routercnc_MK3

[routercnc]

Hi Jonathan,

Thank you for taking time to write that comprehensive reply. I'd done a bit of thinking on the series connections and quickly realised that one way would be ~0V and the other way ~50V AC ! So thank you for confirming that. I'll try connecting as you suggest - its either one way or the other and at least being series it won't matter if I get it wrong.

Had another look at the regulator board and I can now see some tracks but am not confident I could make a safe job of modifying it.




So I've also decided to de-mount the capacitors and build a more conventional system. I've ordered:
Solder sucker (had one years ago but can't find it ~£4)
35mm diameter capacitor clamps (6off ~£1.50 for a pair)
400V 35A rectifier centre bolt mounted (~£3)

Thank you for the kind offer to swap / supply dwgs for the capacitor circuit but I'm going to go for the traditional route of mounting each capacitor in the plastic mounts (inverted) and solder cables between the pins. It looks like there will be about 3mm of pin remaining from the top of the caps which should be enough to a cable get onto. The rectifier will be the traditional 4 spade pin type. All bits will be mounted to a ~4mm aluminium plate for heat dissipation and general sturdiness.

That's good background info on the ripple voltage. I had started to wonder about what an acceptable level would be. More capacitors will be better with lower ripple but I guess gives more in-rush current so more chance of tripping the MCB on start up. At least I can play around with the number of capacitors if I have that problem. But if it just works then I'll leave it and get on with using the machine.

For info this is my 3rd generation of cnc machine, with many minor iterations of development along the way, but it remains my only cnc machine and is currently out of action until the PSU if fixed.

[Jonathan]

Don't you just hate black soldermask :|

If you remove three of the capacitors that are in parallel, then put them back in the other way round, you will have one terminal of all 6 connected, so just one piece of wire will be needed to connect the '+VCC' and '-VCC' terminals to have the capacitors in parallel. If you can break one track on the PCB and add one piece of wire, then you can use the existing diodes.

More capacitors will be better with lower ripple but I guess gives more in-rush current so more chance of tripping the MCB on start up. At least I can play around with the number of capacitors if I have that problem. But if it just works then I'll leave it and get on with using the machine.

Yes, we'll see - you will have a lot of capacitance. I looked at that situation recently with my motor drive - that's got 660uF @ ~340V and with direct connection to the mains the scope showed that the current peaks at about 15-30A depending on the timing. That still doesn't blow a 6.3A slow blow fuse though, as the duration is only a few milliseconds.

[routercnc]

Hi Jonathan,

Thank you again. I had to draw out your idea of reversing the capacitors before I could see how it could work. I considered it for quite a while but in the end was concerned about breaking the tracks and adding flying leads. I've had enough of blue flashes and bangs.

I've managed to remove the capacitors from the board. Looks like there is plenty of remaining pin to solder the cable to:


Capacitor mounting brackets and a new rectifier should be here by the weekend so I will lay it all out on the mounting plate if I get the opportunity.

I've decided for reasons of space (6off 10,000uF caps with mounting brackets takes up a surprising amount of space!) and in-rush current to go with 40,000uF as a starting point.

I'll report back with success or more questions . . .

One more thing, I've added a safety latch relay for the spindle inhibit. This makes sure that if there is an e-stop (which stops everything including the spindle) that when the e-stop is cleared the spindle does not fire up until the start button is deliberately pressed again. Ordered 2 pole DIN relay for this job. Drawing updated:

[routercnc]

Based on a discussion I was having in another thread (but didn't want to hijack) my plan is to remove power directly from the drivers in the event of an e-stop via relays.

But as the steppers a likely to be moving then is this is problem for the drivers? I'm sending an e-stop to the BOB at the same time so the motors should be in the process of stopping.

Would it help if I added a capacitor across the relay coils to slightly delay driver shut down prior to the motors coming to rest - just a few milliseconds? Or am I worrying over nothing?

Does anyone else remove power to the drivers using relays in the event of an e-stop?

[JAZZCNC]

Or am I worrying over nothing?

Does anyone else remove power to the drivers using relays in the event of an e-stop?

Yes your worrying over nothing.!!! . . . If you remove signals so no pulses are still getting thru while caps drain then just turning off the power will be enough and motors will come to stop quick enough.

Edit: If you do want to go a little OTT then have e-stop turn off drive Enables so motors are still under torque and stop quick then remove power second or two later using a timer relay.!!

[routercnc]

Hi Dean

In my layout (see post above) my relays are AFTER the PSU so there are no caps to drain and the drivers loose power immediately. So I either leave it, change to a relay BEFORE the PSU on the mains side which I think is what you are referring to, or add delay / capacitor across the existing driver relays on the DC side to hold them closed for a few milliseconds. Hope this distinction makes sense.

[JAZZCNC]

Well my way is to remove power by dropping out a contactor to transformer and send signal to control informing about e-stop so halting pulses.!

But in all honest your setup won't be a problem unless you intend traveling at high velocity and have a heavy gantry. So if you have it wired then wouldn't worry.!