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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Lighting Forum => Topic started by: Mike Karseboom on May 09, 2015, 10:04:14 am

Title: Two Dimmers in Series?
Post by: Mike Karseboom on May 09, 2015, 10:04:14 am
I have some rope light that I can dim with an elation DP-415 dimmer pack.  Due to a lack of channels, I would like to dim both the rope light and a PAR56 fixture with the same dimmer channel.  However, I need to be able to balance the relative level of the rope light to the PAR56 lamp.  The rope light needs to come down a bit from full on.


I also have a conventional incandescent 120v home depot type dimmer mounted in a self contained box with power cord.  This is the "old" kind, not like newer ones that can dim LED or CFL lights. 


Can I use this second dimmer just ahead of the rope light to set a relative level for the rope light then have the rope light dimming follow the PAR56 dimming from the control board?  I realize the dimming curves might be different but the control would be basically on or off, not slow fades or changing levels.


Just to clarify - one channel of the DP-415 dimmer pack has duplex outlets.  The PAR56 would plug into one outlet and the home depot dimmer for the rope lights would plug into the other outlet.  Then the rope light would plug into the  second dimmer. The total wattage for that channel would not be exceeded.
Title: Re: Two Dimmers in Series?
Post by: Tom Bourke on May 10, 2015, 09:35:56 am
At best it will sort of work.  Dimmers like those you mention are SCR or triac based with no to minimal filtering.  They work by turning on partway into the sine wave and then conduct till the voltage crosses zero again.  If the load is not linear they will not turn off properly.  Worst case one or both of the dimmers burns up.

In your case maybe adding a DMX relay pack to turn the second set of lights on and off may work.  You can set it to the same DMX channel as the existing pack.  If your DP-415 came with an SDC6 you may also have a couple of unused channels of DMX anyway.
Title: Re: Two Dimmers in Series?
Post by: Scott Hofmann on May 11, 2015, 07:21:52 pm
At best it won't work. At worst it will damage both dimmers.
Title: Re: Two Dimmers in Series?
Post by: Stephen Swaffer on May 11, 2015, 07:44:03 pm
Another approach would be to  use a variac or rheostat between the dimmer and the rope light-depending on the rope light wattage.
Title: Re: Two Dimmers in Series?
Post by: Mike Karseboom on May 11, 2015, 07:55:57 pm

Well I did try it briefly and it seemed to work OK on various combinations of dimmer values at both dimmers.  Nothing broke but this was not for very long and I agree that it sounds like a bad idea.


These are LED rope lights that operate on 120V. They are a "warm white" and, as it turned out, were not too bright at 100%.  I did hook them to a channel of the DP-415 dimmer pack so I could black them out with the rest of the lights at the end of a song.


Hopefully that type of connection is OK and I was not "just lucky" for the evening.  I am not sure what they do on the inside for voltage / frequency, etc.  But they seem to respond well to both dimmers individually and in series.


I don't know much about dimmers but when I put my "kill-a-watt" meter between the wall outlet and the old school dimmer leading directly to the rope light I got some strange readings for frequency.  If the dimmer was basically at zero - with no light in the rope light - the line frequency was showing about 60Hz as expected.  As I increased the dimmer to brighten the rope light the frequency reading gradually dropped all the way down to around 20Hz.  Is that normal for a old school non-LED home depot type dimmer?  Or is this a function of the Kill-a-watt giving me an invalid reading?  Very strange.  I would think frequency would always be 60Hz.  I mean what does that do to the AC line in walls if the reading at the outlet is 20Hz?


Thanks for the advice on this.

Title: Re: Two Dimmers in Series?
Post by: John Roberts {JR} on May 11, 2015, 08:49:14 pm
Most thyristor based dimmers work by only tuning the voltage on for a fraction of a waveform cycle. Stacking two in series, will not give a fraction of a fraction but ignore the dimmer that thinks it's still turned on, while the other one says no it's off. In theory they won't hurt each other, but I don't feel lucky. At best you will get the lower light output that either one commands, at worst, smoke and/or funny operation.

If you had an old school variac transformer dimmer, you could stack them on series to get a fraction of a fraction but such variacs would be large and expensive, so not likely to be seen in the wild.

JR
Title: Re: Two Dimmers in Series?
Post by: Frederik Rosenkjśr on May 12, 2015, 03:20:54 am
At worst it will damage both dimmers.

Not "at worst" - at worst it will produce a spark or flame that will burn the house down and start a forest fire.
Title: Re: Two Dimmers in Series?
Post by: Steve M Smith on May 12, 2015, 03:50:14 am
At best it won't work. At worst it will damage both dimmers.
I don't think it will work very well but I can't see how it would do any damage.


Steve.
Title: Re: Two Dimmers in Series?
Post by: Stephen Swaffer on May 17, 2015, 05:53:50 pm
http://www.allelectronics.com/make-a-store/item/sc-3m/3-amp-variable-transformer/1.html

Check out this variac-not cheap, but maybe usable at $65.  360 watts should handle a fair amount of rope light.