Zener diodes for spike suppression on A123 Rx packs? - RC Groups
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Jun 22, 2010, 06:12 AM
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Zener diodes for spike suppression on A123 Rx packs?

Remembering back to my days designing automotive electronics for steer by wire systems, everything we added to the circuit was protected by a zener (usually 16V or so) across the +ve and +ve terminals. Automotive electronics is an extremely noisy environment, BIG dumps from the starter motor stopping and other thingos doing their thing, not to mention the 250A or so the steer by wire servo motor was drawing and dumping, it was always very difficult to get things working nicely, so we always ended up with zeners and lots of big low ESR caps. Looking at some of the long servo leads we use in the big jets, and the high instantaneous currents some of the digital servos draw, I was wondering if there's any benefit to adding a zener across the terminals of a 2S A123 pack to suppress any spikes which may damage the servos or Rx. I've heard about people blowing up (mainly Hitec) digital servos just by powering up when using 2S A123 packs, sure don't want this to happen to me! What do you more learned chaps think?

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Jun 22, 2010, 07:31 AM
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Absolutely do it. Check my post on RCU, youre not alone.
Jun 22, 2010, 11:44 PM
Stu, as you know we use a LOT of the Hitec servos, and we use only A123 packs for our servos (afterall, we are a U.S. distributor for A123). We have never run caps or diodes on any of our systems and we have never had any type of failure. I think when using the 2300mAh A123 packs you are pretty safe from any type of current. Servo pins are only good for 5A and the 2300mAh packs will handle 66A continuous (130A burst). It is unlikley that you are ever going to even create a dip in the power when using these cells. Now, having said that, I don't think you could ever hurt anything by adding the caps and/or diodes.
Last edited by JimDrew; Jun 22, 2010 at 11:51 PM.
Jun 23, 2010, 07:10 AM
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Thanks, guys. Jim, I was more worried about high voltage spikes than power dips. If it works, though, it must be OK. BTW, I have some HS5645 servos which I've set up on the tailerons of my F5. They work at different speeds - one is very slow to centre, as if they programmed a lower D gain in the PID loop, but surely they all have the same parameters programmed into them? I did set the bandwidth to 1 on both of them, BTW. They seem plenty strong enough but I do wonder what this will do in flight as there will be a small amount of roll induced by this. Of course the solution would be to stop being such a cheapskate and get some good JR digitals.........

Jun 23, 2010, 11:03 AM
Stu, for some reason Hitec digital servos always seem to come with different programming setups. It is not uncommon for us to see the deadband on one servo set at 3us and another set to 1us. Likewise, we have seen the speed adjustment off as well. You might want to plug in their servo programmer and see what their settings are. My guess is that they don't match.
Jun 23, 2010, 05:13 PM
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You want the diode near the thing you're protecting, not way over by the power supply.

That being said, most of the things in your setup that you might be worried about are going to have pretty good protection already. Modern receivers are using LDOs designed for very noisy environments (cellphones, etc.) with excellent transient rejection. Servos are their own worst enemies from a noise perspective, so they tend to have integrated regulation/protection for their logic already. Most serious speed controls these days have their own internal power supply and could hardly care less what's going on outside.

There's quite probably an argument to be made for some forms of suppression (chokes on long servo leads, for sure), but a fat, slow zener on your battery pack isn't (IMO) worth the effort.

= Mike
Jun 23, 2010, 08:21 PM
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Thanks, Mike, I think I'll just leave it all alone, seems to work well enough for others.

Jim, I checked both servos with the programmer and they're both the same. I have two other 5645MG's to try, although one's gear train is super sloppy. BTW, the testing I did was on a 4 cell nicad (just what I had lying around), I'll plug in the A123 pack and see if there's a change.


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