# Changing 24v to 12v dc

1. Nov 6, 2007

### sst

what is the simple way to change 24volt circuit to 12volt circuit. i have to plug a converter into a 24volt cigarette lighter to charge battery for computer. the converter only takes 12volts input and will not work with the 24volts the system has for the lighter receptical. so i have to electrically cange the voltage to the cigarette lighter to 12volts.

2. Nov 6, 2007

3. Nov 6, 2007

### sst

that may be the way i have to go...i was trying to do it without a conveter and do it through resistors in the wiring

4. Nov 6, 2007

### tc_kid

????????????? resistors wont lower your voltage, they lower current.

5. Nov 6, 2007

### Averagesupernova

They also lower voltage. Can you honestly tell me that inserting a resistor in series with a load will not decrease the voltage at the load? It will. But, it will not regulate the voltage. As the load changes, the voltage will fluctate with it. It's a pretty easy experiment to do with a small power supply or battery and a couple of reisistors.

6. Nov 6, 2007

### Ouabache

If the output of your cigarette accessory jack is 24Vdc and you want 12V for your converter, you could use a voltage regulator (such as an LM350T). This regulator will supply up to 3A. Check the 'current rating' on your converter to make sure this regulator will supply what it needs.

Last edited: Nov 6, 2007
7. Nov 6, 2007

### tc_kid

really? not according to the formula V=ir... although through energy loss i can see how but that doesnt seem like a very good way to do it.

8. Nov 6, 2007

### tc_kid

why are there 2 of this thread?

9. Nov 6, 2007

### NoTime

Yes really. Exactly by v=ir.
Since voltages sum then V_load = V_battery - V_ir

It's a good simple way to do it if the load presents a constant impedance or if the load isn't fussy about the voltage as when all you are concerned about is limiting the max current in a load.

Efficiency, regulation or allowable heat dissipation requirements in your design may require more active solutions.

10. Nov 6, 2007

### NoTime

You would have to know the details of the device you want to power.

It may be possible (or may not), but you would need the details just to select the proper resistor. For some designs you may not need a resistor and it will work just as well on 24v as 12v.

A guess most likely will destroy and possibly set your unit on fire.

11. Nov 7, 2007

### edmondng

v=ir
do a voltage divider. eg: if you use a pot then the wiper voltage changes if you put one end to voltage and other to gnd
but resistors are not the way to go since heat dissipation, r changes with heat/time
just use a 12v linear regulator you should get 12v from 24v

12. Nov 8, 2007

### sst

thanks , i will try that

there are two threads because i problably messed up putting the thread in.

13. Nov 8, 2007

### Staff: Mentor

14. Nov 9, 2007

### smn

How about a resistor / zener diode configuration?

15. Nov 15, 2007

### likephysics

If you don't want to use a regulator -
Try a Resistor + zener as smn mentioned. Add a transistor at the output.
Or
Voltage divider Plus transistor(CE buffer) at the output. The voltage divider should give an output of 12.6V, so transistor output at emitter will be 12V.
Connect the 24V to the collector of the transistor.

16. Nov 15, 2007

### edmondng

yup resistor and 12V zener should work. use a resistor value that will give the load current you want. put it in series with zener's cathode, and anode to gnd. Your zener should regulate 12v