Fog Machine Problems

[Things]

Hey guys, I just finished building a DMX control remote for my fog machine.

I have the DMX USB Pro interface, and the control works fine, though sometimes when I trigger the fog machine, it somehow knocks out the communication between computer -> DMX USB

Could this have something to do with a small EMP generated when the fog machine pump is switched?

Any solutions you recommend I try?


EDIT: Almost forgot pics

Before ...


After ..


Cheers,
Dan

[Doc]

The problem will be an induced voltage spike when the relay contacts open. You'll need a capacitor across the relay contacts to snub the spike and possibly some ferrite beads on hv cables. I would shorten and tidy up the routing of those cables too.

[Things]

It's not the relay board that's causing the problem, it's the fog machine itself interfering with the USB connection. When I used the Moncha it also dropped out when I used the fog machine. What's standing out is both of these are FTDI devices ....

I tried putting a huge ferrite bead on the fog machine input cable, but it still killed it

Aha, found the problem!

I tried unplugging my laptop, and it worked perfectly, so it seems something is going on thru ground. ANy idea's how I could fix it?

Cheers,
Dan

[Doc]

I would stand by what I've already said here

Switch or relay; it should have a cap across it to absorb the spike, which will be feeding back in to your supply.

[zerowaitstate]

althought it dosnt look loke your circutry supports it, a lot of the problems i have ever have with DMX, ussually stem from dud cable or dud dmx terminator.

[heroic]

I get the same problem with one of my foggers and the USB interface on the FB3. Solution was to replace the fogger, though obviously if there's other sources of noise, it still drops out. Better shielding on the USB cable helps.

[colouredmirrorball]

Had some problems too with my fogger and interference (but it was on my MIDI device). An EMI filter didn't solve the problem but it turned out the MIDI keyboard also reacted to other devices so it is just very sensitive. When I have the funds I'll buy a better fog machine and we'll see what turns out.

[Things]

Weird, I even went out and bought a EMI filter, but it still causes problems!

[heroic]

It's most likely ground level fluctuations. When the fogger comes on it momentarily draws a lot of current, which it dumps into the neutral line. The ground of the DMX lighting is at one point on the graph, which because of the resistance of the line, floats up from true ground potential. This causes current to flow in your signal wiring, which is bad. (It's the same thing that causes hum in audio cables- called a "ground loop".)

That's connected to the DMX interface, which is connected to your USB port, which is connected to the power supply of the computer. Most computer power supplies are actually float-grounded to the neutral line of the mains (the identified conductor, to the Americans in the audience...) because they are double insulated. You could try running the computer through an isolation transformer, getting a USB isolator (though they are expensive), or alternatively, using heavier gauge power wiring. That sometimes solves the problem.

We had this problem with long USB cables when we were running an outdoor event, and nothing solved it; ultimately, we changed to running ethernet because it was just way more reliable, and is isolated by specification.

[Mr.Coffee]

It sounds like you do have some sort of ground loop issue. while you can lift the ground of your laptop, and it might fix the issue, that is not really what you want to do. I'd isolate your dmx signal, either in the usb-dmx box or on your fog machine interface, ideally both. opto isolators aren't that expensive at a component level, and that should solve your issues. You can get the components and install them into your deivices, or you could even try a turnkey solution like this:
http://www.centerstagelighting.com/i...t_detail&p=685

another thing I'm wondering about, you said when you ran off the battery of your laptop that everything worked ok, right? I'm not sure about this next theory, but you could be drawing too much current when sending a dmx signal, which would cause your usb bus voltage to drop and your interface to dismount. this is probably not the issue,I'm just extemporizing.

If there's some sort of local production company around, they may have a dmx opto you could try out, that way you can see if it fixes your issues before you commit to the option.

Also, if it helps, DMX is just half-duplex RS-485.