Choosing fuse and pre-charge resistor for RoboClaw 2x7A setup

General discussion of using Roboclaw motor controllers
jasonwebb
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Joined: Wed Jan 30, 2019 5:00 pm

Choosing fuse and pre-charge resistor for RoboClaw 2x7A setup

Postby jasonwebb » Thu Jan 31, 2019 5:46 pm

Hi! I'm working on setting up a single RoboClaw 2x7A to drive two gearmotors, and I'm having a little bit of trouble understanding and sourcing the fuse and pre-charge resistor that I need to get. I have an intermediate level of hobbyist electronics knowledge (mostly low-voltage digital systems like Arduino and custom PCBs), but I'm not as confident when it comes to analog electronics and theory.

Here are the important bits about my set up:
  • A single RoboClaw 2x7A
  • Using a 24VDC power supply (not batteries)
  • My motors have a stall current of 1.75A
I understand that fuses are not exactly like switches that disconnect themselves instantly at an exact "trigger" current level, so I'm looking for some help in thinking through what the specs should be for my application. My intuition is that since I'll be driving two motors with stall currents of 1.75A, I should maybe look for a fuse that trips at about 3.5A, or maybe a little less to be safe. Is that correct? And should this fuse have a fast response time or slow?

Here is a fuse that I think *might* be appropriate - am I on the right track? It's a 3.15A, 250VAC, fast-acting fuse. Do I instead need a fuse that is rated for a DC voltage? https://www.digikey.com/product-detail/ ... ND/1009002

Also, for my pre-charge resistor I was thinking of using a 1k, 0.5W resistor as described in the user guide, but I wonder if I should use a smaller value since my motors are fairly low lower compared to what the RoboClaw could handle. The RoboClaw 2x7A datasheet says that this value gives a pre-charge time of 15 seconds with 60VDC motors. It'd be nice to not have to wait as long for my setup to charge up before I can start using it, if possible! If so, how do I calculate or choose what the value should be? Or do I really need to purchase a bunch of <1k, 0.5W resistors and experiment?

Please let me know if there is any more info I can provide!

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Basicmicro Support
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Re: Choosing fuse and pre-charge resistor for RoboClaw 2x7A setup

Postby Basicmicro Support » Fri Feb 01, 2019 10:07 am

1. When using a power supply you probably do not need a pre-charge resistor. The power supply will current limit itself(if its a reasonably decent power supply) which will prevent the massive current spike you would get plugging a battery into a drained capacitor.

2. For the fuse you need to look at the timing table from the manufacturer of the fuse. Make sure the fuse will blow in a short period(.1 to 1second preferably) if the power supply were to dead short across the fuse. The kind of motors your use doesnt really matter. The fuse is to protect if something catastrophic happens causing the power source to short across the fuse. So first need to look at the power supply current capability. If the fuse will not blow at the power supply max current then it will never blow. Once you have determined the fuse properly rated for your power source, then look at your motors. Will you accidentally blow the fuse under any coniditons(eg running the motors at full power and running into a fall or otherwise stalling them)? If so you have two choices. Get a larger power supply or make sure you limit the power your motors will draw.

Honestly at 24v on a 2x7a you are on the edge of needing either of these things. The power levels are relatively low and running from a power supply prevvents any of the really massive current surges you can see with a good lipo battery.

One thing you will have consider is motor regen. Power supplies dont support sinking current in general(its very very rare for one to handle it). So you need to handle it. You can either burn off the energy in the motor by braking or you can use an external voltage clamp like the one we describe in the user manual.

The former is the easiest option. In either case you need to set the maximum voltage limit to 1 to 2v above the power supply voltage and I also recommend you set the minimum main voltage to 1 to 2v below the power spply voltage. The former will cause the motors to go into braking mode if regen causes the voltage to rise above that limit and the later will prevent the motor controller from dragging the power supply voltage below that limit(eg prevent brownouts).

jasonwebb
Posts: 2
Joined: Wed Jan 30, 2019 5:00 pm

Re: Choosing fuse and pre-charge resistor for RoboClaw 2x7A setup

Postby jasonwebb » Fri Feb 01, 2019 4:44 pm

Thanks for the great response!! I feel like I have a better idea of what I can do.

1. I will opt to skip the pre-charge resistor and fuse.

2. I'm betting I should also not use the diode that would've been in parallel with fuse since we wouldn't want that regen power to reach the PSU (let me know if that's not the case).

3. I was indeed planning on using the external voltage clamp circuit, but if I can reduce the parts count and complexity by simply enabling braking through the software settings, then that sounds like a better solution!

In total, these changes mean that I will probably not need any external parts or custom PCBs in my application, just chassis wiring. Very cool! Thanks again!

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Basicmicro Support
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Re: Choosing fuse and pre-charge resistor for RoboClaw 2x7A setup

Postby Basicmicro Support » Mon Feb 04, 2019 10:17 am

2. Correct. When using a power supply you either can rely on the Roboclaws braking the motors to disipate the energy or you should use a voltage clamp. The later will prevent abrupt slow downs when slowing down too fast but is more complicated to deal with.

3. Test it. It will depend entirely on your application if that is acceptable or not.

No problem. Let me know if you have any more questions.


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