Re: Voltage regulator help!

<snip>

I was suggested this in another forum and it sounds like a simple and
cheap solution and I do love the KISS principle. A couple followup
questions...

1) Is there any heat concern? 17.5 watts max dissipation according to
my math, should I use a heatsink somehow? How would I accomplish
that?


2) Do I just wire the three diodes in series inbetween the power
source and the component on the positive lead? A multimeter should
verify that its 8.4v alright?


3) I may also do this to run a 6v radio receiver off the 12v battery.
Same deal? 9 .7v diodes (of smaller current) will do the trick or
some
similar configuration thereof?


Many thanks! I don't use Usenet often but it's where I go when I've
exhausted all my other options cuz I always know I'll get the answers
I'm looking for!


Carsyn


You will actually get higher performance from the motor using the full 12v and limiting the current with big
resistors. Although 12v isn't that much above the 8.4v it takes for 5A motor current and you won't see a huge
improvment.

It's a bit tricky to heatsink axial lead diodes even with a PCB, so I'd shop around for some stud mount or
TO-220 case diodes and mount them on some kind of heatsink. If you need to use a high torque stepping sequence
then you'll have to double that power dissipation.

You really shouldn't care as much about the voltage (within reason) as about winding current.
As long as do not excede the rated motor current you will not harm the motor. The motor is an inductive load
and when the current is switched on it will not jump instantaneously to maximum. The change in current over
time is given by di/dt = V/L so for a given inductance the current will ramp up faster given a higher supply
voltage. With .72ohm, (12-8.4)/5, series resistance at time=0 there is no current flow so the full 12v would
appear across the winding and as the current rapidly ramps up the resistor will start dropping the voltage
untill the maximum current of 5A is reached where the voltage will be down to 8.4v. This results in higher
torque available from the motor.
Using diodes is a different matter, the motor never sees the full 12v due to junction drop in the diodes. You
have to dissipate the heat either way, so I'd go for the extra performance.

Just FYI, here's a simple example of a switched mode current limited driver http://www.eio.com/crntctrl.htm
And if you're curious heres a link to an excellent stepper introduction.
http://www.cs.uiowa.edu/~jones/step/index.html

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)
.

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