Comments sought on new build: A CNC Router for RC Gliders and Planes

**njhussey** · 08-04-2015

I'm having the Estops (and AM882 alarms) go through a relay (and Pilz??) which will kill the power, the limits I was going to wire through a relay and to the enable of the drives.....

**routercnc** · 15-04-2015

Hi Neil,

Looking through your build log on post #555 showing the control box -

Looks like the positive red DC output cable from the capacitors is going into an MCB. Is this correct and is this how you are intending to add a fuse? I'm a bit confused as I'd expect the MCB to be on the mains AC input side, and instead to see blade fuses on the DC output. Can you elaborate as I'm about to start my linear PSU build.

Thanks

**njhussey** · 15-04-2015

Originally Posted by routercnc

Hi Neil,

Looking through your build log on post #555 showing the control box -

Looks like the positive red DC output cable from the capacitors is going into an MCB. Is this correct and is this how you are intending to add a fuse? I'm a bit confused as I'd expect the MCB to be on the mains AC input side, and instead to see blade fuses on the DC output. Can you elaborate as I'm about to start my linear PSU build.

Thanks

I'm looking to get some of these but had forgotten about the MCB's. I was going to use them but I'm not sure they're the right thing at all! Lookig at something like these http://uk.rs-online.com/web/p/fused-...inals/8239552/ although I do quite like the idea of the 12V fuse holder EddyCurrent uses but not sure for a work environment?

**routercnc** · 15-04-2015

Hi Neil,

Ok, thanks. Then I'll stay with my original plan which is 6A MCB type D on AC side of PSU, and automotive blade fuses on the 70V DC side. I plan to fit a trial 1A fuse on the DC then jog an axis to deliberately blow the fuse. Then I can make sure that it blows cleanly and there is no residual arcing from the 70V. I'll then fit the intended rating. I think Eddy and Irving both went this way.

But the DIN mounted fuse probably is the way to go for how your machine will be used industrially. Do you know if the 5x20mm fuses need to be specific for DC? This was where I got stuck with this option because reading around AC and DC currents seem to require a different type of fuse and so I went back to the automotive type which are definitely for DC. I wondered if that was why the fuse wire was such a curly shape on the DC blade fuse instead of the straight wires in the AC ones, to make sure there was a clean burn/break rather than continual heating, expansion, and arcing.

I'm not coming from a strong electronic knowledge-base so the above are just my views / thoughts.

As for the rest of your build looking very professional now, well done.

**Jonathan** · 15-04-2015

Originally Posted by routercnc

Ok, thanks. Then I'll stay with my original plan which is 6A MCB type D on AC side of PSU, and automotive blade fuses on the 70V DC side. I plan to fit a trial 1A fuse on the DC then jog an axis to deliberately blow the fuse.

Interrupting the supply to a stepper motor driver (or many drivers for that matter) whilst running can damage them, so I wouldn't recommend that test.

Originally Posted by routercnc

Do you know if the 5x20mm fuses need to be specific for DC? This was where I got stuck with this option because reading around AC and DC currents seem to require a different type of fuse and so I went back to the automotive type which are definitely for DC.

I read somewhere that if the DC rating isn't specified for an AC fuse, you can use it with DC but only at half the AC voltage rating. Don't quote me on that though...
I would trust the 5x20mm fuse more than the blade fuses - running a 240V AC fuse at 70V DC seems more reasonable than using a 12V DC fuse at 70V DC.

**routercnc** · 15-04-2015

Originally Posted by Jonathan

Interrupting the supply to a stepper motor driver (or many drivers for that matter) whilst running can damage them, so I wouldn't recommend that test..

OK good point, but out of interest is that any different to an e-stop? My latest control box (when finished!) will cut power to the drivers via a relay which seemed to be a common approach.

**njhussey** · 23-04-2015

Ok so back on this again after doing work for the last week or two....

Got my estop circuit working using this modified drawing...

I've got to put the AM882s alarm signals daisy chained in the estop circuit but I think I know how to do that using another relay with 2 x NC connections.

Next problem is I keep blowing the 6A C type MCB when switching on....I'm guessing that I need a soft start to stop the inrush into the caps....can someone help with this? I'm guessing a thermistor with a relay/timer?

**fvfdrums** · 23-04-2015

We get this as well but it's not consistent sometimes it'll pop once or twice other times straight away and that's with the caps fully discharged. At the moment I'm living with it but if there is a solution I'm all ears? D type breaker?