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How to power different voltage motors from same power source?

I am taking the "There are no stupid questions" comment to heart here.

I am hoping to power DC motors and servo motors from the same power source. The problem is, the servos want 6V and my DC motors are rated for 1.5-3V. A nice simple solution like a potential divider won't work, since each motor has a non-constant resistance, and a voltage regulator would be hideously inefficient. Adding a separate battery pack seems a silly idea, especially given the Arduino will have one of its own as well.

This sounds like an incredibly common problem to me, but I've looked all over the place and not even found anyone asking the same question, let alone finding answers. It's quite possible that something I've theoretically learnt just hasn't clicked yet.

Could anyone give me any guidance on what I should do, or link me to some kind of article/guide on such a thing? I'm sure this problem exists all over the place, probably even on these very forums, but clearly I don't even know what search terms to use.

Let me know if more information is needed for this to make sense. It should be fairly obvious I'm quite the novice. Thanks in advance for any help - I'm at my wits' end trying to puzzle this one out!

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@Maxhirez: I'm using an Arduino, but I'm trying to steer clear of shields as far as possible (for the sake of gaining a more complete understanding as I go, and with the hope of, when I am a little more knowledgeable, miniaturising and using raw ATmega chips). I've got a SN754410 H-bridge chip which seems to be fine with an output voltage -0.5 - 36, though it's not technically recommended (glad you asked though, I hadn't yet checked). I realise the stall current may be a problem, but for now I will just be keeping an eye on the thing and not letting it run into walls.

Annoyingly, the motors I have aren't the ones I actually intend to use. These are just the Mabuchis that come with the Tamiya twin-motor gearbox. The ones I have on back-order (and will be awaiting impatiently for a few weeks) should be absolutely fine with a 6V supply, making this job much easier! However, given all the other components are here I was hoping to at least make a proof-of-concept prototype at the expense of efficiency.

@PeteH I'll be using rechargeable AAs for convenience, so presumably 5 of them. I hadn't considered drawing from the middle of the supply. I haven't seen this done, are there any downsides I should be aware of (other than some of the batteries dying earlier)? I'll give it a go and see what it's like.

Although it's quite possible I've got my solution, I've also remembered the idea of using diodes for voltage drop. I have a handful of 1N4002s around, which drop 0.6-0.7V each, so theoretically could work. Would this be a terrible idea?

Thanks for the help!

I'm still not clear on what you're planning to do, but the simple answer is that if you have a factory Arduino, then you shouldn't have to worry about the difference in Vcc because you can power the servos off the Arduino Rail and it will take care of voltage regulation on its own. You can even use a single battery to power the motors and controller (as long as it is in excess of 6v) if you shunt a regulated Voltage off of it to feed your SN754410, using say an Lm7803. If you're making your own Arduino, you need some kind of 5v limiter anyway and you can run the servo(s) off that line as well, but even though your servos say they need 6v they'll probably work just fine with 5v, unless you have some combination of very high torque servo and very heavy load on that servo.

What are you hoping to control the motors with?  Will there be a μController of any kind involved or some custom circuitry?  One thing that usually crops up in Arduino is that a motor rated at less than 6v isn't supportable by most shields, for example.

Hi Janvar,

Don't worry - it's not a silly question!

My own preference is to actually use seperate power sources.  This removes power drops on pulling lots of power when the motors switch on.  You have not however stated what is to be your power source. If it will be something like 4 1.5v batteries then another option would be a centre tap on your batteries, use a fairly large capacitor on the motor line, near the batteries to cope with the fluctuations of the motors switching on/off.  The final option is of course using voltage regulators, being careful to ensure that the motor regulator can handle the stall current of all motors combined (worst case scenario!)

Hope this gives some ideas for thought

 

Pete