Servo voltages etc.

I've done a search and can't find anything, but I may have missed it.

Most 'less expensive' servos have a quoted upper voltage of 6v. How critical is this? If I wanted to use either a 5 cell NiMH pack or a 2 cell LiFePO pack will the full charge voltage kill the servo.

Does anyone have any experience of this?

Ian

Edited By Rentman on 10/07/2012 23:29:09

If the servos are rated at 6v then a 5cell NiMH pack is no problem. I'm not so sure about the Lipo. Remember its not only your servos you need to consider its the Rx as well.

I use a 2s Lipo in one of my models and employ a voltage regulator between that and the rest of the system - this sort of thing. It connects between the Lipo and the Rx. Bear in mind if your model is electric powered and you use one of these then you have to remove the red lead on the Rx throttle connector from the ESC.

BEB

The question was asked because, for example Tower Pro, quote 6v as its upper voltage. A fully charged 5 cell MiMH pack is over 6.5 v as is a 2 cell LiFePO pack (LiFePO4 are 3.3v per cell). I suppose that 6v limit has a bit of precaution in it. Thing is I didn't want to test it and ruin a perfectly good servo. I was interested because the LiFePO4 batteries are very compact and exceedingly reliable.

I do take the point of using a voltage regulator. Think I'll get one.

I use Hitec and their receivers are rated at 35v so that isn't a problem.

Ian

The guy to talk to about LiFePO packs is Chris Bott I think, he;ll be here tomorrow I'm sure.

I know for sure that a 5 cell NiMH is no problem as I use then all the time, but I wouldn't use a 2s Lipo without a regulator.

BEB

Be careful, you would be talking about the SPC voltage input on the Hitec Optima series of receivers that is rated at up to 35V. This input only powers the receiver circuitry, not the servos.

Bear in mind that if using the SPC input, you still need to provide a separate battery pack (4.8V, 6V or 7.4V) to power the servos. If using a 7.4V LiPo or equivalent then you need to be using servos rated for 7.4V. Hitec (and most other manufacturers) make a range of high voltage (HV) servos **LINK**.

Most manufacturers specifically advise against using their standard servos with 2 cell LiPo packs - although the nominal voltage is 7.4V, fully charged they will be around 8.4V. Recommend you use a regulator or go for HV servos.

Rob

Its an interesting question. On the Giantcod/shark web page the 5010 servos certainly had a clear warning that they could only cope with 6v.

You are also right that a 5cell NiMh pack kicks out more than 6v when freshly charged. This is, I suspect, more of an issue with the modern low self discharge packs, which from expereince tend to continue holding a very high voltage, even under load for much longer than the older technology NIMH packs. I'm using Vapextech packs and its not unusual to find that they are still way up in voltage after three or four flights.

I have had one Towerpro 5010 fail in such a way that I suspect battery voltage was the issue. Basically it would flutter when the plane was switched on, but if you left the plane on for 20 minutes or so, the fluttering would settle down. Possibly, as the voltage dropped the problem went away.

That said, I've only had one fail in this way. I'm running three airframes on 5 cell, low self discharge RX packs, with a selection of servos. Towerpro5010' s in a WOT4 and a YAK54, and a couple of different Hitec servos, Npower Servos and 'generic' servos in a Spirtfire, and have had no other problems.

Increasingly servos are arriving on the market which are designed for 7.4v [I hope 8.4 in reality] which will make life easier!

Edited By GrahamC on 11/07/2012 07:46:20

When using LiFe cells I put a 3A (or larger) diode in series which introduces a 0.7V drop. Allthough the fully charged voltage would still put this arrangement over 6V they soon drop off towards their nominal voltage. If you were really worried, you could put 2 diodes in series to double the drop.

I'm a bit nervous about introducing a voltage regulator with multiple components from a fault liability point of view - although they are probably very reliable with modern surface mount technology.

For anything critical, I find it hard to fault 2 separately switched LiFe packs in parallel with diode protection which provides a nominal 5.9V with redundancy

With respect, I think this has been covered a few times before. The voltage of a fully charged 5 cell Ni-MH pack can be as high as 7.2 - 7.3 volts, slightly higher than a full charged 2 cell LiFe. But it doesn’t stay up at these dizzy heights for long. Both are fully compatible with 6 volt servos, but the LiFe, for a variety of reasons, and one important one is because they are so easy to charge, I would consider are now definitely the best bet.

A 6V servo is just the nominal voltage, i.e. simply that is what is written on the label. These are fairly flexible, from 3 volts up to about 7.5 volts, or more. However, if it doesn’t say 6V on the label, merely 4.8V, then you might eventually get a miniature barbecue, the photo is a 4.8V servo running on 6 volts. It didn’t blow straight away though, it lasted for quite a number of flights.

Servos that are labelled 6V will definitely be ok with either a 5 cell Ni-MH or a 2 cell LiFe pack.

In the main these days, most servos seem to be 6 volt servos, you might now have to search around somewhat for a 4.8 only flavor; and I suspect they would operate ok from 6 volts downwards.

If you decide to use the LiFe, a Hextronik 6.6V on-board voltage indicator is a really invaluable model saving addition, cost about £2 from HK.

Hope this is of some help.

This one first threw a hissy-fit and then went into self-destruct mode!

PB

No problems to date with either 5 cell NiMh or 2 cell LiFe packs direct through a standard switch, but 2 cell LiPo definitely get regulators of some form. That's with JR, Futaba, Hitec, E-max , TowerPro and various no-name servos.

There are a few servos where the manufacturer/distributor specifically warn against anything more than 4 cell Nixx packs but you would need to check the particular servos you are using. I think JR used to warn against 5 cell packs, maybe they still do but my 591s seem happy enough.

If I were loading digital servos heavily I'd probably stick to 4.8v but use sub-C cells or a large capacity regulator to limit the heat build upo in the servo, but for normal duty most servos seem to cope fine at 6.5 or even 7v.

Woetc. I understand the SPC voltage system on the Optima receiver. The issue is the Optima receiver isn't gonna be bothered by a slightly higher battery voltage.

Martin. I'd forgotten about the forward volt drop on diodes. Its 0.7v for a silicon diode and, if memory serves me well, about 0.3v for a gremanium one. Shottkey diodes its next to nothing.

Peter. I'm sure it has but I did a search and it didn't turn up anything. Thanks also for your advice. Its all useful stuff.

Thanks to you all. I now feel better informed. Time to dig out a cheap servo and power it with a 5 cell pack and keep my fingers crossed. I'll let you know what happens

Ian

Edited By Biggles' Elder Brother - Moderator on 11/07/2012 11:28:30

The GC/GS website has a warning that there 6v servos are only rated at 6v and no more, but I've used several with 5 cell Nimh batteries with no problems (so far).

Ian - As long as you pick a cheepie servo that’s suitable for 6 volt working… … that being so, I’m sure it will be fine. One small point to add, though, when the voltage is too high for 4.8V servos, they often tend to perceptibly jitter before rolling over. Of course, there are also other reasons for this, too, but it’s aways worth checking out.

Fortunately, Bob, I think the system is wide enough to accommodate quite a range of ideas. To some extent it probably doesn’t matter too much what you do, the end result is the same. I’m sure it would be very difficult to distinguish the difference in servo action when flying on say 6 volts or 5.5 volts. With respect, I think that if I felt that digital servos, that were rated for 6 volt working, were getting overloaded to the point of warming up, I would consider upgrading to something a little more substantial. But, again, it’s probably of little consequence either way.

I’ve always felt that the servo loads are not that heavy anyway. I consider that the control surface just changes direction, and the engine/motor actually powers the manoeuver. Rather like steering a moving vehicle, as opposed to turning the wheel when it’s stationary. From the experiments that I’ve done, and also others, I’ve always found that the battery drain is very light, even when the model is flown as hard as possible. The instantaneous servo motor start-up current might be relatively high, but that would be very very short lived; and, indeed, it might be another reason for using LiFe’s, their low internal impedance might be better matched to the motor start current, maximum power is transferred when the load and source impedances are equal.

I think the JR situation has been checked out before, in previous posts, and they are now listed as 6 volt working.

PB