've got the basics filed away in
my brain somewhere, but I feel like I'm just stumbling around here. So
I guess my questions are, for 1) can I build some sort of circuit using
indivdual transistors for each signal from the parallel port to gain
more current capability? And for 2) can I put down a bunch of caps in
parallel with Vdd and Gnd to buffer up enough current to drive the
devices when I turn them on (I'm only ever turning them on
momentarily)? Or, do I have to go to a more polished voltage
generator?

Thanks for any help.
Todd

Hi, Todd. Making an electrical connection between a PC and inductive
loads can cause problems. The voltage transients from commutation or
turn off of an inductor (remember Herr Lenz?) will play hob with your
port, and possibly also with the PC itself.

The best solution is to use a Solid State Relay (SSR) for each bit of
the output data word (assuming your port is set up as a SPP). The
output lines can easily drive the SSR (several mA at 3.3 - 5VDC), and
you can just hook up your loads on the other side of the barrier with
the separate power supply.

DC-in/DC-out SSRs are commonly available with many different current
ratings. If you're not switching anything at a very high frequency,
they'll be ideal for your project (if a little expensive). You can
also roll your own with optocouplers (I like the H11G2 optodarlington
for this) with a darlington transistor. You can set up a medium speed
(a couple hundred Hz) SSR for about $2 or $3 USD per bit, with each bit
being able to switch the rated current of the darlington transistor.

If this isn't clear, feel free to post again with more information
about your project (like output voltage and current requirements).

Good luck
Chris