RE: Votlage Clamp Mosfet Recommendation

General discussion of using Roboclaw motor controllers
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JorgeP
Posts: 14
Joined: Thu Nov 16, 2017 8:16 am
RE: Votlage Clamp Mosfet Recommendation

Post by JorgeP »

I've spent a bit of time trying to find appropriate parts for my application for the protection circuitry on page 8 of the datasheet and Page 40 of the manual. I am using an MCP but I think this applies to ROBOCLAW's as well, so I am posting here because this part of the forum is more active.

Info about my project: Converting a Midwheel Drive Powerchair Base to use the programmable MCP263 motor controller. This particular powerchair has dual 1000 watt (max current) motors as the drive wheels. I have two 12V 73A batteries wired to give 24V.

I am a hobbiest with electronics and not at all an expert, Will the following components and values provide enough safety circuitry even if I do get into a max current stall state? All links are to digikey
Dout isn't really needed to deactivate the motor brake since these motors have a lever to disable the brake manualy allowing the motor to run freely, although I would like to have the circuitry in place incase I change my mind.

I am not sure if it is needed but for R1 and R3 I have rather large heatsinks https://www.digikey.com/product-detail/ ... ND/1625171
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Basicmicro Support
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Re: RE: Votlage Clamp Mosfet Recommendation

Post by Basicmicro Support »

You need to copy the Digikey part number. Copied the links are only valid for the session you were in at the time. None of them work now.

Ive never used a resistor like PF2205-1RF1. As long as its heatsunk you should be ok though using it for R3 in the external voltage clamping circuit(or as R2 in the simple voltage clamp circuit). Using it for R1 however is overkill. R1 can be a 1/4watt resistor. The value of R1 dictates how long it takes for the main battery to charge the caps before turning on the power switch. I would recommend 220 to 480ohms. The point of R1 is to prevent the spark when connecting the battery. It provides a low current to charge the large caps built into the MCP instead of the massive current draw the direct battery connection would provide(hense the spark during the cap charge).

Note, however that the pre-charge resistor will always provide power to the MCP. You wont be able to run the motors but the logic will constantly drain the batterys(at a very low draw). There are complcated circuit with relays that can be setup to engage the precharge resistor for a short period before switching the main switch on or you can setup a secondary precharge switch so you can completely power down the system.

The standard household light switch is not recommended. They are designed for AC circuits of up to 30amps or so(depends on the model). DC amps can go much higher and usually the DC rating is lower than the AC rating. You need a high current relay or contactor. Contactors can be found on ebay for as low as $30 last time I checked.

The diode is ok but the forward voltage drop is larger than I would want usually. Pick a diode that has the lowest Vf(forward voltage) you can get away with. Also pick a diode based on the peak(not repetitive or continous) current. The diode across the power switch/fuse will only activate for a very brief time if the switch or fuse opens while running the motors. In all other cases it doesnt do much of anything except disipate some power through the forward voltage drop.

Note, I recommend the diode bridge across both the fuse and the switch. If the switch is thrown while running the motors it acts exactly like the fuse to you want to have a path back to the battery.

The fuse wont work. The batteries produce a max of 73A. The fuse is rated for 80A. It will never blow. You need a fuse rated so that the timing chart shows it will blow at or near the max current of the batteries in between .1 and 1 second. See the Time-Current Characteristic chart in the datasheet for the fuse you picked. Find a lower current rated fuse that will work or get better batteries.

The MCP voltage clamp circuit shows using an external mosfet. However you can use the DOUT pin directly to disiapte upto 3.5amps and the circuit is much simpler. Just wire R1 directly to Batt+(eg 24v) and the DOUT pin. DOUT turns into a ground when activated(eg when setup as a Voltage clamp).

The MCP manual external clamping circuit however is wrong. It was cut and pasted from the Roboclaw manual and the proper changes were not made in the diagram. Ignore the diagram. The text is correct however. As the text states DOUT should be wired to a 5v Pull-Up. The diagram instead shows a Pull-Down to ground. Make sure you connect R2 to 5v, not ground. 10k is still appropriate for the pull-up though you can use a 1k as well(or anything inbetween).

If I missed something let me know.

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