need help with buck converter circuit

[pomprocker]

ok i'm very rusty with electronics..

The power source will be 12vdc as I'm using a special wal-wart.

I need to get it down to 5vdc to power an ATTINY(45?)

I know this is basic, but again I'm rusty...  

[Spoil9]

You can use a 5V voltage regulator but that will heat up pretty bad.

You can build a DC-DC step down converter. There are prob a hundred circuits for this on google depending how into it you want to get.

My last suggestion, is order a new wal-wart. They are pretty cheap on line if you can't scavenge one locally. Also Radio Shack sells them and they also sell the parts to make your own if you need special requirements like higher amperage or multiple outputs or something.
Hope this helps.

[pomprocker]

You can use a 5V voltage regulator but that will heat up pretty bad.

Yeah thats what I figured

You can build a DC-DC step down converter. There are prob a hundred circuits for this on google depending how into it you want to get.

Sounds about right, can i keep it small?

My last suggestion, is order a new wal-wart. They are pretty cheap on line if you can't scavenge one locally. Also Radio Shack sells them and they also sell the parts to make your own if you need special requirements like higher amperage or multiple outputs or something.
Hope this helps.

I have to use this one.

[SmAsH]

A bunch of diodes to drop the voltage to 7-8v then regulate it with a 7805?

[cyberfish]

But that's still dissipating the power as heat.

Might as well just use a 7805 and get a bigger heatsink for it?

If you are only using it to power a microcontroller, you may not even need a heatsink at all. To get 1W dissipation (max. safe for TO-220 without heatsink) at 7V drop, you'll need to draw ~140mA.

[little-c]

two regulators in in parallel. works fine.

[pomprocker]

ok all different answers so far and not to reassuring  

fyi its not a regulated 12vdc. the power is anywhere from 12-17vdc depending on the load i believe.

[cyberfish]

two regulators in in parallel. works fine.

Usually not a good idea. Regulator voltages will be slightly different, so you could be drawing something like 99% of the current from one of them. It will work theoretically and in simulations, but probably not in real life.

Do you know how much current your circuit will draw?

[duste83]

you can use a computer power supply.  you will need to jumper a pin to ground to turn the power supply on but they are cheap, produce a good amount of current and you will also have regulated 12vdc and 3.3vdc.  you could also take the 5vdc from an extra plug on your computer.  just get a molex connector or splice it.

[pomprocker]

looks like what I need is a buck converter circuit to go from 12-17vdc to 5vdc I will only be powering a hobby servo like the hs-311 and an ATTiny25/45. Using just a 7805 looks like there was around 2.8watts lost at 12vdc and more at 17 which is too much waste.

It looks like a buck converter circuit consists of a schottky diode, an inductor and a smoothing cap. Not exactly sure though, as I've never done this circuit.

[cyberfish]

I think there are ready-made plug-and-play switching converters. They cost quite a bit, though. Will probably be cheaper to buy a new wal-wart.

2.8W is not really that much. A 7805 with a small heatsink will handle it just fine.

[GearMotion]

A National "Simple Switcher" might do you good. LM2575 perhaps.

[cyberfish]

If you do decide to use a switching regulator, it may be a good idea to only use it to regulate down to 7V or so, and use a 7805 to regulate it down to 5V further, because switching regulators have relatively bad transient responses (if load suddenly increases, eg, turning on a motor, voltage will dip, and if load suddenly decreases, it will spike. Microcontrollers probably won't like that).

[Spoil9]

I think there are ready-made plug-and-play switching converters. They cost quite a bit, though. Will probably be cheaper to buy a new wal-wart.

2.8W is not really that much. A 7805 with a small heatsink will handle it just fine.

http://www.wrighthobbies.net/catalog/product_info.php?cPath=49&products_id=143
We're talking a couple bucks here.

Specifications:

Size: .93"x .93"

Input Voltage: 5v to 24v

Output Voltage: 2.5v to 12v

Max Output Current: 500mA continuous, 1.5A Peak

Efficiency: 84% @ 500mA

[cyberfish]

That looks pretty sweet, but 500mA may not be enough (~2 servos + microcontroller?), and it looks like you'll need to do post-regulation with a linear regulator yourself (so the output voltage from the SMPS may need to be considerably higher than 5V so dips won't dip below 7V). If the output voltage from the SMPS has to be, 8V for example, a significant amount of power still needs to be dissipated in the linear regulator.

I'd say 7805 with a heatsink is easier. It's not battery-operated, so power consumption is not THAT important.

[pomprocker]

I might have to think this through a little more...turns out switching power supplies are bad for this project because it dumps excess to ground which interferes with the signals on the powerline. (120khz at the zero crossing). Also the power supply might be too weak as it only supplies 80mA max and it looks like standard servos with no load need approx 180mA.