MMM v3 BEC questions

[BrianG]

Looks like an interesting project! You're right; the Slash is about the only stock vehicle I know of with that much empty space on it.

Originally Posted by coreyfro

...My questions are:
1. Will voltage ever be programmable like the CC dedicated BEC?
2. What available amperages will the BEC provide at 8, 12, and 24 volts (or approximate voltages.)...

1: As of right now, I would say no. The BEC also supplies at least part of the ESC brains with ~6v power, so any adjustments would have to be tolerated by those other components.

2: Not sure what the current output of the built-in BEC is, but judging by the components used, I'd say around 3A is a safe bet. And since it is a switching BEC, that available amperage is good for all voltages from ~7v up to the max of 25v. BEC efficiency doesn't change all that much with supply voltage - it's the load current that makes it fluctuate. And switchers tend to be most efficient at around 75% of their max current rating from what I've seen.

As far as getting stuck in control loops, you outta put in a manual override so you can use a remote (at least for the testing phase). I can just see people running after an out-of-control Slash trying to catch it!

Ahhhhhhhhh, makes total sense now. OK. if I have any requirements for higher voltage, then I'll use my CC BEC. I just want to keep the number of parts low and simple because this isn't for me, I'm trying to make a simple, standard, cookiecutter robot for RC nerds.

Originally Posted by BrianG

1: As of right now, I would say no. The BEC also supplies at least part of the ESC brains with ~6v power, so any adjustments would have to be tolerated by those other components.

2: Not sure what the current output of the built-in BEC is, but judging by the components used, I'd say around 3A is a safe bet. And since it is a switching BEC, that available amperage is good for all voltages from ~7v up to the max of 25v. BEC efficiency doesn't change all that much with supply voltage - it's the load current that makes it fluctuate. And switchers tend to be most efficient at around 75% of their max current rating from what I've seen.

As far as getting stuck in control loops, you outta put in a manual override so you can use a remote (at least for the testing phase). I can just see people running after an out-of-control Slash trying to catch it!

Yeah, I don't have a wireless kill switch yet.

[BrianG]

Originally Posted by coreyfro

Ahhhhhhhhh, makes total sense now. OK. if I have any requirements for higher voltage, then I'll use my CC BEC...

Depending on the actual voltage you'll need, you can probably get away with using a linear regulator since you said you need ~1A. The loss in efficiency will be less and less as the regulator voltage approaches the supply voltage and/or the current goes down.

The losses is simple to calculate: power_loss = (supply_v - regulator_v) X current.

So, if you are running 4s and wanted 9v out, the loss at 1A would be 5.8w. A decent amount to be sure, but nothing that a medium heatsink couldn't handle. As a bonus, linear regulators can be "programmed" (via 2 resistors) to be at any voltage you want.

The disadvantage of linear regulators is power loss/inefficiency. The disadvantage to a swithing regulator (aside from cost) is the potential for it to go bad and send full supply voltage to your circuits. It doesn't happen very often, but is possible. I don't know how expensive that processor/circuit is, but something to think about. Linear regulators usually fail by thermaling, or going into overload shutdown. You have to really try to blow a linear regulator.