You've gotten a bunch of good replies so far, but here's my $0.02 anyway: On 12/14/07, RV9 Factory wrote: > I'm a non-EE hobbyist looking for a little education and Google hasn't > helped much. There's never been a better time to learn electronics, everything is online! But as you've found you have to know what to look for in order to find it. A maddening catch-22... > I'm building a simple power supply and need to caluculate how much power > will be available to me after my power is rectified and run through a 7805. > > The secondary on my transformer will put out 9V at 256ma. That's 9V AC, I assume. Further I'm going to assume that it's the RMS AC voltage, rather than the peak voltage. RMS, or Root Mean Square, is the calculation that tells you what the equivalent DC voltage would be to put the same power into the same load. The peak voltage is higher. You have to take into account both the peak voltage and the RMS - the peak voltage is what your power supply circuitry will see, so capacitors can't be rated at 10V, they must be rated for 15V or better. The rectifier needs to have a breakdown voltage higher than the peak voltage. At the end of the day you can expect 9V x 0.256A = 2.38W of power maximum from the transformer. For the most part you can use the RMS value for your calculations. Your meter's AC reading will be RMS (or an approximation of it suitable for this purpose) and the transformer is usually rated in RMS so you can generally assume the printed value is RMS. Since it's a transformer then the output will always be some function of the input, so if the input is rated in RMS, then the output will be rated in RMS after conversion. AC house voltages are always RMS. If you measure the peak voltage of 120V you should end up seeing peeks at around 169V. Vpeak = RMS * 1.414 (1.414 is an approximation of sqrt(2)) for a single phase sine wave AC voltage. > I'm rectifying > that with a bridge rectifier and dropping it to 5V with a 7805. I need to > figure out how much amperage I can draw from this arrangement. You can't get more current or voltage out of a linear regulator than you put in, so the amperage you can get out will be less than 265mA. The regulator and upstream components do consume some of the current as well, but let's assume here that they consume negligible current. 5V x 0.265A = 1.3W is the power you'll have available. Note that the regulator is dropping 4V, and if you draw a full 265mA through it, then it has to absorb 4V x 0.265A = 1W. This doesn't seem like a lot, but it's a small package and heat builds up over time. Make sure it's well heatsinked and ventilated if you plan on pulling full load most of the time. 1W, given time and fuel, can start a fire, but in general it's nothing to worry about, other than burning your finger from time to time. > There is a > 470uf cap across the output of the transformer before the 7805 as well as > 0.1uf caps before and after the 7805. I'd like also to figure out what my > peak draw can be and for how long. The nice thing about Farad is that it's essentially Farad = Amp x Volt x Second. The 470uF cap therefore will give you 1 second of 0.00047 amp current at one volt. Since it's on the 9V side, and let's assume the regulator can handle a very short 10A peak, then: 9V x 10A x ?S = 0.00047F ? = 5uS Since the capacitor doesn't care about voltage, you can see that putting it on the 5V side will give you more peak power, since the regulator still has to chop off 4V of that peak voltage. Keep in mind that most linear regulators don't like reverse currents, though, so make sure that the capacitor on the input is always greater than the capacitance on the output, or use a diode after the regulator to ensure it doesn't back feed into the regulator. Although in this particular case it's not an issue since there's nothing on the input side that consumes current except the capacitor and diode leakage. There's a lot more to power supply design than this, and I've certainly glossed over and skipped some minor effects and problems, but this should get you started. -Adam -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Moving in southeast Michigan? Buy my house: http://ubasics.com/house/ Interested in electronics? Check out the projects at http://ubasics.com Building your own house? Check out http://ubasics.com/home/ -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist