homemade dimmer switch?

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URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=108597
Printed Date: May 15, 2024 at 7:35 PM

Topic: homemade dimmer switch?

Posted By: twentybelow
Subject: homemade dimmer switch?
Date Posted: November 01, 2008 at 4:19 PM

Greetings,

This question has probably been asked and answered already but I cannot locate it. Here goes..

I own an RV. It has 12 volt lights located all around inside of the living space. The ones in the bedroom are too bright. How do I make a homemade 12 volt dimmer switch??

I have heard some talk of a potentiometer or maybe just splicing in an automobile dimmer switch. I was thinking about hooking up a 3 or 4 pole switch with a different resistor going to two of the legs. I have a bunch of resistor lying around but I am not sure if I am on the right track (e.g. will I burn something up)....any ideas..

Replies:

Posted By: reax222
Date Posted: November 01, 2008 at 6:54 PM

It would all depend on the current. As you add resistance your changing the current and voltage. A 6 watt bulb at 12v needs 0.5amps, e/i=r says 6ohms. If you add 6ohms to the 6ohms, then the 12ohm total changes it to 1 amp and the circuit has 12 watts. I believe the resistor would do 6 watts of work. It's been a while since I've learned this, but at my figure you would need a good resistor to not burn up.

Posted By: Mad Scientists
Date Posted: November 01, 2008 at 8:37 PM

Hmm.. how's this math look;

6w = 12v * 0.5a (calculating how much current a 6w bulb draws)

12v / 0.5a = 24ohms (calculating resistance of bulb)

increase circuit resistance to dim the bulb; add 24 ohms to half the brightness (approximately)

12v / 48ohms = 0.25 amps (calculating current draw of circuit with resistor in series)

24 ohms * 0.25a = 1.5w (figuring power rating of 24 ohm resistor)

Or.. you could build a circuit like this;

https://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#dimmer.gif

Look for '12v lamp dimmer'

Hope this helps..

Jim

Posted By: reax222
Date Posted: November 01, 2008 at 9:18 PM

Yep, I multiplied the 1/2 instead of divided like I should have. I guess I can't reload ammo and do Ohms at the same time.

Posted By: ckeeler
Date Posted: November 03, 2008 at 9:44 AM

twentybelow wrote:

Greetings,

This question has probably been asked and answered already but I cannot locate it. Here goes..

I own an RV. It has 12 volt lights located all around inside of the living space. The ones in the bedroom are too bright. How do I make a homemade 12 volt dimmer switch??

I have heard some talk of a potentiometer or maybe just splicing in an automobile dimmer switch. I was thinking about hooking up a 3 or 4 pole switch with a different resistor going to two of the legs. I have a bunch of resistor lying around but I am not sure if I am on the right track (e.g. will I burn something up)....any ideas..

CARQUEST part number HS-92 is all you need.

Posted By: oldspark
Date Posted: November 04, 2008 at 5:31 PM

A better solution which should also work on 12V fluoros is a PWM (pulse width modulated) circuit driving a FET or transistor.
See "Silicon Chip" Nov 2008 pp30-34 "12V Speed Controller OR 12V Lamp Dimmer" (for 5A motors or 50W lighting). (siliconchip.com.au)
I am building a similar circuit but tuned for 400Hz and driving the MTP3055 FET direct.