Control/Dimming dimmer "sticking" issue

[xander]

I am teching a show right now and I am running into a problem I have never had before.

Equipment:
Lee Colortran 2.4kw dimmers
4 x R40 strips, 600w each

The problem is that when we go from a cue to blackout the strips stay on at a low level. Now, it is an old dimmer and it is loaded to the full 2.4kw, which I don't like to do, but that is what the designer wants so I have to. Initially I thought the dimmer had blown, but it did finally go out and we still seem to have full control over it. I haven't had a chance to troubleshoot it yet, but I would like to try and see if it just this cue and therefore maybe bad data. Has anybody had this issue or know what it might be?

Thanks,
-Tim

EDIT: It has been working really hard, it being tech and all, do you think that it could just be overheated??

 

[NickJones]

Had the same problem with our ancient LSC Dimmers. I did exactly the same thing, loaded it to the max with no watts to spare. The fuses didn't blow because it knew it wasn't over loaded, but the dimmer gets "Jammed" so to speak, you are just going to have to take a few lights off the channels that are getting "stuck" to stop this from happening.
Hope this works for you,
Nick

 

[JD]

Actually, what you have is a dirty low end trim pot. Old analog dimmers had a ramp generator with two calibration points; Top end, and bottom. When the pots (variable resistor, like a volume control) get old, they get may start to shift or jump a bit. If they are accessible without opening the pack, observe where they are set, then move them back and forth a few times, ending up where you started. (With the power off) This usually cleans the trace a bit.

Generally, the problem will show up independent of the loading, so as long as you are within the ratings of the dimmer, you are ok. On the low end trim, the lamp should just turn off as your control hits bottom. The high end trim should be set so that the lamp hits full brightness as you hit 10 on your control.

Most trimmers have a small screwdriver slot in them for adjustment.

As always, anything beyond that should be referred to qualified service personnel.

 

[xander]

Well, here's an update. The SSR, or whatever is in there, blew. They are on at full all the time now. So, now my question becomes, does anybody know where I could find parts compatible with these dimmers?

Thanks,
-Tim

 

[derekleffew]

...does anybody know where I could find parts for these ancient dimmers? ...

I'll answer only if you promise to stop using the word "ancient" when referring to something less than twenty years old. LEE Colortran was formed about 1987, but I thought later than that.

Are the dimmers in question Model D192, ENR, or something else? The SSRs are probably Crydom, and probably have a model number on them. See the glossary. Crydom products are carried by any number of reputable vendors, including Allied, Newark, Mouser, and probably Graybar, Grainger, and McMaster-Carr.

If that route doesn't work for some reason, I'm positive Steve Short at Lite-Trol Service 1-800-LITE-TROL can help you out. (Maybe in turn you could him him with his poor excuse for a website.)

 

[xander]

I am terribly sorry if I offended your delicate sensibilities, but everything I heard about them placed them a lot older than that. Well, that gives me hope that there are parts out there in existence. Thank you.

-Tim

 

[dramatech]

The solid state relays (SSR) haven't changed over the years, so it really doesn't matter how old the equipment is. Now if your system used Silicon Controlled Rectifiers (SCR), That would make a bit of a difference. Dimmer racks if maintained don't go out of date. Even older racks can be updated to DMX, and once that happens, it is just the controllers that get outdated, and that is just because we always want the newest and greatest device.

 

[church]

Finding parts is probably not hard. Once you have the part number you can just search for it on google. The part number you actually want is the TRIAC or SCR manufacturers number which is printed on the device. Some devices used on dimmers built in the 70s are obsolete. However all is not lost as there are equivalents which you can identify through some of the Engineering Databases available onthe Internet. Also a friendly Tech or Engineer can always identify an alternative for an obsolete part. For dimmers manufactured as recently as the ones you mention it should not be a problem.

A good database for obtaining schematics, alternative part numbers and suppliers is:

DigChip IC database

 

[awhaley]

You've already gotten the helpful answers... so I'm going to throw in the academic. The thyristor in your dimmer (Triac or pair of SCRs) is a little solid state switch that switches the load on at various points in the AC cycle. Triacs and SCRs, once turned on, continue to stay on until the current through them gets low enough that they release. This latching mechanism keeps them on until the next time the AC line voltage crosses zero, under normal operation.

This holding current is dependent on several factors, one of which is temperature. I've seen the exact situation you've described happen before when a Triac was being overloaded, because it overheated to the point that it wouldn't unlatch at the Zero Cross... the end result being that once turned on, it would never turn off. Unplugging the load unlatched the dimmer and then you could plug it back in and it would sit there in the off state until you again raised the channel, then again it would stay on.

If you leave the Triac in overload long enough (not the only way to burn one of these up... but...), it can 'fail closed' which is what you described in the update. It stops being a switch and becomes an extra bit of wire that always conducts.

I know you weren't technically overloading the part... but these things do have a lifespan and you can't predict when any given part is going to degrade to the point that it can't handle it's rated limit anymore. It's also possible that the dimmer had been overloaded at some point and a path through the semiconductor had already been partly burned... so it now took less current than the rating to finish it off.

We all keep telling you to track down your Triac and replace it, but I guess we should ask if you need any more basic info about how to do that? If you have the ability to solder through-hold components and operate a screwdriver you really ought to be able to do this with a little coaching, but if you feel over your head, find an electronics guru to help. If you need help identifying the part that needs replacing, feel free to post pictures of the inside of the dimmer! Colortran dimmers may have either a discreet Triac (you can google for 'TO-220 package' to see what this might look like... and it may or may not be attached to a heatsink) or a little block in the middle of the dimmer between the breaker and the two big wirey thingys (chokes for making sure that your dimmers don't make every television plugged into the wall within half a mile go crazy when you run a cue...) which is an SSR module, which contains the SCRs (or triac), and the appropriate resistors and optoisolator to make it all work.

Just shout if you need more help!

Art

 

[JD]

Before you go changing out SCR’s (Triacs, SSRs, etc.) there is something wrong with the pathology of the failure you described. It is true that outright failures often leave the lights full on, (or half way on if only one side (scr) fails), what you described was a “low level.” First, was this only one channel or a group of channels? Second, was it on at a low but consistent level?

If it was a pair or group of dimmer channels, or it (they) was/were on at a low but consistent level, then that points to a ramp generator failure. Older analog dimmers, as well as early DMX dimmers produced an analog ramp. When the control voltage crossed the ramp, the device was switched on. Loss of ramp (open cap, bad pot) would cause the cross to take place right after ZVC resulting in the dimmer being “full on.”

Would hate to have you go to the trouble and expense only to find you still had the problem. There are some simple tests that can be done before spending the money to see if the device is shorted on. As we can’t give repair advice on the board, I would suggest having qualified service tech give it a once over before investing any money. As this is “classic” equipment, an age related failure may indicate there are many more right around the corner.

 

[gordonmcleod]

It isn't ancient by a long shot and even the most ancient dimmer is still usable and repairable in most cases
many dimmers had a feed back circuit to correct for loading because as the load increases the voltage drop across the inductor filter increased. Now this did not make the dimmer as load depended as a old resistance plate dimmer but did cause issues with the overall curve
The feedback was usually applied as a negative amount to the control circuit
because all analogue devices drift with age and temeperature they do require adjustment and calibration which is nice because at least someone can go in and adjust them, that many digital circuits can't be adjusted just replaced when they fail,is there short coming in the long run

As for the triac SCR or SSR block any similar replacement will do but watch out on SSR blocks some have the optocoupler built into them others do not and some also have the zero crossing built in which means they can only be used as a non dim device