VFD interference problem

[eurikain]

You dorks :D

Well, a clue could have been that when the 5V PSU was off, its power LED was slightly on (not fully, but you could see some light), indicating that power was flowing from THE PC through to BOB to the 5V PSU (receiving -5V I guess).

Anyways, problem fixed, let's now move on to cutting !!!!

[cropwell]

You dorks :D

Well, a clue could have been that when the 5V PSU was off, its power LED was slightly on (not fully, but you could see some light), indicating that power was flowing from THE PC through to BOB to the 5V PSU (receiving -5V I guess).

Anyways, problem fixed, let's now move on to cutting !!!!

Had you originally connected V- on your 5v PSU to GND ?

On your BoB the terminal marked PC GND is actually the current return path and should not be connected to GND but only to V- on the power supply. The BoB itself has a GND plane connection next to the 12-24v terminal, which is the current return path AND GND for the BoB electronics.

[eurikain]

Yes that's what I had done. PC GND to 0V of the 5V PSU, and PC +5V to +5V of the 5VPSU.

Can you provide a quick diagram on how to do what you're explaining ?

[cropwell]

Yes that's what I had done. PC GND to +5V of the 5V PSU, and PC +5V to +5V of the 5VPSU.

Can you provide a quick diagram on how to do what you're explaining ?

I trust you mean PC GND to -5v on the power supply.

On your power supply check with your meter for continuity between -5v and the earth terminal. It should not be connected internally in the power supply. If it is you will have a ground loop. These power supplies are generally not connected between the -5v terminal and the Earth terminal. Also make sure that if you use the 5v power supply that -5v is connected to PC GND on the BoB and nowhere else (I am assuming that you do not have any other requirements for 5v in your control box).

The Earth connection on the power supply should be connected to the star earthing point to provide shielding for the power supply.

In a conversation with Graeme, he thought that the power supply, if under loaded, may react badly to interference from the VFD and cause all manner of effects, in fact it could go unstable by itself. So even if you get the earth and current return paths right, you may still have problems as you are only driving the optos on the BoB.

Jazz hit on a solution, and the PC USB supplying 5v to the BoB is workable, but I do not like using USB ports to provide power as they have limitations.

If it is working now don't muck about with it, but I still don't believe that we have got to the root cause of the problem.

Cheers,
Rob

[Clive S]

Rob I agree with you re not loading the power supply enough it would help if you put some sort of load on it even a 12v bulb would do it.

[cropwell]

Rob I agree with you re not loading the power supply enough it would help if you put some sort of load on it even a 12v bulb would do it.

Well, it was Graemes idea, and I thought that a load like a bulb would shed some light on the problem.