Author Topic: Servos for use with Lipos  (Read 5099 times)

EI1638

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Servos for use with Lipos
« on: January 30, 2013, 23:05:58 PM »
Folks,

I'm clearing the decks to start on one of Freds mini quarks and I've been thinking about the controls. I can get a receiver that will run off of a 2S lipo directly (great, no regulator required).

Except that's not much good if I can't get servos that will run off of 7.4V (2S).

I've spent a goodly portion of my lunch time and this evening staring at servo data and I've yet to find a suitable servo that will run directly from 7.4V. If I was flying 1/4 scale it looks like it'd be less of an issue, but I'm looking for something around 5g and so far I've come up blank.

I've seen stuff on the interweb that says certain servos have been observed to work off 7.4V, but so far I've yet to find any info from a supplier or manufacturer that their servos will.

Does anyone out there hae any knowledge/experience of this?

For my part all I want to do is reduce the number of things that can go wrong in the model especially since its small. I know a 7805 linear regulator or equivalent with appropriate wires isn't particularly heavy, and in TO220 package should be capable of handling the worst this size model can throw at it, but thats 3 additional solder joints in the power line, to flex/crack, especially since the wires would be soldered directly to the 7805  pins, rather than havcing everything on a printed circuit board.  

I know I could use a switching regulator (hobbyking can build them much more cheaply than I), but again, its just something else to go wrong.

All advice greatfully received.

Chris

Happy Days

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« Reply #1 on: January 31, 2013, 00:09:28 AM »
Okay Chris. As I read it there are two issues here.

1/ Your faith, or rather lack of it, in electrical systems.

2/ Your desire to use a two cell li-po as a power source.

To my mind the easiest solution for you is to use a standard 4, (or 5) cell Ni-MH battery
The 4 cell will produce a nominal voltage of 4.8v, and the 5cell a voltage of 6v. Both are within the range of most small servos. (Certainly 4.8v will work all small servos)

I don’t know the weight of a (say) 800mAh 4cell Ni-MH pack (Pack of 4 AAA size cells) but they are not much more than a Li-po of the same capacity. In fact you would probably still need to add some extra weight in the nose of the Quark to balance it. So instead of adding lead, why not take the easiest answer to you voltage problem and fit a bigger (heavier) Ni-MH battery?

As for voltage regulators. Linear regulators can sometimes have thermal issues. (They can get hot and fail when handling high current loads, like when using lots of servos for a lot of the time during the flight.) This can be overcome by allowing good air cooling to pass over their heat sink.

Switching regulators, (as most UBEC’s are) run much cooler and although they used to cause RF interference problems with the old 35Mhz rx’s, if you’re running 2.4Ghz they will give no problems.

If I may say so Chris, your concern about electrical failure by virtue of the number of items in circuits is not born out in reality.
I too have read and heard of cracking wires, so called ‘black wire’ corrosion, dry joints, ect yet I’ve been flying model aircraft for 7 years now (gliders and electrical powered planes) and never had any such problems. Modern day electronic devises are very reliable.

For instance, the miles of wire and circuits in your car are very complicated, with hundreds of joins, exposed to all weather conditions yet still it starts ‘on the button’ doesn’t it. (The lights work, so do the windscreen wipers & washers, indicators, heater fan, engine cooling fan, stereo system, brake lights, horn, heated window, ect ect.....)

If you’ve already bought the Li-Po then I’d get a 4.8 or 6v UBEC. If you haven’t got the Li-po yet, I’d go for the 4 or 5 cell Ni-MH. You wont be disappointed. :wink:

Keith
Try not to run out of airspeed, altitude and ideas....... all at the same time.

Fred

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Servos for use with Lipos
« Reply #2 on: January 31, 2013, 07:21:48 AM »
Hi Chris,

For power, for small servos etc, just take the issue differently :)

Use a 1 cell lipo  :D

Most of small receivers will work in 3.7v so as most of small servos  :D

As for electrical things in our planes and gliders, almost 30 years in flying them, and I never had an electrical failure. Only stupid things like flat batteries etc, but that will be it.
The main thing, like everything else, is to give your equipement a proper check once in a while (winter - start of the flying season), even more after a first flight to make sure that nothing went slightly unplugged or moved. But after that, it is more like "general" maintenance.
Education is important, but flying RC planes and gliders is importanter!

andrew wallace

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« Reply #3 on: January 31, 2013, 21:45:46 PM »
Chris
I use a 7.4v 350mah lipo with a 5v regulator in my mini quark I used one of the orange reciever's off eBay, I was able to fit the regulator and the receiver side by side and I put the lipo in the canopy and it works well.
With regards to the servo's I put micro servo's in and the first flight the elevator servo stripped in flight and got away with no damage then I stripped the aileron servo's more than once, I ended up buying small wing servo's off eBay that done the job, with the ribs being so small it's hard getting a servo that fitted
The mini quark flys well but it takes a good bit of wind
Hope this helps  :D

Andrew
i\'m an ordinary man nothing special nothing grand

EI1638

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« Reply #4 on: January 31, 2013, 23:56:21 PM »
Thanks for the feedback. A few things to think about there.

Keith: Its not that I don't trust modern electronics, as they pay my mortgage, but generally the simpler a system the more reliable it is. Maybe thats why I fly CL so much.
I'm guess I'm a bit pessimistic because of the nature of the stuff I work with, but that is rather specialist in application.

After checking my stock of bits I've got a 7805 regulator, a 350 mAh 2S lipo and a few 3 and 5 amp BECs.After reading Andrews post I may consider using some 8g servos I have in a test jig (my 'is that mix right' test jig).

Thanks for the advice,

Chris

billscottni

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Servos for use with Lipos
« Reply #5 on: February 01, 2013, 08:48:37 AM »
Most "electrical" failures I've seen have been switches. They seem to be the weakest link in the system.
Only ever seen one instance of black wire and that was the NIMSA Chairman Mike Leonards thermal model. He was getting intermittent contact and when he jiggled the wires, the black wire fell apart  :shock: