Hello Lindy, There are many versions of AC. When listening to sound, if it comes from an artificial source (speakers) there is an amplifier involved some where. The signal that drives the speakers is AC (not quite as potent as the wall socket) unless its in the 300 + Watt output range. The Transistors, resistors and capacitors in the amplifier are all DC components that are arranged in a way to take a small signal of several millivolts, and produce an output of several volts. In this instance DC input power is used to amplify the AC signal and to produce AC output power. A PWM signal is a series of DC pulses that change their pulse width based on an AC modulating source. Lets say the Source signal is 1 Khz, as in Dave's illustration. With no AC input signal the PWM signal is a square wave with the on time and off time being equal. So when there is no modulation the output is 2.5 volts DC if smoothed by a lowpass filter. (This is similar to the filter capacitors on your DC power supply for your AC powered PICs walwart). In this rest position (no modulation signal applied) the output is at a 50% duty cycle (the square wave) when a modulating signal is applied, then conditions change. If we are to modulate the PWM output at 100% modulation, what happens is this: The pulse with at 1 extreme, peak (lets call it the negative half cycle of the modulating AC signal) the DC output of the PWM will have 1 100% duty cycle (0 Volts DC) this is an instantaneous condition as the modulating signal is dynamic and the next PWM cycle may be at 99% duty cycle. when the AC signal crosses Zero again the PWM duty cycle is again at 50%. Then the Modulating signal starts its positive half cycle and the PWM duty cycle approaches and goes to 0% at the positive peak it then drops back through Zero and repeats. For Audio use we don't want to listen to the high frequency PWM DC pulses. We want to hear the original 1 Khz signal. The use of a low pass filter will smooth the DC pulses leaving behind the average DC value of the PWM signal. This represents the 1Khz signal that was used to modulate the PWM signal. It is a varying DC voltage from 0 to +5 Volts following the original 1Khz Sine Wave. This is an AC signal superimposed on a DC voltage. To recover the AC signal, and eliminate the DC voltage component a Coupling capacitor of sufficient value to pass the 1Khz signal is used. This effective block the 2.5 volt average DC voltage and couples the 5 volt peak to Peak 1Khz sine wave to an output circuit (speakers, headphones, amplifier etc.) I hope this wasn't too long winded, pictures and illustrations would make this a much easier thing to comprehend. Hope this helped, Vern ----- Original Message ----- From: Lindy Mayfield To: Sent: Friday, June 18, 2004 5:23 PM Subject: Re: [EE]: Beginner help in understanding circuit > > > A PWM Signal is a turning on and off of a DC voltage, in the > > case of a PIC, > > 0 to 5 volts. A coupling capacitor is used to block that DC > > component and > > pass the AC component. > > Yes, you are correct. I really need schooling in electronics. If you are correct in what you say, and I of course assume that you are, then I need to learn the basics better. I work full time, so my only recourse for learning is spare time, books, internet, lists (like this), and the kindness of strangers. > > But just to show you how ignorant I am about electronics, I think that PICS and things with batteries are all DC and that AC is another beast that I only have to worry about if I want to use the power from the walls. (Which I'm afraid of for personal safety reasons.) > > But how a PWM can be AC and DC I just don't get... yet. I will. Thanks for your help. > > --- > Outgoing mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.699 / Virus Database: 456 - Release Date: 6/4/2004 > > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body