Repairing DMX Switch Packs

[Cheeseweasel]

Hi all,

 

I'm working my way through a pile of broken DMX switch packs and have come across a fault which is stretching my fairly limited knowledge of electronics.

 

They are 4-channel triac-controlled DMX switch packs (Botex DSP-405), and the issue with some of the units is that they have an output which is permanently 'stuck' on. I have eliminated the output triac itself from the equation - the issue seems to be that 240V is continuously present at the gate of the triac, even when the output is supposed to be 'off'.

 

The triac's gate is fed via an optocoupler (Fairchild MOC3041), which interfaces between the DC side of the unit and the mains AC side. Initially I thought this must be the source of the problem, but after swapping it out for a replacement, the fault persists. The input to the optocoupler's LED seems to be ok (as far as I can tell with my basic multimeter), flip-flopping between about -1V and +5V DC periodically with the chase sequence I'm using for testing. Yet there is always 240V present on both of the optocoupler's main terminals (when I would expect it to switch on and off, mirroring the DC input). The second main terminal is currently disconnected, so I've ruled out the possibility of anything shorting.

 

To me it seems that for some reason the optocoupler isn't switching, as everything is apparently working up to that point in the circuit. I have tried two replacements to no avail. Could it perhaps be that the DC input is not switching from -1V to +5V quickly enough and the optocoupler is sensitive to dV/dt? Or is that not how optocouplers work?

 

I'd be grateful for any suggestions.

 

Thanks,

Tim

[timsabre]

I think it's correct to measure 240V on both optocoupler outputs because the triac is normally connected in line with the live so its gate will be always at or near live potential.

There's a typical triac drive circuit here (for some reason I can't include it??)

 

http://3.bp.blogspot.com/-IiQvg1EweFs/ThviB3fqUdI/AAAAAAAAAxU/azFA0eH-viw/s1600/MOC3021+Inductive+Load.bmp

 

 

 

When you say you have eliminated the triac, does that mean you've tried replacing it? Cos it really sounds like a failed triac.

[Cheeseweasel]

I think it's correct to measure 240V on both optocoupler outputs because the triac is normally connected in line with the live so its gate will be always at or near live potential.

There's a typical triac drive circuit here (for some reason I can't include it??)

 

http://3.bp.blogspot...uctive+Load.bmp

 

 

 

When you say you have eliminated the triac, does that mean you've tried replacing it? Cos it really sounds like a failed triac.

 

Thanks, I have tried replacing the triac - with a brand new triac, and a known working triac from a different channel. I'm measuring 240V on both opto outputs, with nothing at all connected to the second output as I have removed the triac completely.

 

I did knock up a quick schematic but can't find a way to attach the image.

[DrV]

Did you replace the triac after you fitted a new opto? If so it is perfectly possible that you've damaged the opto because there was a faulty triac still in circuit. If the opto is ever asked to drive a circuit containing a faulty triac it can find itself trying to deliver all of the load current on its own. Replace the triac AND the opto with new at the same time or at least rig up a test circuit to check them out off the board.

Dave

[dmxlights]

You should also make sure you have the channel your testing loaded with a min of a 60 watt light bulb as you can get a false reading when testing triacs that are not loaded. I have been caught out by not loading one before and also I have seen what DrV has said happen

[AdrianW]

You should also make sure you have the channel your testing loaded with a min of a 60 watt light bulb as you can get a false reading when testing triacs that are not loaded. I have been caught out by not loading one before and also I have seen what DrV has said happen

 

+1 for this. A triac is a current switch not a voltage switch. You need a load in order to make the triac turn off properly.

[timsabre]

Also the moc3021 is an opto triac so likewise needs a load, of you disconnect the main triac from it you will not learn anything useful.

[PaulDF]

I doubt the opto coupler would be sensitive to dV/dt , well I doubt it very much at 50hZ. They are used to switch things a lot faster than this.

All an opto coupler consists of is a source such as an LED and some sort of light/IR dependant switch, basically to create a barrier between the ELV side and the mains side of the circuits so you can't feed mains back into the ELV circuitry. If things do go wrong then the output side of the opto coupler can be destroyed but leaves the input side intact.

If the input is changing state correctly then as others have said, it is probably worthwhile swopping out both the opto coupler and the triac.

[timsabre]

Some optocouplers have a transistor on the output side which is the type you are describing, but the MOC3021 (and family) has a low powered triac on its output side and is often used to control the main triac in dimmers and switch packs.

[Cheeseweasel]

Thanks everyone for your suggestions.

 

I eventually got to the bottom of this - it turns out there was some arcing going on between two tracks right underneath a capacitor. Checking the DC resistance using a multimeter wasn't indicating a short. It was only happening when the triac was under load, and since I was doing most of my testing with nothing connected to the triac, I wasn't picking up on it. Chopping out the damaged track and bypassing it with a suitable bit of wire has sorted it.