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Old Aug 27, 2002, 06:51 AM
F1B-flyer
Tapio's Avatar
Helsinki, Finland
Joined Mar 2001
32 Posts
Best micro servo for low voltage?

What kind of servos would work on a single lithium cell as power source? I'm seeking possibilities to cut weight on my digital timer in a free flight model, and the kokam lipol cells seem ideal. replacing a 15 gram 4-cell 50 mAh Nicad with a single 4 gram lipol... The microcontroller seems to be happy with less-than 4 volts provided by the cell, but the servo (Simprop SES85) moves pretty slowly, especially in cold (I need to be able to run the setup down to -20C, so I'm currently testing the setup in a freezer )

I think many people run their HLG's on three cell NiCads, and this gives about the same voltage as a single lipol. They also mostly use SH50; is this servo intended to be run with only 3 cells? Are there others that go that low?
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Old Aug 27, 2002, 08:58 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
I think the HS50 is good in terms of current draw. The Westechnik linear servos would be lighter. But, I've heard they have some voltage spikes. However, if you are going to use a single 140mAh Kokam LiPoly you will have more than enough capacity. Is weight your biggest concern? If so, Wes has a new (not on their site yet) linear servo that weighs 2.1g (probably more with wire and plug). Also, Kokam has a 40mAh LiPoly cell that weighs about 1.9g or so. I don't know anything about the current draw of your timer. But, it could turn out the cell would be sufficient for a timer and then running a Wes servo just once (I'm guessing it just goes to one extreme to kick the elevator up). I think if you could go from a 4-cell NiCd to a Wes servo and a 40mAh LiPoly your weight savings could be significant. Your wallent will be lighter too after buying the Wes servo.

Gordon
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Old Aug 27, 2002, 09:17 AM
F1B-flyer
Tapio's Avatar
Helsinki, Finland
Joined Mar 2001
32 Posts
Quote:
I think the HS50 is good in terms of current draw. The Westechnik linear servos would be lighter. But, I've heard they have some voltage spikes. However, if you are going to use a single 140mAh Kokam LiPoly you will have more than enough capacity. Is weight your biggest concern?

<snip>

I think if you could go from a 4-cell NiCd to a Wes servo and a 40mAh LiPoly your weight savings could be significant. Your wallent will be lighter too after buying the Wes servo.
Gordon,

weight is one important concern, that's why I'm trying to replace the 15 gram Nicad with a single LiPol cell. So far testing has shown that servo performance in cold may now be a factor, as there was no problems of slowing down with 4-cell NiCads.

I did not know there are Kokams as small as 40mAh. Those would be sufficient, and with that I could put two in series and get rid of any woltage problems.

The timer draws a few milliamps when flying (servo on), more than that for a couple of seconds when the servo is moving, but most of the time it is idling, servo power turned off, and drawing just microamps. Thus 40 to 50 mAh is quite sufficient capacity for my needs.
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Old Aug 27, 2002, 03:29 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
Here's the link to Kokam's site. http://eam.fub.com/default.asp

The 40's are a different series than the 135's (140's) and do not allow as many times their capacity to be pulled from them (i.e., they don't perform as well as the 135's). But, they are probably plenty for your use. I don't know where you will be able to get them. You might check with FMA Direct to see if they are getting any of the 40's.

Gordon
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Old Aug 28, 2002, 08:04 AM
Crash Master
Gene Bond's Avatar
Indianapolis, IN
Joined Sep 2001
16,632 Posts
My testing showed that HS50's were much better on a single Li-Ion cell than Pico's. Both glitched a little at the low end (3V), but so did the gws 2a esc w/ lvc disabled. The gws pico rx ws much better than the feather at the low voltage.
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