Strange problem with projector lamp circuit.

Thread Starter

daba1955

Joined Apr 27, 2019
119
Hi everyone, hope you are all keeping well in the current situation.

I have a weird situation with, let's face it, the simplest of circuits - not my design, I might add.

The circuit attached is from a stage/DJ lighting fixture (Gobo) that has been installed to replace the original ballast/choke circuit that drove a Mercury Vapour lamp. This apparently is a common modification done to these fixtures, perhaps because the MV lamps are expensive, or perhaps when they "blow" fragments of hot glass pebble-dash the inside of the fixture ! I have 6 of these fixtures, 4 were already modified, I modified the 5th, and one still has its original MV lamp and driver hardware.

The PSU is one of those M.I.C. "Universal Power Supplies" - and that would probably be OK on its own, except that (presumably) the lamp inrush current is occurring before the PSU has had time to wake up properly and stabilise, so it goes into protection mode and "sits down". So the person who modified these added a FRM01 Universal Timing Module (again M.I.C.) to keep the lamp disconnected until the PSU had "joined the party". The timer module is set to 5 seconds, then the on-board relay is energised.

On one of the fixtures, what I am seeing is the PSU sitting down immediately it is powered on, indicating that somehow, it is detecting a large load, but the relay is not being energised as far as I can tell - I can't hear, feel, see or smell any indication that the relay is being pulsed by the microcontroller of the timer module. However if I disconnect the lamp, the PSU fires up properly. All the other 4 modified fixtures are working as they should.

The relay n.o. contacts on the timer module are 100% n.o. and have a resistance too high for me to measure with my multimeter, which is what I would expect to see on a n.o. contact.

I have an oscilloscope, but frankly I'm at a loss to know what to look for.

Is it possible for 24V dc to "arc" across n.o. relay contacts ?

Could such an arc (if it is possible) produce sufficient current to make the PSU sit down ?

Any help would be much appreciated, it's not making any sense to me.

TIA
 

Thread Starter

daba1955

Joined Apr 27, 2019
119
Hi everyone, hope you are all keeping well in the current situation.

I have a weird situation with, let's face it, the simplest of circuits - not my design, I might add.

The circuit attached is from a stage/DJ lighting fixture (Gobo) that has been installed to replace the original ballast/choke circuit that drove a Mercury Vapour lamp. This apparently is a common modification done to these fixtures, perhaps because the MV lamps are expensive, or perhaps when they "blow" fragments of hot glass pebble-dash the inside of the fixture ! I have 6 of these fixtures, 4 were already modified, I modified the 5th, and one still has its original MV lamp and driver hardware.

The PSU is one of those M.I.C. "Universal Power Supplies" - and that would probably be OK on its own, except that (presumably) the lamp inrush current is occurring before the PSU has had time to wake up properly and stabilise, so it goes into protection mode and "sits down". So the person who modified these added a FRM01 Universal Timing Module (again M.I.C.) to keep the lamp disconnected until the PSU had "joined the party". The timer module is set to 5 seconds, then the on-board relay is energised.

On one of the fixtures, what I am seeing is the PSU sitting down immediately it is powered on, indicating that somehow, it is detecting a large load, but the relay is not being energised as far as I can tell - I can't hear, feel, see or smell any indication that the relay is being pulsed by the microcontroller of the timer module. However if I disconnect the lamp, the PSU fires up properly. All the other 4 modified fixtures are working as they should.

The relay n.o. contacts on the timer module are 100% n.o. and have a resistance too high for me to measure with my multimeter, which is what I would expect to see on a n.o. contact.

I have an oscilloscope, but frankly I'm at a loss to know what to look for.

Is it possible for 24V dc to "arc" across n.o. relay contacts ?

Could such an arc (if it is possible) produce sufficient current to make the PSU sit down ?

Any help would be much appreciated, it's not making any sense to me.

TIA
Ooops forgot to attach the circuit ....

2021-01-10_095410.jpg
 

Thread Starter

daba1955

Joined Apr 27, 2019
119
With the lamp disconnected, check what the FRM01 relay does when the supply is switchedd on.
AFAIK it does nothing - no sound, no movement, but I'm just setting up now to monitor the disconnected open contact with my scope. I'll have to make sure that the connected pole of the relay is going to the +24V .... They've used 2 brown wires, so I'll have to bell it out.
 

Thread Starter

daba1955

Joined Apr 27, 2019
119
With the lamp disconnected, check what the FRM01 relay does when the supply is switchedd on.
Well I'm confused now - didn't find anything like the relay being pulsed, so decided to re-connect the lamp, and it works fine now. The only possibility is I've got the COM and NO connections on the relay reversed, but can't see how that can possibly make a difference !
 

Thread Starter

daba1955

Joined Apr 27, 2019
119
Just switched the relay connections back, and it still works OK - tried it about 20 times and no failures ....
 

Ian0

Joined Aug 7, 2020
1,172
Are you sure it's a mercury vapour lamp, not a metal halide? A mercury vapour lamp has much too large an arc to be used as a gobo projector.
Neither has any inrush, but both need high voltage superimposed on the supply to strike them. A hot-restike metal-halide needs >10kV.
 

Thread Starter

daba1955

Joined Apr 27, 2019
119
Are you sure it's a mercury vapour lamp, not a metal halide? A mercury vapour lamp has much too large an arc to be used as a gobo projector.
Neither has any inrush, but both need high voltage superimposed on the supply to strike them. A hot-restike metal-halide needs >10kV.
No, you've misunderstood - the circuit I posted has replaced the whatever lamp and ballast circuit - done by others ...
 
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