Trying to make a usb car charger

Question

I'm working on a project to build usb car charger for my phone. The goal is to do it using only off the shelf parts (radio shack) and as cheap as possible. The size is also a bit of an issue but I'll deal with that later. I expect the phone to draw about 500mA and my car puts out a pretty steady 14.1V from the cigarette lighter.

I came up with the design below but I though of one big problem. IC1 is a 7805 5V regulator (sorry forgot to label it). It will give off somewhere around 4W, which is way too much and right on the edge of what it's rated for.

I set it up on the bread board and confirmed that it's very hot. Then I put the diode in to try and help distribute some of the heat but still way too hot. Any ideas? It seems like this will work as long as I can take care of the heat. There's not enough room in the case for a heat-sink either.

Answer 1

As noted by retro - you can buy converters that will probably do what you want from Amazon or ebay or similar for a few dollars.

And you could use a linear regulator (with series input resistor as per my comment) and a heat sink. But, if you WANT to make your own and learn quite a lot and hav something that will be useful for many other tasks, then an MC34063 is a cheap and easy way to do it.

Agh!!! - just had a look at my answer of May 2nd. Just do this :-}

Ancient but glorious MC34063 + big brother app note

Claims to assist in component selection

Another version fromhere but its a stolen 2006 Silicon Chip circuit. Bigger version here

For inductor they say 75 turns of enamelled 0.5mm dia wire on Neosid 17-732-22 powdered iron core.

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The diagram and notes below are from figure 11, page 7 of the MC34063 data sheet from OnSemi. Digikey $US0.48/1 for SOIC-8 and $US0.62/1 for DIP8.
You also need:
A 1N5819 1A Schottky diode $US0.40 !!!!/1 but far cheaper in modest quantity.
A few R's and C's.
An inductor. 220 uH 1A say

In the circuit below L = 220 uH and Vin = 15-25V. Iout = 500 mA at 5V. It will operate below 15 V in - a smaller inductor would be better. Say 100 uH. The circuit is very forgiving of inductor value.

Answer 2

Here in the UK, we have pound shops - similar to a dollar store in the US. I happened to buy a Signalex USB in-car charger in one today for £1. This shows that your project must be possible!

I just cracked the thing open for you, and there's one IC in there - MC34063. Other than thatm there's an inductor, a diode, 5 resistors, 2 electrolytic capacitors and 1 ceramic. And 2 USB sockets. Oh, and I just noticed a fuse that fell out.

Just one question - at that price, do you really want to build one yourself? Here it is on Amazon.

Photo added. RM:

Answer 3

4.5 W is what it should do: (14 V - 5 V) x 0.5 A = 4.5 W.

The simplest solution is a 3-pin Plug-in-Power part, e.g. PT78HT253 from TI. It is an integrated switching power supply and has much better efficency than 5 V/14 V =36% that you will see with a linear regulator.

If you need higher current, as @Kortuk suggested (probably for USB 3.0), the TI PTH0800W is good to 2.25 A.