Can one dimmer switch feed multiple relay contacts and lights?

[electricus]

Want to use diagram 1 instead of 2 to save on dimmer switches needed, I did not see a problem with this, does anyone else?

Also not sure how to draw a dimmer switch, anyone know?

 

[Dennis Alwon]

I would not have a dimmer switch before the relay. How many watts are you dimmer and what kind of load.

 

[cmreschke]

Would a dimmer even work on a relay/contactor? I would think it would act the same way that a regular dimmer hooked to a flourescent fixture would act. Once voltage is dimmed to a certain level it will shut the coil off, and release the contacts shutting the fixture off. Imo I just don't think that picture 1 will work the way that you want it to.

 

[raberding]

seems like diag 1 would be fine...think of it as a multiple-lamp luminaire; and the relay contacts are a switch on the luminaire

 

[Besoeker]

Want to use diagram 1 instead of 2 to save on dimmer switches needed, I did not see a problem with this, does anyone else?

Also not sure how to draw a dimmer switch, anyone know?

View attachment 5976

Dimmers are usually devices that control the voltage going to the lamp. In the first circuit you presented the dimmer could be anywhere from fully off to fully on when you close the relay contacts. The dimmers might not care for that particularly with incandescent lamps.

 

[electricus]

I would not have a dimmer switch before the relay. How many watts are you dimmer and what kind of load.

100 watts x 3 lamps = 300 watts on 600 watt rated dimmer. Load = incandescent (LED in the future with LED rated dimmer)

Would a dimmer even work on a relay/contactor? I would think it would act the same way that a regular dimmer hooked to a flourescent fixture would act. Once voltage is dimmed to a certain level it will shut the coil off, and release the contacts shutting the fixture off. Imo I just don't think that picture 1 will work the way that you want it to.

Please look at the diagram again, the dimmer load (switch leg) wire is going through the relay contact, not the coil, the contact would basically be like having a single pole switch down stream of the dimmer.

seems like diag 1 would be fine...think of it as a multiple-lamp luminaire; and the relay contacts are a switch on the luminaire

Good point with the "multile-lamp fixture...switch on luminaire" comment, that is exactly what it would be like

Dimmers are usually devices that control the voltage going to the lamp. In the first circuit you presented the dimmer could be anywhere from fully off to fully on when you close the relay contacts. The dimmers might not care for that particularly with incandescent lamps.

Please correct me if I'm wrong, I thought dimmers controlled the lamp brightness by chopping the sine wave to be more of a square wave at the peaks. Would this then reduce RMS voltage?

Turning on the incandescent lamps: 1 lamp on, then 2 lamps on, then 3 lamps on should not affect the voltage of the circuit, even it is less that the normal 120 vac due to the dimmer. The lamps are wired in parallel, voltage stays the same, current will add every time another lamp is turned on.

 

[Besoeker]

100 watts x 3 lamps = 300 watts on 600 watt rated dimmer. Load = incandescent (LED in the future with LED rated dimmer)

Not a good arrangements with incandescent lamps. See below.

Please correct me if I'm wrong, I thought dimmers controlled the lamp brightness by chopping the sine wave to be more of a square wave at the peaks. Would this then reduce RMS voltage?
Most dimmers are "leading edge" in that they chop a bit out of the earlier part of the sine wave on each half cycle like this:

There are trailing edge dimmers that work by chopping off the later part of the sine wave. And yes, both work by reducing the RMS voltage.

Turning on the incandescent lamps: 1 lamp on, then 2 lamps on, then 3 lamps on should not affect the voltage of the circuit, even it is less that the normal 120 vac due to the dimmer. The lamps are wired in parallel, voltage stays the same, current will add every time another lamp is turned on.

Steady state, that's true. But what that misses is the inrush current taken by an incandescent. I've just measured one. The cold resistance is 18 times lower than what the hot resistance would be with the lamp operating at its rated power. The inference is that inrush current at rated voltage would be 18 times higher at the point of switch on. The semiconductors in the dimmer might not like that.

 

[electricus]

Besoeker:
Steady state, that's true. But what that misses is the inrush current taken by an incandescent. I've just measured one. The cold resistance is 18 times lower than what the hot resistance would be with the lamp operating at its rated power. The inference is that inrush current at rated voltage would be 18 times higher at the point of switch on. The semiconductors in the dimmer might not like that.

You make a valid point, one I didn't consider, the inrush. With that being said I will plan to have a dimmer switch after each relay contact controlling the light or lights in parallel on that branch (line). On that note one more question:

Would it be a problem to have the dimmer turned on all the time set to a reduced setting say 60%, and then switch the relay contact feeding it on (closed) and off (open). With this scenario there is still inrush, but there will always be even if the switch in the dimmer is used to turn the lights on/off. Will it make a difference if the power feeding the dimmer is being turned on/off instead, like in diagram 2? I agree diagram 1 is not usable, is diagram 2?

 

[Besoeker]

Would it be a problem to have the dimmer turned on all the time set to a reduced setting say 60%, and then switch the relay contact feeding it on (closed) and off (open). With this scenario there is still inrush, but there will always be even if the switch in the dimmer is used to turn the lights on/off. Will it make a difference if the power feeding the dimmer is being turned on/off instead, like in diagram 2? I agree diagram 1 is not usable, is diagram 2?

The short answer is that I don't know. But I think the arrangement in diagram 2 ought to be OK.
If you have a power outage you would not normally expect dimmers to be destroyed on restoration of power regardless of how they were set prior to the outage.
As a company, we make dimmers, mostly for the high end residential and commercial sectors. I know that those we have in our offices fade up or down depending on what scene is selected from the control plate so voltages don't change abruptly. I think it's the same on restoration of power which is comparable to what your diagram 2 does. And, logically, it's what needs to happen. And I'm pretty sure it does for our stuff.

My niche is mostly industrial and I don't often get involved with lighting projects except when there is a power system problem - and then having to fire fight after the event. Talk to me sooner guys and........rant over.

I'm sorry that it isn't a definitive answer but I'll check with one of my guys tomorrow.

 

[curt swartz]

I don't understand why switching the incandescent lights on an off with a relay would damage the dimmer. Most dimmers have a simple switch connected in series for the on/off function. How would relay contacts be any different? The dimmers circuitry is obviously designed to handle these current spikes. 99% of lamp replacements at businesses is done with the circuits on. Screwing an incandescent lamp in to an energized socket would be about the same as a relays contacts closing.

 

[electricus]

Besoeker:

Thanks for the short answer, I understand, and will have to try the circuit out and see how it holds up over time. I'm not using electronic dimmers, just the old fashion click on/off rotary knob style.

Curt:

Good point with the lamp replacment example. Your point about the relay contacts being just like the small switch inside the dimmer, is that same one I was trying to point out earlier. In theory it seems that this is the same and there should't be any problem. Time will tell once I get it wired up and try it.

 

[Besoeker]

Besoeker:

I'm not using electronic dimmers, just the old fashion click on/off rotary knob style.

Unless it's a (bulky) rheostat, it's electronic.