MLV dimmer multi purpose?

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AZsparky

AZsparky

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Scottsdale, AZ
Anyone run into any difficulty using a MLV dimmer with magnetic LV recessed cans AND 120 volt incandescent halogen loads at the same time? Got a customer who has one switch leg feeding (2) 120 volt sconces with incandescent A19 lamps and (2) 4" recessed cans with magnetic ballasts and she wants a dimmer. Has a few locations set up the same way. Thoughts??? TIA.
 
141210-1630 EST

What is a "can" with a magnetic ballast? I doubt that a "can" has anything to do with your question. What is in the "can" is the import information? Is it a fluorescent or something else?

A magnetic ballasted fluorescent with only two input wires can not be successfully dimmed.

Unless RF excited a fluorescent needs electron emitters within the glass tube.

Practically these emitters need to be heated. To make a dimmable fluorescent you need a separate constant excitation to the filaments, and adjustable current to the conducting gas. Without separate excitation fluorescent bulbs use self heating of the emitters from the arc current flow in the bulb. If you lower this current to dim the bulb, then there is a point where there is insufficient arc current to adequately heat the emitters.

Also are your dimmers a three wire type, meaning require a neutral, and an additional wire for full voltage to excite the filaments?

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Not that complicated

Not that complicated

The recessed lighting is a standard 4" LV (MR16) with a magnetic transformer. The other fixtures (wall sconces) are simply 120 volt A lamps. But there is only switch leg controlling them all. So, the customer asks if we can control them both with the same dimmer. Will an MLV dimmer work with both types of fixtures?
 
There should be no issue to dim both of these together. You may not get the same level on the lights but it should work
 
Thanks Dennis. I did think that the MLV was needed for the Magnetic transformer, but that the incandescents should dim just fine, just wanted another opinion. Thanks again.
 
AZsparky said:
The recessed lighting is a standard 4" LV (MR16) with a magnetic transformer. The other fixtures (wall sconces) are simply 120 volt A lamps. But there is only switch leg controlling them all. So, the customer asks if we can control them both with the same dimmer. Will an MLV dimmer work with both types of fixtures?
Click to expand...

Dennis Alwon said:
There should be no issue to dim both of these together. You may not get the same level on the lights but it should work
Click to expand...

If the 4" LV MR16 & incandescent will not dim to similar levels, I sure as heck would not put both on the same switch. The caliber of residence where we would install LV MR16's this set up would not fly.
 
141210-2344 EST

AZsparky:

In your first post calling a normal transformer a magnetic ballast was totally misleading.

If your "cans" contain a closely coupled normal transformer with the output connected to incandescent lamps, then the input to the transformer, to a major extent, will look like a resistive load. Thus, the behavior with a good phase shift dimmer should not be much different than if an equivalent power incandescdent replaced the transformer primary.

Why don't you run your own experiment on the bench and see what happens.

I tried a three wire dimmer with a transformer, 180 VA, with a 250 W heat lamp as the secondary load. With full 120 V to the transformer the bulb was an orange-red glow, and power input to the transformer primary was 30 watts at an 0.6 power factor. With the dimmer as the source nice adjustment from full brightness to no visible output was achieved. This is sufficiently good results to indicate no likely problem.

Adding full voltage bulbs in parallel with the transformer inputs will only improve the operation. You should check that with your total expected load that dimmer RMS current is within spec. There will be some inductive current, but not much if the "can" transformer is being operated near its full load rating. Note: in my experiment that I was at about 1/6 full load of the transformer.

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141211-1300 EST

Time limit problems.

An additional point. Make sure that the DC component of the dimmer output current is negligible. In my experiment I had about 0.06 A DC with 0.25 A AC. DC added to AC thru the primary causes the magnetic circuit to be unbalanced. Thus, increasing magnetizing current. What is really important is the primary RMS current. Any phase shift dimmer driving a transformer primary needs to be evaluated for DC primary current and any increased heating of the primary.

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141211-1152 EST

Some additional experiments, and a potential big problem.

Experiments using a Kill-A-Watt EZ for most measurements.

Same 180 VA transformer as above, but with no secondary load, and no parallel load on primary side.

No dimmer and direct from AC line ---
123.1 V, 0.24 A RMS, 5.0 W, 30 VA, 0.16 PF

With dimmer, at full on, and measured at dimmer input ---
123.1 V, 0.32 A RMS, 6.3 W, 39.7 VA, 0.15 PF

With dimmer, at full on, and measured at dimmer output (transformer input) ---
121.2 V, 0.23 A RMS, 4.6 W, 28 VA, 0.16 PF

Next I set the dimmer to mid position and current went wild. Somewhere above 2.5 A for the AC RMS current component. I did not measure the DC component. A 25 W bulb on the primary side made the dimmer work correctly. Previously I had shown no problem with secondary load. I don't plan to investigate this further. I believe a phase shift dimmer can be designed to prevent this problem, and possibly some presently exist.

Since loaded there is no problem, then why worry? Because burned out bulbs could cause this problem, with resultant transformer failure.

You are better off paralleling incandescents with the transformer can lights, than separating them.

The best solution for the can lights would be Variac control, but expensive, large, and a big knob.

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I am quite sure we have done this before and it worked out fine. What I am not sure as whether or not it was a LV dimmer or just a standard dimmer. I know that some elv tranies can be dimmed with a standard dimmer - not certain about the magnetic ones.

Gar-- Lutron sells low voltage dimmers that are either magnetic or electronic.

http://digital.turn-page.com/t/23303/83
 
141211-1323 EST

Dennis Alwon:

Thanks.

I could not read the very small printing, could not expand it. Thus, did a search and this document was found http://www.lutron.com/en-US/resourcelibrary/362219.pdf . The critical point in here is that the low voltage bulb fixture have an input fuse. That should solve the problem of bulb burn out and high unloaded transformer primary current in a partial dimming position.

One error in their description of a normal dimmer was that turn off occurs at a zero crossing, that is true, but no mention was made of what zero crossing. The average reader probably thinks this means the voltage zero crossing. It really means the next current zero crossing. With a highly inductive load this means turn off does not occur until way into the next voltage half cycle. Probably causes either Triac or firing angle triggering problems.

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