Low Freq. Capacitor Charger
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From: Martin Brown on 24 May 2010 09:37 On 24/05/2010 14:05, Jeff Niles wrote: > On Mon, 24 May 2010 01:52:55 -0700, mike<spamme0(a)go.com> wrote: > >> If you need more than a third left, you need proportionally higher >> equivalent leakage resistance. >> > > The charge circuit will be disconnected from the cap after charging, > thereby leaving the cap fully isolated. I was also planning to use a > low leakage variety, if they are available at 250V rating. Given this, > what would be the longest interval I could expect before the cap > discharges by half? Unless it is in evacuated chamber it will be gradually attracting any charged dust and ions from the air to the open capacitor electrodes - in much the same way that old CRT screens do. How long it will hold its charge depends on the leakage rate of the storage capacitor, quality of construction of the jig and the relative humidity amongst other things. A 100uF capacitor at 250v will give you a nasty bite. You should make provision to earth the thing when not in use with a few meg across it. > >> As for the power supply... >> Last I checked, places that develop pictures from disposable cameras got >> paid 22-cents for each used camera they returned to the factory for >> refurbishing. > > For this application, it must be charged with the 10Hz oscillator as > described in my OP. It's part of an experiment. Why 10Hz? It would be easier with a faster oscillator and a flyback step up. And do you want regulated 250V DC on the plates at the outset? I suspect you will find capacitor leakage current a problem. Why can't the plates be actively maintained at 250V DC ? Regards, Martin Brown
From: John Larkin on 24 May 2010 10:05 On Mon, 24 May 2010 06:32:18 GMT, jellniles(a)optolux.com (Jeff Niles) wrote: >I need to build a circuit that will charge a 100uF capacitor to 240V >with a _10 Hz_ pulse, and then have the cap hold the charge for at >least 24 hours. The purpose is to establish a static field via a pair >of plate electrodes connected in parallel with the cap. > >I am thinking of an LM555 feeding a 10% duty cycle pulse to an 8R:1K >miniature audio transformer, or two of the latter with output sides >wired in series to increase voltage. The cap would be connected across >this, and isolated from the charging circuit with a switch after >charging. > >Can anyone please any suggestions or improvements regarding this type >of circuit? > >Also, what would be the best type of cap to use? > >Thank you, > >Jeff Niles > Film capacitors - mylar, polycarb, like that - can have self-discharge time constants of years. But 100 uF is a huge heap of film caps. A 1 uF cap would be about right if the purpose is to hold a charge on well insulated plates. Why 10 Hz? John
From: Oppie on 24 May 2010 10:21 Sounds like we are being asked to assist with a homework (or extra credit) problem... Typical student type (or Physicist) with little understanding of practical electronics. Oppie
From: John Devereux on 24 May 2010 17:05 jellniles(a)optolux.com (Jeff Niles) writes: > On Mon, 24 May 2010 08:22:08 +0100, John Devereux > <john(a)devereux.me.uk> wrote: > >>jellniles(a)optolux.com (Jeff Niles) writes: >> >>> I need to build a circuit that will charge a 100uF capacitor to 240V >>> with a _10 Hz_ pulse, and then have the cap hold the charge for at >>> least 24 hours. The purpose is to establish a static field via a pair >>> of plate electrodes connected in parallel with the cap. >> >>"Static" == "DC" so what *exactly* do you mean by a "10 Hz" pulse? >> > > Sorry, if I did not explain this fully. The 10Hz charging circuit > would be disconnected from the cap after charging, thereby leaving the > isolated cap holding a static charge. Sorry Jeff, but specifying "10Hz" with no other contraints is non-sensical. "10Hz" could be a single high energy pulse charging it in a few ms, or a train of current pulses taking an hour - indistinguishable from a linear ramp. I doubt anybody is going to take this requirement seriously unless you are able to explain better what it is you want, or perhaps explain the background to the "experiment" so we can work it out ourselves. -- John Devereux
From: David Eather on 24 May 2010 17:11
On 24/05/2010 4:32 PM, Jeff Niles wrote: > I need to build a circuit that will charge a 100uF capacitor to 240V > with a _10 Hz_ pulse, and then have the cap hold the charge for at > least 24 hours. The purpose is to establish a static field via a pair > of plate electrodes connected in parallel with the cap. > > I am thinking of an LM555 feeding a 10% duty cycle pulse to an 8R:1K > miniature audio transformer, or two of the latter with output sides > wired in series to increase voltage. The cap would be connected across > this, and isolated from the charging circuit with a switch after > charging. > > Can anyone please any suggestions or improvements regarding this type > of circuit? > > Also, what would be the best type of cap to use? > > Thank you, > > Jeff Niles > > ETI and EA magazine both did zener diode testers using a circuit very similar to what you seem to be suggesting (their output was clamped at 75 volts). Perhaps that is a place to start your search (use http://www.siliconchip.com.au - both ea and eti are now defunct but all their published info is kept by Silicon Chip magazine) Others have suggested the right types of caps to use. |