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Current Draw Question

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Old 03-16-2014, 07:48 AM
  #1  
im_a_rcav8r
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Default Current Draw Question

I am planning the electronics for my 1/3 scale Fly Baby and I have a question about Current Draw. When you look at the specs on a servo it tells you the current draw at idle and at zero load travel. I want to know the peak draw so I can plan my receiver power distribution properly.

I stumbled across a website that had several sample "peak servo current tests" done with an oscilloscope. One of the samples used a HS-925MG servo and it showed a peak draw of 1700mA. I looked up the servo specs and I found the following:

Current Drain (4.8V): 5.3mA/idle and 400mA no load operating
Current Drain (6.0V): 6.6mA/idle and 500mA no load operating

Assuming the test was done at 4.8v it looks like the peak is just over 4 times the no load operating (3 times at 6v)

The servo I will be using is a 200oz servo and it draws 8mA idle and 800mA at zero-load operation. I will be using a 2100 LiFe 10C/30C Peak battery to power the servos (receiver will have its own battery).

Using the sample above I expect to have a peak current draw of 2400mA-3200mA per servo. If all 5 servos are activated at the same time (not including the throttle servo) I may be looking at 16000mA or 16A. At 10C my battery will deliver 21A (63A momentarily)

So am I over thinking this? Will my 2100mA-10C battery be up to the job? Am I missing something? I would also like to know the average draw so I can calculate how much time I can safely fly before the battery starts to get too low.

Anyone want to give this 20 year pilot some pointers on flying the big boys?

This is my first 1/3 scale - in the past with my 1/4 scale and smaller planes I did not worry so much about the draw but I was only using 98oz servos then. I worry about the 200oz servos and how much current they will use.
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Old 03-22-2014, 05:59 AM
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KW_Counter
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How often and for how long do you plan to have all servos in extreme draw?
I use a 2000mAh NiMh in my sport planes and never need to charge at the field.
I do believe your battery is up to the task.
However, I would do some testing to find out what you are using per flight.
Do one or two normal flights on a fresh battery.
Then recharge noting how much energy you put back into the battery.
From this you could calculate safe number of flights per charge.

Good Luck,
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Old 03-23-2014, 11:51 AM
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jester_s1
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You'll never pull that many amps in flight. Those peak currents are what the servo will draw when it is stalled, which will not happen in flight unless you've chosen servos that aren't strong enough for the application. At most, you might stall one servo for a few seconds, likely the rudder servo in a hard maneuver. I'd be really surprised to see you go over 3 amps in normal flight, maybe 6 amps for half a second if you decide to do a snap roll or flatten out a spin.
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Old 03-27-2014, 07:22 PM
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You are over thinking it but not by too much.

If you have a voltage regulator on board that can only deliver 10A then 99.999% of the time you be fine until every servo wants full current at the same time. You will then be relying on the output capacitors in the regulator, and any filter capacitors in the servos and rx to make up the current difference. The one saving grace is that as the voltage drops under load, the amount of current the servos can draw will reduce, so while they may draw 16A in total at say 6V (?) that 16A peak might just drop back to 10A as the voltage sags below 4V.

All of the above can happen in the blink of an eye and you'll need an oscilloscope to spot it, but one microcontroller I used to work with would intitate a brownout reset if the supply voltage dropped to ~2V for longer than 100us (0.0001 seconds).

In practice this means that it's highly unlikely a Li-Fe pack will get pulled down to a low enough voltage for a long enough time to cause a problem, but apparently in the early days of 2.4GHz, a nearly flat 4-cell Ni-Cd pack + digital servos didn't leave you with much voltage overhead and brownout's did happen. I tend to size regulators for about 1/2 of worst case current draw and battery capacity enough for about 5 safe flights.

I have a 10A reg in one plane (6 brushless digitals of various sizes) with a 1.3Ah Li-Po for example and have flown the same plane for 1 flight with a 450Mah Li-Po.

As a side note, a servo that is stationary but unloaded will draw "stall" current until it gets moving, meaning the current peaks a servo is drawing while humming away in the pits is the full stall current but only for a fraction of a second. It's when those fractions of a second all happen at the same time that something bad can happen, but it's very rare. I watched the current draw of my two elevator servos as they buzzed away under the weight of the surface and there were two little 1.7A hills on the trace which joined up and become a 3.4A mountain, then split back in to two little 1.7A hills as the 300hz control frequency in each servo must have been very slightly different. Fun to watch if you have nothing better to do??

Last edited by bjr_93tz; 03-27-2014 at 07:35 PM.
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