VERY simple dc motor drive circuit?

Discussion in 'Electronic Basics' started by John E., Mar 2, 2007.

1. John E.Guest

5vdc/670mA max miniature gear motor (load is such that current will be far
below 670). I want to set speed with pot control and leave it (may replace
pot with fixed R when best speed is discovered). Forward only, no reverse.
Just need single transistor, if possible, to do the work. Will use wall wart
p.s. for power.

Speed accuracy, etc. not necessary. This is for friend's art exhibit.

Google turns up an extraordinary number of patents, data sheets for quite
bright ideas using controller ICs, H-bridges, digital controls with feedback,
etc. Much more than I'm looking for.

Your ideas or pointers to designs greatly appreciated.

Thanks,

2. Tim WescottGuest

Set up a 555 timer to be PWM controlled from the pot (there are circuits
floating around, ask back here if you can't find one).

Drive the transistor from the 555. If I wanted the World's Easiest
Circuit I'd use an N-channel MOSFET and drive it slowly (100 to 1000Hz)
-- that way I could go straight from the 555 output to the gate of the
FET. If you use a 12 volt supply you won't even have to get a special FET.

+12V
---
|
o----------o
|
- motor
PWM ^
generator |
o----------o
.--------. |
| | |-+
| | |
| |------------->|-+
| | |
'--------' |
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

3. PalindromeGuest

How about a length of resistance wire and a crocodile clip slider? When
you find the right point, chop the spare resistance wire off and replace
the crocodile clip with a permanent connection. Or a loudspeaker
volumer control rheostat? Or add rectifier diodes in series, one by one,
until the desired speed is obtained. Or a few 1.2v christmas tree light
bulbs, adding more until the speed is good - they can even be "power
indicators". Or some quantum tunneling tape or pills and stick weights
on it until the speed is right?
http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=44202&doy=2m3

It really doesn't need an active device acting as a variable resistor
when a real variable resistor will do the job nicely.

4. John E.Guest

Palindrome sez:

If I could just put a pot in series and adjust that, I'd try it. But it looks
like whatever controls it will have to dissipate ~5w. A bit much for a cheap
pot. I could just buy a bunch of hi-W resistors (or -- as you brightly
suggest -- diodes) and mix and match to see what speeds I could cobble up.

But I was hoping for a bit more elegant solution which will allow the artist
the option of choosing the proper speed without me needing to change the
resistor (or diode) so she may see "how that looks".

Once a speed is settled on, it will probably remain fixed for good, but
getting there -- I'd like to have a bit of adjustment available, so I don't
have to be around during the trial.

Thanks again,

5. PalindromeGuest

12w 1 p rotary switch + a handfull of diodes/resistors?

You can get 5W* pots, no problem - as I suggested, have a look at
speaker volume controls. * and higher..

6. Guest

Use Tim Wescott's ckt.

7. Phil AllisonGuest

"John E."

** Use an LM 317 voltage regulator IC after the wart.

Allows the voltage to be smoothly varied from 1.2 volts up.

Works just as good as PWM methods - but with more heat loss.

http://www.national.com/pf/LM/LM317.html

........ Phil

8. PeterDGuest

One (power) transistor, one small pot. Think 'adjustable voltage
regulator' the first power supply every experimenter builds.

9. jasenGuest

here's one I have used in the past, only 2 parts

+V --+---. _.------ > to motor
| \ /|
| \ /
| ------- large NPN
| | (or mid size with heatsink)
| |
| V
`--/\/\/\/\/--.
1K lin |
|
gnd ---------------+---------- gnd

not particularly well regulated
I used a surplus 2n3055 transistor and a 1K pot and was able to slow down
the motor in a tape deck and record stuff at a lower speed....

if you need more precision you may consider using a variable voltage
regulator or a 555 based pwm.

Bye.
Jasen

10. MarraGuest

I would use a PIC with a PWM output driving the motor through a
transistor pair.

11. Rich GriseGuest

You can get high-power rheostats, but they're spendy (~US\$30.00 new at
Digi-key).
Maybe shop around your local surplus dealers.

Good Luck!
Rich

12. Rich GriseGuest

There's a new convention around here. Henceforth, any person posting "Use
a PIC" without providing your code, circuit, and development kit is
to be automagically deemed a "Troll".

Cheers!
Rich

13. Ken SmithGuest

You may be pushing the limits on a LM317 depending on the wall wart you
use. Fortunately they protect themselves quite well.

The LT1083CP is a lot easier to get the heat out of and doesn't need as
much head room. The parts count to use it is very low. You have to have
10uF at least on the output.

14. Ken SmithGuest

No, you only need one transistor. You can use a LM555 to level shift up
to drive the gate of a MOSFET. The voltage is much too low to get an NE-2
into the circuit too.

15. Sjouke BurryGuest

Put an emittor follower behind the potentiometer.

16. bwGuest

You want one part called a rheostat. Assuming the motor near full load
at 500 mA then the resistance will be E/I = 10 ohms. Filament control
rheostats were often zero to 25 ohms at around 5 watts. Odds are you
will end up using a 10 ohm resistor. If thats not enough, try 20 ohms
or so.

17. JamieGuest

That's all well and fine how ever, a must smaller and more
ergonomic approach would be to use a 555 driving a Hex Fet which
can all mount on a back of a panel mount pot.
More than likely the Fet will not need a heat sink.

18. John E.Guest

Jamie sez:
Looks interesting.

Thanks,

19. Jim ThompsonGuest

The fundamentals of constant speed motor control are shown at...

http://analog-innovations.com/SED/MotorDriver-Simple.pdf

The only caveat is that the external RM may not track the internal RM
over temperature.

Some day, when I'm in the mood, I will ponder that problem ;-)

...Jim Thompson

20. JamieGuest

I don't have any ASCII translation program here to post
prints in the NG.
It's a very simply circuit using a 555 timer varying the
TH (threshold) charging time via a POT to vary the time
width of the timer.
Output can directly drive a Hex fet gate. The source
would be on the common side (-) and the Drain on one motor
Also, make sure you have a protection diode across the motor
leads for reverse discharge from the coils of the motor to protect
the FET.
Look at Digikey or mouser for Hex fets or Logic Fets in the
N channel family.
you want one that can turn on at aprox 50% of your Vcc.
Below links points to a print that uses a non logic level
fet and some bias resistors.
http://www3.telus.net/chemelec/Projects/PWM/PWM-1.jpg

THe 555 timer will handle up to 200 ma output, so if you're
driving a light load, you may not even need a transistor.
also, you could use a NPN transistor in Emitter flower mode
to drive the motor..