# Help with dropping 20 volts for 8kw AC or DC

Discussion in 'General Electronics Discussion' started by Richie8Mann, Nov 19, 2012.

1. ### Richie8Mann

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Nov 19, 2012
The project I am building needs to turn 120 volts AC into 100 volts DC while being able to withstand 8 kw of current. I already have a 10 kw bridge rectifier. The 20 volt reduction could happen before the rectifier by using a transformer or after the rectifier using a voltage divider circuit (using two resistors). The problem is, either one of those options is not possible due to cost. Transformers in that power range are extremely expensive and even 1 kw resistors are \$70 each at best from what I could find. Does anyone have any other ideas on how to drop those 20 volts? I have plenty of area for this project, so I was even thinking of some thick wire run for a distance or maybe some pieces of graphite from the art store to create a crude voltage divider. I have thought of different cooling methods that would need to be employed using water, oil, or fans. Finally, I was thinking that furnace heating element resistive wire might work in place of resistors in the divider circuit. I think it can handle 220 volts at 800 watts (of course multiple strands would have to be paralleled), but I think that would be better for lowering the amount of current available instead of lowering the voltage.
Thanks in advance for any ideas.

Last edited: Nov 19, 2012
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
8 kW isn't current, and a bridge rectifier isn't rated in kW either.

Perhaps you can describe it in terms of current.

i.e. how much current from your 100V DC supply? (80A? really?)

Also, how good does the voltage regulation need to be, and how much ripple is acceptable?

There's more than 20V, 120VAC gives you 170 VDC.

Knowing what this power is required for might help too.

3. ### Richie8Mann

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Nov 19, 2012
Yes, it's 80 amps - I though that was obvious from my post. Ripple is not important for my project and I would have stated so if it was. I was simply trying to see what other options are out there for reducing voltage. And yes, the bridge rectifier I have was not rated in kilowatts but I was stating that for simplicity - my bridge rectifier is 100 amps at 120 volts if you need me to be simplistic about it. I have no idea where you get 170 volts - I'm trying to go from 120 volts to 100 volts.

4. ### Richie8Mann

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Nov 19, 2012
This is the problem with the internet today. I remember in the 90's when if someone had an idea outside the mainstream it was encouraged, not derided. Now days it's more likely to have someone try to make you look foolish to make themselves look superior than to have someone offer help. Never mind, I'll find the answer myself as I always do.

5. ### Harald KappModeratorModerator

12,455
2,984
Nov 17, 2011
The most simple way is to use common terms and expressions and not leave it to others to figure out what you might have meant.

For the voltage:
120 V AC = RMS
120 V AC = 120 V *sqrt(2) peak = 170 V peak

The rectifier will give you the peak value and that is what a smoothing capacitor will store. You will have to regulate from 170 V to 100 V. At 80 A that will give you a loss of 5.6 kW!
Apart from the technical difficulties of dissipating that amount of energy - it is pure waste and not a sensible way of handling energy in our times. Who is going to pay for that energy? How many barrels of oil will be burnt for nothing but heat?

I think for this application soem kind of switch mode regulator is the appropriate solution.

Harald

Last edited: Nov 19, 2012
6. ### duke37

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Jan 9, 2011
A voltage divider will waste more energy than a simple resistor and so generate more heat.

You have not said what the load is or how much ripple is permissible. A controlled bridge could be made to chop the output waveform but the ripple will be very high unless a capacitor half the size of a house is used. Also, the neigbours may not be very happy with the amount of interference put into the mains.

With single phase you get two pulses per cycle, with three phase you get six pulses per cycle this reduces any capacitor requirement