View Full Version : Supplemental power for servos?

03-04-2010, 08:21 PM
I am using a Roboard to control my hexapod, the deal is that 18 645mg's draw 8.1A under worst case. As I don't really want to try to push that much current I came up with a simple circuit to put the current draw on a second battery (don't hate on the MS paint wiring diagram).

I set this up on a breadboard with a single servo, and it worked fine. I am looking to put together 3 boards with 6 servos each on them. My question is will this scale up? Should I add diodes just in case? Anything I am not thinking of that could blow something up? Thanks.

03-04-2010, 09:20 PM
A 3300 battery can easily take the amps. If its a lipo its going to be 20c or 30C. Either one your never going to come close to that unless you sit on your hex as it tries to stand. If its NiMh you have a little higher rating but along the same lines. No need for the other battery as it just adds weight.

You need a min of 6v to run if the battery gets to that point its time to recharge either way.

I run 1300 Lipo's 20C on my hexs which power everything and they lst a long time.

03-04-2010, 10:12 PM
That circuit looks fine by me. If you want to add additional servos don't add additional batteries. Just use a single large one - many batteries connected together will be continuously charging and discharging into each other.

I assume that you are adding the second battery to protect your controller board from very high/unstable currents. In most cases, it's not necessary to separate the power sources for the controller and the actuators but I imagine that you have a good reason.

Having said that though, the traditional method of preventing current overload is to add a fuse to your circuit.

03-04-2010, 10:51 PM
Practically instant gratification here.... thanks.

Quantum, basically what you're saying is that 3300mAH is plenty to control a hex for a while, and that the extra is overkill? The batteries I'm using are 6V NiMH's by the way. I think that you are right, but 8 amps seems to be a lot to potentially run through my control board.

Badcommand, I don't really have a good reason except that I tend to overbuild everything, and yes I was planning on using the one pack for all the servos. Fuses go without saying (now that you've reminded me;)), I'm trying to provide plenty of current in this case as opposed to limiting it. I have read that the Roboard will reboot occasionally running 18 servos

The other kinda' reason that I wanted to do this was that at spec the roboard draws 450mAH, so with other stuff powered off of it probably 4-5 hours runtime. With the separate pack I can swap servo power without restarting.

Glad to know that the circuit probably won't destroy anything, thanks.

03-04-2010, 11:28 PM
Make sure to use a slow blow fuse. Servos will eat fast blows like candy.

03-04-2010, 11:34 PM
No need for any fuse. And batteries don't charge and discharge in toeach other. Has anyone ever charged a battery by discharging a battery into another? Thats a short. And things blow up when this is done.

The amps dont run thru your board. Your board draws as much current as it needs. My guess is around 500ma. At most. Your servos are the power hogs. They draw the current they need and your board draws as much as it needs.

If your servos draw 100 amps that will never run thru your board since it only needs what it needs.

03-04-2010, 11:39 PM
Common guys there is no need for a fuse. Thats crazy talk. His board can handle more than the voltage that is applied by the battery. Up to 24V. There is no way that board is going to draw that much current to fry it. The servos will but they draw power off the battery dirrectly. Which the battery can handle. I see no problem running servos of the battery as well as this board with no issues.

I run 7.4V Lipo when fully charged these guys are around 8.4V. I then run the battery thru a UBEC bring it down to 6V. 6V to the servos which is right where they like it and the voltage to my controller runs of this. The controller board my own doesign then brings the 6V to 3.3V. But that dosent matter since your board works to 6V and that wouldnt be an issue. A good Ubec will start to warn you that you are at this pouint your screwed up your battery.

03-04-2010, 11:56 PM
Just so it's clear, I'm very new to all this.

The batteries are rated to discharge at 38 amps which means I could probably weld with them. So no, battery supply is not a problem.

My understanding is that for example, if the board needs 500mA and one servo needs 500mA then in order for the board to receive its own power and pass the required power to the servo then the total combined passing through the board is 1 amp.

Please correct me if I'm wrong. I'm just trying to make sure that I end up with a stable platform, and I have been unable to find any docs on how much current each PWM can pass on the board. This way I figure I eliminate any potential problems. If it is really overkill, or I have power supply problems I can easily add or remove the secondary battery.

Just making sure that I have all my bases covered, no sense frying a $250 board for not asking "dumb" questions.

03-05-2010, 12:02 AM
Quantum I don't think that is correct. I believe the servos connect directly to the board and the batteries plug up to the board. This means that the current for the servos has to travel across the board via the traces.

03-05-2010, 12:03 AM
"My understanding is that for example, if the board needs 500mA and one servo needs 500mA then in order for the board to receive its own power and pass the required power to the servo then the total combined passing through the board is 1 amp."


Board will draw 500mA from the battery and the servos draw 500mA from the battery. The total load is 1 amp on the battery but that dosent go thru your board.

Your board will only draw the amps it needs. And since its basicly a small computer it draw more than the usuall MCU.

03-05-2010, 12:12 AM
Jes even if this the case the power from the batteries runs via traces directly to servos. Signal lines run from processor to servo usually buffered depending on voltage levels. I run my boards based of the prop from parallax. This is a 3.3V chip with a resistor to handle the current from servos via signal lines to prop you could blow the pin out if a resitor is not there. The Resistor is needed to bring the the signal current down almost like a voltage translator chip. In his case I am almost postive that he could run those servos at 10v and the servos would blow/melt before the pin that it's connected to for the PWM signal would.

03-05-2010, 12:29 AM
Ah, I see where you are confused. It's not the pulse pin that's the question. The question is the power supply pins. The PWM pin will only be a couple of mA. i don't have a roboard so this is pure conjecture:

Lets assume a row of servo pins all side by side. The PWM pins will all go to the individual pins on the micro (perhaps through a resistor). The traces on the power pins for the servos will all be run in parallel. They will probably go to a single trace and then to the battery. That single trace is the question. They are usually designed with a certain max current in mind. Those are the traces in question.

With all of that said, I'm fairly certain they will be fine.

03-05-2010, 12:33 AM
So the long and short is that I should quit worrying about it and just plug everything up and go.

03-05-2010, 12:37 AM
His schematic shows power going directly to the servo and not to the board.

03-05-2010, 12:42 AM
Correct. I was explaining what his drawing was supposed to solve.

03-05-2010, 01:03 AM
Jes Im not confused i understand what your talking about. I looked at the info from the Robart websites it's vague no schematic. I cant trace the servo power in. Im assuming its coming of the power plane but its not clearly posted.

On my board I've dedicated the whole power plane to servos aswell. Run it theu A UBEC your good.

Usually the trace for the servos should'nt be one trace but a plane. Thats the inner layer of a min 4 layer baord. Or trace about .150 thickness but thats about a 8 amp rating. Which maxes alOt oif robot setups.

03-05-2010, 01:18 AM
Billy yes it shows power going directly to the servo There is no reasonn it can't parrallel to the board. There are companies that suggest a seperate power supply for logic. Logic takes less voltage that the servos so why? So you buy an extra battery? Its just crap.

Just run the thing off one battery.

03-05-2010, 01:26 AM
I agree with the single battery. There was a concern about the board traces, but if he wired as in his schematic the traces would not be an issue.

03-05-2010, 10:14 AM
I think your worst case current estimate is a bit low. My bot can draw 8 amps with only seven 645mg.
With that many servos, you better have thick power traces on your board.

03-05-2010, 02:36 PM
Thanks for all the info ya'll, I think I'll just put my system together. I probably don't need it, but it won't hurt anything either. If weight ends up being a huge issue I can eliminate it no problem.

Thanks again.