Low Freq. Capacitor Charger

[Jeff Niles]

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

 

[John Devereux]

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?

What voltage waveform do you want to appear on the capacitor?

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?

If you just want to end up with a charged capacitor you probably want a
flyback convertor circuit.

Also, what would be the best type of cap to use?

This is a dangerous circuit. Defibrillators work both ways you know...

 

[mike]

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

Before you build the supply, you should do some math.
Do you care how accurate the initial voltage?
What's your definition of "Hold the charge for at least 24 hours?"

If you're content to have a third of the voltage after 24 hours,
that's
one time constant. 86400 seconds / 100uF --> 864 MegOhms of equivalent
leakage resistance. Even if you ignore the leakage in the cap and the
surrounding circuit, depending on the environment surrounding your
plate electrodes, you might find that difficult to obtain.
If you need more than a third left, you need proportionally higher
equivalent leakage resistance.

Look into the technology used for radiation dosimeters.

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.

My local Walgreens sold me 10 of them for $3. The flash
power unit is just about what you need.

mike

 

[Jeff Niles]

"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.

Jeff Niles

 

[Jeff Niles]

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?

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.

Thank you for your reply,

Jeff Niles

 

[Martin Brown]

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.

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

 

[Oppie]

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

 

[John Devereux]

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 Nagle]

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.

I get that feeling too. It's straightforward to build or get
a photoflash-type switching power supply, but the "10 Hz" business
just gets in the way. Normally, one does step-up applications
like this at a few KHz, using a tiny transformer.

John Nagle

 

[Uwe Hercksen]

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.

Hello,

you should do the necessary math before. Calculate the needed current to
drive a 100 µF capacitor to a voltage of 240 V 10 Hz sinus and calculate
the necessary primary current to the transformer to get the secondary
current to the capacitor. 10 % duty cycle requires even more current.

Bye

 

[Uwe Hercksen]

I get that feeling too. It's straightforward to build or get
a photoflash-type switching power supply, but the "10 Hz" business
just gets in the way. Normally, one does step-up applications
like this at a few KHz, using a tiny transformer.

Hello,

a minatur audio transformer is not useful with 10 Hz anyway.

Bye