simple dc-ac convertor diagram and transformer stuff

[daguuy]

since i don't have a dc-dc convertor yet (will get one eventually), i want to built one of my own for an appatizer untill i can get one. what i plan to do is convert DC into AC, use a transformer to get 3.3v, and use a bridge rectifier to change it back to DC. then get a dc-dc convertor after that fails . here's the questions:

1. what's the simpleset way to make AC with just using parts i can get from old junk? i don't care about frequency as long as it can power a transformer. how efficient are transformers anyway?

2. how do you determine how many coils on each end of transformer you need to step 8.4v down to 3.3v? is it just a simple 33/84 ratio or is it more complex?

i already have a high-power bridge rectifier, all that i need to make a transformer, and tons of junk to get compenents from, just need the info. i googled and searched but all i found was complex stuff for get 120v from 12v and step up stuff like that; that's not what i need

[timmeh87]

youre going to have fun trying to run a portable off that, the power will be incredibly noisy.
it might work...

i guess you could power a dc motor that runs an ac generator

[daguuy]

LCD backlights are powered by transformers that need AC so there must be a way to do it with transistors or oscillators or whatever it is that does it without a motor
EDIT: oh yeah i forgot i have something like this bookmarked in my other browser anyway, here's a 12v->120v AC invertor circut that could probably be modified to output 3.3v. would i need to change any of the resistor values? i don't really know much about center tap transformers so would the 1st or second one be right?


i have a hunch it's the first one, is it? and how many coils would i need?

[usbcd36]

I'd start with an existing transformer, unwind the secondary and find the amount of turns. Set that aside and unwind the primary. Count the turns from that one too. Then use your ratio and find the max amount of turns you could have (exactly like you would in a stoichiometry problem where you find the limiting reactant) and wind both back in their respective spots. Basically, take the secondary turns and divide that by 33, write down the result and divide the primary by 84 and write that down too. Whichever one is smaller, that's the amount you'll need to do. Truncate the decimal and multiply that by 33 for the number of turns in the secondary, then by 84 for the number of turns in the primary.

Yes, it's a little complex, but it's a heck of a lot easier than making your own transformer from scratch.

[superdeformed]

Something to keep in mind if you go through with this is that the output voltage is going to be very uneven. It will have to be higher than 3.3V as the bridge will cause it to drop slightly (you can find out how much by looking at its spec sheet) and you will also need a fairly large capacitor after the bridge to keep it up at the voltage you want. Generally if you were using a transformer it would be followed by some form of regulator to keep the voltage exactly where you wanted it, but if you were to do that then you might as well cut out the middle man and just use the regulator.

[codeman]

um...i dont get it. Why would you invert a DC voltage just to step it down to another DC voltage? There are a lot of DC-DC converters that use transformers, but the only point to them is to provide isolation, because they're converting semi-regulated AC to a regulated DC voltage, and no one wants to get fried if they touch something grounded and the positive terminal of the device you're powering. Anyway, the website that you mentioned shows a half-bridge inverter, if you're interested. Also, you could get it to output 3.3v, but since the switching frequency and duty cycle of the switches would be constant, the output would drop along with the battery voltage, unless you used some sort of control system which is a real pain in the butt.

Anyway, for those of you looking to build a DC-DC converter of some sort, the easiest is a buck-converter. The best one by far is called the SEPIC, look it up. It can step the voltage down and up. So if you have a battery that supplies 14v or something, and you need 12, it will step it down to 12 while the battery is above 12v, but will then step it up to 12v when the battery voltage drops.

have fun

[sniper_spike]

This guy sounds like he knows what he's talking about daaguy, I think you should listen to him.

[marshallh]

Converting to DC - AC -DC is inefficient and a roundabout way of solving a problem. You need a straight DC-DC converter like codeman said. Why don't you consider the National part no# LM2670 , it can output in 3.3v. You'll need some supporting circuitry from digikey but I can help you with that.