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Old 11-03-2019, 02:31 AM
  #1407  
Looping31
 
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Originally Posted by ww2birds
The CB200 is a good product, I have several of them in service.

The only thing to consider is the fact that the CB200 uses polyfuses on the servo power outputs. These are similar to physical fuses in that they open when too much current is passed through them, but unlike physical fuses, they close again when they recover. They are tripped by heating, and once open, the source of heating stops and they re-close. If you are curious, see: https://en.wikipedia.org/wiki/Resettable_fuse

The idea is that a shorted servo does not take down the power supply to the entire aircraft, a laudable goal. The issue is that with very large jets or large 3D aerobatic airplanes, the approx 4A trip points of the polyfuses could be hit in extreme flight operations (e.g. flaps full on large jet at too high a speed, huge aileron or elevator deflections in 3D birds).

I've made measurements on the CB200's polyfuses and find that they take a current a little beyond 4A applied for 10s of seconds before they open. You can get close to this on the flaps for example on an Ultra Bandit with full flaps put down at a little too high a speed .. I've got the measurements for this (!) .. and they polyfuses did not pop. But if they had it is likely on flap would "pop" before the other which could be a problem...

So I think it's safe to say that for most of us, this is not likely a concern .. just something to be aware of. I first noticed this when Jeti published update notes on the CB 200 that suggested it would be appropriate to parallel the power and ground (NOT the signal!) wires from two outputs of the CB200 for possible high-draw situations.

I really appreciate Jeti making us aware of this in the release notes and applaud them for giving us a choice in the new central box.

For my larger planes, I use a CB400 but don't use its servo power outputs (red and black wires to servos) for possible high-draw surfaces directly .. I supply power from another source (e.g. the BEC on the CB400) to avoid this risk.

As you have seen, Jeti will have an option for polyfuses or not on the new CB which is really great .. then we can make a choice based on the requirements of a particular install.

Dave
Hi,
Thank you Dave for your interesting contribution.
So, I guess that the new CB210 has no polyfuses, so it has no overload protection in case of stuck servo or elec short ?
Arnaud