Centroid Community CNC Support Forum

The ferrite bead (FB6) on the Acorn PCB went up in smoke and support's email response was:
"This blows when there is a voltage difference between com and ground. Try connecting a jumper between the 0V common and ground on the power supply. With the Jumper, the Acorn might function, even with the blown Ferrite bead (FB6)"

I am wanting to understand the issue some more so I either do not do further damage to the existing PCB or the potential replacement PCB.
Setup
1. This issue happened when I started to commission my touch probe (plate).
2. My spindle shaft is earthed through its chassis, so I connected Acorn common (0v) to the spindle chassis then the probe plate to input 7
3. The spindle itself is earthed to mains earth
4. All seemed to work as Acorn was responding to the touch off (Z axis)
5. About 2 min later I was trying to set X,Y axis touch off the using a second touch off plate for testing when I smelt electrical burning and powered down

You have suggested to place a link between Power Supply Common (0v) to ground (Main’s earth)
1. I would have assumed from the spindle chassis (effectively mains earth) to Acorn common is exactly the same as your suggestion?
2. There is no earth link to common described in the hook up diagram?
3. As there are multiple power supplies in the installation all commons have been tied together so you are suggesting tying them all to mains earth?

Thanks in advance.

The Acorn schematic for Basic Power and Communication Connections, Bench Test (basic schematic S14952.DWG) shows that Earth Ground and DC Ground are connected by the trace under the terminals on H9. Chassis, COM and COM share the same trace. The schematic shows that Earth (Green wire) and Chassis (Green Wire) are connected to the same terminal on the Meanwell RD35B power supply.

Earth Ground does not mean 0 volts unless ALL earth grounds are connected to a common point by wires of sufficient gauge to show 0 ohms resistance between all earth grounding points. My old Shopbot was set up in my garage. Until I ground off the paint on a chassis grounding point and then connected a 10 gauge ground wire between that bare spot and the controller's chassis (an earth grounding lug inside the controller), there was significant resistance between the machine's frame and earth ground. That resistance increases the current flowing where no current should be flowing. Current X voltage = watts. Watts = heat.

Tell us which schematic you used to set up your touch plates. Or, sketch how you wired them up.
Please take a picture of your Acorn power supply. Pictures of your control cabinet, pictures of your machine, spindle etc.

You indicate you used two touch off places, show pictures of the first one that worked fine
Show pictures of the "Second One" that did not and indicate how you wired it up.

More information will get you better suggestions.
Marty

Earth ground, chassis ground and DC ground are common to each other when properly wired. In other words, if earth ground, chassis ground and DC ground are NOT connected to each other, then they are not necessarily at the same voltage potential. Tying them together is considered best practice. Letting any of them "float" means that there is no "common" point of reference, which can lead to the kind of problems that you've experienced.

The damage to FB6 proves that the earth/chassis and supply common are not the same potential.

2. My spindle shaft is earthed through its chassis, so I connected Acorn common (0v) to the spindle chassis then the probe plate to input 7

There is some problem with this step. Either the ACORN 0v / COM is not connected to spindle chassis, or the ACORN EGND and spindle chassis are not the same potential.

The easiest step is to install a jumper wire between earth/chassis ground and COM terminals on the ACORN power supply.
The grounds should still be checked with a continuity or resistance meter when the system is off and voltage measurement when the system is on. If there is something pulling earth/chassis ground and COM to different levels, the jumper wire will just cause an overload somewhere else. If earth/chassis ground and COM are not connected together and floating to different levels, then the jumper wire will be a good solution.