MDC2230 wiring questions

Hi all,

I have a few questions about wiring the MDC2230.
It's taking 24V power from two car batteries in series, and driving two wheelchair motors with brakes.

1) I have long wires to my batteries - total loop length is about 10m of 16mm2 wire between the Vmot+ & Vmot- terminals, including the batteries half-way along it. The user manual (p.28) says to avoid long wires to power source - and if I can't avoid it to use a ~10,000uF capacitor near the controller. Does this 10m loop count as long and so do I need to add the capacitor?

2) Pre-charge resistor across SW2 - I've fitted a 1k resistor to prevent the SW2 arcing as recommended. But I've realised that I'm losing 0.6W across this resistor, which is significant as I'm trying to minimise all current draws to maximise the lifetime of my battery. What is the maximum recommended value of this resistor I can use to reduce my lost power? Can I replace this resistor with a flyback diode, that will prevent arcing on disconnect, but not connect?

3) The Advanced Digital Motor Controller User Manual says I should include flyback diodes across relay coils driven by outputs (p.43). But SW2 is not shown with one. What determines whether I should include one or not?

4) Diodes across fuses: There is a diode across the main fuse to protect the controller against regenerated current if the fuse blows. I take it I need diodes across all fuses in the circuit feeding the controller - and also any master on/off switches or relays in-line too that might trip for any reason?

I'd be grateful for any help you can give.

Best,

Frank

Hello Niko,

Thanks for your help in this. I understand your answers to 1 and 4, but I think I'm a little confused about 2&3:

I'm implementing SW2 as a relay, energised by an circuit through an E-stop button and into the Digital Output controlled by “No
MOSFET Failure" - the circuit shown at the bottom of p.38.
In this case the coil doesn't have a flyback diode across it, when I get the feeling it probably should? Because if the MOSFET failure happens or (more likely) someone hits the Estop, then the motor controller output will see a spike in voltage due to breaking the relay coil current.

Thanks for pointing out the resistor won't dissipate power most of the time - should have seen that myself! But why do I need to keep the controller switched off when the E-stop contactor is off? I was hoping to leave the controller to keep comms links up and running.

Best,

Frank

Before using a flyback diode on a high current DC relay, I suggest that you search out the Tyco web site and carefully read two application notes:

"Coil Suppression Can Reduce Relay Life"

and

"The application of relay coil suppression with DC relays"

Using just a diode can cause severe contact erosion if the relay is switched under load and several other solutions can be used to both protect your electronics and the relay's contacts.

Ciao,
Lenny

P.S. I will try to attach copies of those notes here, but I don't know what kinds of files this forum accepts, so that might not work.

Thanks for this Lenny, really helpful. I had no idea the diode suppression could have such a significant effect.
I'm breaking 100A so it's worth doing right. I'll have a think about what components I can realistically fit in.
Regards,
Frank

Hello Niko,
Thanks for this.
All the best,
Frank