Wiring home and limit switches

[Danielroyal25]

Cheers Dean! That's a big help.

I don't know if this should be posted in another section really, but its all related.

When switching off the 70v supply should it be switch before or after the transformer or does this not matter?

Do many people have an input to the 24v system from the pc incase the computer crashes / turns off for some reason acting as an e-stop or would the code just stop running the anyway not harming the machine?

So that the drain resistor isn't constantly connected during the 70v supply is it connected via a relay so that when power is removed the resistor is engaged to drain the motor supply system?

[HUKBMBEAR]

Thanks Irving - I di think that the kill switch in series with the limits looked funny - effectivley actinas another limit switch and relying on the catches in the switch to keep the power off (dangerous if it failed.) so the second diagram (without the kill switch at present is the next step) - going by the other suggestions in this thread I should use NC contacts. The Home switches look like they are using separate connections with the board numbers mimicing the port outputs.

For the kill switch - would I be OK fitting that to the mains side as a breaker to cut all power to the controller and motors and leave the PC on.

Thanks

[irving2008]

Yes the normal arrangement is to use a contactor with a low voltage (under 50v, typically 12 or 24) relay coil. The contactor is normally open, and you close it manually which energises a small transformer which provides power to the contactor coil via the kill switch, thus holding the contactor closed. Hitting the kill switch interrupts the power to the coil causing the contactor to drop out. This arrangement prevents it being re-energised accidentally in the event of a power failure. You have to manually close the contactor and it wont stay closed unless the power is on.

Another, cheaper, arrangement uses a 10A 240v NO relay (4 contacts) with a 12v coil and a low voltage transformer as used for lighting. This requires a start switch to energise the coil which is then held energised by the relay via the kill switch.

[HUKBMBEAR]

Thanks Irving for all the advice its been a great help in clearing this up - just have to put it to work now

Cheers Ken

[Danielroyal25]

I'm just about to order my cy cable for the limit switches. If using a multi core cable I.e 4 core, is only one single core used to physically wire the switches together and the other cables are left unconnected?
Is the minimum 2 core cable that you can get?

[EddyCurrent]

You don't need to use CY cable for your limits assuming it's a 24v circuit. I would use twin cable and bring them back to a junction box where you can series them up. If later on you want to change how they are connected then you won't need to rewire them, just change how they are connected in the junction box. The limits circuit is low current and low voltage so I plan to use twisted pair CAT5 cable, because it's twisted it is effective at screening out noise.

[cropwell]

My only caution is that CAT5 cable tends to be single core and that would not be suitable for constant flexing. You would have to use the type suitable for patch cables. I wish I had thought of it, I have an oudle of it (15+ metres) and I bought cable from BRIMAL (Welcome to Brimal Components) They have a good range of interesting bits at keen prices.

[Danielroyal25]

Thanks guys. Just another stupid question: as the x axis is slaved do I only need limits on one side only or both. I have homes on both to ensure the gantry is squared.

[JAZZCNC]

Thanks guys. Just another stupid question: as the x axis is slaved do I only need limits on one side only or both. I have homes on both to ensure the gantry is squared.

Just one Side Dan. Think about having the Switch travel on the gantry with Triggers at each end to save on wiring and switches.

[Ahmad T]

Thanks.