ST Discovery burn out

[Allan Bennett]

"... What if the cut off for the u or s bec is higher than the esc cut off because of quality control issues with the esc? ..."

So far as I know, stand-alone BECs don't have a low-voltage-cutoff per se, though they will stop working if the main battery voltage goes below the voltage they're trying to supply to the radio. But, assuming one is using a main battery that's 3S or more (I'm not sure about 2S), the motor will have noticeably lost power -- whether or not the ESC's lvc has kicked in -- long before the battery voltage is anywhere low enough to affect the BECs output.

"... I’d consider that disconnecting the red wire, this seems to be the popular notion on the matter, is simply an option rather than a requirement. The system will work equally well either way, that is until the going gets tough, then actually the battery will always deliver. ..."

I'm sure there are many instances where BECs, or BECs and receiver batteries, have worked in parallel without any apparent problem. But, without knowing the design of the specific BECs -- do they have back-current protection, can they tolerate a back-current if the other device is putting out a slightly higher voltage, etc.? -- it must be safest to disconnect all but one of the devices, by pulling out its red wire.

[Peter Beeney]

All the voltage regulators that I’ve played about with in the past do not cut the supply off at all, when they get down to the drop out voltage, that’s just above it’s output voltage, they just becomes a straight through connection, and conduct the battery voltage to the load. But I can’t vouch for any of the modelling types, I’ve never tried them. The reason being, I guess, is because as you quite rightly say, if the battery ever gets down to these dismal depths, you are going to be in more trouble than enough long before this anyway, so it’s not really even really worth considering.

With regard to the BEC and the red wire, my view is this. The BEC’s sole aim is to supply a very steady constant 5 volts, irrespective of what the input voltage is, within reason of course, and however the output load varies, again within some defined parameters. The way it does this is by constantly monitoring it’s own output voltage, and should this drop it will increase the current to raise the voltage, if it goes high it reduces the current to lower the voltage. So this makes it a constant voltage device, but if it’s severely abused in anyway way it does have some safeguards. At least, they normally do, I can’t vouch one hundred percent for model trade types, such as those on ESC’s. The voltage of a charged four cell nickel battery will be in the region of perhaps 5.3 - 5.4 volts, so if this connected in parallel with a BEC it will simply just shut down, and it stays that way, it simply thinks it’s supplying too much current. The battery will supply all the current that the BEC was supplying, including that used on the ESC to drive some other components. This will happen seamlessly, that is, without anyone noticing.

But now the difference is the battery is a variable voltage device. As soon as it starts to discharge, the voltage starts to fall. At some point the battery voltage will reach down to 5 volts, and at this point the BEC will start to awaken. When the voltage falls to fractionally under 5 volts the BEC will fire up and supply some of the load. The nett outcome of this over time is that the battery will never go flat, that’s 4.4V in my book, if there is a sudden large surge on the load the battery makes up the shortfall; again seamlessly, but as soon as it’s gone the BEC will raise the voltage to a steady 5V.

If we disconnect the red wire so that the battery has to do all the work, we need a sufficiently large capacity to power the radio and servos for a sensible time, but if we share the load we can get away with a much smaller and lighter battery, here. It goes without saying that it does still need proper care and attention though, plenty of TLC, but if I didn’t do this I would consider I was just asking for trouble anyway! I personally consider it to be a small price to pay for the benefits received.

The whole crux of the matter could be those safeguards. As we mentioned, there is a thermal cutout on the BEC and if this is operated, for whatsoever reason, the device shuts down; in self protection. It soon resets, but by then the model is probably stuffed. If a battery is connected there are no such restrictions, it will supply the current come what may. Exactly what we want for our model, it’s as safe as we can make it. Or it is for me, at least.

I’ve related on the forum a couple of times before about the model that was totalled because on one single occasion this parallel battery was not switched on. But it was unusual circumstances, though, many pilots have many varying ideas about all this electricity stuff.

If I may, and with respect, regarding the back-voltage, not current, when it’s shut down it’s acting like a diode, and there is no protection, other than self-protection, exactly like that of a diode. It’s more likely the components on the ESC that are most at risk. They will certainly stand a back-voltage of up to 8 volts, though, I’ve tested this, and unless you were using the new high voltage servos it’s much more likely you’d zap these instead, rather than pop the ESC. I suspect they might withstand up to around 15 volts, but that’s total supposition, I don’t know this for sure.

I’d certainly be quite happy to ignore the slight voltage difference in this instance, it’s never going to be an issue. Not ideal, perhaps, a bit of a cobble up, but so is the concept of using a device with a limited output, the regulator or BEC, to drive an unregulated load, the radio and servos, in the first instance anyway! As far as I’m concerned, that is. This has always been problematic, hence all the various ways of now providing an alternative supply, such as the UBEC. And again, I’m sure this works just fine, for many folks probably forever without giving any trouble, but if the main battery goes out for any reason, say an open connection, all is again lost. Likely to be a rare occurrence, but it’s happened to me twice when nicads went open circuit, burnt out due to abuse, I guess. Fortunately on both occasions I had that receiver pack installed...…

Just my little perspective, as always definitely not any sort of instructions on how to do it, but I’ve found it easy enough to crash models without anything going wrong at all. So I just make every best effort to at least try and eliminate anything that is likely to give any grief……

PB