Help with building a 24 volt to 12 volt 1amp converter

[goatboynsac]

I have looked all over the net but can seem to find a good enough converter. Im looking for a circuit to pull 12 volt 1a through a 24 volt truck ignition system. And it has to be the ignition system. Preferably a circuit that wont get too hot and wont need a large expesive heat sink. Cost is very important in this situation.
Thanks for all your help.

[Chris Smith]

I assume your trying to reduce your already 24 volt system, down to 12 volts at one amp?

[goatboynsac]

That is correct, I have a line of trucks that i need to develope a converter to convert the trucks 24 volt supply to power my 12 volt 700ma electronics. And I have to pull it from the ignition system, so pulling power off of one battery is not an option.

[philba]

you can get modules that do that for around $40 or so. for example, http://www.mouser.com/catalog/624/1462.pdf the wpn20r looks like its a fit. there's a decent price break at 10 units. there are 10W units that are cheaper but run below your spec of 1A. If 700 mA is an upper limit, you might be able to use the 10W units.

With more work, you can build your own step-down converter - national and linear both have lots of chips as well and fairly good design support on their web sites.

<small>[ January 17, 2006, 01:44 PM: Message edited by: philba ]</small>

[ecerfoglio]

Preferably a circuit that wont get too hot and wont need a large expesive heat sink. Cost is very important in this situation

If the cost factor is important, you may try a single 7812 regulator (with a pair of decoupling capacitors at its input and output).

It will dissipate 12 W @ 1A (8.4 W @ 700mA), but you may heat sink it to any metal part of your truck (perhaps with a small alluminium angle or sheet stock between them)

The 7812's tab is at ground potential, as is the truck's body. So you don´t need to insulate your heat sink.

<small>[ January 18, 2006, 02:27 PM: Message edited by: ecerfoglio ]</small>

[Chris Smith]

You can also use two small 12 volt batteries in series, and then tap off one of the small ones for the 12 volts.

There are many small batteries that will do, anything from Motor cycle batteries to Cam corder packs and even battery packs that you can design on your own to be even smaller.

This method is a no loss method because None of the current is lost in the translation, thus there is no heat build up and you can even steer both batteries back through the use of diodes to power your system or use one in reserve.

<small>[ January 17, 2006, 02:31 PM: Message edited by: Chris Smith ]</small>

[dyarker]

" ... a line of trucks ... "

Do not use 2 12V batteries in series.

If you use the upper (12V to 24V) battery to power something, sooner or later some mechanic who doen't realize what you've done will connect the negative side of the upper battery (which is at 12V) to the chassis thinking he fixed a grounding problem. Poof! (see quote above)

If the upper battery is not loaded the same as the other battery it will over charge. Sizzle! (see quote above)

Go with philba or ecerfoglio. I prefer ecerfoglio's suggestion a little better, with the addition of an aluminum case power resistor to drop the 24V to 17V or 18V.

Cheers,

[philba]

I'm not defensive about my solution but be aware that if you go the linear approach you will need to really understand the thermal issues - a pretty significant heat sink and placement is critical.

Typical linears have a Theta junction-case of 3 C/W. Dropping 12V at 1A yields 12W dissipation (I think it's more than that, though) plus an elevated ambient for the auto environment needs to be considered. Max temp for linears is often 125C so the following inequality will need to be valid:

theta[jc]*theta[heatsink]*Watts + ambient < 125

Assuming max ambient is 30C - probably too low. then we have

3*theta[heatsink]*12 + 30 < 125

or

36*theta[heatsink] < 95

thus

theta[heatsink] < 2.64

it's not hard to find a heatsink with that but it also needs good ventilation since it will significantly raise ambient in an enclosed space.

Finally, what is the max ambient you will see in your trucks? I suspect that 30C is way too low and more like 40C is a good design point. Could even be higher - think about how hot your car gets after sitting in the sun in summer. Get a hot day and you can cook your power supply pretty good.

The power resistor will help somewhat but all that heat still needs to go somewhere. Placement of the resistor has the same consideration as the heatsink.

The advantage of the DC-DC converter is that it is a nice compact unit that doesn't have much thermal generation and can thus be installed directly with the equipment.

[edit] oops, I was wrong on the Theta[junction-case]. It's higher. T0-3 is 4 C/W and TO-220 is 5 C/W. That makes the calcs even worse. The TO-220 needs a heatsink with a Theta of 1.5 and the TO-3 needs 2.0

<small>[ January 19, 2006, 11:39 AM: Message edited by: philba ]</small>

[Chris Smith]

Two sealed batteries [in a container] cant be tampered with if its done right, and no loss or heat requirements to deal with.

Tap out of the center with the correct color coded terminals that cant be swapped, reversed, or tampered with and power your device.

Make it Polarity coded and add in a diode if you like just for extra safe measures.

If you do it correctly only a bad mechanic with a pair of wire cutters can screw it up, and then you send him the bill for touching your equipment in the first place.

[goatboynsac]

I appreciate your input Chris, but why would i want to spend the extra money on other batterys that i will need to change in time. I like philba's approach though. I went online and am going to test out one of the dc to dc units they have.

However, i would love to be about to make my own dc to dc converter. Probably with a 7812 regulator. Biggest problem there is heat. Can i through in a few more resisters of something to bring down the voltage, so that the 7812 doesnt heat up at much?

[philba]

The resistors will generate heat as well. You might be able to more easily dump the heat by spreading them out physically but I'm biased towards a compact design. Note that big power resistors aren't cheap.

I'd suggest you look at national and linear tech's web pages. They both have pretty complete canned designs for switching converters which will run much much cooler. For example, using National's web bench, I entered a 20-30 VDC input range, 12VDC output at 1A and it spat out a complete design based on the LM5010 which is $3.20 q1 from digikey. one inductor, a diode and a few caps and resistors. Probably less than $10 of parts. This took me about 5 minutes.

<small>[ January 19, 2006, 01:24 PM: Message edited by: philba ]</small>

[Chris Smith]

Thats the whole reason I mentioned the no heat, no loss system. No electronics involved. Im all for electronics but ..... You set the goals.

"Preferably a circuit that wont get too hot and wont need a large expesive heat sink. Cost is very important in this situation"


Small batteries will last for years, they are cheap, and all you need is the wiring and a housing.

As long as the battery is a one amp hour or greater, your covered when your running and stopped.

I always take the simplest and most reliable approach first, then get complicated as the project dictates.

12V 2.3AH BATTERY, SEALED LEAD ACID $12.00

<small>[ January 19, 2006, 04:27 PM: Message edited by: Chris Smith ]</small>

[ian]

goat.......I have a better solution than all those mentioned so far...........

Your post states "to 12 volt 1amp converter".

If you're just driving a lamp then pulse width modulate the 24V through the lamp with a 50% duty cycle. Use a 555 or something and drive a TIP122 transistor. There will be almost no power lost in the transistor since it's always fully on or fully off, it won't heat up, and the lamp will get the equivalent of 12V "average".

Cost will be about $2.00.


It's also possible you could pulse the lamp for
1uS at 700,000 amps but there are design considerations, please reference the "Perfect current limiting LED challenge" thread.

<small>[ January 20, 2006, 06:00 AM: Message edited by: ian ]</small>

[philba]

Originally posted by ian:
goat.......I have a better solution than all those mentioned so far...........

Your post states "to 12 volt 1amp converter".

If you're just driving a lamp then pulse width modulate the 24V through the lamp with a 50% duty cycle. Use a 555 or something and drive a TIP122 transistor. There will be almost no power lost in the transistor since it's always fully on or fully off, it won't heat up, and the lamp will get the equivalent of 12V "average".

Cost will be about $2.00.


It's also possible you could pulse the lamp for
1uS at 700,000 amps but there are design considerations, please reference the "Perfect current limiting LED challenge" thread.

Normally, I'd just ignore a post like this but in your case, I'll make an exception. I can only conclude this is a lame attempt at a joke since the posting's text was extremely clear, all 5 sentences of it...

<small>[ January 20, 2006, 09:33 AM: Message edited by: philba ]</small>

[ecerfoglio]

Philba: I think Ian has misread the thread's tittle, the 1 amp (one amp) is simmilar to LAMP....