Transformer attached lighting

[Joren_Wendschuh]

I'm slightly confused about a new lighting instrument that I've aquired. Its essentialy a 120vAC to 12vac transformer, with a single 12vlamp attached to the output of the transformer. Would the load as seen by the dimmer be an inductive load, or a resistive one?
Would the dimmer not like the transformer in there, or would it just not care?
Thanks.

[Mayhem]

Hi Joren,

It is a little late at night for me to remember the load type the dimmer sees (I think inductive).

However, I tried this once (to control pin spots) and was unable to get the dimmer to run them. Some people have told me that they can, but I wonder if that is just using a dimmer to switch the unit on and off. In which case you should use a switch pac as I am sure that it isn't good for your dimmers.

Not sure that this is of any help

[ship]

Good question. Very good question. Above my head also somewhat.


The lamp itself is resistive and as Mayhem mentions, unless the resistive load is sufficient (normally 75w) for the dimmers to work with, it won't without a ghost loading somewhere around to make them function on most electronic dimmers but not all types of dimmers especially variable resistance types. A household dimmer being a variable resistance type for the most part should work given it's quality or range. At lower rates, that dimmer also will not be sufficient to incandess the lamp without going to at least almost full first. Were that load on a SCR dimmer or of sufficient load on a resistance dimmer be it from other fixtures on the dimmer of sufficient load or it's own lamp wattage than it would be effective no matter the dimmer type or transformer. (Chopping of the sinewave verses reducing the voltage.)

On the other hand, the transformer is a inductive load as classically stated in that it induces a lag time on the lights effected by the dimmer and further disrupts phase harmonics in much a similar way to the dimmer itself which is for all intensive purposes a transformer. It is also possible that this lag, phase harmonic shift and other variables on the load side of your electronic dimmer can be harmful to it by way of extra heating onto it and other reasons. It's also possible your dimmers will conpensate for this especially if not as high of a resistive loading is in effect. Such inductive loading is evidenced by if you send a low voltage lamp to full on a light board and it's going to take a moment or three longer than a line voltage lamp to go to full by way of the transformer. The lag time is inductive loading and also measured by some amount of heat or loss in the transformer. Max out the loading in wattage on a dimmer without accounting for transformer effects on it and it's very likely you will not blow it out but overheat it.


A final factor is in the low voltage lamp itself. A common low voltage lamp requires full voltage for optimum operation of the halogen effect gas within it. Otherwise, that gas when operated at temperatures less than design will attack the pinch seal of the lamp itself an not prolong it's life. (The other than noble gasses in the lamp prolonging normal filament life will seek out and eat away at the cooler parts of the lamp thus the pinch seal.) Some question of the effects of series operation of low voltage lamps on dimmers but not a transformer, or stage lighting lamps such as in a strip light, but for the most part, your 12v lamp when dimmed will not act as per halogen lamps. They will operate at the expected halogen effect color temperature the lamp is designed for as per a normal halogen lamp, but for the most part only have the life of a incandescent lamp operated as it were over it's rated voltage. A low voltage lamp on a dimmer frequently will not be a very good long term option for stage lighting use. For a specific show sure, but other than that is not much to work with in efficiency. The life will be greatly reduced.

The volume in line voltage verses low voltage power is normally sufficient, no matter if it's voltage chopping or voltage dimming that the halogen effect on the lamp's filament is not having such problems with dimming.

[ship]

A reply from a friend of Wolf and I in response.

Hello Brian;

Re: The 12 volt lamp with the transformer.

The dimmer won't care but the transformer will.
This is a large part of what got Colortran's ENR dimmers in trouble in
Florida and 'Vegas.

The transformer depends upon it's inductive reactance to limit it's current
to safe values.
When the dimmer is set to less than full, it's chopping out pieces of the
normally complete sine wave.
This is what drives the transformers nuts as they can no longer properly
develop their inductive reactance and thus end up consuming far more power
than they were ever intended to.
Most of the more modern state of the art dimmers have various control curve
options designed to work with cold cathode loads, transformer loads, yada,
yada, etcetera.

Some transformers are rated as "impedance protected", many are not.
In many theatres operators turn off their control consoles at night but
leave the power for their dimmer racks on. In these situations there is
often enough trickle current still available to smoke some of these little
non-impedance protected transformers.
One of our local road houses smoked several rain light transformers over
night for just this reason.

Best McGiver work around:
Add a dummy load in parallel with the transformer's primary.
This will normally save you from smoking the transformers.
With the 1990 Strand CD80 racks, I was successfully utilizing dummy loads as
small as 120-S-6's, little 6 watt, 120 volt lamps although 40 and 60 watt
lamps are more commonly utilized.

Toodleoo Brian!
Ron