View Full Version : Servo Power Regulating

11-24-2009, 03:32 PM
My robot is going to have 10 standard HS422 servos and I am planning to power it with 2 Li-Po batteries which add up to 7.4V.

I am wondering how much current will all those servos draw. That's because I would have to use a regulator to step-down the voltage to 6V for the servos and I need to know if I should have one regulator per servo or what the current limit do I need for my regulator.
I heard that the servos can draw up to 3A each when stalling. So with 10 servos, I might have to deal with 30A??
How do people normally deal with so many servos in one robot?


11-24-2009, 04:21 PM
This Battery Eliminator Circuit is used a lot in the RC market. It will handle the Li-Po batteries and regulate them down to 5+ volts and 10Amps. Those servos draw 150mA with no load, about 600mA with load.


11-24-2009, 05:51 PM
Second the ccbec! I have three and could be very happy with half a dozen more. Invaluable devices.

Make sure you get the programmer for it if you think you may want something other than 5v at some point.

11-24-2009, 10:52 PM
Thank you very much.

For that, I don't really have to follow the wiring diagram right? The user manual doesn't have much information in it. Do you have datasheet that talks about what those input and output pins are?

It said that the non-pro one can only continuously supply 5A current at maximum. And I don't know if there would be chance that each servo draw more than 1.5A. With 10 servos, would it be safe just to use the PRO version then?

And thanks, I would probably want to drive the servos with 6V instead of 5V. And also, I am wondering if it would be okay to put a 5V linear voltage regulator to step down from 6V supplied from the CCBEC to power my camera and scoring module? This way, there would be lower loss in energy then stepping it down directly from the li-po battery. Or is there other way to do it?

Lastly, I have never worked with li-po battery before. Are there special things that I should aware of when using li-po battery? I do know that I would need a special charger to charge it. And do I need voltage balancing for two cells (7.4V)?

11-24-2009, 11:02 PM
Linear regulators have a drop-out - a certain amount of voltage that gets lost in the process. It's tough to find even a LDO (Low Drop Out) that could provide enough amperage for a wifi camera and xbees with a 6v input for 5v output.

You'll probably want to put your regulator as a second input from your battery. Choose at LEAST 2A, 3A preferable.

There would absolutely not be a lower loss. There is loss in every conversion in physics. Converting something converted is lossier than just converting it, in general (with exceptions, for example, a regulator that was close to its input limit and so bleeding substantial heat)

The ccbec would also be driven at least a couple amps closer to its limit which would be unwise.

You need to monitor the output from the battery and make sure it doesn't fall below its minimum voltage per-cell during use. Failure to observe that can lead to catastrophic (but exciting) failures. ;)

11-25-2009, 01:40 AM
Or, you could just hook 2 high-power diodes in series, getting a voltage drop of ~1.4v.. but then it's not regulated, so the voltage will be dependent on both current draw and battery capacity.

11-25-2009, 04:27 AM
Yeah, I'd definitely want to regulate the power going into the camera. The Trendnet, for example, has a regulator in the camera, but it's touchy as all get-out - hence the reason it's using a regulated wallwort instead of an unregulated wallwort. Generally, if the device comes with a [more expensive] regulated power supply it's a good idea to feed it regulated power unless you really understand what's going on inside the black-box. That's my take on it anyway.

11-25-2009, 04:37 AM
Completely read over the entire camera part. I was under the impression it was only relating to servo power supply.

Please ignore my previous post :)

11-25-2009, 09:00 AM
And, I was thinking that unregulated power might be better for servos since I don't really need a precise voltage level and this way can eliminate the need of one high-current voltage regulator.

So at this point, my battery (2 cells 7.4 Li-po) will be powering 1 TrendNet IP camera (2.5A, 5V), 10 HS-422 servos (600mA, 6V), dsPIC33F (3.3V), and a 6V motor for electric airsoft gun.

Those could add up to about 10A. After checking the max current level for the Li-Po battery that I has in mind (http://www.sparkfun.com/commerce/product_info.php?products_id=8483), it only supports up to 2A max. Are there high current Li-Po battery or I just have to connect 5 batteries pack in parallel?!!

And also, according to Castle Creation, I couldn't connect more than one of its switching regulator in parallel. So I would probably have to use one regulator for all the servos, and some how use linear regulator for all the other things including camera? Or do you think that I would be better off leaving the servo unregulated using two high power diode and regulate the camera with the swtiching regulator and other with linear regulator. (Is it really true that I can't have more than one switching regulator without major noise?)

This is so much more complicated than I anticipated!!

Thank you very much for your help!!

11-25-2009, 10:16 AM
I actually figure that I have to use higher power battery now...
But I still don't know the second part of my questions..

11-25-2009, 11:15 AM
You could power one set of servos from one power supply and another set from another power supply - just make sure the grounds are common.

The camera actually draws around 800mA, 1A drop-dead-peak.

The servos would only draw their stall current if they were all stalled simultaneously.

I'd put a 3.3v regulator coming straight off the main battery for the processor.

Robot Dude
11-25-2009, 11:24 AM
The 2.5a on the trendnet camera is simply wrong. It draws about 600mA.

You CAN use the HS-422 at 7.4vdc directly. It's much less twitchy as more powerful servos run at 7.4vdc. Ultimately it's up to you as it will decrease the life of the servos, but I have had great "luck" doing this in the past. However your milage may vary.

We have used a 6.0vdc NiMH for the servos and guns directly, and used a lipo with CC vreg down to 5vdc for the camera and other electronics. Works very well.

Hope this helps...

11-25-2009, 11:44 AM
I differ slightly with the "simply wrong". Any decent engineer doubles the current capacity of their power source. And it will peak to about 850ish mA in bad coverage, even a little higher with quality/size settings maxed doing picture-push (as the onboard processor is loaded up to its max and pushing more current as heat). So 2A is a minimum for best-practices for consumer electronics design.

11-25-2009, 12:57 PM
I sort of want to keep it running using single battery pack, so i can just have many of the same batteries as back-up.

After thinking about it, I am thinking that I would use a 6V NiMH battery pack. This way, I won't need a high current switching regulator and just leave the servo unregulated. This may actually save weight. Then, I can get two other regulators to provide to power to the camera and other stuffs. Also, NiMH is easier to deal with since I don't have to have another sensor to monitor the voltage level and watch its charging.

What do you think about this? But one thing that I am uncertain is the size of the battery pack that could provide that much amperage, because I looked at a few batteries and they didn't provide the max discharge current.

And right, I would rather be safe and prepare for 2A max, but now I at least don't need a regulator that provide 2A continuously for the camera.

11-25-2009, 02:43 PM
I absolutely promise you that you are not going to save weight replacing a LiPo + regulator vs NiMH. Average energy density is something along the lines of 4:1 with a 2:1 density being the minimum.

As an example: A Team Orion 6v 1000mAh NiMH weighs 145g where a Mystery 11.1v 2200mAh LiPo weighs 158g. That's in the neighborhood, off the cuff, of a ~4:1 energy density comparing watt/hours to watt/hours. **

Internal regulation on the Trendnet camera, depending upon the series, is 1-1.5A. 2A for the IR version, which probably also explains the larger power supply - economies-of-scale would dictate the use the larger supply for both.

** n=1 each. First batteries I grabbed. Accuracy of balance reported as 0.15g, uncertified. For a more exhaustive exploration of comparative energy densities of the various chemistries, web research advised.

Robot Dude
11-25-2009, 04:26 PM
Internal regulation on the Trendnet camera, depending upon the series, is 1-1.5A. 2A for the IR version, which probably also explains the larger power supply - economies-of-scale would dictate the use the larger supply for both.

Yeah, that's the direction I was leaning. If they want to stock one supply for all their offerings it would make sense to print the current of the supply on the camera case. Even if the camera doesn't need 2.5 amps. The bottom line is the camera we have uses about 600mA. plan for 1.2 amps if you want, but 2.5 amps is overkill.

11-26-2009, 12:38 AM
But then, for a match that's about 2 to 4 minutes, I don't think the battery capacity is that critical.
Like this battery (http://www.robotshop.ca/lynxmotion-bat-03-battery-3.html) has 1600mAH. Even if I consume 10A, 1.6/10*0.7 = about 6.5 minutes. Of course, I really won't be consuming 10A all the time or even ever reach that much at all. Using that will save me quite a few things to buy and set-up, such as battery protection and regulating. (Or I can use the a slightly bigger battery with 2800mAH)

With that 6V battery pack, I can power all 10 servos unregulated. But I have question regarding the efficiency of using switching regulator. Now, I have a 5V camera, and a 3.3V microcontroller to power. The MCU doesn't take much current and need power will little noise, so linear regulator should be used to power that. And the camera, I am only stepping it down from 6V to 5V. Do you think using a switching regulator in that case is a bit unnecessary?

Also, I am wondering if anyone has airsoft guns that run on 6V. That's because all the one that I found run at 9V or 7.2V. In that case, I would have have a separate battery pack to run it since I can't really find a step-up regulator that go up to 9V.

11-26-2009, 01:36 AM
You should be planning on closer to 15 minutes. And you're assuming you can drain a battery to 0v. The "max energy" printed on the side of a battery is as much marketing as it is technology... A 2800mAh battery is very heavy by comparision. It's like those of us running lipos trying to use a 10,000mAh or higher battery.

Switching regulators are very efficient when compared to linear power supplies - they don't bleed off the conversion in nearly as much heat.

What linear are you going to use that's taking 5.2v input (the battery in just a few minutes) and putting out a constant 5v?

Any airsoft gun will run on 6v. It's just not going to cycle as fast. It's nothing but a motor that actuates a gear train that pulls a plunger back against a spring. At some point it hits a flat spot in the gearing and that releases the tension. As long as there's enough torque to draw the plunger against the spring - use whatever voltage is under the max rating of the motor's windings. There's no magic to an airsoft mechanism.

To step up, you're looking for a "buck-up" or "bucking" regulator.

11-26-2009, 11:54 AM
Most of the boost regulators are SMD, but a few nice breakout boards are out there. Sparkfun and Pololu both sell 3.3V and 5V versions of NCP1402 breakouts. Pololu also has this 4-25V output adjustable stepup (which I believe is the same regulator they use on the 3PI) http://www.pololu.com/catalog/product/799 Claims to be 2A output, which would be greater than or equal to what the SN754410 motor driver I used on Issy can source, so I'd imagine it could drive a tank gun (but not a defender).


11-26-2009, 12:57 PM
Thank you for all your advices!
I just have one more question. Is there any bad consequence if I use two switching regulator in one circuit?

11-26-2009, 01:42 PM
Assuming you mean put them in parallel: Yes. Don't.

There's no [practical] way (on a tiny commercial embedded device) to lock their cycles together and that's not a good thing.

You can have two [or more] on one battery feeding multiple circuits however.

11-26-2009, 02:36 PM
I see, I always get that confused and thought I can only have one switching regulator feeding from one battery.

After thinking about it, maybe Li-Po battery does seem like a good idea. However, do you have any recommendation for li-po batteries that can power so many servos while being small enough to fit in a biped mech??

11-26-2009, 02:46 PM
That's a substantial advantage. The energy density is 2-4:1. So the same power battery as you NiMH will be 1/2-1/4 the size.

Something like this: http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXUUE4&P=FR comes to mind.

11-27-2009, 10:28 AM
I am planning to get the FlightPower EONX lite 2500 7.4 LiPo battery (http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXXLS8&P=0#) and regulate the servos using Castle Creation BEC. (http://www.castlecreations.com/products/ccbec.html)To use that, it said that I need a balancer (http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXRWF6&P=V). Is this balancer going on the robot? Or is it for charging only?

And I think I also need a low-voltage cut-off device. Would a Dimension Engineering CellShield (http://www.dimensionengineering.com/CellShield.htm) do the job?

And is there any affordable and safe charger that you could recommend? I have this one (http://www.robotshop.ca/multiplex-ln-5014-charger.html) in mind.

12-04-2009, 09:35 PM
I am regulating my bot's servo power using the CCBEC and was having a lot of problems with transient current draws being higher then the BEC could handle and the BEC would cut out, making the bot fall over. I solved this problem with a ultra cap array. I got some 2.5v, 4.7 Fared ultra caps off ebay and put three in series for 1.5F at up to 7.5V. I forget what the internal resistance of the caps was, but in any case it is low enough that I could not make the bot reset even when stalling all the servos at once.

NEC makes a raid backup ultracap that would go up to the voltage needed in just one part, but it was expensive and hard to find. Its easy to build a series array out of 2.5v parts, just keep in mind that that the internal resistance is tripped, which may cause problems if your ultracaps were the high resistance type.

As far as cells, i am using 12 AA NiMH right now, but i may switch to 4 26650 lipos. The A123 type do not require as careful charge regulation as the others.