Intensity vs. Current for ETC Sensor?

[mnfreelancer]

I am training some people who are less familiar with lighting control at work and pointing out that the relationship between dimmer intensity (%) and the current running on the feeders is not one of a linear nature. Someone asked the obvious question I would have asked if I were in their shoes; what is the specific nature of the relationship and where are the cutoff points? So, can someone point me to a reliable source of either a graph that shows the % dimmer intensity (on a per-dimmer basis or averaged across an entire rack) and current drawn on the feeder; or real-word (or theoretical) data to draw my own conclusions? I would like to create a more empirical graph of % intensity vs. % current based on the rack at 100% intensity.

(before someone points it out, yes I considered that in a sense I want to compare intensity to intensity as current is represented as "I" meaning intensity...hopefully I was clear in what I'm really after in the above post.)

[derekleffew]

See page 49 of the attached document for CEM classic.

OR
page 67 of this document for the CEM+: http://www.etcconnect.com/docs/docs_...anual_revA.pdf.

edit: upon re-reading your post, the above may not be exactly what you want, but I don't know of any graph of feeder currents and, as we learned in this thread: http://www.controlbooth.com/forums/q...ok-inputs.html, measuring exact feeder current, particularly on the neutral, is rather an advanced subject.

[David Ashton]

here is a short excerpt from the pre-heating debate [which got shut down just as I was winning] however it shows how you can make your own graph very easily, your meter will be a little inaccurate at low current but not enough to be significant, and a little demonstration is worth a thousand words

Please consider, a 1000 watt 240 volt T19 on a dimmer{for 120v thinkers just halve all the figures} It has a cold resistance of 3.8 ohms and a hot resistance of 57.6 ohms. A typical dimmer will feed 1.65v at "0" and drive .35A putting 2.24 W into the lamp of preheat.
@ 13V you can see the barest glow in the filament and are drawing .96A and 12.6W
@30V you can see a real glow and are drawing 1.5A and 45W
at 60V you begin to get some output but at 1/4 voltage you are drawing 2A or nearly half the "full" current
@80V which is 1/3 voltage you are drawing 2.3A which is more than half the full load current and is also the point at which the dimmer starts to interact with the other dimmers on other phases.
This is why running all your dimmers at 1/3 is the worst thing you can do to your neutrals.
hope this helps

[garyvp]

Great question. My issue with techies is that they think that during long rehearsals that they are using 50% of the power by setting the grand master at 5 when in fact they are using 80%. It takes a lot of current just to light that filament.

I have installed digital ammeters (a good idea for $1000) on each leg of my dimmers in our 3-phase system (6 legs, 2 per phase to monitor) to help techies balance the grid and to make them aware of consumption. It is also interesting tool to observe the 'cascading' nature of SCR dimmers. A single 500w lamp consumes about 4.2 amps at full glow. At a dimmer setting of 1, the current is about 2 amps, at level 7 it is almost 4 amps. The other discussions that follow are perhaps a better explanation.

[STEVETERRY]

Originally Posted by garyvp

Great question. My issue with techies is that they think that during long rehearsals that they are using 50% of the power by setting the grand master at 5 when in fact they are using 80%. It takes a lot of current just to light that filament.

I have installed digital ammeters (a good idea for $1000) on each leg of my dimmers in our 3-phase system (6 legs, 2 per phase to monitor) to help techies balance the grid and to make them aware of consumption. It is also interesting tool to observe the 'cascading' nature of SCR dimmers. A single 500w lamp consumes about 4.2 amps at full glow. At a dimmer setting of 1, the current is about 2 amps, at level 7 it is almost 4 amps. The other discussions that follow are perhaps a better explanation.

The ammeters are a great idea for any derated feed to a dimmer-per-circuit system!

And of course, they are True-RMS-responding ammeters, right?

ST

[garyvp]

They are clamp based Murata digital RMS meters - the readings match my RMS Fluke clamp. The observed change in amperage across the 3 phases, and especially the oversized neutral, highlight the effects of subtle changes in the local line voltage and the non-linear behavior of the SCR dimmers.

[David Ashton]

There may be a misunderstanding here, it's not the dimmers which are causing this very non linearity, it's the lamps, and the low resistance at low voltage which increases the current.Remember that the cold resistance is 10 times less than the hot resistance and this is what causes this distortion of the curve.You get the same effect with any type of dimmer.

[mnfreelancer]

Originally Posted by allthingstheatre

There may be a misunderstanding here, it's not the dimmers which are causing this very non linearity, it's the lamps, and the low resistance at low voltage which increases the current.Remember that the cold resistance is 10 times less than the hot resistance and this is what causes this distortion of the curve.You get the same effect with any type of dimmer.

I realized this when I looked at the document Derek cited that contained voltage curves. I stopped and thought about how lamp filament resistances change with temperature and how that directly affects current due to ohm's law...

[derekleffew]

See here.

I may now have to buy a Variac and conduct this experiment on my own. Where's my Kill-A-Watt and Amprobe?

Danger. Electricity kills. When in doubt, consult a qualified professional.

[David Ashton]

Of course, one should mention that those using carbon filament lamps will have no problem as the resistance goes down as the temperature increases, all we need now are some theatre grade carbon filament lamps.