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#### TuscanT

##### New Member
hi guys

im tryin to figure out what type of 2 pin Rheostat i would need to variy the voltage of a 12v 15w bulb from 6v - 12v.

what type, ohm risistance, etc exactly would i need?

im not doing anything complex so i would NOT really want to use anything else i.e digital controllers

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#### ericgibbs

##### Well-Known Member
TuscanT said:
hi guys

im tryin to figure out what type of 2 pin Rheostat i would need to variy the voltage of a 12v 15w bulb from 6v - 12v.

what type, ohm risistance, etc exactly would i need?

im not doing anything complex so i would NOT really want to use anything else i.e digital controllers
hi,
It can be done, BUT its not very elegant.
A 15W lamp at 12V passes a current of 15W/12V =1.25Amp.
To drop from 12V to 6V [ie; 6Volts] at say 1.25A = 4.8R

The power dissipated by the resistor would be 7.5Watts,, it would be HOT.

Expensive:
https://uk.farnell.com/jsp/search/b...k=gensearch_001&Ntt=5R+variable+resistor&Ntx=

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#### TuscanT

##### New Member
I see

are the any other in-line (2pin) simple alternatives that i can have analoge/manual control to vary resistance?

#### ericgibbs

##### Well-Known Member
TuscanT said:
I see

are the any other in-line (2pin) simple alternatives that i can have analoge/manual control to vary resistance?

Do you have any power transistors or regulators in your odds/ends box.?

#### TuscanT

##### New Member
i believe so. the source of power is a car 12v battery. the power is regulated if i recall correctly via the relays on the car.

#### ericgibbs

##### Well-Known Member
TuscanT said:
i believe so. the source of power is a car 12v battery. the power is regulated if i recall correctly via the relays on the car.
hi,

Between the 12V car battery and the lamp you need a power transistor, mounted on a heatsink. The base voltage of the power transistor would be controlled by a standard variable resistor.

Do you have any experience with transistors?

##### New Member
A transistor is DEFINATELY the way to go. so simple to integrate. I would suggest a NPN Power transistor. If you want to continue, lemme know and I will draw you a schemmy of it to help you.

#### TuscanT

##### New Member
aside from building simple subwoofer crossovers and i only have basic knowlage of transistors etc. any schematic is welcome. all i want as aformentioned is just to have and inline variable manual control of the bulb (varying the value between 6v-12v)

##### New Member
There you go. Just connect the rheostat any way. It has no polarity.

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#### audioguru

##### Well-Known Member
The transistor will have the full supply across its base-emitter diode when the variable resistor is turned up in your circuit. SMOKE!

Mine is an emitter-follower.

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• dimming.PNG
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#### TuscanT

##### New Member
o.k

so what exact components/value/type etc would i use to do this

#### Leftyretro

##### New Member
TuscanT said:
o.k

so what exact components/value/type etc would i use to do this

Well a 2N3055 power transistor would work fine and is an easy cheap transistor to find. However like any power transistor, you will have to mount it to a heat sink of sufficient size.

As far as the rheostat I would prefer to see a fixed resistor, to limit the maximum current draw, wired to the pot for decreasing brightness from that set maximum. I'm sure someone here can 'spice' simulate the resistance values for you.

Lefty

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