Yea, that was I kinda thought after I looked up prices forsome of those components. I don't know what your talking about (Willis), about using a light bulb to sense load output (sounds pretty "Rube Goldberg'ish" to me). From reading my own post over about ADC voltage sensing speed, syncronizing the ADC timer to take a sample 20us after the triac is turned on seems like a pretty good idea. As long as the triac turn on times don't vary much, I should get pretty accurate readings. I'm still looking around for that waveform of a triac turning off that drives and inductive load (to see how fast they turn off). Heh, reminds me of when I used to go up to my old programming instructor, ask him a question, and then say "never mind, I just figured it out" ;-). -----Original Message----- From: Dave Dilatush To: PICLIST@MITVMA.MIT.EDU Date: Thursday, May 03, 2001 4:54 PM Subject: Re: [EE]: [PIC]: AC Voltage Compensation Bob wrote... [snip] >A big concern is cost. Ah, OK. Back to the drawing board... >If you take out the cost of the digital circuitry (which >both methods would have), how do the basic components that make up a switching >regulator of this sort compare to basic guts of a phase control circuit >(basically a triac $0.89, opto coupler $0.35, some resistors <$0.10, and a >capacitor $0.28)? My current transformer cost is around $5.50 to $5.75 (120 & >240v capable). A switching regulator is going to be significantly more than that: several dollars each for the switching regulator chip, the power MOSFETs, low-ESR filter caps, and most especially the inductor. The main advantages of the switcher are its precise regulation against both line and load variations, its high efficiency, and its behavior during current limiting (when done right, power consumption drops to a very low level when the output is shorted; after all, 0V x 20A = 0 watts). These come at a cost, though, as the components would be more expensive than the triac-related stuff you listed. Take your pick: if the performance of a switching regulator would be worth the extra bucks, do it that way; you'll get excellent results. If not, it's back to the PIC and some sort of sensing scheme, as we've been discussing. Here's an idea. What better way to sense the power being delivered to a hot wire heating element than with another hot wire? Connect a small light bulb (like the bulb out of a two-cell flashlight) across your 2V heater output at the transformer secondary. Position the light bulb so it shines on a cadmium-sulfide photocell. Connect the photocell and a fixed resistor (you'll have to experiment to find a good value to use, or use an adjustable resistor) as a voltage divider between your PIC's +5V supply and ground, with the photocell at the +5V end. Connect the photocell/resistor junction to one of the PIC's A/D converter inputs, and you're in business. Here's how it works: The more RMS voltage that's delivered to your heater load, the more light the lamp puts out (and they both respond to RMS, by the way, without regard to the waveform). As the lamp brightens, the resistance of the CdS photocell drops. As the photocell resistance goes down, the voltage at your PIC's A/D input goes up. The PIC measures this voltage and compares the reading with the value it knows it "should" be reading for the selected power level, and adjusts the triac firing angle accordingly. As to how often to do the A/D conversion, once per line cycle would suffice; no need for high sample rates here. You could do it on an interrupt triggered at the line zero crossings. I think that's about as cheap as it can be made, Bob: a flashlight bulb, a CdS cell, and a resistor. Dave -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.