Dead Dimmer Channel

[Neil Hampson]

During a rehearsal last night a 1Kw Lamp blew, no problem, replace the lamp, however when plugged up again the dimmer channel is now permanently at full up, regardless of the control signal (The LED indicator responds to the control level). I have read somewhere that this is a symptom of a blown power Triac, Can a lamp blowing cause the triac to fail, and is this generally considered as a DI repair, or should it go back for repair. (I have a fair amount of electriacl experiance through my day job involving Industrial switchgear and instrumentation, but have never repaired a dimmer before)

 

The pack in question is an Anytronics 194 series 6x10A unit with a DMX card added. The pack has been repaired before, having had a new DMX card and 'Ramp generators' amongst others. The pack also does not like the cold, it would have one channel creep to ~50% but be controllable between 50-100%, as soon as the rom warmed up it would be fine. thie unit was returned to Anytronics but the fault could not be replicated. (I found the fault when I was working in a church and I had moved the dimmer from a nice warm area of the church and put it on a cold marble slab to bring it closer to the truss. Putting the dimmer on a piece of foam cured the problem!)

[pritch]

It's not unheard of for a lamp blowing to take the triac with it, as the fuse doesn't always blow in time to protect the triac. I'm not familiar with the pack in question, but replacing triacs is usually a fairly simple job if you're used to working on electrical equipment.

[Nicktaylor]

Very much so! In my experience overloading the circuit can damage the triac before taking out the fuse. I know Zero 88 specify quick blow fuses but I also know some people think it better to use anti surge. As to changing a triac, it is not so much doing the electrical bit, rather finding out which part of the cae you have to take to bits to get to it!

 

Reading the original message, triacs do age so it may have been on the edge of blowing, I have had a triac go closed circuit ie on, between the flash through and the start of a show!

[Neil Hampson]

Thanks for the replies so far, The pack is being used in a village hall panto until Sunday, and I an coping with 17 channels for the rest of the run (I have just remved the fuse on the affected channel). I'll open it up and see if I can get to the Triacs easily without tearing the whole thing to bits, if I can see them, it would seem that I can replace them , if not I'll let someone else do it rather than totally destroying the thing. looks like a little trip to the RS catalouge is in order.

[StevieR]

I've seen this often - normally in cheaper dimmers. When a lamp blows there can be a quick current surge which can overload the triac and short it open. When you're replacing the triac, try to select one with a higher current rating. This seems to lessen the chance of this happening again.

 

Steve

[David A]

Easily the most common cause of blown triacs is the high currents made by the arcing when a lamp blows, high speed ceramic fuses will offer some protection but I frequently find car fuses in dimmers which offer very little protection.I would agree with the suggestion of fitting the highest rating triac you can but triacs don't ' wear out ' they will function for decades.

[Light Console]

Anytronics dimmers are very easy to repair, but I can't for the life of me remember how to get into them, sorry.

[MarcT]

After a quick google I found the product spec:

 

Anytronics Series 194 Dimming Pack

 

It apparently has a plate fitted to the rear of the chassis to facilitate triac changing, so it sounds like they've thought about this job in the design and hopefully made it easy. Why not give them a call (the number is on the bottom of that PDF) and find out what part you need & how to do it?

 

If its easy to do, you could also try swapping the triac with one from a channel that does work and confirm the fault moves. If it does not, the driver circuit is shot and that could be a return to base. Chances are though that the triac which has failed short circuit, especially given it went when a lamp blew.

 

Best Regards,

Marc

[Suzette]

Can a lamp blowing cause the triac to fail?

 

Yes. I used to have triacs blow in old Strand STM dimmers when lamps blew in the rig.

Not every day,thank God, but enough times to be able to answer - Yes they can!!

[Neil Hampson]

Got the dimmer back over the weekend and swapped the Triac out from a known good channel. The fault stays on the same channel, thus the Triac is actually OK, I have removed the Demux board from the main PCB and the fault remains, therefore the fault must be on the main board, between the 10V signal input and the triac. I am now a bit out of my depth in terms of fault-finding beyond simple substitution of major components (although my electrical is OK, electronics is still a bit of a black art), the only other main component that I have not swapped out is the power transformer prior to the channel output, but I cant think of a fault on a toroid transformer that will give a full up output fault.

Any ideas greatfully recieved, also if anyone can recommend a repair base (preferably local to Manchester/Liverpool)

[musht]

At a guess opto triac or its load resistor that drives the big triac. But if your not sure what it looks like take it somewhere with electronics knowledge.

[danjshelton]

Hi Neil,

 

If your at the end of what you think you can do with the unit in terms of fault finding and repair then I would recommend sending the unit back to Anytronics. A company that I work for a lot has a lot of anytronic packs (Series 192, so I would guess it's a similar layout inside looking at the pictures from the link) and although they are very user servicable for the bigger jobs (and more fiddly) it's easier to send it back, also everything that we've sent back has been turned around very very quickly and at a very reasonable rate (A lot of time cheaper than sourcing the parts, waiting for the parts, fitting the parts yourself, testing the unit).

 

Cheers

 

Dan

[dbuckley]

Humorously, the saying goes "it's amazing how often a triac will lay down it's life to protect the fuse". You were unlucky; both went. Normally it's one or t'other.

 

This is, of course, just cost engineering; by fitting appropriate triacs they would not fail under short circuit conditions.

[Meduza]

Thats why I love Okero dimmers, they are super-stable built with 40A, 700V rated Triacs for the 13A 240v channels and such things ;)

 

EDIT: On the other hand... they are not cheap... their 6-channel touring dimmer is abut £700 + VAT

[Stuart91]

Thats why I love Okero dimmers, they are super-stable built with 40A, 700V rated Triacs for the 13A 240v channels and such things ;)

 

In a similar vein, Zero88 Alpha packs apparently use the same triacs as the Beta packs, despite only carrying half the load. Ours have suffered some terrible abuse at times, and we haven't blown a triac yet. (Touch wood)

 

However, we did have one pack die when it was trodden on, causing the case to short onto the PCB tracks...