At 05:53 PM 9/16/2011, V G wrote: >I want to be able to generate a low current isolated supply (for driving m= y >solid state relay MOSFETs) gate-to-source. I googled and found this: >http://www.maxim-ic.com/app-notes/index.mvp/id/1932 > >1. How does this circuit work? I'm guessing the 74HC14s give a sharp signa= l >to U1 whose outputs oscillate signals into high pass filters C1 and C2. At >that point, I'm guessing the signal looks more like a sine wave which pass= es >through the full wave rectifier comped of those 4 diodes and the capacitor= .. >Tell me if I'm wrong. Nope - you are pretty much bang-on. However, the signal coupled=20 through the capacitors is a square wave, not sine. You can think of the coupling capacitors as high-pass filters but=20 that is NOT their intent and might cause confusion at a later=20 date. Rather, they present a low impedance at the high switching=20 frequency used, and therefore couple most of the energy to the load. The coupling capacitors present a fairly high impedance (~13k) at=20 line frequency (60 Hz). There are several down-sides with the circuit as presented. 1) the rectifier diodes drop the available voltage=20 significantly. Note that the app note states that this circuit will=20 supply 3v3 from 5V input. You would have to use a voltage=20 doubler-type circuit if you needed to get an output somewhere near=20 5V. Or: run the left side of the circuit from a supply voltage=20 significantly higher than 5V. You would need to change the 74HC=20 parts to 74C or 4000 family parts if you do that. The MAX628 is most=20 likely rated to run from 12V or higher. 2) the coupling capacitors will couple unwanted high-frequency energy=20 across the isolation barrier. 3) the amount of energy coupled across the isolation boundary at line=20 frequency is actually fairly high. If you were relying upon this to=20 provide isolation from a line-operated supply, the leakage current=20 could be as high as 1mA from a 120V supply. That is well above the=20 'ouch' level. (assumes 10% accurate capacitors) >2. Is there a simpler way to generate an isolated supply? Several. 1) Use a transformer. I routinely use really inexpensive Common Mode=20 chokes as isolation transformers. Advantage: really high isolation=20 voltage, really low coupling capacitance. Disadvantage: drive=20 circuit can be complex. However, google for "Build Your Own=20 Ultra-Low-Cost Isolated DC-DC Converter" for a really low cost=20 implementation:=20 =20 Disclosure: its my design - and it works REALLY well. 2) Use photo-voltaic opto-isolators. Feed current into the input=20 LED, get up to 10 or 12Vdc out at several uA. Advantage: really high=20 isolation voltage, really low coupling capacitance, really=20 simple. Disadvantage: only capable of supplying uA current levels. dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .