Cheapest 24-12V DC converter - Page 1
Forum section: electronics / projects

author: AFH
author_rank: Newbie
timestamp: August 21, 2012, 06:37:11 pm
content: 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..

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 21, 2012, 07:00:23 pm
content: QuoteThat'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.

author: AFH
author_rank: Newbie
timestamp: August 21, 2012, 07:13:40 pm
content: 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.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 21, 2012, 08:23:02 pm
content: Quotesure...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.

author: poptones
author_rank: Frequent Contributor
timestamp: August 22, 2012, 08:26:47 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: August 22, 2012, 09:29:00 pm
content: Haha, not very efficient thoughyou can get them off eBay for as little as 15 pounds

author: Psi
author_rank: Super Contributor
timestamp: August 23, 2012, 02:07:07 am
content: Quote from: poptones on August 22, 2012, 08:26:47 pmThe 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.

author: poptones
author_rank: Frequent Contributor
timestamp: August 23, 2012, 02:34:26 am
content: 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.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 25, 2012, 06:50:16 am
content: QuoteOf course it works, no one doubt it. And if it didn't work I would not believe in the god of electronics anymoreBut you cant call this a voltage regulator.

author: poptones
author_rank: Frequent Contributor
timestamp: August 25, 2012, 02:25:26 pm
content: 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.