Concerns over circuit protection with 2x160 34VDC Roboclaw

General discussion of using Roboclaw motor controllers
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Damien
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Joined: Sun Apr 21, 2019 3:08 pm

Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Damien » Tue Aug 13, 2019 2:34 pm

Hello All,

I'm inquiring as to recommendations on the fuse, power diode and pre-charged resistor for this circuit. I'd be using the 2x160A 34VDC Roboclaw Motor Controller
UPDATED CIRCUIT SCHEMATIC
Circuit.PNG
Circuit.PNG (101.96 KiB) Viewed 483 times
Motor Specs: MMP D40-675C-24V GP81-014
Rated DC Voltage -> 24VDC
Rated Continuous Current -> 23.1A
DC Armature Resistance -> 0.15Ohms
Stall Current -> 24V / 0.15Ohm = 160A

Battery Specs: Turnigy Graphene 5000mAh 6S 45C LiPo Pack w/ XT90 (Two of these in parallel)
Capacity -> 5000mAh
Voltage -> 6S1P / 6 Cell / 22.2V
Discharge -> 45C Constant / 90C Burst

I selected these batteries for safety, as the safe continuous current load would be 225A and the safe burst current would be 450A which is above the combined stall current of 320A for both motors.
I plan on using 10AWG wire for the entire circuit above since that is what the batteries and motors both come with by default, but if anyone thinks that could be an issue I'd like to address it before hand. My total travel length (one-way) won't be more than 2 feet, tops.

For the pre-charged resistor:
Based on what BasicMicro says in their resource page...
If I want a charge time of 1 second then (1/3) / (2820uF) = 118.2 ohm
Should I increase my charge time? Is a pre-charge resistor simply a resistor? Is it a power resistor?
Note: I really want to use these XT90-S connector that prevent sparking
https://hobbyking.com/en_us/xt90-s-anti ... s-bag.html
Would these be akin to a pre-charge resistor? I think they have about 8ohm resistance as you plug them in.

For the power diode:
I understand I need something with a reverse voltage of 22.2V and the forward current should be what exactly? Again, BasicMicro says that anywhere from 1A to 3A works for most cases.

For the fuse: I want a fast-blowing fuse, which I understand. So the current the fuse should blow at would be what exactly, the stall current of the combined motors, i.e. 320A? This would mean the fuse would be rated at a current much much less than 320A.

Note: I know I have two switches in the circuit, SW6 and SW5.
I'll be using these switches https://www.gigavac.com/catalog/power-p ... t-switches
Which range from a continuous current rating of 200A up to 400A.
(I know, overkill..)

Alternatively, I've used a voltage clamp circuit for another project because I was using a DC power supply. Could I use the voltage clamp that BasicMicro provides in place of the power diode? I'd still want the pre-charged resistor and fuse though wouldn't I?

I'm concerned about safety, so any advice is welcome!
Last edited by Damien on Wed Aug 14, 2019 5:21 pm, edited 2 times in total.

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Basicmicro Support
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Re: Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Basicmicro Support » Wed Aug 14, 2019 10:37 am

1. A 1/4watt resistor will be fine. For a 1 second charge time, it will not dissipate anywhere near more power than it can handle in that time period.

2. The fuse should be in series with SW5 but still parallel to the diode. If the fuse or SW5 is opened the diode will continue to provide a path back to the battery for regen energy.

3. You can also put the precharge circuit parallel to the diode. Its less of an issue since the pre-charge resistor would still provide a path back to the battery for moderate regen.

4. Yes, the resistor value is correct but you can safely use a 100ohm(easier to find).

5. The fuse should be selected to blow quickly at some point below the maximum battery current rating. Since the battery limits the peak current in a short circuit situation it dictates the fuse requirements. Keep in mind a damaged motor controller will not cause a dead short so you want to be somewhat below the maximum battery current. Also batteries maximum current will reduce while in use due to temperature. But you also need to be sure the fuse isn't going to accidentally blow under normal motor load current. It is somewhat subjective. You need to take into account your application, your battery specs, the fact a damaged controller will be somewhat less than a dead short. Just a reminder, the fuse isn't there to protect the controller. It is there to protect you. The fuse isn't going to come into play unless something gets damaged.

6. You can use a voltage clamp instead of the power diode. If you do you will not be recharging the battery when you are in regen. Instead, it will burn off in the voltage clamp. This will reduce the efficiency of your system quite a lot depending on the application.

Damien
Posts: 8
Joined: Sun Apr 21, 2019 3:08 pm

Re: Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Damien » Wed Aug 14, 2019 1:19 pm

I'm glad to hear that I'm on the right track with picking out these components.
I'll be sure to make a user-project post once it's all done.

Thanks for the quick response!

Damien
Posts: 8
Joined: Sun Apr 21, 2019 3:08 pm

Re: Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Damien » Wed Aug 14, 2019 5:30 pm

The application in question is a robot for competition. The robot will have to drive around for 5-10 minutes at a time between it's rounds. The robot itself will be just under 140lbs, so it's quite the load.

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Re: Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Basicmicro Support » Thu Aug 15, 2019 9:42 am

For battery-powered things like that, I don't recommend voltage clamps. You'll want as much of the regen energy back in the battery as you can get I expect.

Unless you're going to use the 90C limit of the battery I'd probably set the fuse so it would blow around 300amps(about mid way between the 45C continuous and 90C peak limits of the battery). That is of course unless you are using considerably less than that to drive the robot. Then you can set it lower.

If you are planning on using near peak power from the battery that's where selecting a fuse gets complicated.

Damien
Posts: 8
Joined: Sun Apr 21, 2019 3:08 pm

Re: Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Damien » Wed Aug 21, 2019 8:17 pm

Accidentally posted a snippet of the comment below, please ignore this comment.
Last edited by Damien on Wed Aug 21, 2019 8:33 pm, edited 2 times in total.

Damien
Posts: 8
Joined: Sun Apr 21, 2019 3:08 pm

Re: Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Damien » Wed Aug 21, 2019 8:32 pm

Well at most we only want to see right below the stall current of the motors, which combined is 320A. So your assessment of about 300A being the blowing point of the fuse is spot on. We don't expect to reach that level of current though. I suspect we will see around 40-60A continuous current, even over duration. I'm trying to make this as safe as humanly possible though, I feel as though I and past members in my group may have been far to lenient on safety in the past.

I suspect this because our former motor controller was a Roboteq XDC2230
I noticed a few things recently about our setup.
1. The power/safety circuit that is in the data manual for the Roboteq was never made fully, we had a cutoff
switch, but no one ever did the pre-charge resistor nor the diode!
roboteq.PNG
roboteq.PNG (191.76 KiB) Viewed 13 times
2. Incidentally, we actually we have extensively ran the robot without that protection and the motor controller is still working (thank goodness).
3. I believe this happened due to a lack of knowledge from previous members and myself.
4. Now I know better than to risk ruining things.
I've opted for the Roboclaw because I genuinely like the product more. I've used the smaller 2x30A ones and have been thinking about replacing the roboteq for sometime since then.
5. Old roboteq will be recycled for robots that don't have the budget to buy new motor controllers.

Based on the stats of the Roboteq in comparision to the Roboclaw 2x160A 34VDC I'd say with the additional protection we will surely be fine. I do plan on testing the actual values from within the Roboclaw to validate everything: no voltage spikes, running the motors to see the current load at full speed.

Overall I'm quite excited to put together this new motor controller for our robot. It's going to be a tight fit since the Roboclaw is slightly larger in volume than the Roboteq but nothing a little bit of wire management and detailing won't fix.

We will most certainly not use the voltage clamp. I was able to find some diodes in the range that should be suitable, as well as some blade fuse rated at 15A, 20A, 25A which through the data sheets showed them blowing at 0.01 s around 100-300A, respectively.

I'll take advantage of the built-in functions of the roboclaw to also set appropriate limits.
PS. Just received the molex connectors for the roboclaw, those are going to be fun when it comes time to crimp wire connectors into them! ;)

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Basicmicro Support
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Re: Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Basicmicro Support » Thu Aug 22, 2019 9:22 am

The protection isn't required, as you found, but is recommended.

I usually recommened the pre-charge resistor(or a more complex circuit) when running 24v or higher. Rarely will the inrush cause damage even at 24v but if you power cycle enough from a cold start it can eventually damage something. It's much more of an issue at higher voltages.

P.S. I hate crimping wires for the connectors. :)

Damien
Posts: 8
Joined: Sun Apr 21, 2019 3:08 pm

Re: Concerns over circuit protection with 2x160 34VDC Roboclaw

Post by Damien » Sat Aug 24, 2019 12:39 am

HAHAHA
I only bought about 3 extra crimps per holes on the connector so I do hope I can get it right the first time.
I'm glad to hear that my approach on safety will make us better off, in the event that something can go wrong.
Just gotta remember steady hands, steady hands, steady hands!

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