On Sun, Jan 23, 2011 at 6:20 PM, RussellMc wrote: > LED and load swapped and slightly altered. > Cap is charged by D2 and pumped by FETS. > LED load is driven by D1 or by Cap via R1. > > *** NB NB NB *** > Electrons can jump over small breaks in circuit in this tolopogy :-). > eg like one immediately under LED string. Ah, wireless! > Cap charges via D1 when Q2 is on > Cap -ve pulled to V+ when Q1 is on and Q2 off. > Cap+ now discharges via R1. > R1 limits flash brightness, duration and phase of Moon. Ok, I understand the basic concept of how the cap is being switched. I think I'm a bit confused in how D1, D2, and R1 interact. When Q1 is on and Q2 off (flash), what stops C1 from discharging via D1 right back into itself? > Not that FETs as shown need to be N Channel so that body diodes behave. > This means Q1 gate needs to be above Vin but can be biased from Vout. > Gets messay albeit cheap and easy enough. I'm not sure I understand this. Q1's gate needs to be above Vin so the body diodes conduct in the correct direction, right? I'm not sure how to achieve that by using Vout. This circuit won't neccesarily be operated in a continuous flashing cycle. In fact I'd guess it's more likely that it will sit charging the cap (Q1 off, Q2on), wait some arbitrary amount, then a flash. One thing I like about this setup is that I think I'd be able to add another MOSFET and PWM the LED directly off the 12V line for times when constant light is required. These would be two very different modes, so I could ensure there would be no crossover with Q1/Q2 and the charge pump circuitry. > A simple inductive SMPS gets very attractive. > My often mentioned olde MC34063 & friends IC will do what you want with 1= IC > and a few glue parts. > eg take fig6 here and add one extra diode, add an extra diode from Vin to > Vout and adjust R2 to suit. > Inductor also to suit. > Enable high voltage operation as required by enabling/disabling smps by a= ny > of several means. > Switch is good for 1.5A AFAIR BUT do design for package dissipation > eg =A041/ =A085 / 97 degrees C/Watt =A0 DIP / SOIC / QFP packages > Efficiency is lower than modern ICs and opn at not much over 100 kHz mean= s > inductor is larger than some BUT quite likely OK in this application. Is this a suggestion as an alternative or adjunct to your charge pump? Thanks! Josh --=20 A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. =A0 =A0 =A0 =A0 -Douglas Adams --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .