VERY simple dc motor drive circuit?

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,
--
John English


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John E. wrote:

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)
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John E. wrote:

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&ModuleNoD202&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.
--
Sue


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Palindrome sez:

Thanks for your reply, Sue.
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,
--
John English


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John E. wrote:

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..
--
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One (power) transistor, one small pot. Think 'adjustable voltage regulator' the first power supply every experimenter builds.
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Use Tim Wescott's ckt.
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"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
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I would use a PIC with a PWM output driving the motor through a transistor pair.
www.ckp-railways.talktalk.net/pcbcad21.htm
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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.
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In that case, from now on I will be suggesting the use of AVRs! I wouldn't want to be a "troll".
More seriously ... while I agree that using a uC to generate an unvarying square wave is overkill, I think you're diluting the term "troll" into meaninglessness ... it doesn't just mean "someone who posted something I don't agree with", you know.
Actually, the uC approach might not be so silly. DigiKey shows about the same prices for PIC10F200s as for 555s, and the PIC would require fewer external components (after programming, of course). Ditto the ATtiny11 and some of the Zilog Z8s. Sheesh!
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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.
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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
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jasen wrote:

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.
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Jamie sez:

Looks interesting.
Wiring details, please?
Thanks,
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John E. wrote:

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 lead while the other motor lead is connected to your + supply. 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.. it's your pick.
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On Sat, 03 Mar 2007 10:46:57 -0500, Jamie

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
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Jim Thompson wrote:

And the magnets will vary in strength over temperature.
I understand that there are also PWM driver circuits that read the motor voltage when the driver amp is high impedance, basically using the motor as a tachometer. This doesn't solve the magnet's variance with temperature, but that's not a big issue normally.
I haven't tried it, but someday...
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wrote:

Now that you mention it, I remember the trick of reading the back EMF.
However, where does the current go ?:-)
Then there's EC motor (electronically commutated) controllers like I designed for Bosch. The speed is a function of the clock rate.
I still have one of their motors here that is as big as an alternator ;-)
...Jim Thompson
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Jim Thompson sez:

A straightforward solution.
What is U1? Any op-amp?
Is Rext chosen to be the same R as the motor?
To make speed variable, should R1 be replaced with variable?

That's OK. I think that speed regulation is not so critical. The goal is to be able to adjust full range. If it's noticed that speed has changed due to thermal reasons, can easily be manually adjusted. It should settle down once motor has warmed...

If you would be so kind... (c:
Thanks,
--
John English


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