In-Car MP3 player charger

Hi there everyone, new member!

I did GCSE tech (Electronics) and im doing A level physics but I figured it'd be a good idea to ask some people who knows what they're doing about this.

I have a Zune MP3 player which is only available in the states right now, and im getting my first car soon, and would like to create an all in charge and audio cable. Ive pretty much the got the pinouts figured, and im soldering the connector together, so im just left to figure out the charging side.

The car is either gonna be 12 or 16v, no idea on the current (Not even a vague idea, how much current would it usually put out given the devices already installed?)

I need to whittle that down to 5VDC and either 0.5A or 1.5A (It can charge with either, 0.5A is preferable however as im not sure the cable im using can handle 1.5A), as well as to around 1.75VDC with the same ampage.

How would you guys recommend I go about this?

Cheers all!
Ben Wedlake.

EDIT: What do you guys reckon to something like this: **broken link removed**

(Looking at the TS78M05 obviously)

Hi Ben,
Welcome to our forum.

A car battery can supply up to hundreds of Amps so use a 2A fuse in series with your 5V power supply project.

The Zune will draw only as much current as it needs.
A 7805 IC is a 5V voltage regulator with an output current up to 1.5A. If the car battery is fully charged at 13.8V and the Zune is drawing 1.5A then the 7805 heats with 13.2 Watts which is a lot of heat. The regulator will need to have a pretty big finned heatsink. If the regulator gets too hot then it shuts down.
The regulator needs to have an input capacitor and an output capacitor as shown on its datasheet.

What do you need 1.75V for?
An LM317 is a voltage regulator with an adjustable output voltage from 1.25V and up. Its max output current is 1.5A. It will heat with up to 18W so it will also need a pretty big finned heatsink , two capacitors and two resistors to set its output voltage.

The 1.75V is all to do with the audio decoder chip in the MP3 player itself.

Rather than having a charge cable and a standard headphone cable (3.5mm jack) in the top, I want to just have the charge cable which will output audio too. To activate the audio outputs on that cable you have to put 1.75V through two pins (At least, we're pretty sure its 1.75V give or take).

Could I not just output the 5V then use a potential divider to bring it down to 1.75V, use the 5V where needed and likewise with the 1.75?

Furthermore, I don't really know what the hell all that stuff on the datasheet means So what value capacitors + resistors are we looking at. And how hot is this thing likely to get? Am I gonna need like a real 'this-could-keep-the-poles-from-melting' heatsink?

As I said if there's a way to limit the current totally to 0.5A on that 5V line it would be great becase the cable im using isnt that thick and id be a bit worried about pushing 1.5A down it - doesn't make much difference to the Zune as that's the peak output of a USB cable, it accepts 1.5A for when your using the wall charger.

Oh and thanks for the 2A fuse thing, I did wonder if there was a chance of my little circuit being melted xD

Cheers for the reply, sorry for being a n00b!
Ben.

if you do use a linear voltage regulator than the amount of heat generated will be the input voltage minus the output voltage multiplied by the current (13-5) x 0.5 or 3.5watts of heat. The fuse isn't to protect the circuit rather the car, I once hooked up a 12V fan thru a cig lighter but didn't include a fuse, had a short at the switch near the fan, all the insulation melted off the wires.

Without a fuse, if something happened to short the wires then the red hot wires will set the car on fire.

You need to determine how much current the Zune will use. If it is 1.5A then the ragulator needs a pretty big heatsink. It the current is only 500mA then the heatsink can be 1/3rd the size.

A voltage divider made with two resistors has an output voltage that changes when the load current changes. Then the circuit it is powering will either blow up sometimes or won't work sometimes.

Well that uhh....sounds like a lot of fun.

As I say the Zune draws either 500mA or 1.5A, it just draws whichever it can get. But I am just a bit concerned about the cable im using as to whether it could handle 1.5A, it's not exactly sturdy stuff. Plus if it's gonna generate a load more heat, I would entertain any method of limiting it to around 500mA

If a potential divider could make things blow up, how would you suggest it's done? Another IC like that to take it down to ~2v and generate ridiculous heat or what?

Nothing draws the amount of current it can get. It draws the amount of current that it needs. If you limit the current and it needs more then the current limiter reduces the voltage then the circuit doesn't work anymore.

I already mentioned using the LM317 adjustable output voltage regulator set to 1.75V.
The heat is determined by the voltage across it (the car battery voltage minus the regulator's output voltage, times the current). The LM317 also shuts down if it gets too hot.

Ok ok ok.

Well all I know is a USB cable can only supply about 500mA tops, and my PC will only supply ~400 due to the amount of devices sharing current. Yet if you buy the AC adaptor that pushes out 1.5A to the device. Apparently anyway. I live in the UK and imported my Zune so im not in the market for buying accessories .

Could I use the LM317 to do the initial downplay to 5V? If it's gonna shut down if it gets too hot, surely that's better than using the linear converter that - I assume - will just fry. I see it can go from 1.25 - 37V, and pushes 1.5A.

I will actually be getting the car tomorrow - im not sure of the voltage but ill find out tomorrow since ive gotta wire in some speakers anyway, so ill get the multimeter on it.

Ive nearly got the connector soldered together so this circuitry to power it all is really the last thing I need - then just hook it in to a power line and the stereo.

The cable im actually using to make this is sort of bog standard general purpose stuff, which I originally made a SCART lead out of. So im not sure how it's gonna hold up to this job, what sort of thickness or whatever should cable to carry 5V/1.5A be?

--- On a completely different subject, anyone recommend a PCB design software? So I can throw all this stuff together and get a board etched.

what sort of power connector does a Zune have? Most of these players seem to be headed towards a USB connector of some sort. Just checked out the Zune website and they look like they have gone down the i-pod path. There were a few 12V adaptors listed on the website.

Well the end of the cable that goes to the PC is a USB, but the actual connection to the device is a 25 pin beast of a plug. I basically got one of these 25 pin Zune plugs for about $2, with solder points, and im assembling my own cable to charge and put music along, alot like the home dock thing you can buy for the Zune.


That's the sorta thing im seeing using an LM317. Where in that would I put a fuse, and...well does it looks right-ish

Sorry bout my scribbly diagram xD

So a car cigarette to USB charger adaptor would probably work. The fuses should be on the input side, additional protection on the output is optional.
You could just use a 5v regulator as the output voltage on the above circuit with your calculations will give an output of 5.25V, the LM317 is a linear reg so no great changes to go straight to a 7x05

Sorry, is the 7x05 actually a 5v regulator chip?

I googled it and got...well nothing.

(Yeh I know this is an old thread but I only just got round to this again)

The 7805 and LM317 have a typical output current of 2.2A but it could be as high as 3.4A. Then the regulator and maybe even your Zune will get very hot.

The 78M05 has a typical output current of 1A but they don't say how much is its max current. Use the TO-220 package with a medium size finned heatsink.

So you reckon just a 78M05 in TO-220 form with a heatsink and it should be cool-ish?

wetlegs6 said:

Sorry, is the 7x05 actually a 5v regulator chip?

I googled it and got...well nothing.

(Yeh I know this is an old thread but I only just got round to this again)

Click to expand...

Not as old as some that get resurrected.

Yes, the 7x05 are 5-volt regulators. The 7805 provides a positive regulated voltage and the 7905 provides a negative regulated voltage.

One quick rundown on different members of the family: http://www.scienceprog.com/7805-voltage-regulator-for-your-design/

You can find more information by googling "7805 tutorial" or similar. Also google "7805 datasheet" and read one or two of those. They should include example circuits in the "Application Notes" section.

One thing you said in an earlier post:

Well all I know is a USB cable can only supply about 500mA tops, and my PC will only supply ~400 due to the amount of devices sharing current. Yet if you buy the AC adaptor that pushes out 1.5A to the device. Apparently anyway. I live in the UK and imported my Zune so im not in the market for buying accessories .

Click to expand...

I don't know if it's just the way you phrased it but the AC adaptor isn't "pushing" current anywhere. The 1.5A rating just means that it can provide up to 1.5A at the given voltage rating. If the device it's feeding doesn't need 1.5A, it won't draw that much. And as many adaptors are not regulated, they will actually show a larger output voltage when the current draw is below the rated amps. i.e. I have a 120VAC to 12VDC, 1A adaptor which provides 13-odd volts when loaded with ~1A and 17-18 volts when feeding a load of ~100mA.

If I'm misreading what you wrote then ignore that last paragraph.


Torben

If your alternator over-charges the battery at 14.4V and the 78M05 draws 1.2A then it must dissipate 11.3W which is not cool.

Uhmm.

Well, what that meant was, a USB port on a PC can only supply a max of 500mA. I know the wall charger charges the device a heck of a lot faster, so it stands to reason the device draws more.

I dont really know what I meant with that.

You've succeeded in confusing me there, with my own words

Anyway, from the looks of that link you just pin the 7805 in and put capacitors on the in/out (for smoothing im guessing?).

wetlegs6 said:

Uhmm.

Well, what that meant was, a USB port on a PC can only supply a max of 500mA. I know the wall charger charges the device a heck of a lot faster, so it stands to reason the device draws more.

I dont really know what I meant with that.

You've succeeded in confusing me there, with my own words

Anyway, from the looks of that link you just pin the 7805 in and put capacitors on the in/out (for smoothing im guessing?).

Click to expand...

I was just talking about the word "push"--to oversimplify, voltage is "pushed", and current is "pulled". But yeah, if the charger can draw more than 500mA when available, it will, and it would presumably use that to speed up the charging.

The input-side cap is sometimes called a "bypass" capacitor and you use it to help stabilize the voltage if the power supply is not very close to the regulator. The output-side cap helps keep the regulator from oscillating and also helps transient response.


Torben

The capacitors must be very close to the pins of the regulator. They keep it from oscillating.

You never said how much current is needed at 1.75V. If it is low then two resistors can make it from the 5V supply.