Cheapest 24-12V DC converter

[AFH]

Hi there,

I'm new here, looks like you guys have a nice place here. Looking forward to be a part of it.

I need several DC-DC converters, and in this respect I am looking for the cheapest way to do such.

I've had a look at the MC34063, and I made a nice schematic. But because I want to draw min. 1.5 A out of it, I have a mosfet or transistor on the output, and then the circuit become twice as expensive if you use a IRF4905.
My schematic is attached.

Requirements:
-Input: 20 to 30V.
-Power: Min. 1.5 A
-Should not create much heat, as the PCB needs to be built into some shrink tubing, and hidden behind the dashboard of a truck.

Do some of you guys have a nice idear how to build a really cheap 24-12V converter?


Hope for your reply..

[PSR B1257]

and hidden behind the dashboard of a truck.

That's a problem. Not the dashboard, but the truck itself. Because in trucks, as well as in cars, you got anything, but no constant 12V/24V in the on-board power supply.
There are voltage peaks about 200V or even higher (due to load-rejection), which can easily destroy your DCDC converter (and any other not explicit protected electronic).

So you should better looking for a vehicle-specific DCDC converter.

[AFH]

Hi PSR B1257,

Thank you for your quick reply.

There is nothing a diode is unable to do. 

In fact, some cars / trucks have a particularly stable supply for ceiling lighting and lighting of the speedometer, and if so, the driving computer.

[PSR B1257]

There is nothing a diode is unable to do.

sure... 

But these overvoltages can be negative and positive with a duration of up to 300ms or so, which is effectively DC to an IC.

So you need a input protection circuit, which can handle pulses. A single diode would be most likely not enough.

[poptones]

I did this once for a fellow who wanted to put a car stereo in his tractor, which had a 24V electrical system.

A string of 18 3A diodes cost maybe two bucks. There ya go.

[Simon]

Haha, not very efficient though  you can get them off eBay for as little as 15 pounds

[Psi]

I did this once for a fellow who wanted to put a car stereo in his tractor, which had a 24V electrical system.

A string of 18 3A diodes cost maybe two bucks. There ya go.

The problem with that is the forward voltage varies with current.
For a silicon it could be 0.6V to 0.9V, so assuming 18 diodes..
You could have 0.6x18=10V or  0.9x18=16V drop which means the output might be 8-14V.
And of course there's also the heat that it generates.

[poptones]

Sure there's all kinds of niggles. For the application I had I used 12 I believe, and I potted it to give a place to mount it and to help dissipate heat if need be. But it does work... like any design, depends on the real world parameters. The question didn't mention the specific application.

[PSR B1257]

But it does work...

Of course it works, no one doubt it. And if it didn't work I would not believe in the god of electronics anymore 

But you cant call this a voltage regulator.

[poptones]

The title of the thread doesn't mention regulation. And the car has a regulator, so it's easy to do the math.

Like I said: it works fine for the application. And this thread didn't mention that, either.

'Scuse me for contributing to the conversation.