this is repost from r/AskElectronics because I was told they only deal with individual electronics component, not my kind of question. So here is the post (here is the old post, because there was already an answer by someone for reference) :
so I have some questions about the “correct” way to wire the battery, the DC motor controller and the DC motors for a “heavy” robot (about 70kg).
For DC motor controller I have a MCP233 whose datasheet is here and whose User Manual is here.
My concerns / questions are regarding PDF page 7 of the datasheet and PDF page 34 of the User Manual.
I am using 24V batteries.
So the basic points that I am wondering about are:
– pre-charge resistors
– high current diode
– relay / power switch
– emergency switch (you know the big, scary red button on every robot)
So their datasheet advise to always keep a return path to the battery when power is switched off, in case the robot is being pushed and the motors are generating power. As this is very likely to be my case, I need to get this right. From what I understand, this is role of the high-current diode named D1 in their diagram, put across the fuse. I understand this all right: if the fuse blows, the diode will keep a return path opened for regenerative power. No problem here.
Problem begin now: apparently the R1 resistor in their diagram is the precharge resistor, to limit inrush current on startup. That’s fine, but what I don’t understand is this R1 resistor being put across a simple switch like they have drawn. Based on this diagram alone, it means the power switch cannot turn the MCP and the motors off, because even when opened, current can still flow through the resistor (and by the way, if this is the case, and some orders are given to the motors, this would destroy the resistor through heat).
I would expect the precharge resistor to be on a multi poles switch / relay, at least 3 poles: one to connect the battery through the precharge resistors (let the capacitors charge “slowly”), then when it’s done, switch to next pole that connects the battery to the MCP directly (bypassing the resistor) and finally, the last pole should disconnect everything so that we can cut the power (more importantly not keeping the precharge resistor connected). Why isn’t this the case here? Am I missing something?
And if my understanding is correct, this means we have to put another high current diode across this switch, right? Because when the switch is opened, we still need a way for current generated by the motors to return to the battery, am I right?
And last point of concern: in the paragraph above the diagram, it is said that “an external main power cut off is required for safety”, I suppose they mean that we have to add another switch, right? The one represented on their schematics, is certainly not the emergency switch, right?
So! If I add another red, big, scary emergency switch (very close to the battery), should I also add a high current diode across it? Because otherwise what happens when the emergency button is punched (circuit is open) and the robot is pushed and thus the motors generate power?
On the other hand, it looks weird to me to add a diode across the emergency switch, because there’s always the possibility of this diode to fail in short-circuit, and then I have no other means of cutting the power.
So can you help? What’s the correct way to wire this DC motor controller?
Thanks in advance for your time!