Converting 24v to 12v for truck accessories

[Peter Pan]

What is the easiest way to convert 24v to 12v without buying a store bought do-hicky. I am running into a problem trying to power things like cd players and lights on my truck. I would be greatful for any advice

 

[chroot]

A resistor is undoubtedly the easiest way, though not at all the best.

What kind of current do you need to pull at 12V?

- Warren

 

[Peter Pan]

I need it to be DC. Would a resistor just burn of the 12 unneeded volts as heat?

 

[chroot]

I figured you wanted DC...

And no, you don't "burn off" volts, you burn off power. If you only need a small amount of current, a resistor might be a reasonable solution. If you need a significant amount of current, you'll need to use a more sophisticated approach.

- Warren

 

[Peter Pan]

i need about 600 watts.

 

[chroot]

That's an awful lot. You're going to have use a regulator. I suggest just buying a "store bought do-hickey."

- Warren

 

[cider_drinker]

Under no circumstances would I suggest a resistor drop for items such as CD players. The power requirements for such devices is not continious, and the rated power of the devices, e.g. 1A, is the maximum, not continious. A CD player requires more power when the motor is going, for example when a cd is present. When no cd is present, the power requirements drop off, and so does the current.

If a resistor was used, when the current drops off the voltage dropped by the resistor decreases, and the cd player will see more voltage! POOF, there goes the cd player.

For 1 Amps or less use a 7812 3pin regulator. Should be able to get these from Radio Shack in the US, or Maplin or RS in the UK. For bigger loads, use a commercial converter.

 

[dduardo]

cider_drinker, yes a 7812 would do the trick.

Peter Pan, if you do decide to use a 7812, don't forget to add a capacitor to the output, you don't want any feedback from the motor to break the IC chip.

 

[chroot]

There are a billion and a half linear IC regulators that can source < 5 amps. That's great and all, but Peter Pan needs TEN TIMES THAT MUCH CURRENT.

- Warren

 

[cider_drinker]

More than 5A for a CD player, unlikey.

I note Peter that you mentioned lights. Are these 12V spots on the outside of the truck. If so, then you can wire two similar rated lights in series and connect them directly to the 24V supply. If they are rated the same, then they will have similar resistance, and so will work as a voltage divider, one light being the reistor for the other, both dropping 12V across their terminals, and all the power going into lighting up the road!

Note that the lights must be of a similar power rating, i.e you can group 2 spots of the same type in series. If you need I can supply you with a wiring diagram. This way you don't need anything at all, just a diferent wiring configuration.

NB, you can't do this with two CD players or other types of 'intelligent' electronics, just with two passive loads.

 

[cider_drinker]

PS, the capacitor on the output of the 7812 is to supply any sudden power requirements from whatever is connected to it so that the voltage does not fall off, not to prevent damage to the chip! I.e. is surpresses ripples in the supply. And by motor, hope you mean the CD motor dduardo.

 

[Peter Pan]

thanks cider! I have already bought a store bought do-hickey from radio shack. It works well, but i still have the reciept. I would be greatful if you worked up a wiring diagram.

I do have one concern though. My buddy tried hooking up lights the same way(in series). They work, but the battery dies all the time. Is this something I sould be worried about?

 

[Cliff_J]

What battery is this?

Wiring in series is going to be the most efficient method to hook up the lights, anything else will have losses that do anything but create more light.

Cliff

 

[flexifirm]

High current 12V regulator

I don't suggest the resistor idea either (I really don't know how that works, nevertheless, i don't think it will either)

You can't get high currents (over 1.5A) 'from' a 7812.

But! You can get high currents 'using' a 7812 (even 50A).



Just use a bunch of pnp power transformers to turn on when the current goes near the 7812's limits. These "pass" transistors will supply the additional current.

Put a resistor before the input of the 7812 (choose it's value so Imax * R = turn on voltage for the transistors).

Now connect the left end of the resistor to all the emitters of the pnp BJTs and connect the right end of the resistor (the input of the 7812) to the bases of the BJTs. Now all the collectors of the BJTs are connected to the output of the 7812.

There you go, I've seen as many as 7 PNP power transformers used over the same regulator.

Have fun and remember to wear your safety glasses.

 

[cider_drinker]

Wiring same rated lights in series is the way to do it. .. But, the alternator or generator on your truck can only supply a limited amount of current, and if you are drawing more current than the alternator can supply while the engine is on, the rest of the current will come from the battery, draining it even if you have the engine running.

It is common sense that any large loads will discarge the battery quickly when the engine is not running. Your friend must have one of the following conditions for the battery to become discharged.

- A very large current draw from the lights, more than the alternator can supply,

- Running the large load when the engine is switched off,

- Has a problem, e.g. an exhausted battery ( because some battery power is required before an alternator can generate current), a faulty alternator, or faulty regulator on the output of the alternator.

How much current do the lights draw, or what wattage are they?