Charging Circuit
Charging Circuit
(OP)
Please forgive my ignorance in the relm of electrical engineering as I am a mechanical designer. I would like help with a project that I am working on. The description is as follows. I have a flash bulb, that I would like to flash with 1100 joules of power. The charge bank consists of 3 capacitors 1200u, 500V, and 3 caps 1600u, 500v. I would like to charge these capacitors after flashing in under 10 seconds. Can anyone help me with this circuit?
Sincerely,
Mark
Sincerely,
Mark





RE: Charging Circuit
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Charging Circuit
RE: Charging Circuit
So...1100 joules of energy to return to the caps in 10 seconds.
Q = C V......
Q = 8.4mF * V
Cap energy is 0.5 * C * V^2.
Sounds like you want a controlled boost converter to pour charge back into the caps.
RE: Charging Circuit
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Charging Circuit
A quick search (Digikey) uncovered the Linear Technology LT3420, and the data sheet even has examples of paralleling multiple chips to charge larger capacitors quicker - but still not as large as you indicate. (Digikey also has a Allegro Micorsystems A8439 as well, but given it's small surface-mount package, it might be hard for you to use).
Check out Linear Tech, Micrel, Maxim, National Semi, Fairchild Semi, and TI. Look for 'power management' ICs, boost-type switcher, PWM chips, and/or for photo flash chips, application notes, or circuit ideas. If you get lucky, you might find one of these companies has a demo board available.
RE: Charging Circuit
First of all, 1100J out of a total of 8400uF. That requires 512V. The capacitors are being used above their 500V rating.
512V in 10 seconds takes 430mA average, probably in the neigborhood of 1A peak. That's a 500W peak switching circuit....not trivial.
If someone asked me to design that, I'd give them an estimate of 80 hours for analysis and a crude breadboard, then several more weeks to fine tune and get boards layed out.
It just doesn't sound like a simple circuit that someone can "help you with" on a chat room.
Darrell Hambley PE
SENTEK Engineering
RE: Charging Circuit
RE: Charging Circuit
Linear Tech Application note 118, some of which appeared in an article this month - Electronic Design News, Aug 7, 2008, "High-voltage, low noise dc/dc converters" . These were in the pile of papers on my desk when I read your post.
RE: Charging Circuit
If you're using a regulating circuit, even a bang-bang regulator, you can safely charge the caps to their full 500 volt operating voltage. You will get up to 10% more energy during a pulse discharge such as that through a strobe lamp than the rated capacity of the caps would indicate. The discrepancy exists because the standard capacitance measurements are performed using a sine wave at 120Hz, and the discharge 'wavefront' is much faster than a 120 Hz sine wave.
A correctly designed line operated multiplier should recharge your cap bank in about 2.5 seconds, with an initial surge current of around 20 amperes lasting for the first few ac half-cycles, then tailing off to less than 2 amperes by the end of the cycle. Be sure to use zero crossing control on any thyristor you might use to control the multiplier.