Author Options:

How do computer power supplies convert wall power to logic-useable power? Answered

Although I have some fair experience with electronics, it has all been battery-powered. I know that a transformer can step wall voltage down, but in doing so raises the amperage, correct? I know that the forward current limit on a 555 timer is lower than most wall-wart tranformers supply, and therefore lower than what a power supply can supply? Thanks!



Best Answer 10 years ago

The typical unregulated AC-DC wart uses a transformer with a ratio of high-side wire windings to low-side windings. The ac current as well as DC changes based upon the load on the DC side. This is a topic best addressed by you reading some books on basics of electronics, but briefly what you need to do is is take a transformer with at least a margin above the 555 timer operating voltage, then use a regulation circuit (generic example would be an LM7805 circuit per it's datasheet) to arrive at the correct regulated voltage. Current is not a problem, the resultant current depends upon the voltage, current does not rise based on what a power supply is capable of, current flows based upon the potential (voltage difference) in that circuit. So for example, if your 555 chip needs 5V, if your power supply was rated for 5V and 100 million amps, this would not be a problem at all because it is the voltage that determines how much current flows, and/or damages a chip if it's too high. I applaud your efforts to lean more about electronics, but suggest that you don't yet have much experience with them and would benefit greatly from reading some basic primers, library books or college 101 level course textbooks on the subject - it will have a lifelong benefit, well worth the time even if you don't ever make a career out of it.


Answer 10 years ago

However, if you are using an unregulated wall-wart AC-DC supply, then if it is rated for 5V at (whatever current, any wall wart is capable of far more current than a 555 timer chip needs), some current, then because it is unregulated, the less current the load uses, the higher the voltage floats above it's rated value. For example, if you had an unregulated wall-wart rated for 5V, it might end up closer to 6.5V powering only a 555 timer chip because the transformer alone is a VA device, the lower the current used the higher the voltage rises. This is why a regulation stage afterwards is so important, or a power supply that regulates itself so the circuit voltage does not fluctuate based upon the load.


10 years ago

Another good source is Half-Priced Books. (Think CHEAP...;-)) I've been partial to just about anything that the ARRL publishes (Amateur Radio Relay League) as Amateur Radio has always been a hotbed of experimenters and DIY kinda people. They love to share knowlege and tech with others, and can explain things in layman's terms (as opposed to the professor who has tons of theory and couldn't explain how to tear a wet paper bag).