Hey, i'm making a phone charger for in my car and was just wondering if this principle would work.
The input will come from a 12 volt dc cigarette lighter power point in the car. Then by using resistors i will step the dc voltage down to the 5 volts dc needed to charge the phone. So basically a resistor divider circuit.
Would this work? I'm just wondering if i'd need any more components than the two resistors i plan to use to step the voltage down.
Thanks for any help.

 

Won't work. Get yourself a 7805 voltage regulator, and follow the datasheet example circuit. Mount the 7805 on a heat sink. I used an Altoid tin for the circuit, and mounted the 7805 to the wall of it. It gets hot to the touch but has lasted a long, long time.

A resistor divider won't work because the voltage drop across any resistor depends an the current going through it. This will not be consistent for the devices you might be charging. It WOULD work for something like a light bulb that is predictable.

Be sure you know what your device needs in order to charge properly. An iPod, for instance, needs the right voltages on the USB data pins or it won't draw power from the power pins.

 

thanks for the advice i've just ordered some 7805s.

 

Did you notice the application circuits also use capacitors? And maybe you'll want an indicator LED (with current limiting resistor), as well as a jack on both sides of the enclosure, for attaching cords?

My point is, you'll want to order all the stuff you need at once, to avoid multiple freight charges.

 

Yeah i noticed the capacitors. I can get most components from college so that isnt a problem. Just out of curiosity, what are the capacitors for?

 

The caps help quiet things down, in response to transients or any tendency to oscillate. If you were operating off only the battery, it wouldn't be as big an issue. Car electrical environment is very noisy.

 

Thanks for the help

 

Good luck with your project. I tried that and found my particular phone requires a smart charger (one designed specifically for that phone) or it refuses to charge and the front panel lights up with a message that says something like "UNRECOGNIZED CHARGER". Even to use a USB cable from my PC to charge my phone, I had to get software to load on the PC from the phone manufacturer before it would charge. Again, good luck with your project and I hope it works better for you than it did for me.

 

The solution with an iPod is to put a voltage divider onto each data pin, to get the voltage into a range where the iPod will accept it as a charger. It can take a bit of fiddling, but you can find the starting values to try with a little searching.

If the device requires a genuine handshake - to enforce buying the blessed charger - you may be outta luck.

 

okay.. so a charger for charging a phone huh? very nice

Here are your options:
1- Use a voltage regulator..Lm317s, 7805a, etc
2- Use a buck converter IC
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Advantages/disadvantages for voltage regulators:

1- simple to use. So if your new to electronics.. stick with this one. The data sheet are easy to follow. You dont need to be worried for the inhereted issues with regulators--like drop out voltage etc.

2- 12V to 5V is a big change. U'll need a decent size (not a big one) heatsink.

3- Due to this, the housing in which you'll keep your project will have to be changed.

Advantages/disadvantages for buck ICs

1- Very small packages. You can make your circuit small and wont need a heat sink

2- The data sheets are just a tad bit more complicated as compared to the regulators

3- They use passive components-- so the component count might be a bit more.

4- Very good load regulation. Line regulation does not need to be addressed here.

5- The project housing can be small so it'll end up looking 'elegant'.

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If the device your trying to charge is an smart phone, you WILL need to look at the voltages at the data pins of the USB. Standard usb have 4 pins which are power, ground,D+ and D-.

IF you read up on your device, you'll find out the voltage levels for the D+ and D- pins. The power pin is at 5V standard.

Be sure you know what your device needs in order to charge properly. An iPod, for instance, needs the right voltages on the USB data pins or it won't draw power from the power pins.

Wayneh is correct to point it out.

Let us know what you've decided to come up with. Best!

 

Thanks for the tips
@ BillB3857, what phone did you try? I'm using an iphone.

 

You should go read this, and Google "iphone charger circuit".

 

Thanks for that. Looks like the two data lines of the usb require around 2 volts. According to various sites, a line a of resistors will do that. I'll just step down the voltage from the 5v from the 7805 chip to the necessary voltage.

 

That should do it.

One thing I never knew when I made mine is the effect of the voltage on the data pins on the charging rate. When I made mine, I was just concerned with getting it to charge at all. The MintyBoost article says that 2.8v and 2.0v could allow charging at 1A. I suspect mine never does that, and might overheat if it did. Dropping 10v from the car's 15v while running to 5v at the output, at 1A, would generate 10 watts. Without a genuine heat sink, that's a lot of heat. I wonder if mine ever gets to even 0.5A.

My point is, you may want to specifically avoid charging at high current if your 7805 is not seriously heat-sinked.

It'll be kind of a pain to test, but you might want to measure current flow before the final solder connections are completed, so you can insert an ammeter in series with current path.

 

Of course the standard car chargers nowadays pretty much all use the MC34063 or some variant.

The basic circuit I have seen uses only 11 components:

47uf 35v capacitor for the input
16v 100uf or 220uf on the output
1N5819 diode
1.5k and 4.5-5k for feedback resistors
470R resistor for LED
LED
1000pf, 681pf, or 470pf for the timing cap
1W .22, .33, .39 ohm for the current sense resistor

The only real variable is the inductor. Most inductors are the bobbin or "dumbell" shape and are not marked so its hard to tell what their inductance is by just looking at them... and I don't have an LCR meter so I have no way to really find out. The only clue I have found is with one toroid... it is type 26 material yellow/white powdered iron core with around 60 or so turns on it. Guessing it is around a size 40 something... definately smaller than a T-50 toroid. These type of powdered metal toroid cores are easily scavenged off of junk power supplies and what not. The green/blue type 52 material cores should work as a substitute too since they have more or less the same inductance just a different temperature rating. Best of all there are calculator programs floating about that once you know what core material your working with its possible to just punch in the inductance you need and they will poop out the number of turns you need to make a core.

And if you substitute the 4.5kish resistor with a 10k potentiometer you can have the power output adjustable from around 2v all the way up to 10v. Or you could possibly hook up a rotary switch and by using different resistors have multiple preset output voltages. And if you really wanted to get serious you could drive a mosfet with the chip and use a larger inductor and diode to put out several amps of power.