StaticDet5 wrote:Some of the MOSFET conversions advertise an "active braking" component. Is that the second MOSFET with the opposite appearing gate (I'm guessing a P-Channel MOSFET)?
You're correct that Q2 is a P-chan MOSFET. When turned on it'll apply a pretty hard short across the motor. The motor, since it and the gear train have mass, has inertia. Even when the power is turned off (via Q1) it'll keep spinning until friction and the energy needed to compress the spring stop it (which I'd have guessed would have been sufficient). A motor (w/o power) and spinning makes it a generator. It'll produce a voltage at it's terminals. Any electrical load at those terminal will dissapate energy which comes from the inertia in the system. A big load, like a short circuit, "steals" that energy very quickly and so brakes the motor (and gearbox). I'm just surprised it's deemed necessary.
Looking at the circuit the brake can only be on for a short time, perhaps 0.3 secs or less. I guess that's long enough to do the job.
StaticDet5 wrote:It probably is a braking circuit. But how do they know when to brake? I don't see anything in the schematic that could tell you what's going on in the gearbox. UNLESS! Unless PB2/ADC1 (I'm betting ADC1 is Analog to Digital 1) measures.... something. It looks to me like it would only measure voltage. Is voltage going to change through the firing cycle?
Well obviously the brakes would be slammed on only after Q1 has been turned off (otherwise you'd place a short across the battery terminals and that would be baaaaaaaaaad !). So they might just apply the brake some short period of time after they turn off Q1 and be done with it. But I think you're thinking they brake to stop the gearbox at a good point, when the spring is relaxed. I don't know how they'd know that. I'm not sure what purpose(s) the R5/R6/C5/PB1 circuit does. With the motor off and stopped, it'll measure the batt voltage but that's an odd place to put it. Besides they already can do that via the trigger circuit so why bother to do it again ? With the motor on and running, Q1 should be shorting that - motor terminal to ground very effectively. The R5/R6 voltage divider would further reduce any small voltage present. I might guess there would be some spikes there as the motor turns but that would be indicative of the brushes making/breaking contact. I don't see how that would be useful. I guess it might be a failsafe ... to know that the motor has been turned off when commanded ? Or that the brake "engaged" ? But of what practical use would this be ? It'll already be obvious to the person and the MCU controller can't do more than it already has (command Q1 off, perhaps Q2 on). Got me on this. A peek at the source code might illuminate the thinking behind this circuit.
StaticDet5 wrote:For what it's worth, I've pulled their code and emailed the designer. I don't know if he'd want to give hints on this.
Probably not but no harm in asking. Perhaps ask if the controller intelligently parks the gearbox to save the spring. Right now I'll guess it does not.
StaticDet5 wrote:It's really hard for the sailor in me not to start swearing at moments like this. I'm so close... I grabbed some terminal blocks from Radio Shack today. I'm going to build a big, blocky prototype system on a piece of 1x6. It will keep things organized, and give me a little more fun if everything catches on fire...
So just take it in pieces, implement the easy and needed functions first and add in the neat and not-strictly-needed stuff afterwards. RIght now I'm more puzzled by how you've managed to smoke 2 of your FETs. There's nothing, obvious to me, that you've done wrong. Look at the basic circuit used by those guys, it's the same as you have now. Do you have an DMM/ohmeter ? If so measure the DC resistance across the motor + and - terminals with everything else disconnected. Perhaps rotate the motor so you have that measurement for several of the coils. This should be the limiting resistance in the motor drive circuit. It might be smaller (< 1 ohm) than your DMM can measure though.