# need variable DC volt regulator gizmo

Discussion in 'Electronic Basics' started by Michael Eisenstadt, Jan 14, 2004.

I'm not sure that sci.electronics.basics is the right SIG
for this question. If there is a more appropriate newsgroup
I would appreciate your pointer to it.

I have a project where a 12V DC motor and gear box
slowly turns a circular disk. I want to have the disk
turn as slowly as possible. I discovered that this 12
DC motor/gear box which has a plate RMP of .45 can
be slowed down by supplying it with less voltage. In
the case of one of the motors, 4.5 V (from 3 flashlight
batteries) turns the motor nice and slowly. The other
motor requires around 6 V for it to turn. The motors
take .3 of an amp at 12 volts according to their specs.

I am looking for DC voltage regulator that I could
put in line with batteries so that I could more
exactly choose the speed of the motor.

I looked in the Grainger catalogue for something
that might do this and just visited a local Radio
Shack but I don't know exactly what I am looking for.

Please tell me what is the name of the gizmo I need
for this project. And where I might buy one.

2. ### John PopelishGuest

Probably the simplest way you can get a stable regulated voltage is to
use an LM317 regulator IC mounted on a few square inches of sheet
aluminum or some other heat sink.

Here is the data sheet:
http://cache.national.com/ds/LM/LM117.pdf

These cost about a dollar at Radio Shack, and the data sheet shows how
to connect a little adjustable resistor, a fixed resistor and a
capacitor to precisely lower the 12 volt supply down 10 to 1.2 volts.
See figure 1. If you have any questions after reading the data sheet,
I will try to answer them.

4. ### Greg NeillGuest

How about a series resistor? You want to drop 7.5
volts at 0.3A in one case, and 6V at 0.3A in the
other. The maximum power to be dissipated is 2.25W,
so you'd need something like 5W resistors.

4.5V/0.3A = 25 Ohms

6V/0.3A = 20 Ohms

Greg,

Wouldn't this solution require a steady source of 12V DC?
I have been using D batteries to power the motor because
in this application it is preferable to be independent of
house current + transformer.

I may have not made this clear in my original posting.

Now Im not sure if the solution suggested by John Popelish:
will maintain a specific voltage supplied by a 12 V battery
as the battery slowly loses voltage seeing as the LM317 solution
will require resistors.

Thanks in advance for any and all suggestions.

6. ### Greg NeillGuest

Yes, it assumes a steady supply.
Since your required voltages are significantly below the
nominal 12V supply, the regulator will be able to provide
a steady output until the batteries are almost exhausted
and they fail.

7. ### Ross MacGuest

If you aren't comfortable building a motor control circuit and don't mind
spending a few bucks, try this place http://www.minarik.com/pwm.html . I
linked you to the pwm modules. These folks are very helpful and will help
you with your application over the phone. Good luck with your
project....Ross

8. ### John PopelishGuest

I was not paying close attention to your intended source of power when
I suggested the LM317 regulator. It is pretty wasteful and must
discard 2 volts or so to maintain good control of the output voltage.
If you don't mind the short battery life and have at least 2 volts to
spare, it is still workable and will provide better speed control than
a series resistor.

A much more efficient solution would require a switching buck
regulator that takes quite a few more components to get working well
than the LM317. But it would have something like 80 or 90% efficiency
and would work much better with a battery that had double or so the
needed motor voltage. For instance, if you had a 12 volt battery (two
6 volt lantern batteries, for example) and a motor requirement of
about 6 volts at .3 amperes, the switching regulator would provide
this while drawing less than .2 amps from the batteries.

But unless you want to purchase a packaged switching regulator, this
is a pretty big first electronic project. If you try the LM317 and
get satisfactory speed control, but would like to upgrade to a
switcher, I'll see what I can do to get you started.

9. ### CFoley1064Guest

Three options:

1) Quick & Dirty -- Use your four "C" batteries, and put a power rheostat in
series. This extra resistance will drop some of the 6 volts, leading to a
lower voltage across the motor, and lower speed. I happened to find a 15 ohm,
15 watt rheostat at All Electronics Corp for \$1.65 USD in single quantities
which might fill the bill:

http://www.allelectronics.com/cgi-bin/category.cgi?category=470&item=RHE-1
5&type=store

This is the simplest solution. This power rheo would be at least \$20.00 from
Newark, so it wouldn't be the most economical if the rheo wasn't available
surplus.

2) A series rheo will have very non-linear control of speed. John has
suggested using an LM317 with a couple of caps, a couple of resistors, and a
pot to set output voltage. Since the dropout voltage of the LM317 is about 3V,
you will either need 6 "C" or "D" batteries instead of 4, or you might want to
duct tape together 2 6V lantern batteries to get 12VDC.

3) Even with precise, linear control of the voltage like an LM317, you still
might have problems at low speeds, because torque on those little DC motors
drops off a lot at lower voltages/RPMs. You may have trouble getting it to
stay at the right speed without stalling out. One potential solution to that
is using Pulse Width Modulation to switch your 9V or 12VDC on and off very
quickly (several KHz) to get a lower average voltage while maintaining torque.
That will tend to allow the motor to go a lot slower before it stalls out (a
potential problem with the LM317 solution). If you've got a little time, you
can order the Velleman K8004 DC to Pulse Width Modulator kit from Jameco as
their P/N 120539 for \$22.95 USD. This kit is made precisely for what you want,
it's probably the best solution from a technical standpoint, and is made so a
beginner can do the job well. It's also probably the most expensive of the
three.

Good luck
Chris

10. ### MichaelGuest

The LM317 will maintain its set output voltage, until your battery
voltage drops to around 2.5V more than the output voltage.

For example if you set it to 7V it will stay at 7V until you 12V
battery drops to about 9.5V, thats the whole idea of a regulator.

A couple of pointers: The caps shown in the application note need to
be very close to the regulator pins, and the tab on the top of the
package is the same connection as the middle pin of the package, so
don't short it to ground.