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Here's the circuit I'm working on, it's a robotic stringed musical instrument thing, that uses 6 DC motors and an Arduino Duemilanove -

Circuit

The resistors are 0.25w 1KΩ The transistors are TIP122 Darlington PNP The diodes are N5401

The motors require 3v and 1.17A each, but the Arduino likes to work on about 6v-9v and only a few mA. I'd like to know the easiest way to use a single DC power supply unit (Of any appropriate spec), to power the whole circuit.

Cheers.

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    \$\begingroup\$ Remember that voltages of a supply and load need to match, while the current capability of a supply just needs to be larger than the draw of the load. In other words, it's not like a 2 amp supply is going to shove the full 2 amps down the poor little Arduino's throat. It's going to supply a voltage, and the Arduino will only draw as much as it wants. \$\endgroup\$ – endolith Feb 15 '10 at 21:59
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If you are sure of the motor voltage and currents and all the motors may need to be on at the same time you really want two voltages.

Using a single a 7V supply you would need to drop 4V to power a 3V motor. 4V * 1.17A * 6 motors = 28W. That is a lot of power to dissipate.

You can get dual output wall warts. Get one with a 7-10V output and a 3-5V high current output. If size is not a factor you could get a PC power supply. Use the 3.3V for the motors and use the 12V for the Arduino with an regulator to drop the voltage to 7V or so.

You could also get a single 3.3V supply and use a boost converter to generate 7V. It would be a small boost converter since the Arduino wouldn't need much power. These are easy to build. Something like a MAX34063 or LT1302 would work.

I would do the boost converter solution.

One more note -- I would not use Darlingtons to drive the motors. Darlingtons have a very high saturation voltage and you are going to dissipate a lot of power. A better choice would be a MOSFET. The IR IRLB8748PBF is a 40A MOSFET with an Rds(on) of 7mOhms (at logic levels). At 1A you will dissipate around 7mW compared to over 1W in the Darlington. They are about 90cents from Digikey.

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    \$\begingroup\$ +1 for mentioning MOSFETs rather than darlington PNPs \$\endgroup\$ – Jason S Feb 15 '10 at 23:25
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You could use a small DC-DC converter to convert the 3V supply to something the Arduino can use.

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