Servo Controller battery power supply

sylvie369

Okay, I promise that I used the search engine first to look for answers to this question...

I've been learning to use servos and servo controllers, always using a wall wart power supply for both servo power and controller power. I'd like to learn how to use a battery power supply. I bought a 7.2V NiCad pack,and then noticed that the servos I'm using say that 6V is the max. I built a little adjustable power supply to bring it down a little under 6V using a LM 317, but under load it drops down to something like 2.5V, and of course at that voltage doesn't drive the servos. It drops down to that point even when I just turn on the servo controller (Parallax Serial Servo Controller) without any servos attached.

I see in an earlier (2005) thread that Ken Gracey suggested that one can just use a 7.2V battery pack directly into the servo power inputs, and the documentation for the servo controller shows that as well. But that's 7.2V, over the 6V max on the servos' instructions. In addition, I'm actually getting a little over 7.2V, and the documentation says "7.5V max" for that input.

Am I going to fry my servos if I use a 7.2V NiCad pack?
Am I going to fry my controller if that pack is actually putting out (say) 7.85V?

And while we're at it, am I right in assuming that the reason my little LM 317 power supply's voltage drops so dramatically is that I'm trying to draw far too much current through it?

If the 7.2V pack can't be used directly, is there a better voltage regulator/power supply circuit I could use to provide adequate voltage and current using that 7.2V NiCad pack?

W9GFO

I would remove one of the cells from the battery pack. Depending on the pack you should be able to remove one cell from the back end of the pack, opposite the power leads, and then lay the neighboring cell on it's side. Use side cutters to cut the tabs, cut close to the cell that is being removed. Then you just solder a short 12 ga wire between the two tabs. Frequently the connecting tabs are long enough so that you may not have to cut the tab on the cell that is laid crossways on the back of the pack.

I'd post a picture but my pack is zip-tied in my ProtoBot (Propeller Protoboard BoeBOT)

Rich H

SRLM

Yep, I've had this problem. How to get the 7.2 volts safely to my servos? My solution? Get a bunch of 5V 3A LDO regulators and put them together onto a board (The GG version). Basically, all that it has are the regulators with inputs for signal and output to servos. If you don't plan on heavily loading your batteries, you could make a similar circuit for your project. Otherwise, you can always find a higher current LDO regulator. As a side note, the higher the voltage given to a servo the faster the wear.

sylvie369

Ah. Okay, I see there's an "LM 1085" that might fill the bill, and could (I think) be simply dropped into where I currently (no pun intended) have that LM 317 in my circuit:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM1085IT-5.0-ND

(though I might have to replace the capacitors as well). Is that reasonable?

I'd rather not hack apart the battery pack - I'd buy a 6V version instead before doing that. I can certainly make other use of this 7.2V pack.

kwinn

You could always put 2 forward biased diodes in series with the battery to drop it to 5.8V.

Prettybird

Agree with kwinn. A med amp diode and figure .7v drop per diode. Do the math. Cheap way is best way.

Prettybird

·Forgot to mention. LM 317 regulator is ratted at 1.5 amps if heat sinked properly. It is short circuit protectted internally and will totally shut down if too much current. The most dangerous point is running just below 1.5 amps. Probally the transformer is too small. Check the va or ma ratting. Most people think in voltage but forget the current. Ohms law always works. lol I have an ajustable bench power supply that has lasted for years but has a hefty 12 vac 4 amp rating. Comes to about 16v after a full wave bridge rectifier and big capacitor