LED Tape and dimmer packs

Robert F Jarvis

Well-Known Member
We are running several LED tapes to backlight some 'windows'. One set is controlled from a DMX controller powered by a 12v power brick and another two sets straight from simple 12v power bricks.

All three 12 volt supplies are fed from a ADJ DP-415 dimmer pack so we can control things from the booth. The DXM controller is powered up at the beginning from one of the packs four outlets (#1) and the other two bricks are fed straight from the 2nd and 3rd outlets. When up and running all work very well. But, once all the dimmer pack channels are pulled down and the console closed, strange things happen: All the LED tapes will flash about once every 10 seconds!

It is as if the dimmer pack is releasing a power burst on these occasions. The dimmer pack is normally run in "Switch" mode but was turned to "Dimmer" to test. There was no difference. So now we plug the DMX controller's dongle into a wall socket and whilst the other two have to remain on the dimmer (to keep control) we have to disconnect the mains from the pack between shows. Finally, a tungsten lamp was connected as a test and we did not observe any flashing.

On the other side of the stage I have a 20Amp 12 v supply that feeds a 30 channel DMX controller in turn feeding 3 LED tapes and do not have nay problems. So some thing to do with those power brick coupled to the dimmer outlets. Ideas?
 
Switch mode isn't a switch. The ADJ DP-415 is leaking current that trickle-charges a capacitor. Every now and then the charge gets high enough to discharge through the LED. That's your intermittent flashing.

It's not usually a good idea to power a switched-mode power supply, like the one in the LED power brick, from a dimmer, even one in switched mode. Some power supplies are designed to handle it but many (most?) aren't.
 
That relay pack would be a good solution. It says 12V working voltage, but I suspect that's just the input voltage to the control electronics part of the board - the relays are probably rated at mains voltage (you should check - it will be written on the relay) so could switch the incoming main power to the power bricks.

The leakage current is via the snubber network of the triac in the dimmer. When you have a large load, such as a 20A 12V supply, or a dummy load such as a (relatively high power) tungsten lamp in parallel, there isn't enough current passed to charge up the capacitors in the switched mode supply. With a proper switching module, where there is a relay/contactor instead of the output triac, you don't get this problem.

Essentially, don't connect anything other than an incandescent lamp to a triac dimmer, even if it says it has "switch" as a mode, unless you know for certain that it has a physical relay when it's configured to switch, except where the load you are connecting specifically specifies that it can be dimmed (e.g. dimmable "electronic transformers" on birdies).

Edit: There is an exception to this (isn't there always?) - you can usually connect non lamp loads to suitably beefy sine-wave dimmers. Whether they behave the way you expect is moot, but you won't suffer from the snubber leakage problem you get with triacs.
 
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Switch mode isn't a switch. The ADJ DP-415 is leaking current that trickle-charges a capacitor. Every now and then the charge gets high enough to discharge through the LED. That's your intermittent flashing.

It's not usually a good idea to power a switched-mode power supply, like the one in the LED power brick, from a dimmer, even one in switched mode. Some power supplies are designed to handle it but many (most?) aren't.
This. Use a proper LED driver and shoot DMX into it.
 
It sounds to me as if Robert already has a DMX driver for the LEDs to look after the dimming, but what they're also doing is trying to switchthe power that is input to both the controller and the LEDs by DMX too. So he needs a DMX relay to turn the input power off and on and a DMX LED driver/dimmer to actually control the LEDs.

Does that sound like what you're doing, Robert?
 
We are illuminating several faux windows with LED tape. Each tape has one of those little wall wort 120/12 volt dongles. To ensure power cut at end of show we plugged them into a couple of 4 output, DMX controlled Dimmer/Relay packs. But started seeing 'flashes' soon after powering down. An expert here noted the packs were 'leaking' current which, in turn was charging capacitors until they fired thus causing the visible flash. So my question is are these dimmer/relay boxes really pure relays or merely putting the dimmer circuit to 100% and allowing current to leak?
So we are looking for pure relay packs. We have some small pcb board relay ones but want something more robust we can just plug our dongles into.
 
One rather obvious way to procure a more robust relay box is to take one of the various relay boards and mount it in your own robust box with appropriate gazintas and gazoutas. If it's a metal box, and assuming you're controlling 120V devices, make sure the box itself is properly bonded to ground. Fuses or circuit breakers for the individual relay controlled circuits would also be a very good idea if the relay contact ratings are less than the nominal current ratings of the controlled circuits (e.g. less than 15A if your input(s)/output(s) are normal Edison plugs and sockets).
 
Several years ago we bought some Milford Instruments 8 way DMX relay boards - I believe they are still available from a supplier in The States - and mounted them in instrument cases with a power cable in and tails out terminated in 15A BS546 shuttered sockets, so we can plug in things that won't like dimmers (such as LED wall warts, TV sets, Hi-Fi units) so they can be turned off and on under lighting board control.
 
Yep, I have two such boards but being as lazy as sin thought I could get commercial in a rugged box. But I think I will look more seriously at this now a show is going and I have some time. Thank you
 
Yep, I have two such boards but being as lazy as sin thought I could get commercial in a rugged box. But I think I will look more seriously at this now a show is going and I have some time. Thank you
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