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class="style10">Headlight Dimming

>Published : 28th Aug. 2006


>I have an upcoming shoot where I need to show a parked cars headlights dimming/flickering like when a car battery is just about dead. The car is a 1998 Ford Explorer. The shoot takes place in a studio. Any thoughts on the best way to achieve this effect? Do I just hook up a double Variac rig to the existing headlights and play with the dial. I don't want to risk overvolting anything electrical in the car even if it is an old gas hog.


>Ted Wiegand

>Disconnect the headlights from the car's circuits and wire directly to a 12v dimmer and battery.

>Mitch Gross

class="style11">>Do I just hook up a double Variac rig to the existing headlights and play >with the dial.

>Simple, controllable, effective. I would absolutely go this route with the following notes :

>As you probably know, Variac's are variable transformers and absolutely will not work with DC power. Headlights (except those fancy xenon jobs nor normally found on '98 Explorers) are pure resistive loads and do not care whether they are fed AC or DC. I assume you intend to feed the headlights AC stepped down to 12volts and then lower by the two daisy-chained Variac’s.

>I would go to an auto parts store and buy a couple of sockets to plug on the backs of the headlights so you do not have to mess with the car's harness at all. If you want to have the running lights dim and flicker with the headlights, you can buy a couple of sockets for them as well.

>Wire everything in parallel and have at it.

>Note that if the headlights are high beam and low beam in one bulb, there are three wires - you only need two - you can use a voltmeter stuck in the socket of the car's harness to figure out which two give you low beam.

>There are enough magic lumps of clay in a modern car that I would not suggest feeding AC through the wiring harness itself but rather to wire the lights you need directly as described above.

>You will probably need at LEAST 2k Variac’s to handle the amperage of headlights - you might actually need 5k Variac’s if the headlights are really high wattage(unlikely, but do your math) Alternatively, you could put each headlight on its own daisy chain of two smaller Variac’s.

>A 60w headlight draws five amps at 12 volts. The primary side of the Variac won't be drawing that much, but the output of the side will be putting that out, and I think the fuses are on the load side (they should be.) I suppose you could use a 1k Variac to step down part way and then a 2k Variac to go the rest of the way? I have to think about this, but the safest thing to do, of course is to get a pair of 2k Variac’s and then you know you are safe.

>The math is easy - West Virginia or Watts = Volts X Amps

>find out the wattage of the running lights and headlights and add them all together and divide by 12 volts. The answer you get is the amperage that you will need at 12 volts - or what has to squirt out of the Variac.

>Mark Weingartner
LA based

>Or how about I sell you a few dead batteries cheap. As a favour.

>-- Steve Braker - WORTHWHILE FILMS - media production for --- nonprofits
-- +001 608-635-4040 WI USA

class="style11">>As you probably know, Variac’s are variable transformers and >absolutely will not work with DC power.

>A gaffer I know has a little rheostat device that hooks onto the load side of the fuse block in the car and attaches to the existing lighting circuit.

>It's like putting a dimmer in-line for the headlights. Works great and have used it several times but I don't know the name of the manufacturer.

>If someone doesn't know it here I can make a call and easily find out.

>Jim Sofranko

>Never mind all these transformer schemes. With modern headlights you could probably just pull the 12V bulbs out and stick 120V AC quartz bulbs into the backs of the headlight reflectors. Probably 50-to-75-watt peanut-style household halogen bulbs would be plenty bright enough for this gag. Just make sure you use the proper sockets and your wiring is well insulated.

>Depending on the style of headlight you might even be able to light the headlights from the front, using tightly focused and/or flagged or goboed beams.

>You could also just put an old battery in the car and let it run down. Do your flickering in post. Might take less time and futzing than all these sophisticated strategies.

>Dan Drasin
Marin County, CA

>Hi Ted,

>Here's an easy solution: give me your address, pay for shipping both ways, and I'll send you a headlight dimming box that I've used for years. I built this contraption back when I had time -- it consists of a resistance dimmer wired to fuses that bypass the car's existing fuses. Simply remove the low-beam fuses from the panel, and insert the bypass fuses.

>The circuit will be redirected to the dimmer, and presto, you have control.
Takes all of one minute to wire. Works with both glass fuses and blade type. It generally will give dimming control only down to about 90%; not enough resistance to dim out, but you can just pull a built-in line fuse to accomplish blackout.

>Chris Mosio

>Thought I would follow up on what I ended up doing for the Duralast Battery shoot.

>Chris, thanks for the car gag offer, but there wasn't enough time. My whole crew offered the dead battery solution, thanks Steve,Dan. I took Mark W.'s advice and purchased the wiring harnesses for connecting the bulbs. This made the wiring simple. I ended up purchasing a 300 watt outdoor lighting transformer from home depot that converts 120v to 12v as the power source. This fed into 2 Variac's which controlled the intensity of the headlights. We ended up only using only one Variac because the output light levels from the headlights never equalled 100 percent. Actually the top intensity was more like 50% output even when bypassing the Variac altogether. Also touching bare wire to 12v DC. battery contacts the bulbs would not light up 100%. Could this be a result of the third wire ( not used ) in the harness that controls the high beams? Perhaps a portion of voltage from the third wire "helps" the low beam

>Either way the gag still worked as we were working in low light levels and needed the headlight to dim down to a zero-30% range.

>Thanks for all of the offers and suggestions.

>Ted Wiegand

>Ted Wiegand wrote:

class="style11">>Also touching bare wire to 12v DC. battery contacts the bulbs would not >light up 100%. Could this be a result of the third wire ( not used ) in the >harness that controls the high beams?

>Hi Ted,

>It sounds more like you hooked up the battery across the high beam hot and low beam hot and did not connect the lamp common.

>You in put the filaments in series with each other. This put only about 6v across each.

>Richard Bakos
Studio One Inc.
25833 State Road 2
South Bend, In 46619-4736
VOICE 574-232-9084
FAX 574-232-2220

class="style11">>Also touching bare wire to 12v DC. battery contacts the bulbs would not >light up 100%.

>Thanks for letting us know how it went.

>One possible explanation for the fifty percentness of the output may be if you wired from one hot to the other hot on the bulb and not from one hot to the return,

>The filaments share a ground so they look like this


>If you hook up to the H and the N, the high beam filament lights
If you hook up to the L and the N then the low beam lights
If you hook up to the H and the L then the high and low beams both light but at approx half the voltage because they are in series (doubling the resistance)

>You might have been hooked up between the H and the L.

>You might also have been dimmer if you were using thing gauge wire between the 12v transformer and the headlights. At 12 volts, there is so much less "push" behind the current that the voltage drop from the resistance in the wire is greater...the wire might have acted as a resistance dimmer.

>Either way, if you got the shots, that is the important thing – glad it worked out.

>More nearly useless information :

>On many types of dual beam headlights, the wattage of the high beam and low beam are actually the same (or almost the same) and the high beams aren't really "brighter" as such, but since the high beam filament is closer to the focus of the parabola of the reflector, the beams are more spotted down, whereas the low beam filament is not in the focus so the beam is more flooded out.

>Mark Weingartner
LA based vfx

class="style11">>Also touching bare wire to 12v DC. battery contacts the bulbs would not >light up 100%.

>Could you have been running the high and low beam filaments in series with the ground wire floating?

>Clive Mitchell