View Full Version : Circuitry and componentry protection

16-10-2022, 05:31 AM
I've finished my electronics cabinet but have fitted no circuit breakers or fuses yet.
What should I use and where in the systems


I've used aircraft plugs to connect to my stepper motors and limit switches
My limits and probe wired as recommended by makerspace
All leads shielded and grounded as well as the chassis


Though two limits per axis as shown in the AXBB-E manual


I'm using hall sensor proximity switches


Any advice would be appreciated

AXBB-E controller
DM542 drivers
Nema 23 Bipolar 3Nm (425oz.in) 4.2A
57x57x114mm 4 Wires Stepper Motor CNC

16-10-2022, 09:23 AM
For fault protection, I'd personally look at a chassis-mounted fuse holder with an anti-surge (time-delay) fuse suitably rated for the combined power capability of the various PSUs. So, let's imagine (I can't see from the photos) that your 5/24/36/36 supplies are rated at (respectively) 20W, 240W, 600W, 600W, then that's 1460W, or around 6A. I'd start with an 8A fuse for that (a little headroom). You could look at individual fuses for each supply on the basis that only one is likely to fail at a time, use the above notion of working out the expected mains-side current and add a suitable head-room (+10%) and then head to nearest conventional rating. Note, fuses have a fusing factor - they don't blow at a given current, but are designed to fuse at a given current multiplied by the fusing factor (>1) over a given time period - keep that in mind when looking at available values.

What the supply fuse is intended to do is ensure that no fault on your device is going to overload the supply/distribution (i.e. mains cables - either on the ring, or the equipment mains lead) that could cause overheating resulting in fire.

One fuse versus many?, One protects the whole system, but might not catch a smaller supply soft-failing. Individual fuses are more likely to catch individual faults but you may want to consider additional circuitry to ensure any single failure causes an E-Stop. Any supply could fail and let the magic smoke out and not pop a fuse. But, as earlier, the fuse is not protecting the control box, it's protecting you against potential overheating/fires resulting from failed circuitry.

You've not indicated your grounding strategy - and maybe you don't know if the PSUs internally bond their 0V to chassis ground (I'd expect them not to) but that can be considered as an electrical noise management strategy. My concern would be the 24V alligator clip floating about - that PSU looks like it can deliver a whack (and an unexpected spark can lead to comedy flailing of limbs near a machine capable of eating your arm). I'd be looking to current-limit the alligator clip. Not by a fuse - that's not the point here - but just with e.g. a series resistor mounted within the box. The AXBB instructions give you hints here - the inputs have a series 2k2 resistor ("which limits the current into the optocoupler.... function between 10 and 30Volts...") which gives me great confidence that you could place a series resistor within your control box of 1K and not worry at all about the influence on the probe, but you've limited the "fault" current on the flailing 24V line striking a grounded bit of metal to 24mA by doing so, and protected your 24V supply in the process.

My assumption here is that the machine is connected to mains via an earthed, fused outlet, and that is on a circuit protected by an RCD.

That's the limit of what I'd consider necessary for safe operation.

**EDIT** That advice is worth exactly what you paid for it. Solicit the advice from other, more learned, forum members, and ultimately refer out to your national electrical standards (or discharge this through a competent design engineer)... phew, that should cover my arse

17-10-2022, 12:55 PM
Away from my PC again but you can see my setup at 1:09 in this video https://m.youtube.com/watch?v=AWvzFnInIRw

I have circuit breakers on the mains side for the stepper PSU and the logic PSU. I canít remember the values but the stepper PSU needed a slow reaction type (D?) as I have a toroid supply.

Then on the DC side I have a fuse(canít remember the amps) for the logic and for the steppers I have a 5A fuse on Y driver, 5A fuse on the Z driver and a 10A fuse to both X drivers. I think these are a bit high but have not changed them.

If doing again I would go with a single fuse for all steppers and try something around 5 or 6A. I think the amps are per phase and so you donít get the full amp draw stated (is my understanding). You can get resetting fuses so experiment with lower values and if they pop during use you can reset to get going then get some higher ones or maybe add an ammeter?

Iím not an electrician either so take that for what itís worth too!