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Low Voltage Dimmers

Published : 4th December 2006

You can use a simple hand squeezer. A hand squeezer dims by adding resistance to the line. A variac has a transformer in there. I've used simple hand squeezers for the 12v globes run from a battery belt. I also understand that is what most reality shows do when the have a walk & talk. [with a ball on a boom or something]

Just my two cents.

Jared Hoy
Gaffer || Best Boy
Los Angeles, CA


Does one need to use a low voltage dimmer when dimming a 12 volt halogen bulb from a 12 volt DC source? Are those slightly pricey low voltage dimmers only necessary when you are starting with a 120 AC source to a transformer, regulator, or whatever it is that out puts the 12 volts to the 12 volt bulbs ( as in track lighting)?

Can I just use a cheap and compact squeezer to dim a 12 volt bulb from a battery source?

Nils Kenaston
DoP DIYer NYC, LA


class="style15">>>Can I just use a cheap and compact squeezer to dim a 12 volt bulb >>from a battery source?

Nils

Any dimmer will dim the 12v bulbs. The problem with 120v dimmers is you can overcrank them and destroy the bulb quite easily. I prefer to use a variac that has a big dial on it with corresponding numbers that I can easily tape off to prevent over-voltage.

The problem with a cheap house-hold dimmer-"Squeezer" is that it is a small dial not easily controlled with any accuracy, and therefore prone to failure.

Mike Ambrose
Gaffer
LA Based


class="style15">>Does one need to use a low voltage dimmer when dimming a 12 volt >halogen bulb from a 12 volt DC source?

In a 120 volt AC configuration it is necessary to maintain the transformer integrity properly. You sound like you want to dim one from a 12 volt source with no AC is that correct?

If so that is not the kind of dimmer you need.

Walter Graff
BlueSky Media, Inc.


Nils Kenaston writes :

class="style15">>Does one need to use a low voltage dimmer when dimming a 12 volt >halogen bulb from a 12 volt DC source?

Dimming a DC load run from battery power requires a PWM dimmer (Pulse Width Modulation) which is completely different from a mains style PAC dimmer (Phase Angle Control). I'm not sure who does these, but they must be available. I'm guessing that camper shops might do a version for caravans?

I really should make a simple DC dimmer kit some time. I toyed with doing one in the past that offered a simple dimming mode (up, down and store) and also some effects like candle glow simulation. I did a dedicated dimmer module like this for the "mask of knowledge" on the BBC series Shoebox Zoo.

Clive Mitchell
http://www.bigclive.com


Those cheap compact AC dimmers, are electronic dimmers. They do not work on DC.

A much more expensive autotransformer dimmer, (often called a Variac, after one of the brand names) ALSO does not work on DC.

There are rheostats (which are variable tap resistance dimmers) which will work on DC...they are sometimes used as dimmers in news station video vans like the ones Wolf Coach used to build)...they work on DC. There may also be some DC dimmers available, but the household ones definitely want to work with AC, not DC. You might try an RV (recreational vehicle) supply place for DC dimmers for 12v. lighting.

Mark Weingartner
LA based VFX


class="style15">>Those cheap compact AC dimmers, are electronic dimmers. They do >not work on DC.
>A much more expensive autotransformer dimmer, (often called a >Variac, after one of the brand names) ALSO does not work on DC.

I have used the Vari-Ac dimmers with 12V "grain of wheat" bulbs that are commonly used in model train sets--although commonly controlled by DC the bulb itself doesn't care whether you feed it AC or DC just like any light bulb--therefore it can be done--however if 12V batteries are the source power--than a DC dimmer will be required.

Mike Ambrose
Gaffer
LA


It's been mentioned many times here but just for the record, you can gang two Variacs in line together for finer dimming control when using AC. This is especially useful in low voltage situations. Set the first one near the voltage you are using and tape it off. The second one will fine tune with greater latitude than one dimmer alone.

Jim Sofranko
NY/DP


class="style15">>two Variacs in line together for finer dimming control when using AC.

Excellent point! I had forgotten about that technique.

Thanks Jim!

Mike Ambrose
Gaffer
LA


class="style15">>those cheap compact AC dimmers, are electronic dimmers. They do >not work on DC.A much more expensive autotransformer dimmer, >(often called a Variac, after one of the brand names) ALSO does not >work on DC.

When using 12V (or 30V) source you usually are mobile (like in a long walk and talk) and carrying a Variac is troublesome (even if its works with DC). I've been looking for 30V DC dimmers at rental houses for a while and no one seems to have them. So I'm looking in making one.

The best and most power efficient (low heat created) way is through PWM (Pulse Width Modulation) technology. What it does is pulse the light (or motor) at a really fast pace (800MHZ) to reduce brightness or speed (motor) to a DC load. Isn't this the hertz KinoFlo ballasts "flicker" at? (making them flicker-free) The filament has such a relatively long decay time (for it to not give light) that to our eye and hopefully to video/film it is a continuous light source. With tungsten this mean dimming without shifting colour temperature. PWM is used by most DC lights like Anton Bauer and Frezzi that dim tungsten filaments.

Anyone have any experience with PWM dimming and film? I plan using it with a Mole Teenie Weenie (with a DYG "30V 250W") with a Chimera Lantern, and a 30V belt like used with Jokers and readily available.

Possible pre-made kits:

$18, says its for 12 or 24VDC (will 30VDC burn it up?)

http://store.qkits.com/moreinfo.cfm/MX033

$40, for any voltage up to 36VDC (great for our 30V belts) Also the fact the "pot" isn't attach gives me more versatility to place it)

http://cgi.ebay.com/PWM-DC-Motor-Speed-Control-robotics_W0QQitemZ6055017407QQcategoryZ2563QQrdZ1QQcmdZ

ViewItem#ebayphotohosting

Roberto Schein
Chief Lighting Technician
www.robertoschein.com


class="style15">>The best and most power efficient (low heat created) way is through >PWM (Pulse Width Modulation) technology. What it does is pulse the >light (or motor) at a really fast pace (800MHZ)

It most certainly doesn't happen at 800MHz. It's more like a few hundred Hertz to maybe a kHz for tungsten loads. The colour temperature does not remain constant when dimming with PWM. At the reduced duty cycles the filament loses temperature and drops in colour temperature just as with a normal dimmer.

You may be thinking of the sustained torque experienced with pulse width modulation of motors.

Clive Mitchell
http://www.bigclive.com


It most certainly doesn't happen at 800MHz. It's more like a few |hundred Hertz to maybe a kHz for tungsten loads. The colour |temperature does not remain constant when dimming with PWM. At the reduced duty cycles the filament loses temperature and drops in colour temperature just as with a normal dimmer.

I misread, most work at 800Hz (like the one I just bought from eBay). Is this high enough of a freq to be flicker safe? This article makes it seem that PWM gives less of a colour temperature shift than rheostat dimming, but it is talking about dimming 50W to 20W output with just a shift from 3200 to 2900 Kelvin. On a 250W globe, if I could dim over a full stop and only loose 300 Kelvin I would call this a success.

Article on building a PWM dimmer for on-camera light
http://www.ctshooter.com/dimmer.html

Roberto Schein
Chief Lighting Technician



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