disco dimming

i wondered why the 8 channel dimmer needs two feeds and why they have to be on the same phase. well now i know the answer to one of the questions, if you use two different phases the loads do a disco impersonation. why is this and why two mains terminals

 

These two issues are connected in as much as the two feeds are useful under some conditions and because of that the phases connected to each need to match.

Firstly the two feeds, The top right feed powers the cbus electronics (including the C-bus power suppy if this is a dimmer with a sourcing power supply) and the drive circuitry to the TRIAC (the component that effectivly does the dimming). The top left feed takes power to the TRIAC to be passed through to the loads.

On a big site you may want power to be dropped from some dimmers in the event of a power cut and an emergency generator starting. If you dropped the power to the TRIAC's and the C-bus electronics and power supply you could loose sufficient C-bus power being sourced to the network and the rest of your network would not function.

So splitting the feeds allows a seperate circuit (as long as it is on the same phase) to run to the C-bus power supply which allows the power to be dropped on the circuit to the TRIAC but remain on the power supply.

In most domestic cases this is not necessary as they wouldn't have a back up generator so you simply loop acrosss from one feed to the other.

As far as the 'Disco' effect is concerned the TRIAC control timing signal is based on when the sine wave on the feed to the top right terminal passes through zero crossing. So if you have a different phase actually going to the TRIAC the control is going to fire the TRIAC at what it thinks is the right point (calculated from when IT last passed through zero crossing) but the Voltage level at the TRIAC will be quite different (based on when it last passed through zero crossing - which could be 120* before or after the phase on the control). (I hope that makes some kind of sense but it would be easier to explain with a white board and a couple of coloured pens!)

The good news is that you havn't done any damage to the dimmer by connecting it this way.

 

My understanding is that the top left feeds C-Bus electronics and power supply and top right takes power to the TRIAC to be passed through to the loads.

allgo

 

allgo,
If this was true the world would be a better place...but i'm afraid our good friend uncle dick has the good oil on this one.

 

Not that it adds to the thread!

well it appears that way on mine

supply for the load is on the top right and the supply for the C-bus electronics is on the top left (on my din rail dimmer anyway)

 

Ok I was wrong!

Sorry Guys you are right I got my left and right mixed up - all the other stuff is correct though (I hope)

 

All phases, red, white and blue are 120 degrees out of phase to each other. The triac in the electronics curcuit can not see the 120 degrees out of phase. SO.... each time the dimmer increases or decreases, the two phases counter act each other at their cross point (looking at a graph of the three phase waves) therefore giving you that "disco" effect, and/or turning them off automatically and not giving you correct operation. The phases are counter acting each other.

This is very basic 1 st year trade school theory.........

This was stipulated and clearly stated in the Basic Intro Course for C-Bus!!!!!!!!!!!!

 

yep i get the deal with the phase angles although you would have thought that it would have been possible for the control signal to be phase independent and all the zero crossing and supply to be done at the TRIAC side and then you could have dimmer channels on the same unit on different phases with in and out terminals with seperate mcb's for each channel, how about that for an idea . the bit about emergency power explains the why which is what i was after i mainly do domestic so back up gen sets etc didn't occur to me