Calling all electronics technicians! Need help!

[HondaCop]

I am going to use automotive, 12v bulbs for my MIP's backlighting. The bulbs are these: http://store.qualitydist.net/gel-194na.html

My question is...

1. Seeing that these bulbs are 3.78 watts, do I need to make sure that the potentiometer I use to control the voltage into the bulb is at LEAST a 4 watt potentiometer?

2. How many Ohms should my potentiometer be?

I was looking at these potentiometers: http://www.mouser.com/Search/Refine.aspx?N=254355+4294576007 but I cannot figure out which one would do the job for me.

BTW, the bulbs will be assembled parallel to each other.

Thanks for the help!

[Mike.Powell]

I suggest against using a potentiometer to dim your bulbs. It's much better to use a dimmer circuit which is basically a variable power supply. There are several simple circuits that are easy to build, for example see http://www.reuk.co.uk/LED-Dimmer-Circuit.htm Alternatively you can buy something ready built. I haven't used this, but from a once over the specs, it looks like it would do the job http://www.ecolightled.com/product/pwm_led_light_dimmer_knob/pwm_led_light_dimmer

[davek]

I bought one of those (the last link) and they work well. I am using it for LEDs.

[HondaCop]

I suggest against using a potentiometer to dim your bulbs.

Hi Mike,

Why would you recommend against it? What are the pros and cons of using a potentiometer? If you were to recommend one, which one should I use? Just asking so I can learn. ;)

[kiek]

with a potentiometer you are waisting electrical energy (you transform the electrical energy into heat).

with a variable power supply you only use the energy that is needed.

[HondaCop]

with a potentiometer you are waisting electrical energy (you transform the electrical energy into heat).

with a variable power supply you only use the energy that is needed.

Thanks for clearing that up, Kiek. Now I have another question, which I know its so silly, you guys will be laughing! :D

Can I use one of those wall dimmers found in homes? Let the laughing begin! :p

Also, will this dimmer work? http://cgi.ebay.com/Dimmer-switch-for-12v-LED-yacht-lighting-dim-DC-lights_W0QQitemZ130320092922QQcmdZViewItemQQptZBoat_Parts_Accessories_Gear?hash=item1e57aecefa&_trksid=p3286.c0.m14

Even though it says it's for LED lighting?

[Mike.Powell]

Hi Mike,

Why would you recommend against it? What are the pros and cons of using a potentiometer? If you were to recommend one, which one should I use? Just asking so I can learn. ;)

There are multiple reasons. Kiek is correct in that a potentiometer will waste energy. Additionally, except for small power loads, it's more expensive. There are performance issues like linearity and stability that are probably not a big concern here, but have resulted in using a variable resistor as a dimmer or motor speed controller becoming a generally abandoned approach.

Variable resistors designed to handle large amounts of power are called rheostats rather than potentiometers. Once the power handling capacity increases beyond several watts, it's cheaper to use an electronic dimmer.

A house light dimmer is designed for use with 120 volt AC lighting circuits. Depending on the dimmer you might be able to use it with a transformer to drop the voltage to something your bulbs could use. I really don't know. Just to safe, I'd stick with a dimmer designed for 12 volts.

[HondaCop]

There are multiple reasons. Kiek is correct in that a potentiometer will waste energy. Additionally, except for small power loads, it's more expensive. There are performance issues like linearity and stability that are probably not a big concern here, but have resulted in using a variable resistor as a dimmer or motor speed controller becoming a generally abandoned approach.

Variable resistors designed to handle large amounts of power are called rheostats rather than potentiometers. Once the power handling capacity increases beyond several watts, it's cheaper to use an electronic dimmer.

A house light dimmer is designed for use with 120 volt AC lighting circuits. Depending on the dimmer you might be able to use it with a transformer to drop the voltage to something your bulbs could use. I really don't know. Just to safe, I'd stick with a dimmer designed for 12 volts.


Great stuff, Mike! Thanks! I think I will follow your electronic dimmer approach and find me one of them dimmers on eBay.

And I thought it would be simply a matter of using a potentiometer to control my back lighting! hahahah Gotta love this hobby, you learn something new EVERY DAY! :)

[fweinrebe]

I am using 220V dimmers on 12V 50W downlighter bulbs in my house. The dimmers are on the 220V side of the 220V/12V transformer. Some are 500W max and the other types are 800W max output.

You should be able to buy these at your local hardware shop.

I would rather go for LED's since they draw less current and are less prone to fusing. There are a LED dimmer circuit on Mike's website that you can try out.