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Constant power in dimmer circuits?

Discussion in 'Lighting and Electrics' started by Sayen, Mar 4, 2009.

  1. Sayen

    Sayen Active Member

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    Just a quick question - maybe a 'duh' moment, but...

    I'm helping a local school troubleshoot their theater. They have a Strand CD80 dimmer rack, and a number of circuits which do not seem to work. Sending signal from a 1-1 patched console turns on about half of the circuits. However, plugging a plain old fashioned Home Depot circuit tester on a 2P&G adapter shows constant power on every circuit - just enough to light up the tester. Is that normal? I've used the tester in other theaters and have never noticed it lighting up even when the circuit isn't switched on by the board.

    There is not enough power to turn on a light without the console.

    The company that did the install was somewhat less than reputable, so I'm not sure where to begin the investigations.
     
  2. icewolf08

    icewolf08 CBMod CB Mods

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    Yes, it is normal, there is usually a little bit of voltage leakage even when the dimmer is at zero. As you said, not enough to light a lamp, but enough to trigger some electronics. It is usually no more than around 3 volts, but on older dimmers or dimmers where the trims are off you could get a little more.
     
  3. xander

    xander Well-Known Member

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    How old are the racks? When dimmers get old the pots can leak. The leak might be enough to power a very small lamp.

    -Tim
     
  4. Sayen

    Sayen Active Member

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    Thanks!

    Couldn't be that old...maybe 10 years or so? There aren't any records for the theater that we could find.
     
  5. Footer

    Footer Senior Team Senior Team Premium Member

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    Most dimmers always have power on the line until there is resistance on the line. This is why you have to put "dummy" loads on flicker lamps. This is also the reason why you should not use off the shelf circuit testers on dimmers. Either use it in conjunction with a 100w light bulb and a two-fer or buy a tester made for theatrical use. These testers, such as the Gam-Check or generic tester such as the one below have a resistor in line with the tester circuit.

    [​IMG]
     
  6. jerekb

    jerekb Member

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    So for the longest time every one said that our SL floor pocket containing channels 18 & 19 didn't work. I decide to test this one day and found that they were just labeled or wired wrong cause if I brought up 18 then 19 came up and vise versa. now i just make sure and patch them switched to avoid confusion. so when you test them try testing them all at once or something like that.
     
  7. willbb123

    willbb123 Active Member

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    I know our ETC Sensor dimmers can be patched at the dimmers. I would assume you can do it with most dimmers. That way you dont have to worry about when you make a patch sheet and patch the board.
     
  8. willbb123

    willbb123 Active Member

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    I am designing for a Community Theater right now and we are renting some 6 channel Leprecon dimmers. They have the extension cables that have the little LED that lights up when powered. When have the dimmers powered, but not on (from the board) there is enough current to power the LEDs in the cables. As soon as I plug in a light, the LEDs go out. So I'm thinking the same thing is happening to you with the tester.

    I have read (no idea the source) that most dimmers "leak" by design. It is not enough current to make the lamp glow, but it heats it. That way it lessons the chance of heat shock and the lamp blowing when you bring it up to full.

    As to why your dimmers are not working.... I have no idea... :rolleyes:
     
  9. Sony

    Sony Active Member

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    Which half of the circuits light up? Is the rack addressed properly? What model rack is it? What board is it connected to? Have you tried it with a different board?

    I have a Strand CD80 Supervisor rack at work, we may be able to help you out more if you give more information.
     
  10. seanandkate

    seanandkate Well-Known Member Fight Leukemia

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    Yea, the dimmers aren't leaking. That's what's called their "trim" level -- and it's settable (I remember setting them on old CD80 portable packs). It's one of the many reasons you unplug an instrument before relamping regardless of whether or not the fader is at "0".
     
  11. renegadeblack

    renegadeblack Active Member

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    Enough to make the LEDs that they just installed as house lights on an SCR dimmer to flicker like you're in the twilight zone :evil:
     
  12. SteveB

    SteveB Well-Known Member

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    The theory being that they went to very expensive LED's to save on power, which they will, only to have to plunk down a ton more cash in the not-so-distant future, for new LED's to replace the fried units that were powered off of dimmers ?.

    Hmmm.... whose "bright" idea was that ?.

    SB
     
  13. gordonmcleod

    gordonmcleod Active Member

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    Also under no load SCR and Triac dimmers will almost always show line voltage on there output even with the control set to 0
     
  14. Van

    Van CBMod CB Mods Premium Member

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    I think the main questions have been answered already but I want to respond to something that struck me as I read the original post and some of the answers.
    Sayen, I can not remember your particulars, God knows there are so many members now I can barely remember who I am, so don't take offense at my first statement here as I am not directing this at you, rather at any one else looking for help

    Maintenance on Dimmers, Electrical devices, cables, should only be performed by QUALIFIED individuals. If you don't know what qualified means check the Wiki here. You will find the NEMA definition there.

    Of course a couple of folks here responded properly and with good explanations as to why there is the presence of voltage in a dimmer circuit even when the dimmer is set to "0", that's because they are Qualified. "Having a Meter does not a qualified electrician make"
    < you can quote me on that>

    Sayen, it's great that you are troubleshooting with someone, and again I'm not directing this at you in particular but in reference to other young folks who might think, " Hey, I read about this stuff on CB....." When trouble shooting it's good to be completely familiar with the equipment. In reference to the original issue, I can think of two things right off the bat that don't involve checking the voltage; 1. Since Dimmer racks can be powered by a single phase 220v source, and half the pack is out my first guess would be blown breaker or fuse in the distro or switch feeding the dims. 2nd would be patch, how many channels of control do you have? if you have 48 channels of control and 96 dims then depending on the board it will either auto 1:1 patch dims 1-48 to channels 1-48 and dims 49-96 to 0 or it might double up and patch dims 1-48 & 49-96 to channels 1-48. or it might patch 1-96 to channels 1-96, with no physical way to access channels for which no physical sliders exist.

    Again this is more for you "young 'uns" who are all fired up to waltz in and be a hero, 'cause you got your new induction current tester to check Which wires are hot in a 3" conduit........ There's a reason Why Pros only use induction testers for limited purposes, it's because they're qualified...
    Ok Safety lecture over.
    Be careful out there.
    :mrgreen:
     
  15. Sayen

    Sayen Active Member

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    I think you're dead on, Van. For me, testing the system and having to ask the constant on question pretty much puts me at my limits for this type of work, so by 'troubleshooting' I'm just trying to isolate where the problem is, and then send that up the line.

    I should post pictures - the grid almost looks home made, although the teacher assures me it was "professionally" installed. Conduit raceways along every pipe, terminating in boxes with two homemade circuits. One circuit out of every pair doesn't work, one box almost broke off while I was testing, and on one box the plug pulled clear out when I tested it. I swapped a few working dimmers around to check those, but it's a control issue somewhere, along with other issues. My suggestion was not to touch the grid again until a professional has been in to service it.

    Thanks all, and thanks Van.
     
  16. JD

    JD Well-Known Member

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    Just for semantics, "Leak" would not be the right term. The Triac, SCR, SSR, IGBT, or whatever is truly off. What is happening has to do with the design of the trigger circuit that fires the thrysister. It bridges across the device and requires a little power of its own to run. (very little) Because it is in parallel to the device, and in series with the output, a voltage will be present on the output when there is no load in place. Dimmers actually have a second rating that few look at. It is the "minimum load" rating. (Usually around 50 watts)

    Just this last week I ran into the typical scenario that brings this to light. I had to control an LED rope light that was part of a stage set. As it was below the minimum load, it would light dimly even when the dimmer was at 0. The answer was to plug in a droplight and hide it in a off-stage closet. This is the true "ghost load" ;)
     
  17. FMEng

    FMEng Well-Known Member Fight Leukemia

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    The fact that you have leak voltage into an open circuit at all of the outlets suggests that the wiring is intact from the outlet to the dimmers, and that the dimmers themselves all have power on the input side. That eliminates a lot.

    The control card for a CD-80 racks has three, semi-independent sections, one for each of the three phases of incoming power. This remains true even when the rack is fed from a single phase source. So, some kinds of control card failures would lead to 1/3 of all the dimmers failing to work.

    My advice is to call a knowledgeable dimmer rack service technician.
     
  18. jms7805

    jms7805 Member

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    I'm sure you got it corrected by now or walked away. I had the same problem (CD80) system. Someone moved around some "contactor only non-dim" packs and got them in a dimmer slot. The console basically took it out of service until the right dimmer type was re-assigned for that slot. I really don't know how the rack controller senses it or why it would matter.

    Also check, dimmers can be "parked" and then assigned to a Mux channel number. This is done on the rack's computer. You can't turn them on from any console unless you address the "assigned" Mux channel number. Can be very troublesome since you have to go to the rack's computer controller to see it;).
     

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