converting 12vdc to 5vdc

[protocell]

hi. i just wanted to build a travel charger for my psp. i want to use the 12-volt supply from a car. i already searched the net for the schematics and now i'm having problems with the output rating of the circuit. i saw on my psp charger that it outputs at 2000mA while the rating i found on the net has only a max output of 600mA. how could i get it to up to 2000mA? another question, will the input current rating of the 12-volt supply from the car affect the output of the circuit?

here is the link to the circuit diagram i found:

2-transistor Black Regulator

thanks for any reply.

 

[colin55]

You can get a 3-terminal 5v regulator 5 amp such as this:

LM338K STEEL, 5 Amp Adjustable Regulator, TO-3 for $12.00

or put two in parallel of:

LM317 3-Terminal 1.5Amp Adjustable Voltage Regulator for 70 cents each

 

[tracidfish]

LM7805ct, of the top of my head it can hanlde 2 amps. cost about £1.50 from rs

 

[Birdman Adam]

Yep, LM7805 can handle up to 1 amp without a heatsink.
Once you add a heatsink, you can go up to 2+.

Its very easy to use, only three pins: Input, Output, and Gnd.

You should also place some smoothing capacitors befor and after the regulator, to make sure your device gets nice smooth DC.

 

[protocell]

so sir it means that the circuit on the link is impractical to use for my travel charger?

 

[crutschow]

so sir it means that the circuit on the link is impractical to use for my travel charger?

It could be used, and would have higher efficiency and consequent lower power dissipation than a linear regulator, but you would need to modify it to carry the higher current you need. Perhaps some Black Converter variation such as one of these which use a MOSFET to increase the output current would work.

 

[Diver300]

Most cheap mobile phone chargers can be modified to give out 5 V, if they aren't already at that voltage.

 

[crutschow]

Most cheap mobile phone chargers can be modified to give out 5 V, if they aren't already at that voltage.

I doubt that any of those are capable of 2A output.

 

[ambriel]

It's the 2 amps that's the killer. The 7805 is the best bet. Circuitry is simple - any number of examples on the 'net - and with a decent heatsink will manage the current just fine. If you fit it inside a metal case (tobacco tin, etc) you can use it as the heatsink.

I use a similar setup to feed a wireless webcam with 9v from a 12v car battery connected to a small solar trickle-charger, although thhat only needs .3 of an amp.

 

[Birdman Adam]

Yep, a good old 7805 will work just fine.

The more energy you put through it, the hotter it gets dissipating the extra heat made in the conversion of voltage.
The 7805 already has a little metal tab on it. That's why you can take it up to 1A without a heatsink.

After that, you need to dissipate more heat. I have taken a 7805 up to ~3A, its really just a matter of dissipating the heat!

Here's a typical setup for a 7805.
The capacitor are there to smooth out any ripples in the DC.
You could also add an LED to show it's working:



And for heatsinks, here's a link to some that may work.
Bigger is always better when dissipating heat.

TO-220 Heatsinks

 

[crutschow]

The 7805 can only carry 1A without a heatsink if the voltage drop across the device is not much more than a volt. In this application it will drop up to 9V, which will generate up to 18W when supplying 5V at 2A from a 12V (14V with engine running) source. Thus it's going to need a pretty healthy heatsink, probably one with no more than about 2.5C/W thermal resistance to air.

 

[Birdman Adam]

Hey! Here's an idea:

Why not use two 7805's at once? Seems to me it would work...

I would still use a small heatsink with both the regulators.

Or maybe you could just chain a whole bunch of diodes before the regulator(s) to bring down the voltage?

 

[Grossel]

Why not use two 7805's at once? Seems to me it would work...

You know better, right?

One will take the most load, just like two diodes in parallell (when producing heat).

So no, two 7805 in parallell is a bad idea.


A better idea is to use a PNP power transistor like this:
**broken link removed**

 

[crutschow]

And two regulators or a regulator with a transistor still have to dissipate the same total amount of heat so you still need the same amount of heatsink capacity.

 

[mneary]

I think you should try to modify the original Black Regulator for 2A.

Change Q1 to another PNP with a 6A or more rating (such as TIP42), D1 to a higher rating, and get an inductor rated at 3A or more.

Use LTSpice to test your changes before building it.

 

[MikeMl]

If you have a mobile charger rated for 500mA at 5V, it will current limit to protect itself, but still charge your mobile device. It will just take four times longer to charge your device than a 2A charger could recharge it.

 

[Grossel]

And two regulators or a regulator with a transistor still have to dissipate the same total amount of heat so you still need the same amount of heatsink capacity.

Yes, that's true - but now the transistor tends to take most load when output current is high

 

[mneary]

When the transistor takes most of the load (using the 1R resistor shown), then it will be dissipating 12W while the 7805 dissipates 5W (and the 1R dissipates 3/4W).

This is still destructive to all components involved unless the design allows for safe removal of 18+ watts. I see no good reason to build a 35% efficient power supply when it's not so hard to build one that's 90% efficient.

 

[Grossel]

When the transistor takes most of the load (using the 1R resistor shown), then it will be dissipating 12W while the 7805 dissipates 5W (and the 1R dissipates 3/4W).

Removing the resistor to the base of the transistor will cause transistor to take more load and less for the 7805. Same effect will also occur if the other resistors value increases.

This is still destructive to all components involved unless the design allows for safe removal of 18+ watts. I see no good reason to build a 35% efficient power supply when it's not so hard to build one that's 90% efficient.

It's very easy to build. There might also be a balance between procurement cost and energy cost over time. If this device is intended to be used just a few times, it's probably the cheaper solution.

However, I'm agreed that it's a waste of power.

 

[mneary]

I'm not so concerned about waste as I am of the problem of finding a safe location for an 18W heat source in an automobile cabin.