On Fri, Sep 16, 2011 at 9:20 PM, Dwayne Reid wrote= : > At 05:53 PM 9/16/2011, V G wrote: > >I want to be able to generate a low current isolated supply (for driving > my > >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 > signal > >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 > passes > >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 > through the capacitors is a square wave, not sine. > > You can think of the coupling capacitors as high-pass filters but > that is NOT their intent and might cause confusion at a later > date. Rather, they present a low impedance at the high switching > frequency used, and therefore couple most of the energy to the load. > > The coupling capacitors present a fairly high impedance (~13k) at > line frequency (60 Hz). > 1. How do you calculate the impedance of a capacitor at a particular fequency? 2. I thought the definition of a filter is to present low/high impedance at low/high/high/low frequencies. There are several down-sides with the circuit as presented. > > 1) the rectifier diodes drop the available voltage > significantly. Note that the app note states that this circuit will > supply 3v3 from 5V input. You would have to use a voltage > doubler-type circuit if you needed to get an output somewhere near > 5V. Or: run the left side of the circuit from a supply voltage > significantly higher than 5V. You would need to change the 74HC > parts to 74C or 4000 family parts if you do that. The MAX628 is most > likely rated to run from 12V or higher. > > 2) the coupling capacitors will couple unwanted high-frequency energy > across the isolation barrier. > > 3) the amount of energy coupled across the isolation boundary at line > frequency is actually fairly high. If you were relying upon this to > provide isolation from a line-operated supply, the leakage current > could be as high as 1mA from a 120V supply. That is well above the > '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 > chokes as isolation transformers. Advantage: really high isolation > voltage, really low coupling capacitance. Disadvantage: drive > circuit can be complex. However, google for "Build Your Own > Ultra-Low-Cost Isolated DC-DC Converter" for a really low cost > implementation: > < > http://electronicdesign.com/article/power/build_your_own_ultra_low_cost_i= solated_dc_dc_converter.aspx > > > > > Disclosure: its my design - and it works REALLY well. > > 2) Use photo-voltaic opto-isolators. Feed current into the input > LED, get up to 10 or 12Vdc out at several uA. Advantage: really high > isolation voltage, really low coupling capacitance, really > simple. Disadvantage: only capable of supplying uA current levels. > Wow, that photo voltaic isolator method was right under my nose all along. Perfect for driving my MOSFET SSR. One chip should provide the voltage necessary to drive a few SSRs (I'm guessing at least 10) at slow speeds. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .