Custom power distribution

I’m working on a simple power distribution layout with 3 Victors in low-load applications, powered by a 12V SLA battery like this one. To do so (without using the full-scale FRC control system), I’m connecting the motor controllers in parallel to a terminal block. A 4th load will be powered through one of the old 12V-5V converters, also hooked up to the parallel circuit. I’ve attached images of the circuit below.

Full circuit, without 12V-5V converter
Terminal block closeup

Is there any way that this circuit can damage any of the hardware (especially the Victors)?

I am assuming that you are working on something other than an FRC robot.

It would be best if you could insert a fuse between the battery and the terminal block. Those batteries can source enough current to do some damage if there is a short circuit in your system. Choose the fuse rating to be 125% of the total expected current, or the next higher available rating. You may still have to go up a size or two if the expected current is low. The motor controllers will consume some current even when they do not have a load connected. If the total current is less than 10A, you can use an inline fuse holder like the one linked below. They are available at Radio Shack and auto parts stores.

It would be best if you replaced the un-insulated crimp lugs with insulated crimp lugs. The positive and negative lugs are pretty close together and it would be easy to short them together. The bottom of the two lugs can be flipped over so the two lugs do not have to splay apart. You may also want to change how much insulation you strip off the wires. The two red wires at the lower left of the second picture are too short. The single black wire at the right is a bit too long. The single red wire, second from the right, is about the correct length. Make sure you do a pull test after making each crimp.

Is that a breaker or a switch in the bottom left corner? Also what gauge of wire are you using?

The idea is safe as long as you are using properly sized wire. Three victors should pull less than 30 amps continuously, although they may spike higher. So for that setup I would use 14 gauge for downstream (12 in enclosed spaces) and probably 8 gauge for up stream (6 in enclosed spaces). And a properly sized breaker (80-100 amps)

It looks like you may be using 12-14 gauge for the whole thing, that’s probably not big enough.
That being said I have powered three motors off of 12 gauge before in a pinch, and have destroyed wires. But if you are only running it for a short time, don’t mind smoke, and have a fire extinguisher nearby, you can probably get away with this setup and it probably wont do anything too bad. But I wouldn’t recommend it.

Breaker/fuse size should be selected based on the wire used, not the expected current draw. Overcurrent protection is used primarily to prevent the wires from overheating and setting things on fire. If you follow the FRC robot rules for matching wire gauge to fuse/breaker rating, then you should be safe.

If you use a single fuse or breaker, it should go between the battery and the terminal block. In this configuration, all wires should be appropriately sized for the rated current of the fuse/breaker.
If you put a fuse/breaker on each branch, then the wires between the battery and the terminal block should be sized to handle the sum of the rated currents.

The text that I made bold above is not quite accurate. The current draw of three victors is indeed far less than 30A, but one must realize that the current draw of a victor isn’t nearly as important as the current draw of the device on the other side of the victor. For example, at their rated “maximum efficiency” condition, 3 CIMs should draw ~60 amps continuously (and 81 amps under “normal load” conditions). At stall, the same motors draw a combined 399 amps. This is why the breakers used on FRC robots, and Victors, are designed to be relatively tolerant of overcurrent conditions.

I would recommend using the same breakers used in the PDP since history shows that they do a good job of protecting victors while still allowing for the large current spikes that occur with powerful motors.

That is really building wiring philosophy or for situations where you don’t know what the downstream loads are going to be. The assumption there is the machine designer will design his equipment to handle the over current. Even there I have downsized breakers to match the equipment where I specified over size wiring because of the voltage drop. In machinery where you know the current demands, good practice is to size the fuses for the load. Cars are a good example of this.

A little late now, but you can buy fuse blocks so you can individually fuse the circuits. ATC style fuse blocks can be had for cheap. Cross the Road recomends strongly that their motor controller have circuit protection.

Something a friend from 193 and I have been tinkering with is putting together parts lists for smaller custom robots: Yesterday we were doing the power distribution parts so why not share…

They will take the FRC fuses (up to 30A, recall this is small) and are pretty low cost.

Thanks for the pointer – I hadn’t seen something like that before.

I was looking through the 2015 rules, and it seems like they would be legal for FRC use as part of a “custom circuit”, for example to put a bunch of low-load custom circuits on one PDB circuit.

Given that the current draw of the loads(motors?) connected to the victors was not given, I assumed that it was not known.

Moreover, the protection devices need to be sized to prevent an unsafe condition if there is a failure (i.e. a motor fails short, terminal block is shorted, etc). So, you can pick a wire gauge and size the fuse to it, then pick the load (but the load may not be able to draw all the current it wants or needs to). Or, you can pick a load, size the fuse to that, then select the wire gauge based on the fuse (so the load can draw all the current required). The former is often the case in FRC. The latter is the more real-world design methodology.

Really, your earlier statement about sizing the fuse to the wire encapsulates the broader idea that if a failure occurs the elements of the system need to mitigate the failure so that an unsafe condition is not reached. That safety concern is primarily thermal (not having a fire, preventing the insulation from melting, etc), but can also deal with other areas (like protecting the battery). In many cases the mitigation means a fuse, but there are other mechanisms (like foldback circuits) that can be used to prevent the unsafe condition due to too much current draw.

The simpliest for this application is probably the inline circuit breaker or fuse between the terminal block and battery that has been previously mentioned.

Several approaches to this. Fuses only trip once. That can be annoying and or useful. So for a fuse I would size it either at or somewhat over the motor stall current. For motor I would use a slow blow fuse. A lot depends on the inertial load. For a self resetting breaker closer to the actual motor load. In any case I would size it to protect the motor controller.

All the Blue Sea Systems stuff is just perfect. Almost makes me want to buy a boat.

Hey NWChen

Please clarify what you mean when you say you are using the “3 Victors in low-load applications”. Please also state what wire size you are using.

The current draw of the motor control controllers is the load current plus some small quiescent current. I doubt that the OP would consider 30A per Victor a low-load application.

If the OP is running small motors that only draw a few amps in a non-FRC application, then selecting the fuses to protect the load is a valid way to go. From the photos, it looks like he is already using 14AWG or 12AWG wire which would be more than enough for a few amps.

The fuse holder Andrew linked to with resettable breakers would be a nice touch but the breakers might not be available in ratings that are low enough. The Blue Sea bus bars are nice too but the terminal blocks he is using would also be just fine. We use many of those in the equipment we build at work.

The fuse holder was chosen because I’m FAIRLY certain it will work with FRC style Snap-On resettable fuses. And those are available from 5-40A (though the holder is only rated for 30A per branch. I’ll let you know if this is true on Friday when one is delivered.

I assumed the terminal block he’s using was fine, I just linked the bus bar in case anyone was looking for something like it. (And I was hoping someone else would do a sanity check on if it would work for smaller than FRC applications)

The circuit pictured uses 16AWG wire (oops).
The loads on the Victors are an RS775 in a 1:188 gearbox, a Globe Motor, and a Denso window motor. Each motor (with the exception of the Globe, for which I couldn’t find a performance curve) is drawing no more than ~10A.

Thanks Andrew - recommended parts lists for smaller robots would be pretty helpful.

The globe pulls ~21.6a at stall

Globe_Motor.pdf (41.3 KB)

Globe_Motor.pdf (41.3 KB)

When I said 30 amps, I meant 30 amps each.
Also I agree, a properly sized circuit breaker is based solely on the wire size or maximum power of any device on that branch, whichever is lower. Example, if your wire can handle ten amps but your DC DC converter can only handle one and will fry if overloaded, you should use a breaker sized for the converter. But if your wire rated for ten amps is used with victors and motors that can handle 100, you should use breakers sized for the wire.

The block that Andrew references is similar to the type used on FRC robots prior to the PD. It uses the same breaker size as the 30, 20 amp breakers that are legal on FRC robots. What is better is that the power distro inside the block is sufficient to keep losses at a minimum. The terminal block wired as you show, drops voltage across each wire and connection from left to right in your photo. You can help things if you move the input to the center of the terminal strip instead of the end. That splits the load equally.
I really recommend another breaker or fuse near to the battery. It should be sized such that a dead short on the battery would trip the fuse/breaker before the battery is damaged. The limit on the battery you reference is the current that the terminals can handle. I would shoot for 100 amps max. Anything more than that is likely to cause the terminals to fail.