Film-Tech Forum ARCHIVE: De-rating dimmer for halogen bulbs?

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Posted by Andy Frodsham (Member # 4061) on 07-02-2007, 03:22 PM:

We are in the process of installing a dual-channel electronic dimmer (Panalogic DM260) for use in our theatre. The unit will be used to supplement our existing houselight dimmer and will be used purely for 'dressing' our screen between shows.

We are planning on using PAR 64 lanterns on the screen.

I read somewhere that halogen bulbs present a particularly difficult load for dimmer circuits and when using these bulbs one should de-rate the dimmer by about 50% when calculating the maximum load! Is this also the case when using professional dimmers like the Panalogic?

For your information, the DM260 should be able to support approximately 25 amps per channel!

Posted by Gordon McLeod (Member # 33) on 07-02-2007, 03:27 PM:

If they are not a transformer low voltage halogen then the loading to the dimmer is the same
It is the general rule of derateing if transformer loads are connected

Posted by Andy Frodsham (Member # 4061) on 07-02-2007, 03:32 PM:

Gordon, all the bulbs are mains voltage (230 volts over here in the UK).

Various electrician friends have told me that domestic (wall plate type) dimmers must be de-rated when driving halogen bulbs - even mains operated non-transformer types. Anyone know why? And would this need to be born in mind whe loading the Panalogic?

Posted by Robert Minichino (Member # 3495) on 07-02-2007, 03:45 PM:

I agree, no need to derate dimmers for mains/line voltage halogens; they're resistive loads just like ordinary incandescents. The cold current for halogens can be about twice that as incandescents, but well within the capability of an ordinary dimmer, mostly because it doesn't last as long.

Posted by Steve Guttag (Member # 268) on 07-02-2007, 06:00 PM:

In many residential applications, the halogen lamps are low-voltage lamps and as such there are transformers and such driving the lamp, thus the problem with the dimmer circuit. In your case, you are driving the lamp directly so there should not be any derating necessary.

Posted by Gordon McLeod (Member # 33) on 07-02-2007, 06:18 PM:

The reason that transformer loads must be derated is that the current and voltage isn't in phase due to the inductance of the transformer and that will cause a issue with the dT/dV (delta time/deltavovoltage) rating of the power device
It can also cause a dimmer to flicker depending on the coupling between the power device and the control circuit
Dimmers that use an optocoupler with a snubbing network typically need a different value for the two resistors and cap that make up the output section of the optocircuit
Ones that are pulse transformer drive typically are more tollerent

Posted by Mark Gulbrandsen (Member # 72) on 07-02-2007, 10:00 PM:

Ah Gord.... Does a transformer exist where the current and voltage are perfectly in phase? I don't think so.....

Mark

Posted by Louis Bornwasser (Member # 3063) on 07-03-2007, 06:03 AM:

Only if it is a non-inductive one! Louis

Posted by Gordon McLeod (Member # 33) on 07-03-2007, 03:10 PM:

All transformers exhibit phase distortion
the one I was mentioning was a pulse transformer that drives the SCRs typically that driver design is based on a voltage controled oscillator driving a UJT compared to most opto coupled designs that are a dc comparator to a sawtooth wave

Posted by Mark Gulbrandsen (Member # 72) on 07-03-2007, 04:18 PM:

Gord,

Most really decent dimmers are all digital now. Kelmar's are an exception to that in the well made dimmer department, I think they are minus the pulse tranny too if memory serves me correctly. Most of the high end dimmers use Solid State Relays that are opto-coupled to the rack computers to actually handle the load... SCR's are for the most part obsolete as far as dimmers go.

Mark

Posted by Gordon McLeod (Member # 33) on 07-03-2007, 06:02 PM:

Actually many high end dimmers still are dual SCR but in a encpsulated case like a SSR
The most interesting is the use of IGB and FET devices in power dimming

Posted by Mark Gulbrandsen (Member # 72) on 07-03-2007, 06:41 PM:

While electrically they may be drawn similar they just don't work like a pair of SCR's do. An IBGT is a fet of sorts... has a gate just like a fet does. Using an IGBT eliminates the need for an output inductor in smaller dimmers. They are mainly used in florescent dummer modules as they can be set to keep the filaments of a florescent lamps warm so you don't get the start up flashing that many standard dimmers have. This allows a wider fade range than normal.

Mark

Posted by Gordon McLeod (Member # 33) on 07-03-2007, 07:12 PM:

Actually I know several dimmer makers that use an encapsulated dual SCR block and they are discrete SCRs in the block with seperate terminations and gates