[lordwispa]

I'm trying to put a variable resistor in a car lighter circuit and need to know the specifics, I want to be able to set the lighter to any temperature from naught to red hot. The resistors I tried have either burnt out or stopped the voltage entirely.

[Credendovidis]

Hello lordwispa

The problem here is that the supply voltage is only the 12 Volt from the battery. And at 12 Volt, to get a cigarette lighter warm you require at least several Amps (say 5 to 6 Amps).
To lower that current your variable resistor should be in the range of maximum 10 Ohms only.
But - and that is important : that resistor shoud be wire wound and of the 50 Watt type. And it will get warm indeed, as it has to dissipate at all the energy you do not want to go to the cigarette lighter !

A better solution would be a voltage switcher, like the ones in power supplies, where voltages of up to hundreds of volts are powered down to 5 and 12 volts without hardly any heat loss. That is because these do not function on dissipating heat, but on time switching of a supply voltage.

Explanation of that : imagine you have a 12 volt car battery. You mount a switch in the line to a cigarette lighter. Than you start switching on-off very fast. Depending on the ratio On versus Off the voltage at the lighter will reduce in efficiency to any value below 12 volt. Without any heat, other than in your fingers that operate the switch. In reality several other components are mounted too, like an electrolitic capacitor and some electronic circuitry, to keep the outgoing voltage more stable.

Hope this helps.

:)

[KISS]

I agree with C... in some respects. Lighter has 10-15A fuse, so R=V/I. So, I'd estimate R to be 12/10 or less than 1.2 ohms. I might reduce C's value of the variable resistor to 1 to 5 ohms. P=V^2/R; at 1 ohm, that's 200 W.

These are estimates. A 0-12V 10 Amp regulator switching or non-switching won't be easy.

However, this part may work: Jameco Electronics Education & Hobby: VELLEMAN INC.: K8004

It's a Pulse Width Modulator good for 6.5 amps. It could be modified easily for higher currents. Most likely by changing the FET snd the sense reistor network. You vary the duty cycle applies to the resistor and that changes the temperature. It's in kit form and works well.

As long as the lighter is not powered when the ignition is off you'll be OK will techniqies. Only the resistor technique will use no power unless the lighter is on. The terms are adjustible resistor which usually has a band that changes the resistance value or rheostat. Both are going to be BIG components and both will heat up. The last high powered rheostat I replaced was about 3.5" dia x 1". I have some 8 ohm 200 W fixed resistors which are about 8" long and 3/4 dia. They could eaily be in adjustible design. And for fun, I bought a 0.5 ohm, 1000 W adjustible resistor. It's more like 1.5 foot long and 3" in diameter. Sources US - Electronic Components Distributor | Newark.com

Actual measurements of the current will almost require a DC clamp ammeter. (easiest), but an estimate of 5-10 Amps is resonable. Resistanceof the lighter may change with temperature.