Discussions

Power supply problem using an external BEC

Posted by Neel on October 4, 2010 at 5:17pm
Hi all,

I am facing a power supply problem which I hope someone can shed a light on with their experience.

This is my current setup:
- 3S 4000 Lipo flight pack powers my ESC (Castle Creations Thunderbird 54)
- I have an External BEC (6V ParkBEC, 1.25 Amps) connected between the ESC and APM board. This BEC also draws power from the Lipo and supplies 6V to the APM.
- Throttle out of APM board is connected to the External BEC
- APM board powers Rx, servos, Xbee, GPS

Whenever I power on my system, the servos are twitching a lot even when the IMU is calibrating (powering up in hardware Manual mode). Once the IMU is done calibrating, the throttle comes on by itself and goes to full power even without giving any throttle command. At this point, I cannot move any servos with Tx (even though I am in manual mode).

However, when I remove the External BEC and power my system using the ESC, I get a very clean power up -servos don't move, throttle doesn't move - it's perfect. I can move the servos around with no jitter in Manual mode. Stabilize mode works well on the ground.

This leads me to believe my External BEC is somehow not supplying clean power to my system. However, I checked the voltage and the current draw from the External BEC and everything looks normal (6V and about 400 ma current draw).

My questions are:
1. What could be causing the issue that I am seeing? I can post a video of the issue if that is helpful. Is 6V too high a voltage for the APM board? The internal BEC of my ESC supplies about 5V.

2. The main reason I am using an External BEC is that I read somewhere that powering the autopilot + servos in Auto mode using an ESC will tend to heat up the ESC a lot. Is this a valid concern?

Thanks!
Neel


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Replies

  • Developer
    Mark Colwell October 5, 2010 at 5:25am
    6V ParkBEC, 1.25 Amps, is to low amperage for 3-4 servo operation get the Castle Creations UBEC at 3 amps with adjustable output from 4.5v to 12v about $20, a very good product.
    Check your BEC output when you wiggle both sticks, it must not dip below 4.8v or get noisy.
    To disable your Thunderbird54 Linear regulator BEC remove red wire from Throttle connector and insulate with tape or heat shrink. I would use 5v max for APM, and sensors, if you want 6v for servos check out wiring diagrams on forum, servo +6 bus must be isolated from APM +5v bus.
  • Paul Thompson October 5, 2010 at 3:55am
    The Thunderbird54's have a 3amp Linear regulator BEC, I dont think it would be a problem, I am going to use the same ESC. Some of the BEC's particularly cheap ones can have a fair bit of electrical noise. Also you would have to disable the BEC on the ESC (if you wanted to run an external one) which I dont think you can do on the thunderbird range. Having two BEC's supplying power (which is what I think you have happening) to the same device will cause problems. The heat generated from the BEC on the thunderbird is nothing campaired to the heat generated from supplying power the motor. Just keep some cooling airflow to it.

    Cheer Paul
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