On Sunday 20 December 2015 16:42:48 John Kasunich wrote: > On Sun, Dec 20, 2015, at 03:45 PM, Gene Heskett wrote: > > On Sunday 20 December 2015 04:27:39 Gene Heskett wrote: > > > > [... back to readable size, on my charge-pump detection to run the > > SSR's] > > > > > Has anyone else concocted such a circuit, and were you able to get > > > enough pump power out of it running the BoB on 5 volts, with the > > > pump at 500 hz, to run an SSR without resorting to an external > > > current booster? > > > > Now I am both bumfuzzled and confused. There really ought to be a > > formula for calculating the size of the capacitors needed, but there > > is absolutely zip in ITT #5, nor in the hambook of about the same > > vintage. > > > > So, I guess its build it, and test it by the old fashioned cuss & > > cry method. Seems to me there really ought to be, in the year 2015, > > a plug in the blanks formula for that. Sigh... > > My gut feeling is that you won't be able to do it. > > First, lets make sure we are talking about the same circuit. A google > image search found this schematic of a charge pump, which is the > circuit I normally use. > http://www.zen22142.zen.co.uk/Theory/diode_pump/Image152.jpg > (refer to the circuit on the left)
Yes, thats the one John K., except the circuit ground is the + terminal, and the diode and cap pols reversed. The idea is to make the 1st cap's available charge so low at powerup, that there will not be a 1 or 2 cycle on time at powerup. > > Assume that the input is swinging from 0.0 to 5.0 volts. Also assume > that the loading is negligible. When the input is low, the voltage on > the left side of Cp is 0.0, and the right side is -0.7V due to the > drop in D1. When the input goes high, the left side of Cp is 5.0V, > and the right side is 4.3V, assuming zero change in the voltage across > Cp. Because of the drop across D2, the output is 4.3-0.7 = 3.6V. > Pretty marginal. And that is with zero load, and an output that > swings a full 5V. No real output will swing all the way down to 0.0 > or up to 5.0V, so you lose another couple tenths of a volt. Unless cmos drivers are used, true. > The above assumes that there is NO CHANGE in the voltage across Cp. > That means no current flows into or out of Cp, and the circuit > delivers no output current (I said "assume loading is negligable"). > > To deliver any load current, the voltage across Cp has to change, and > that will further drop the output voltage. Say you need a load > current of 10mA. That is 10 milli-coulombs per second. If the input > frequency is 500Hz, then each cycle needs to deliver 0.010/500 = 20 > micro-coulombs. If you need an output voltage of 3V, and the ideal > no-load output is 3.6V, then the voltage across Cp can only be allowed > to drop 0.6V when 20 micro-coulombs of charge is extracted from it. C > = Q/V = 0.000020 / 0.6 = 33uF. That is a HUGE pump cap, and will > badly load down whatever is driving it. That is the reasoning behind what I was trying to do, but all my reference books are apparently too prehistoric to know about coulombs. > When I make a charge pump, I usually think of something like Cr = > 0.1uF, and Cp = 0.01uF. Charge per cycle at 0.6V droop is 0.006 > micro-coulombs, and I run it at 10kHz instead of 500Hz, for a total > load current of 60 micro- amps. Put a 100K load resistor as a load > and drive the gate of a logic level MOSFET and it works fine. But > getting milli-amp currents out of a low-powered charge pump is much > much harder. The mosfet isn't a problem, nor would a small sig npn transistor, but power for it is. Not because its needed, but because of the external powers on-off transients feeding in and causing a few cycles of "on" before the rest of the circuit is ready. That will obviously defeat the purpose unless that external supply is itself fully floating. I do have a smallish 12.6.ct.12.6 tranny, good for 200 mills or so, that I can make that psu with, and be completely isolated from a ground reference except for the capacitance in Cp, and of course the leakage capacitance in the transformer. But theres no base thread, so I'm stuck at 500 hertz. This almost sounds like a good place for a cmos 7556 if I can figure out how to disable it until the psu is up and stable. I am home from the woofs cataract surgery, which I was up well before breakfast to attend to, so I'll be later this afternoon before I am able to play. Right now I feel a catchup nap coming on. :) Thanks for confirming my worries. > John Kasunich > > > > > > ---------------------------------------------------------------------- >-------- _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Some mill pix are at: Genes Web page <http://geneslinuxbox.net:6309/gene/GO704-pix> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
