Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

enter image description here

share|improve this question
If you MUST use a linear regulator, as a series input resistor to drop most of the voltage. Say you need 8V at regulator input (wort case) then R = V/I = (14-8)/0.5 = 12 ohm. Wattage = V x I = 6 x 0.5 = 3W - use 5W or 10W auir cooled R. – Russell McMahon May 15 '12 at 4:01
See answers to this similar question. – m.Alin May 15 '12 at 8:21

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.

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.

enter image description here

share|improve this answer
Thanks. I took apart an old car charger I had and found that it has a MC34063 in it. I'm going to try and get that working. It's kind of a black box to me so I like the idea of learning how to get it working too. – JDD May 15 '12 at 16:24

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.

share|improve this answer
I think you should explain why and how it solves the problem. – stevenvh May 15 '12 at 8:27
A good charger needs to supply 1.8A also, as the charging spec requires. – Kortuk May 15 '12 at 10:58

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:

enter image description here

share|improve this answer
There is this certain happiness when building things on your own and that is priceless. ;-) – shimofuri May 15 '12 at 9:47
Awesome! I opened up an old car charger I found and it also has a MC34063 in it. I just de-solder the chip and use it in my own design. – JDD May 15 '12 at 16:22

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.