Demonic dimmers - weirdest freakin' thing ever

[Josh Bass]

So I came across a new one today. I put some dimmers on some lights while on a shoot, as is my wont, and when dimming them, it MADE MY VIDEO MONITOR FLICKER. WHAT THE HELL????

Dimmer was not on the same strip/cord as monitor, even tried moving to different wall sockets, still happened.

I would start dimming, and at a certain point the monitor would start flickering, and if I persisted it would cut off 'til I raised the dimmer back to a certain point.

I have never encountered this. It's utter madness. Right up there with cats and donkeys breeding together.

Dimmers are those router speed control things from Harbor Freight that are recommended by some folks due to cost and usability. Is that the issue?

Monitor is a $3K Sony LCD monitor, 8/9" or so. Don't know model #. It is a pro production monitor.

[Allan Black]

Sounds like flakey dimmers with questionable house earthing. Try better quality and/or put AC filters in circuit.

If it is an earthing problem and the house is 3 phase, put extension cables out to the other end of the house and try running the dimmers off those. HTH

Cheers.

[Josh Bass]

Is earthing the same thing as grounding?

I couldn't comment on that. No idea.

The dimmers have come with general recommendation from many in the industry, probably some on this board, as a low cost alternative to purchasing $80 and up 1KW dimmers. I've never heard of this issue with them. . .heard about 'em getting hot, heard about the fuses having issues, heard about them not being able to dim down to 0% brightness, never heard of this.

I'm pretty sure I switched one out with a dimmer built by DVI member (former member?) Walter. . .something. NY Guy. Had a company called Blue Sky or Skyblue or something that built several types of accessories for the video production you could buy. Anyway, I think even THAT dimmer caused problems, though they weren't as bad.

What do you mean by "AC filters"?

Thanks.

[Chris Soucy]

As the dimmers were turned down, although the length of the pulse going to the light filament per half mains cycle was reducing, due to the cooling of the filament, the current pulse drawn by the lower resistance of same would increase.

That spike would have rippled through the entire system and the resultant voltage drop would have coincided with the firing cycle of the swich mode supply of the monitor (they both sharing the same 60z mains).

The more you turned the lights down, the worse it got till the monitors power supply shouted "Uncle" as it wasn't able to get adequate power through to power the screen.

It does raise the question, however, of what gives with that particular circuit, a fault? heavilly loaded? over long cable run?

There had to be some reason why the circuit behaved so poorly.


CS

[Josh Bass]

Who knows? I'll probably never shoot at that location again, so no chance to find out.

I don't know how the circuits were laid out. . .I tried to distribute power to different outlets around the room, no idea if we were on different circuits.

We had a 1K fresnel, 650 fresnel and 650 tota going, and never blew anything.

[Greg Boston]

Josh,

There is also the possiblity that you had EMI radiating from one of the cables and the monitor was picking up on it. When that's the case, simply putting more distance between the two can alleviate the issue. Improper circuit grounding can cause issues like this as well.

-gb-

[Josh Bass]

Any of this could be true.

Funny thing is the monitor was rented from another frequent client. I do shoots with that monitor all the time, and we use one of those router speed control dimmers on those shoots too sometimes (it's not one of mine, belongs to same client). I've never seen the issue come up 'til today.

Of course, those shoots are almost always in the office buildings of a major oil company, so maybe that makes all the difference.

[Paul R Johnson]

If you buy a TV spec lighting dimmer it costs a heck of a lot more than a router speed control! For good reason - you are paying for decent filtering. I bet the monitor has a switch mode power supply - which works by using just a small portion of the mains power cycle. Your dimmer on lower level has also chopped out a bit of the mains cycle too. So the lights are using their full wattage for a percentage of the time. If the building power supply is working hard, it shouldn't matter as long as it can supply the full demand, and your power requirements are not huge in the great scheme of things. However, all that's probably happening is as has been said - the waveform is being distorted by the dimmers without filtering, which also produces harmonics which travel back through the system - and you've just got dirty mains voltage - The monitor power supply just doesn't like it. If the mains power system is well designed then it's better able to cope with these kinds of problems - worst case is when everything is powered from one source in the building at the end of a long cable.

This is the kind of issue you do have from time to time with kit pressed into service it wasn't designed to do. I doubt a router has problems with a choppy harmonic ridden power supply, but your monitor does - and also any audio equipment powered from the mains could also have huge amounts of nasty hum induced - and this is even worse because the hum sound is a nasty buzz that changes as the dimmers are used. I've never come across the monitor problem, but have had the sound version hundreds of time.

Earthing/grounding is different. It's essentially a safety system, and should not in normal use be carrying any current. If the earth path is carrying current then something is very wrong - as in danger to life wrong. Mains distribution systems differ from country to country. Here in the UK, each house or building nowadays tends to have the neutral of the incoming mains power circuit bonded to earth at the point it enters the building. In other places earth/ground can be totally separate. It's unlikely the problem being discussed here is earth/ground related as most monitors nowadays (not all) are class II appliances and don't have an earth in the conventional way. Of course others, with metal cases do, so we're guessing.

A solution in this case could have been to run out another longer cable and separate the production equipment from the dimmers - and if this means a different supply, possibly on another phase in a larger building, then the problem would probably go away - but you also increase the voltage between equipment, so electric shocks could become worse, if they happen.

[Josh Bass]

I'll keep it in mind for the future.

I forgot to mention I've used those dimmers multiple times with another monitor, an older Sony CRT production monitor (again, the one I used recently with the problem was a newer LCD). No problems with the CRT with those dimmers.

As for the sound thing, would you consider a mic taking phantom power from the camera, and then camera then plugged into a wall with its AC power, as "sound equipment powered by the mains?"

I was hearing hum yesterday, but it was from the actual lights. . .they hum when dimmed. Or some of them do. Doesn't matter if it's the router speed control or the Walter something professional dimmer. Must be something with those particular fresnels.

[Chris Soucy]

Well, they would..........

they hum when dimmed. Or some of them do.

Of course they hum, they're getting pulses of mains voltage at huge currents at 120hz, no wonder some of them hum!

As for a CRT not exhibiting the same symptoms, different CRT power supply, different site, different everything.

As for getting into a sound system, you betcha.

Man, the problems I've had keeping mains borne interference from getting into CB radio gear over the years, you simply would not believe.

If I had any hair left to pull out, some of the problems would have seen the end of it.


CS

[Paul R Johnson]

The nasty harmonics the dimmers stick onto the mains will often get back into the camera. Many people think their balanced mic pre-amps are to blame, but the interference gets into the camera on the DC supply! This interference is a constant problem with dimmers - and you can try a few tricks - ferrite beads are common (and often already in the DC line to many bits of kit - the odd lump in the cable) - but you can buy these and you wind the mains cable through, clip them together and they help - to a degree. By far the most unpleasant device for inducing noises into sound systems is the Dell Laptop power supplies - sometimes people use the laptop to generate the audio and they plug the cable into the camera and get very strange chirps and hums - that vanish when the computer works on batteries. This kind of mains borne rubbish just superimpose themselves on the cabling you use. I saw a guy the other day with a large circular magnet, removed from the back of a loudspeaker (a big one!) and he had three or four turns of his power cable wound through it then covered with tape. Swore it really cured some buzz problems.