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4hv.org :: Forums :: High Voltage
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Rectifying 120vac for charging my capacitor bank.

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David.Lightman
Sun Mar 23 2008, 02:01PM Print
David.Lightman Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38
Hello everyone. I am looking to build a simple circuit to rectify 120vac to charge my capacitor bank. I have already built one that seemed to get hot. I ran the AC through a 1:1 isolation transformer. Then ran it through a bridge rectifier PB156m. I didn't really build out a circuit as much as just hook it all together. It charged the bank a few times, but then my switched got welded, I think, and it was all down hill from that point. ;)

I now have a few bridge rectifiers KPBC804. I will be using these to build a similar setup but want to know if I need anything more than just the isolation transformer, bridge rectifier, and a simple fuse. I am looking to build something simple, clean, and durable. I also need to build 12vdc into this so I can trigger an SCR.

Thanks, D.L
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Dr. Dark Current
Sun Mar 23 2008, 02:15PM
Dr. Dark Current Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The problem is that when you turn the charger on, the capacitors draw as much current as they can. This can result in a blown fuse or welded switch contact.
You gave to limit the charge current. Two popular methods are 1) Light bulb or 2) AC capacitor, of suitable rating connected in series with your mains input.

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David.Lightman
Sun Mar 23 2008, 03:57PM
David.Lightman Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38
How would I go about limiting the draw from my power circuit to the caps? I like the idea of the light bulb. This I have and can do easily enough. Does it go on the AC outlet side of the circuit or between the circuit and the caps? My only issue with the light bulb is that I want, in the end, to have a self contained power supply. D.L

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uzzors2k
Sun Mar 23 2008, 08:01PM
uzzors2k Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Doesn't matter, as long as it's in the main current path it will restrict the flow. Put it before or after the rectifier so you don't mess with the isolation transformer.
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David.Lightman
Sun Mar 23 2008, 10:29PM
David.Lightman Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38
Thanks. The lamp worked out great. I wired it between the isolation transformer and the bridge rectifier. I like that it illuminates as power flows and gets dimmer as the bank fills. Light goes out when the caps have about 135v in. I let them fill to around 160 before I turn off power. What causes the light to go out? Does it become more resistive as it heats up?

I think I want to find another way to limit the current other than the lamp. It's good, but not so compact. I'd like to eventually build a meter circuit so I can have a meter inline as part of the power supply.

D.L
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Proud Mary
Sun Mar 23 2008, 11:33PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Current will flow through the light bulb, lighting it up, until the capacitor is fully charged.

If we know the size of the resistor (the light bulb) and the size of the capacitor (which has capacity to store an electric charge) we can easily calculate the time it will take for the capacitor to fully charged through that resistor by a simple formula called the time constant which you are sure to come across soon. The fact that a given resistance value will take a definite time to charge (or discharge) a capacitor of a given size is one of the basics upon which the whole science of electronics is built.
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David.Lightman
Mon Mar 24 2008, 02:06AM
David.Lightman Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38
Some pics of what I have done so far. Link2 and Link2

Funny, when I have the 20-2400mfd caps it charges to around 163vdc, but if I add in the 5 6000mfd caps it charges slower which I expected, but it will not charge past 152vdc. Not sure why that is.

Now I need to figure out a way to replace the light bulb with something smaller and add in a 12vdc circuit so I can trigger my SCR too.

D.L
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