I have a MCP233 and I intend to drive a fairly heavy robot with 24V DC motors. I'm reading the datasheet and the user manual http://downloads.basicmicro.com/docs/mcp230_datasheet.pdf
I'm very confused by some diagrams in the sections that explain how to wire relays and emergency switches.
This is on PDF page 7 of the datasheet and page 34 of the user manual.
My main concern is about the pre-charge resistor R1, that is across the power switch. If my understanding is correct, a pre-charge resistor is used to limit inrush current when big capacitors are present in the device, as I'm sure it is in the MCP. So the idea is to limit current to slowly charge the capacitors. This is fine. But when I look in other circuits, the switch that is used with a pre-charge resistor usually has several poles. So the idea to have a sequence like this: connect the pole of the switch with the pre-charge resistors, wait for the time necessary for the capacitor to charge, and then disconnect the resistor and connect the second pole of the resistor.
In this situation, the current goes through the switch, and the pre-charge resistor is left floating. When one want to shutdown the MCP / motors, the switch is opened.
In the diagrams presented in this datasheet and user manual, the schematics makes it look like the switch is single pole, with a resistor across it. Plain simple. But then in this case, it is not possible to shutdown the MCP / motors, because even with the switch opened, current can still flow through the resistors.
What's ticking me even more is that with the switch open (and thus current going through the 1/2 W resistor), if one were to drive the motors, the resistor would die instantly, as it would dissipate an enormous amount of heat (for instance, with advised 220 Ohms resistors, if motors consume 5A, then the resistor will dissipate 220 * 5 * 5 = 5.5kW). Clearly there is something I don't understand.
My second concern is that, if I consider the above-mentioned switch to be, in fact, a multipole switch (or relay), then there should also be a reverse free-wheel diode across it, because then when we want to turn off the MCP / motors, this is subject to arcing, no?
My third concern is that in the text above the diagram, it is said "An external main power cut off is required for safety.". Am I understanding correctly that this "external main power cut off switch" is not represented in the diagram (hence it is not the represented switch with the resistor across)?
If I'm correct, then my question is: should I also place a high current reverse diode across this emergency main power cut off switch also? Because when the main power cut off switch is opened, what happens when the robot is pushed and the motors generate some power?
Sorry this was a long post, I tried to be as clear as possible regarding my mis-understandings. I'm really wanting to wire my battery, motors and MCP together is the best, safe possible case, this is why I need to understand correct.
Thanks in advance for your time!