Idea about how to power in the house

**Rob Withey** · 03-21-2002, 05:36 AM

This is information people may want, not a question.

For ages I've been trying to figure out a cost effective way to power my mp3 box in the house. Up to now I've just been connecting in a regular pc power supply, which is messy because it means I have to open up the box. However I figured that car battery chargers kick out about the right voltage with wedge loads of current, and with the addition of a capacitor to smooth the rectified ac (the charger I have chucks out unsmoothed full-wave rectified ac), you got yourself a fairly cheap unregulated mains power supply for feeding your dc 2 dc.

Because car battery chargers are common as muck, they are pretty cheap too.

Coupled with my new power supply design (finally got round to reducing the psu circuit I have onto a proper pcb, 2.7"x2.3") I may end up with a nice power supply system yet.

Rob

**Rob** · 03-21-2002, 10:53 AM

Also, a pretty good way to test a DC-DC PSU, before you plug it in the car and find it dosn't work!

**jol** · 03-21-2002, 11:28 AM

Originally posted by Rob
Also, a pretty good way to test a DC-DC PSU, before you plug it in the car and find it dosn't work!

use a somehow large batterycharger to simulate the cars +12 and ground

**Aaron Cake** · 03-21-2002, 05:11 PM

Do not use a battery charger. They are unfiltered and unregulated. This means that you will have a horribly dirty 18+ volts going to your DC-DC. Not good.

Try a regular AT power supply.

**Rob Withey** · 03-21-2002, 07:30 PM

Hence the addition of a capacitor to smooth the full-wave rectified ac (that's all an unregulated supply is anyway). My dc2dc supply is a regulator itself. Why on earth would I want to waste time pre-regulating?

I've had the charger apart, I know exactly what it is doing. It's a four tap transformer (with a bunch of switches for 6v/12v, low/high charge rate) into a full wave bridge rectifier. It's perfect for my needs, anyway. The output is no way near as high is 18v, even under no load...!

A regular AT supply has two problems:

First it's 12v, that's not representative of a car battery when being charged by an alternator (which is nearer to 14.4v).

Secondly, if you load up an AT supply on the 12v line only, quite often it will trip on the over voltage on 5v (due to loading). So you need to articficially load the 5v to get 12v out. Not my bag at all. I want a tidy solution.

Rob

**jol** · 03-22-2002, 06:03 AM

Originally posted by Aaron Cake
Do not use a battery charger. They are unfiltered and unregulated. This means that you will have a horribly dirty 18+ volts going to your DC-DC. Not good.

Try a regular AT power supply.

doesn't the sproggys have a input range of 9-14 volts? does it really go all up to eighteen?

**Aaron Cake** · 03-22-2002, 09:56 AM

Most car battery chargers are fairly stupid, only a transformer or rectifier. Therefore, the 12V out from the transformer gets rectified to 18 or more, and is unfiltered. Some chargers are better than others though.

**Rob Withey** · 03-22-2002, 10:24 AM

What rectifier adds 50% on to the ac rms voltage? A full-wave bridge rectifier only goes to 1.41 times, and then you have 1.4 volts drop across the two diodes which are conducting at any time.

My battery charger is very stupid. It is a transformer and a bridge rectifier. The AC taps are 5.7, 6.3, 11.8 and 12.8 v ac. This rectfies to 6.6, 7.5, 15.2 or 16.6 v. Adding a capacitor solves the unfiltered aspect. I would select the 15.2v dc output as the most suitable.

Rob

**Aaron Cake** · 03-23-2002, 04:38 PM

Depends on the charger or transformer used. Some chargers use 18V transformers, some use 13V transformers. Either way, I have measured upwards of 20V with an RMS meter on some of these things...

I'm not saying it won't work, I'm just warning others that some battery chargers may be unsuitable for these types of things.

**Rob Withey** · 03-30-2002, 06:24 AM

Originally posted by Rob Withey
Coupled with my new power supply design (finally got round to reducing the psu circuit I have onto a proper pcb, 2.7"x2.3") I may end up with a nice power supply system yet.

Just updated my website with a new circuit diagram and photos of my new tiny power supply for anyone that is interested.

Rob