I am using a 12 V power supply to run a series of motors in Parallel expecting them to pull up to 25 ampere. The motors are running too fast at 12 V so we need to drop it down to 9 or 10 V, preferably making it adjustable..

I am familiar with voltage dividers and voltage reg. chips, and zener diodes but never worked at such high Ampere situations, we are even looking at lamp dimmers(?)

Can anyone suggest the most efficient and simple means of achieving this without excessive cost? The system does not need of dynamic control, I'm fishing for a hi amp voltage-regulator/ zener-diode type solution, if it exists!

  • \$\begingroup\$ How many motors in parallel will there be? Is the total current consumed by all the motors 25 amps? It would be helpful to know how much current each motor draws individually. \$\endgroup\$ – Nick Williams Jul 6 '13 at 13:38
  • \$\begingroup\$ seven car style wiper motors that measure ~2Amps while loaded \$\endgroup\$ – Oliver Kellow Jul 6 '13 at 13:42
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    \$\begingroup\$ Get proper motor speed controllers (which will be using PWM); cheap prebuilt modules are widely available. \$\endgroup\$ – pjc50 Jul 6 '13 at 13:47
  • \$\begingroup\$ The system does not need that level of dynamic control, I'm fishing for a hi amp voltage regulator/zener type solution but what I am coming across essentially maxes out at 8Amp or so \$\endgroup\$ – Oliver Kellow Jul 6 '13 at 13:56
  • \$\begingroup\$ I'd vote for a simple PWM with a 555 and a high-current FET (I like the IRF3205), maybe with a mosfet driver inbetween. You might not need the control it offers, but my guess is that it will still be the cheapest option. \$\endgroup\$ – Wouter van Ooijen Jul 6 '13 at 20:57

25 Amps is a very large current, I don't believe there is a part capable of handling such current for hobbyist applications. However, you could regulate each 2 Amp branch to step down your voltage.

I found a neat LDO that will drop your voltage down to 9V and can handle up to 2 amps:

DigiKey: PQ09RD21

It is also wired in the familiar fashion for LDOs (Extracted from the datasheet): Extracted from Datasheet

These guys are only a dollar each, buy seven for each motor and wire them in series with each DC motor in the manner shown above.

In regards to your adjustable request: from my experience with LDOs, a resistor may be placed between pin 2 and GND, and another resistor between pin 2 and pin 3. The resistance values will adjust the voltage. Use a potentiometer to adjust the resistance. I'm just throwing out ideas.

I also like pjc50's suggestion on the motor controllers; another neat thing to look into. Best of luck!

  • \$\begingroup\$ very nice! Interesting workaround :D, I think i will give this a try! \$\endgroup\$ – Oliver Kellow Jul 6 '13 at 14:21
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    \$\begingroup\$ 12V in and 9V out means the regulator drops 3V to the output. It does this at 2A and the regulator is disippating 6W. Seven of these circuits and the power is 42W - you'll need a biggish heatsink and a fan more than likely. \$\endgroup\$ – Andy aka Jul 6 '13 at 17:56
  • \$\begingroup\$ thanks, we already bought a few small fans and lotsa heatsinks \$\endgroup\$ – Oliver Kellow Jul 7 '13 at 16:55

Doing this with a linear regulator is possible but you'll need a sizable heatsink and probably a fan to dissippate the 42W wasted in the linear regulator. 42W comes from 7 circuits dissipating 6W each. 6W comes from producing 9V from 12V at 2A.

I'd definitely consider a buck switching regulator. It's likely to be +90% efficient and would therefore be only dissipating about 10W in total. Reducing the voltage to (say) 8V or 7V won't make the dissipation any worse either BUT if it were a linear regulator, the power dissipated would be approaching 70W.

What sort of buck regulator? I would consider using 7 of them like the linear approach but i'd probably go, in the end for 4 x 5A regulators with each driving two motors and the final one driving the seventh.

What about the LT3976: -

enter image description here

The 1M ohms resistor and the 110k resistor set the output voltage and this of course can be chosen to give 9V. This device is not a synchronous device and therefore it's efficiency could be higher if it were BUT this would meet the requirements.

  • \$\begingroup\$ Great use with the switching regulators. \$\endgroup\$ – Nick Williams Jul 9 '13 at 13:08

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