small dc motors

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

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

Last year a guy named Robert gave me this schematic which I have been using for a year now with no problems.

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

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

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

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

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

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

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.

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

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

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

I assume your question applies to the schematic I sent to your e-mail - but an answer that will work 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

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

Trouble viewing? On a Google page:

You can also cut & paste it into NotePad.

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

wrote

That's funny, the datasheet I looked at clearly says on the first page. ;-) TIP31 is NPN, TIP32 is PNP.

Extremely.

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

Agreed there Homer!....Ross

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