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Aug 08, 2017, 09:16 AM
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Careful!

brownout risk deviating from stock


Just got a Tundra or maybe a Timber and servos froze or stripped on your maiden? So your next move is searching the forum for mentions of alternative metal gear digital servos, you find a good deal and buy six. Along the way you pick up a flight controller and a TM1000 (telemetry). I've been there buddy. No big deal, right?

You preflight the plane, run it full throttle on the bench, wattmetering, it all seems kosher. The guys in the forum didn't report problems with the servos, the site from which you bought the stuff has a good reputation, and you range checked after installing the TM1000, so what could go wrong (other than forgetting to turn off your AS3X now that you've added a stabilizing flight controller)?

Brownouts!

Your ESC, if it's a common type, in addition to feeding your motor power from your battery, it also feeds, and at a different voltage and maximum amperage, power to your receiver, which in turn powers your servos, your flight controller and that TM1000, all through three tiny servo pins. The ESC Horizon picked out for that plane was likely designed to be as lightweight as possible while delivering just enough power to the plane without failing.

Now when you switch to non-stock metal gear digital servos, even if they don't have higher torque and speed, there's a decent chance they will draw more power, and a moderate chance they'll draw significantly more power, and a noteworthy chance that they'll collectively spike and eat up more amps than your Horizon included ESC is able to dish out to the receiver. Add in a flight controller, telemetry stuff, lights, camera and camera gimbal servos, that's even less breathing room.

If all these things eat up more power than your ESC can handle, probably 2A to 6A over 5V, not a lot (a camera alone could eat up an amp), while you may be fine taking off and having thirty flights, one day with the right combination of stick movements, your ESC and/or receiver is liable to say "Screw this, I'm resetting myself" which is like you cutting the power and turning it back on. All control is lost. If you're really lucky and at altitude and level, maybe you'll have enough time for the receiver to light up again with the camera drawing less power because it's no longer recording, sync up a new relationship with your transmitter and initialize before you hit the ground. Good luck with that.

RC Plane Servo Brownout and Solution (3 min 2 sec)

Now, odds are you don't own an oscilloscope, but at the very least, find out what your ESC's BEC's max constant output is, and then find out (google, forums, contacting the manufacturer/hobby shop?) the peak power draw of the servos (and multiply that by however many you have), the current your camera burns when recording (and that changes based on things like frame rate), the peak current of your TM1000 (it's not constant, it spikes up when firing out a packet of data) and so forth, and make sure your battery elimination circuit is capable of powering that with good room to spare.

Note also that your ESC's BEC/SBEC might have an exaggerated rating or might be defective -- I just discovered this yesterday (thankfully on the ground) when testing an ESC with an SBEC supposedly capable of putting out 6A/5V, more than enough to handle the plane's configuration, but it browned out repeatedly. Switched in another ESC with a less powerful BEC current rating, no more blackouts.

I'm just a rookie, I don't really know what I'm talking about, I'm not going to get into using a UBEC, I'm only going to advise you to be mindful and seek other advice, there are better ways to ensure this is not something to worry about than simply reading labels and adding up amp numbers or wiring in a capacitor somehow, so before deviating from what came out of the box, search rcgroups and wattflyer for better information on brownouts and how to safeguard your plane, and the people on the ground, from them.

Doug
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