variable relay

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jkantelli

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I am trying to figure out how to mirror a manual resistor with a relay. He is my issue. I currently have an 12volt light switch currently supplying power to 2 1157 automotive bulbs you can adjust the brightness by turning the knob on the switch. The bulbs are in the instrument panel of a car and currently has 2 1157 12 volt bulbs.

I would like to install a relay that would mirror the output of the switch so I can install additional bulbs without burning out the switch. So when the brightness is adjusted on the switch the additional bulbs are also adjusted.

I currently have a bosch relay installed, but no variable brightness.
 
You wouldn't use a relay. You'd use a transistor instead. What electronics parts places do you have access to so we can recommend a suitable transistor?
 

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Too many sites called "electronic warehouse" for me to guess. About the only thing at RadioShack that would work is the old 2N3055. How many extra 1157 bulbs do you want to drive? A 1157 is a dual filament bulb; which filament is going to be on the dimmer? The 27W, 8W, or both?
 
Then the 2N3055 on a heatsink would work fine for that. Don't forget to resize the fuse for the extra current draw from the 3 8Watt lamps (2.5 amps extra. Includes a 0.5A margin).
 
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Hi,

Just a note, if you use PWM you could get away with much smaller heat sinks,
or maybe even no heatsink. It's more complex but may be worth it.

Also, if you use transistor drives for the additional bulbs (good idea btw)
then you should drive ALL of the bulbs that way, including the old existing
bulbs. This way they all get the same voltage quite easily.
 
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Unless the dimmer pot has 3 terminals available, jkantelli will have to have at least one bulb connected directly to the dimmer.
 
actually there are 5 mini 12volt bulbs in an instrument panel. What size heat sink i am I looking at? will a TO-220 Heat Sink from radio shack work?

whats a pwm?
 
PWM = Pulse Width Modulation, by turning the power to the lamp on and off very quickly the brightness can be controlled. The width of the pulse (ratio of on time to off time in each pulse) is changed. It is a more efficient way of controling power to a load.
 
jkantelli said:
actually there are 5 mini 12volt bulbs in an instrument panel. What size heat sink i am I looking at? will a TO-220 Heat Sink from radio shack work?
whats a pwm?
Click to expand...
How much current the 5 mini 12volt bulbs draw will determine what sized heatsink you'll need. What is the part number of the mini bulbs? I doubt that the TO-220 Heat Sink from radio shack will be large enough.
 
Use the car chasis as a heat sink. Get the correct insulator kit for your transistor and bolt it to the metal work.
 
using PWM is a good idea and relatively easy to make using a 555 timer IC driving the 2N3055, mosfet would be even better for power eficiency and producing less heat that needs disipating
 
kchriste said:
How much current the 5 mini 12volt bulbs draw will determine what sized heatsink you'll need. What is the part number of the mini bulbs? I doubt that the TO-220 Heat Sink from radio shack will be large enough.
Click to expand...

The mini bulbs are 1895s and are original. They use the light switch for power.

so what would a pwm setup look like?

my main concern would be how much heat. The wires are all bunched together and I do not want to melt and short any other wire in the harness.
 
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I like Gordz idea. Get a piece of angle aluminum and mount the transistor to it with insulating hardware to prevent the collector from shorting to GND. Then bolt the angle aluminum to the firewall, metal dash, etc as a near infinite heatsink.
Depending on your skills, PWM can be very easy or impossible. If you are comfortable with soldering ICs, etc to a piece of proto-board, then something like the circuits on this page could be made to work with a few modifications.
 
you could produce a simpler and more compact circuit with a 555 I'll try and remember the diagram if your interested

EDIT: I hadn't got as far as the second diagram thats about it
 
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