ceecrb1 Posted July 3, 2007 Share Posted July 3, 2007 can anyone give me a rough outline of the internals of a dimmer..... gotta learn as just discovered our company has LOTS waiting for repair and I aint repaired them before....(mind you I said that about ballasts too and now I´m getting quite good with them!) so far, for me dimmers have always been the magical buzzing box backstage... but this has to change I am an electrical engineer but an informed head start is always a help ta muchly Link to comment Share on other sites More sharing options...
musht Posted July 3, 2007 Share Posted July 3, 2007 http://www.epanorama.net/links/lights.html#dimmer For reapirs big tube of triacs, bigger is always better, just `cos its a 10A dimmer dont mean a 40A triac is too big, unless it physically wont fit, ditto with voltage ratings 400V good 800V better. Link to comment Share on other sites More sharing options...
ceecrb1 Posted July 3, 2007 Author Share Posted July 3, 2007 A truley excelent link!! Lukily it turned out that 2 of the 6 dimmers I need to repair only needed Triacs replaced... yet to discover about the final 4..... Link to comment Share on other sites More sharing options...
Andrew C Posted July 3, 2007 Share Posted July 3, 2007 As an engineer you are probably aware, but others may not be, the can and heatsink on triacs may well be live at 240V! Link to comment Share on other sites More sharing options...
dbuckley Posted July 3, 2007 Share Posted July 3, 2007 Old school analogue dimmers are quite simple things, and usually fairly easy to troubleshoot, even when it isn't a fritzed triac. Link to comment Share on other sites More sharing options...
Don Allen Posted July 4, 2007 Share Posted July 4, 2007 Always best to upgrade to 40 amp triacs as most budget dimmers will have smaller capacity ones. Several different case styles. Some triacs are INSULATED TAB triacs so make sure you put the correct ones in. Print out and keep data sheets for all triacs and scr's you come across to make sourcing replacements easier. Print out and keep tech manuals for dimmers you have in use. A common fault I come across is open circuit gate resistors, usually 10 ohm half watt. replace with same as this is what is classed as a "safety resistor" and dies to protect the triac. If the channel is permanently on, usually a shorted triac. If the channel won't come on, usually the triac driver, MOC3021, MOC3041, BRT12 etc. If the channel is flickering randomly, could be a faulty gate on the triac, partially damaged. Dimmers are grouped in modules of four per phase. If a group of four channels are faulty, look at the phase reference circuit for that group. The epanorama is a good link. Some Theatrelight dimmers have a mode switch on the front panel and need to be set to desk for DMX in to work. Check mode switches and also for correct alignment of the mode switch knob. I use an adaptor to run dimmers of a single phase plug to allow use of an rcd, to pat test etc, some dimmers need three phases for correct operation of monitoring circuits. They will still work single phase but will show a fault condition. Develop a safe working practise when working on dimmers as you will have to test with them live. Hope that helps.Don Link to comment Share on other sites More sharing options...
dbuckley Posted July 4, 2007 Share Posted July 4, 2007 Dimmers are grouped in modules of four per phase.True for twelve channel dimmers, but in the UK most dimmers you come across are six channel, powered from 60A single phase. Frankly, most dimmers you come across in the UK are betapacks... Anyway, on these typical dimmers you have three sets of zero cross detector mechanisms (in case you rewire for three phase), but the underlying point is that if a third of the channels are down its probably a zero cross detect related fault. I use an adaptor to run dimmers of a single phase plug to allow use of an rcd, to pat test etc, some dimmers need three phases for correct operation of monitoring circuits. They will still work single phase but will show a fault condition.I do the same, but I note that many older TLs have a N/E bond inside, which pops the RCD.... Develop a safe working practise when working on dimmers as you will have to test with them live.Thats the most important bit of advice in the thread. I use an isolating transformer, but it's by no means a panacea, an RCD might well be safer. I use the tranny so the scope has a decent ground without any circulating currents or RCD trip worries. Also in the general advice category, note that most dimmers or European origin wired for three phase only have one power supply tranny on one phase, so you need to feed at least that phase or theres nowt to test. TLs have three power supply trannies, much better Link to comment Share on other sites More sharing options...
Andrew C Posted July 4, 2007 Share Posted July 4, 2007 If the channel is permanently on, usually a shorted triac.Unless it is a Beta pack; these fail on when one or more potential dividing resistors fail. Big ones, mains potential. Search the forum for some detail. Link to comment Share on other sites More sharing options...
Floydey Posted July 4, 2007 Share Posted July 4, 2007 I do the same, but I note that many older TLs have a N/E bond inside, which pops the RCD.... I have some Rank Strand Tempus 10A/15 dimmer packs that trip RCD's. Can they be modified to prevent this? Link to comment Share on other sites More sharing options...
Andrew C Posted July 4, 2007 Share Posted July 4, 2007 I have some Rank Strand Tempus 10A/15 dimmer packs that trip RCD's. Can they be modified to prevent this?The smoothing/filter capacitors have almost certainly gone "leaky". Check these out first, they may even leak too much by design. Link to comment Share on other sites More sharing options...
Dmills Posted July 4, 2007 Share Posted July 4, 2007 IIRC from back in the day, there was a mod to the Tempus that added a slide switch to the end of the rack to select wither the RFI filter had an earth return path or not, precisely to allow you to trade off dimmer noise for earth current if required. I **think** it was a Stage Lx hire rack that I first came across this on? Does anyone else remember seeing this on their Tempus hire stock? Anyone have the details? Regards, Dan. Link to comment Share on other sites More sharing options...
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