# small dc motors

Discussion in 'Electronic Basics' started by [email protected], Jan 18, 2006.

1. ### Guest

Hi, does anyone know how I change the start/stop points on small dc
motors? I have a few that I need to control at low revs. When I run
them through a pot, the voltage gets to a point then they just go on at
almost full speed. Any ideas?

Thanks, Raul

2. ### Lord GarthGuest

Assuming you want to control the motor speed, there are several links
to simple pulse width modulators...

http://www.aaroncake.net/circuits/motorcon.htm
http://www.homepower.com/files/pwmhp75.pdf
http://www.solorb.com/elect/pwm/

3. ### ehsjrGuest

Here's a very simple PWM circuit to control your motors
at lower RPM. Be aware that these small motors usually
have some minimum speed that you can't expect to go below.
For example, don't expect to get 1 RPM out of a motor
rated for 12000 RPM.

+12 --+----+-----------+--------------+------+
| | | | |
| | | --- |
[1K] | ------------ /_\ [Motor]
| +-----|4 8 | | |
P | | +------+
50K O<---+-----|7 | |
T | | 555 | c/
| \ / | 3|---[150R]---| TIP31
[1K] --- +-|6 | e\
| | | | | |
| +---+-|2 | |
.1 [C1] | | |
| +-|5 1 | |
| | ------------ |
| .01 [C2] | |
| | | |
Gnd --+--------+-------+---------------------+

Both diodes are 1N4001. Depending on the motor,
you may need to put the TIP31 on a heat sink. On
a heatsink, it's good for up to 3 amps.

Ed

4. ### Guest

Thanks guys, do you think they will work with lower voltages like 1.5v
or 3v? The motors are small.

Ed- is there another schematic of your circuit? I have a hard time
reading mono-spaced font schematics. I'm not that good yet. I'm sorry.

Raul

5. ### Jasen BettsGuest

I've had some success with this circuit

+V --+--. _.--+
| \ /| |
| \ / |
| ~~~T~~~ |
| | |
| | |
| v | _
+----[10K]----+-------[(M)]---+
~ |
----------------------------------+

I discovered it before I understood it (by connecting parts more or
less at random) I used a 2n3055 transistor (because it was the largest
I had) and put the circuit on a cable with a 2.5mm plug and used it to
slow a regular tape recorder to about half speed ...

Bye.
Jasen

6. ### Guest

Last year a guy named Robert gave me this schematic which I have been
using for a year now with no problems.
(http://www.2minclip.biz/controller/1.html)But, when I try to use it on
some motors the transistors get hot and burn up. I have been using it
on a 6v motor out of a cd player with no problems. Can I change this in
some way so it will work?

Raul

7. ### Lord GarthGuest

Use a fixed width font like courier to view his post...

If you use OE, click view, text size, fixed.

8. ### Geir_HKGuest

My guess is that the opamp is very fast, and the transistors is very
slow, resulting one transistor start conducting before the other
transistor cease.

It'll be even worse if that opamp is a type of "voltage comparator".
Then it may switches rapidly between low and hi state (making
transistors hot as I described above) in order to keep the average
voltage about equal to center-tap on the pot.

9. ### ehsjrGuest

I'll send a jpg to your E-mail.
Ed

10. ### ehsjrGuest

Those transistors are too small for driving some of
your motors. Use power transistors like the TIP31C
and TIP32A. Put each transistor on a heatsink.

Ed

11. ### Guest

Thanks Ed, I got it.

Here's my last question then I'll go away until I've built it.

Can I make the circut reversable? So I can control the motor in both
directions?

Thanks, Raul

12. ### Ross MacGuest

If you use a PWM controller the applied DC control voltage, usually +-10vdc
or less will control the direction and speed of the motor. And on top of
that it will stop on a dime and if you add an encoder or tachometer feedback
attached to the motor or device you are driving you will get servo lock at a
perfect zero volts. Add a computer with a DAC card to read the encoder
counts and you can close the loop and have full positioning control.....Ross

13. ### ehsjrGuest

I assume your question applies to the schematic I
with any DC motor circuit is to use a DPDT switch
or relay to reverse the wires that connect to the
motor. The diode across the motor is replaced by
a resistor-capacitor snubber.

I'll send a jpg to your e-mail.

One caveat - there are DC motor assemblies with
gearing that prevent reverasal.

Ed

14. ### Bob MonsenGuest

Look at the output transistors again. It is a voltage follower.

--
Regards,
Bob Monsen

"a scientific man ought to have no wishes, no affections, -- a mere
heart of stone"
-- Charles Darwin

16. ### Guest

To anyone who might know. Ed mentioned switching the transistors to
larger ones, "Those transistors are too small for driving some of
your motors. Use power transistors like the TIP31C
and TIP32A. Put each transistor on a heatsink."

I'm using KN3904(NPN) and KN3906 (PNP), the data sheets on TIP31C and
TIP32A don't say which is PNP or NPN. Does it matter?

Raul

17. ### Anthony FremontGuest

That's funny, the datasheet I looked at clearly says on the first page.
;-) TIP31 is NPN, TIP32 is PNP.
http://users.ece.gatech.edu/~jskenney/TIP31A-D.pdf
Extremely.

18. ### Homer J SimpsonGuest

But then you should use a motor that costs more than \$1 retail.

19. ### Ross MacGuest

Agreed there Homer!....Ross

20. ### Guest

Anthony, you're right. I did'nt see it was listed at the top. Thanks,
Raul