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How would you make a 12V light dimmer? Answered

I have a portable 50W light that is run off a 12V 7Ah battery. In some cases the light is too bright so i was wondering what parts i would need to make a dimmer for it.

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mathews
mathews

11 years ago

Build an oscillator and use it to trigger a transistor base. Connect the emitter to ground. Connect the light between the power supply and the collector.

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steveastrouk
steveastrouk

Answer 11 years ago

An oscillator won't hack it. It'll need to be PWM.

Steve

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mathews
mathews

Answer 11 years ago

If you make a relaxation oscillator, and use a potentiometer and a pair of diodes. This way you can define the frequency using half of the pot value as the resistance in the calculation, and then the mark/space ratio can be adjusted with the pot.

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steveastrouk
steveastrouk

Answer 11 years ago

Neat solution.  Nice lateral thinking.

I know exactly what you mean, but the OP probably won't, and the 555 will be pressed into action again.

Steve

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mathews
mathews

Answer 11 years ago

How about a circuit diagram.

fig3.JPG
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RavingMadStudios
RavingMadStudios

11 years ago

Variable resistor/potentiometer - knob or slider style, with a "click off" position.

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somerockenguy
somerockenguy

Answer 11 years ago

Could you give me a radio shack part number on the correct one to use and how to wire it? also the light that i have can power bulbs from 50w to 100w so i was wondering would the potentiometer be able to be used for a 100w bulb max but still be able to do a 50w bulb? or would the light with a 50w bulb require a diffrent potentiometer then it with a 100w bulb

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RavingMadStudios
RavingMadStudios

Answer 11 years ago

I'm not finding anything at Ratty Ol' Shack that is rated for your application. That's probably a good thing, because the other guys' suggestions are much better than the potentiometer anyway. I was not considering the battery drain aspect that frollard pointed out. That's why he gets all the hot chicks.

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steveastrouk
steveastrouk

11 years ago

You can "roll your own" or you can buy a Velleman kit off the shelf online.

The neatest solution I know of  uses a MIcrel MIC502 and a single transistor.

Steve

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frollard
frollard

11 years ago

As ravingmad suggests, you can use a potentiometer but the downside is you are burning off excess power in the resistor - your battery will die just as fast but your light will be dimmer.

A better, slightly more complex solution is to use pulse width modulation.  This involves flashing the light on and off hundreds or thousands of times per second, varying the ratio of time between on and off.  This is roughly how a 120/230 volt light dimmer switch works.  It will be far more efficient, because the switching circuit is only ever on or off, never halfway (resulting in wasted energy).
The only difference between what you need and a small led pwm driver is the size of the switching transistor.  Your 50 watt light will need a fairly heavy transistor (capable of at least 50/12 = 4.2A @ 12 volts.  I would recommend one capable of at least 5-10 amps.
The pwm circuit can be as simple as an oscillator, a 555 timer, or as complex as a microcontroller (pic, avr, arduino etc).  This site has TONNES of circuits - search for pwm.  The top related ible is perfect:  https://www.instructables.com/id/Very_simple_PWM_with_555Modulate_every_thing/