Old Frsky D8R brown out.

[jhsa]

I have been working on a little project of mine and at the same time taking the chance to learn some more about electronics. Been playing with some Mosfet transistors, a frsky receiver (old D8R) and some servos. As in the past I have nearly crashed a model because of a shorted servo (luckily it shorted before takeoff) I decided to install resettable fuses on my servos by soldering the fuse in line with the positive (red) wire.. Please see the picture below..
So, i was powering a receiver and servos using a 4 cell NiMh battery and through a Mosfet. Then simulated a short circuit on one of the servos protected with the resettable fuse. I have soldered a LED and resistor in parallel with the fuse that will turn ON when there is a short circuit. This particular fuse will trip at 1A and hold the current at 0.5A
Shorted the servo, fuse tripped and LED came ON, but obviously the battery voltage always drops by some degree until the fuse starts doing it's job.
I have noticed that sometimes the receiver would brown out and then immediately turn On again and start working normally, specially when the fuse has been tripped before and it is still warm and I short the servo again. but sometimes, the rx will brown out, then power ON again, but just doesn't work and has both red and green LEDs fully ON. I need to turn the power OFF and ON again for it to start working.
I don't remember seeing this behavior on a frsky receiver before. I have seen receivers stop working with low voltage, but they always resumed normal operation after the voltage got back to a good level. Therefore I'm sharing it here and ask if someone have noticed this kind of behavior?

João

[Bill]

A few things you might look at:
Some of the resetting on power failure voltage glitches etc is done by a watchdog timer in the main microprocessor, if the shorting / brownout caused the microprocessor to jump into a loop where it is still resetting the watchdog there's a problem it doesn't know its not working!
If you don't quite lower the voltage enough for a brownout trigger that won't work either, with your rapid fuse break the voltage may be restored to quickly for the brownout to register, that's another possibility.
What typically happens will be a very rapid spike to ground and back to supply, throwing the micro out of program, sometimes the watchdog detects it sometimes it doesn't.
Do the more up to date receivers show the same behaviour?
Have fun..

[HC1969]

It is very familiar with the circuit.
http://www.rc-miskolc.emiter.hu/rc-misk ... board-fuse

[jhsa]

A few things you might look at:
Some of the resetting on power failure voltage glitches etc is done by a watchdog timer in the main microprocessor, if the shorting / brownout caused the microprocessor to jump into a loop where it is still resetting the watchdog there's a problem it doesn't know its not working!
If you don't quite lower the voltage enough for a brownout trigger that won't work either, with your rapid fuse break the voltage may be restored to quickly for the brownout to register, that's another possibility.
What typically happens will be a very rapid spike to ground and back to supply, throwing the micro out of program, sometimes the watchdog detects it sometimes it doesn't.
Do the more up to date receivers show the same behaviour?
Have fun..

Bill, thank you for the explanation. Yeah, that could be what is happening. I'm sure there is a spike as the resettable fuse takes some time to heat up and react. So until the fuse triggers the circuit is in fact shorted.
I don't have any of the new receivers available at the moment. I'm waiting for two of them to arrive at any time now hopefully. It has been over a month now since I've placed my order at HK
But in any case it is good that is happening on the bench so we have in mind what we can expect when using the equipment. Some of us do power the frsky receivers from just one Lipo cell in some cases and that is already a quite low voltage for the receiver. I have a little glider that is powered this way, so we should have the possibility of a RX brownout situation on the back of our heads when working on this kind of setup. Also for people using resettable fuses as a protection against shorted servos, it is a good idea to use a fuse that matches the servo current specification. A fuse of too higher value will take too long to trip causing a longer and deeper voltage drop on the complete system, and one of too lower value could trip when the servo is in normal operation, leaving us with intermittent servo operation, which is also not good.

I will test the new receivers as soon as I get them

João

[jhsa]

It is very familiar with the circuit.
http://www.rc-miskolc.emiter.hu/rc-misk ... board-fuse

Indeed, it looks like the same/or similar basic circuit.. But I'm using some through hole PPTC and I solder them directly to the servo positive wire. I have seen this somewhere on RCG a while ago and I did like the idea
As I said above, I think the value should match the servo current and power supply specification for a much faster operation.. Or am I wrong here and any value above the servo current, and under the power supply max current could be used?
I think different fuse trip values will also affect the fuse trip time?
I did experiment with a 750mA and a 500mA hold current on one of those little blue servos from HK. The 500mA seems to trip faster, and certainly holds the current lower when tripped. I am waiting for some 300mA fuses. I think they will be about right for these servos, but need testing..

João

[HC1969]

I suggest that at the receiving end of a fairly large booster capacitor to smooth the voltage.

[jhsa]

You mean that the capacitor should be connected to the receiver side??

Tanks

João

[HC1969]

Yes, protect receiver.