# Same Power Source for Multiple Servos

I would like to create a small autonomous robotic arm that will use 5 servos; 4 of the servos will use less than 0.5A (the other doesn’t say, but for sure less that 1A) and all operating between 4.8 and 6V.

If yes how exactly should I link them?
If no, what solution do you suggest?

P.S. It is planned to stay on a tank threaded chassis where there are already 2 kits x 4 AA batteries power the main board and the 2 engines. So I hope I won’t be piling batteries to the sky…

Your “challenge” in using shared power sources on devices like this is pretty much always current draw. Second consideration is noise; engines are noisy, even in servos.

Yes what you are talking about is possible. As in all power designs, you need to consider maximum voltage of your components and current draw at load. Each AA cell delivers 1.2 to 1.5 volts; each pack puts them in series, so delivering a higher overall voltage; in this case it’s somewhere between 4.8v and 6v. You could therefore use just one 4-pack to provide the correct voltage for your servos. Whether that provides enough current to run them all, that’s a question that will be decided when you look at the battery datasheet - and whether they’re rechargeable or alkaline batteries will possibly have a big influence too. You also need to remember that as batteries age they will only be able to provide lower current peaks - and if what you’re powering attempts to draw higher current than they can provide, the voltage will decrease (potentially leading to brown outs on your microcontroller, assuming that you’re going to have one somewhere in the system).

By running two 4-packs in parallel, so -ve and +ve connected together, you stay at the same voltage but give a greater current capability. So if you find you don’t have enough current, this can possibly get you out of trouble.

And if you really want to, and they’re supplying the same voltage (so connected in parallel) you can use the power packs that are powering the chassis. Perhaps even adding a third 4-pack will provide you a big enough current capability - but again you have to make sure that they aren’t configured in 12v (serial) connection. If they are, you could regulate the voltage back to the voltage you want - say 5v - and use that to power the whole thing.

Batteries can be simple but they usually aren’t Yet another option that will give you better current capacity would be to look at radio control (RC) plane batteries. They come in a wide range of chemistries and cell configurations (ie voltages) that also have a wide range of discharge characteristics. Yes, you’ll need specialised recharge tools but they’ll give you much better performance and power-to-space ratio than your everyday AAs ever could

Good ol’ Ni-Cd can provide shocking (pun intended) amounts of current.

@ Brett - thanks.

I’ve started to remember from high school how serial and parallel circuits work; I also can make the software to move only 2 servos at the time so I shouldn’t be in need for so many amps.

Can I power these servos through the main board?
(I know that I’ll have to use the PulseInOut module (http://www.ghielectronics.com/catalog/product/392) )

I was thinking that one power source (with 3x4AA in parallel) should be more efficient that separately (I mean with the same source or the entire system works or it doesn’t).

@ allex4project - No you don’t want to power the servos thru the main board.

As Brett has mentioned use 4xAA Nicad or NiHh rechargeable or if you want to invest in Lipo you have more options, but more expense.

Most boards that allow plugging in Servos or mores will have pins that allow you to plug in an external supply.

@ Justin -

Cool!!!

For the Pulse InOut Module the pins for external supply, are by any chance those two under the 5V mark?

Actually, can someone explain to me the meanings of what is on that board? like what goes on the input part and what on the PWM; or a reference where to read all these things?

@ allex4project - Yes it’s the top 5v pins on the output side.

On the left hand side you can wire in anything that creates a PWM signals like a RC receiver and read the values.

On the right hand side you can wire up 8 PWM devices like servos.

Not sure how mature the driver is an might be more for advanced users but i stand to be corrected

You lost me here; I found what noise in electronics means; but isn’t clear for me what exactly should I do about it…

@ allex4project - What Brett means is you might want to have different supply’s for your servos and FEZ that way if there is noise from the servos it is less likely to cause a problem.

I would start by sharing them, and if it does become an issue then separate them.

You can filter electrical noise by using capacitors or even a voltage regulator will help buffer your sensitive electronics from it. So if you put your Fez on same supply as motors, you might need to ensure you have enough voltage that you can power the Fez via a voltage regulator (the onboard one for a Panda or similar, or DP module if a Gadgeteer board)