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dimmer issue

Discussion in 'Lighting and Electrics' started by dj_illusions, Nov 3, 2007.

  1. dj_illusions

    dj_illusions Active Member

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    hey,
    had a weird issue crop up during a show tonight, never seen it happen before and was wondering if any one else had experienced it.
    in the venue we run about 200 dimmer channels off a strand 520i, majority of the dimmers are jands roadpack's and a solution 12 rack.
    i did a blackout on the desk (cue0 go) and everything went out except for 1 lamp, the desk said nothing was up so did a few checks on the desk etc. everything was fine. pulled dmx and rebooted etc. still the lamp was on.
    went to check the dimmer, it reported no faults, so pulled the dmx there, still the lamp was on. unpatched and repatched the lamp and it remained on. power cycled the dimmer and it still stayed on, so in the end had to repatch the lamp to continue the show.
    anyone know why the lamp got stuck on, or experienced this problem before? i am thinking something in the dimmer rack may be a bit iffy, possibly the processor or something on that individual dimmer channel??

    cheers
     
  2. SerraAva

    SerraAva Active Member

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    Is the dimmer possibly set to be on constantly, aka hot patched? Also, try checking the console making sure the channel in question isn't parked on or the dimmer number isn't profiled to be on at full all the time. You can also try taking out the control card in the dimmer rack and making sure everything looks fine, clean the contacts a little, and then put it back in making sure its seated correctly. The last thing like you said, the dimmer may be bad. I had a dimmer once that would only dim between 0-20%. At 20% it was on full, so it made programing interesting to say the least. People would look at the console and wonder why that channel was at 15% and the others at 75%.

    I know that NSI rack mount dimmers have that problem fairly often, some times the light is on full blast all the time, some times it won't come on at all but will in another dimmer. The fix is just pulling out the control card, cleaning it, and making sure its seated properly. Same thing with the old ENR Colortran racks.
     
    Last edited: Nov 3, 2007
  3. SteveB

    SteveB Well-Known Member

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    SCR and SSR dimmers dead-short fail in a constant on state, which is what this sounds like.

    I could not find this particular Jands dimmer system on their site, but am taking a guess it's a 12 pack non-modular system ? (I.E. the dimmers are not on individual or dual modular/swappable cards). I would open up the pack (cut mains power first) and swap the dimmer card to another location to determine if the problem migrates to another DMX address. If the problem migrates, you have a blown pair of SCR's or one half of the SSR (SSR's are typically one power cube powering 2 circuits). If the problem crops up in the same output circuit, then the trouble is not in the actual SCR pairs or the SSR. Next test would be to swap control modules between packs (don't worry about DMX addressing at this point, to see of the problem migrates with the card - if not, then the problem is external control, etc.....

    As David Fox at ETC told me, look for the simplest solution first.

    Steve B.
     
  4. JD

    JD Well-Known Member

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    Second vote for blown SCR here. When they go, that's what you get, solid on. Plug a 100 watt houselamp into it and listen to the bulb, then plug the same lamp in another channel that is at 100%. If it's blown, you will hear a small amount of "lamp sing" from the bulb on the good dimmer, but nothing on the blown channel. Even at 100% you usually can hear some sing, but when the SCR is gone, its like plugging it into wall outlet. Nothing!
     
  5. dj_illusions

    dj_illusions Active Member

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    Steve, You are right they are a non-modular dimmer, a fairly basic unit with a test function and addressing switch, nothing fancy at all. They are probably about 10 or so years old now aswell, hence not on the website.

    Until I test it, I wont know however I think that you may be right in saying it is a blown SCR - will give it a go and see what the result is, if not will try swapping around the control card.

    I dont believe it is a control issue as the desk was switched off, the dimmers will still hold state but then the power was cycled to the dimmer aswell which means it would loose any held dmx states, because the lamp continued to power I would agree that it is either an scr or control.
     
  6. church

    church Active Member

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    many dimmer cards allow for adjustment of the min and maximum dimmer threshold - via potentiometers. These do become dirty and I have seen dimmers come on and stay on this way, removing the control signal doesn't clear the problem. Once you have checked the SCRs then this is the next likely issue. Sometimes adjusting them solves the problem. But first inspect the pot to make sure they are not cracked or broken. Heat and dirt play havoc over time with these pots.

    Most but not all dimmers using SCRs and a min max pot can be adjusted this way. If you can get an Installation Manual do so and read it carefully. If you are not sure do not attempt this. Caution is always required because there are high voltages in dimmer racks. Always remove your watch and do not wear jewelry. THe following adjustments involve low voltage electtronics but you would not want to accidently catch a metal wristband on something at 120Vac you can loose a hand.

    You need to put a load on the channel typically 1kW is required, then command the channel to 15% and adjust the min pot so the light turns on to about 15% - have another channel on at this level for a comparison. Then set the channel to 90% and adjust the max pot to give approx 90% brightness. If this does not fix it then you need someone who knows electronics and has an oscilloscope. Good luck.
     
  7. David Ashton

    David Ashton Active Member

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    Right,this dimmer has a triac triggered by an opto coupler and I would bet on a faulty opto as a triac usually dies when a lamp blows.While you've got it open check the snubber capacitors as there was a bad batch of them a few years ago which blow up quite frequently.[Client rings up and says there was a bang and smoke pouring out of dimmer but now it's working fine = snubber capacitor ]
     
  8. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    Third vote for a blown SCR. I've had it happen a couple times.
     
  9. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    I guess I've been really lucky. Triacs yes, SCRs/SSRs no, never had one fail in either the open or closed position. But then I've used only ETC/LMI dimmers since about 1990. There, I finally said something nice about ETC! Never replaced an SCR on a Ward-Leonard or Kliegl dimmer either. I may have on a very early CD-80 pack though, now that I think about it.
     
  10. dj_illusions

    dj_illusions Active Member

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    hey all, thanks for your input...

    just thought i would let you know it was the SCR that caused the fault. Dimmer will be sent off to Jands for service tomorrow!

    Cheers
     
  11. David Ashton

    David Ashton Active Member

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    I'm curious as to how you know its a triac, could you clarify?
     
  12. SteveB

    SteveB Well-Known Member

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    Let's see if I remember this:

    - SCR - 2 devices that usually bolt onto/mount on a conducting heat sink. Has a control wire to the top, with a hot/load wire to the lower post (it's been a while - so bear with me). SCR dimmers require 2 such devices per dimmer circuit (one SCR for ea. half of the sine wave) and as such are usually mounted on a single modular dimmer card, usually with a control board full of non-transistor style components - I.E. resistors, capacators, etc... The chokes - big copper coils wrapped around a magnet, are usually not mounted on the dimmer card, but are somewhere else in the dimmer pack.

    - SSR - Solid State Switching Relays. A flat square 2" x 1" approx. (power) cube with 4 wires, 2 as control, 2 as load. A single SSR controls 2 dimmed circuits and are often mounted on a modular card controlling 2 dimmer circuits, complete with the 2 chokes. Control electronics are in the pack Control Electronics Module - to use the ETC model.

    - TRIAC - single device that controls both sine waves. Used in cheaper dimmers, including the common house hold 600w wall dimmer (US practice), thus often has no choke and as a result will usually induce RF noise into the electrical system. In truth I have not - to my knowledge, ever used a dimmer with TRIAC's in a theatrical dimming system, thus cannot remember having to fix one.

    In general, most good quality entertainment dimming systems built in the past 20 years or so, have used SSR's in one form or another, so that's what you are usually going to encounter.

    Steve B.
     
  13. David Ashton

    David Ashton Active Member

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    Well the Australian Jands, Theatrelight LSC Dynalight and others use 40amp Triacs, but if a channel locks on its 1- a circuit board fault 2- an opto coupler fault or 3- a Triac fault and I was curious as to how he knew it was the triac.Bear in mind our "standard" rack is 240v /10a /channel so there is headroom with 40a triacs.
     
    Last edited: Nov 5, 2007
  14. JD

    JD Well-Known Member

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    Hee hee! Actually, Triacs almost made SCR's obsolete. Both are "Avalanche" devices, meaning that a small gate current causes the device to "switch" on and conduct a huge current! An SCR works in one polarity, and a Triac works in both directions. So, why do we even use SCRs? Well, with SCRs, the device is only working 50% of the time. (Thus two back to back) Because of this, you can use two 35 amp SCR's where you would use one 70 amp Triac. The price of these devices goes up quite radically once you cross the 40 amp area. So, it's cheaper to build a 70 amp dimmer with SCR's rather than Triacs. 70 amps, by the way, is the standard for a 20 amp, 2400 watt dimmer. (Safety factor.) Unfortunately, most inexpensive 20 amp dimmers use 40 amp Triacs because that is where the price break is. There is another advantage: With two devices, you can dissipate the heat twice as fast because they have twice the surface area. Control is a bit more tricky with SCRs and you will find four control wires leading from the PCB to the SCRs as compared to two wires on the Triac dimmer. I prefer SCR dimmers over Triacs. I don't know how long we will have the option though! Diesel locomotives now use banks of 4000 amp Triacs. More and more high power motor controls are using them as well. They may go the way of the vacuum tube, who knows.

    An SSR is actually one Triac or two SCRs (depending on manufacturer) mounted in an epoxy case with a photo-coupler, thus eliminating some of the auxiliary equipment. Very handy! Although they are called "Relays" they actually work just as fast as discreet SCRs and Triacs, so there is no problem using them in waveform chopping dimmers.

    How to test all of the above: REMOVE POWER, measure with a multi-meter across the two heavier wires going to the device. It should not conduct outside of a very high k leakage from the control circuit. If it measures as a short or as a diode, remove all connecting leads. If it still measures as a short or diode, it is bad. WARNING: If you are not an electrical technician, or if you have any self doubt about your abilities, refer the work to qualified personnel! This is not a self help post, only an answer to a question.
     
  15. SteveB

    SteveB Well-Known Member

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    Thanks John,

    Excellent description, especially the part about SSR's.

    Steve B.
     

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