Power Supply for mains circuit _linear or switch mode

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Andy_2020
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Power Supply for mains circuit _linear or switch mode

Post by Andy_2020 » Mon Nov 02, 2020 9:00 am

For Nuts and Volts Magazine
Power Supply Question: 11/2/2020
From: Andy_2020

I am building a power supply. The issue is understanding the ripple current for a capacitor. I am building a linear power supply for conversion of 110 AC 12 amps, to 110 DC 12 amps.

Question:
1) How do you select a capacitor when the ripple current appears to keep increasing? When I select a capacitor with lower ESR this causes the ripple current to increase? Again, how do you select the capacitor? Does anyone know where I can find the proper circuit diagram for or where I can purchase a linear power supply for the conversion of 110 AC 12 amps, to 110 DC 12 amps.

Below are my calculations and circuit diagram. I have neglected to include a fuse/breaker and a transformer for this circuit, but the basic explains the issue about selecting a capacitor for a linear power supply.

2) A switched Power supply is known to be very efficient. Does anyone know have a circuit diagram this circuit or where I can buy a switched mode power supply for the conversion of 110 AC 12 amps, to 110 DC 12 amps

From Nuts and Volts Website, we get that the equation for calculating ripple.
We are calculating for three scenarios
C= I load /Frequency *(voltage)
Change in voltage = delta Volts = I load /Frequency * C
I load = 12 amps, Frequency = 120 hertz, Voltage= 120 volts
Solving for two cases
C = 400 UF = .000400 Farad
1 C = 10,000 UF = 0.01 Farad
2 C = 20,000 UF = 0.02 Farad
3 C = 20,000 UF = 0.1 Farad

1) Calculating we get delta volts = 12 amps/(120 hertz) * (.0004 F) = 250 volts
2) Calculating we get delta volts = 12 amps/(120 hertz) * (.01 F) = 10 volts
3) Calculating we get delta volts = 12 amps/(120 hertz) * (.02 F) = 5 volts
4) Calculating we get delta volts = 12 amps/(120 hertz) * (.1 F) = 1 volts …. This a very low ripple
1) Change in voltage = ripple = 250 volts
2) Change in voltage = ripple = 10 volts
3) Change in voltage = ripple = 5 volts
4) Change in voltage = ripple = 1 volts
Using a higher capacitor reduces the ripple volts, reduces the ripple from 250 volts to 1 volt
Calculating ripple Current

I ripple = square root (P/ESR) = square root (1400watts/.062 ohm) = 150 amps and this value is larger than the stated value of 7 amps as stated in the table.

As the capacitor increase from .0004 F to .01 F, the ESR reduces about 61 micro ohms. That is great, but now I am concerned with the rated increase in ripple Current where at .01 uF, the ripple current 7.7 amps, and that appears contrary to reduce the overall ripple.

dyarker
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Re: Power Supply for mains circuit _linear or switch mode

Post by dyarker » Mon Nov 02, 2020 10:11 am

How are you regulating the output to 110VDC?

The peak of 110VAC is 155V. At no load that is what the filtered DC will be. As load current increases, output ripple voltage increases as the filter capacitor supplies the current between the input voltage peaks. The input current is zero till the input voltage is greater than whatever the capacitor discharged down to. As input voltage decreases after peak, the input current goes to zero again when input goes below output. The input "ripple" current surges increase because the available time to charge the capacitor is less with larger capacitor. (power in must equal power out. power out is continuous (droops some between peaks), but input charging only near peak input.)

Use the minimum capacitor that keeps output voltage above ... say 120VDC ... at 12Amps , then regulate to 110VDC. (note that voltage before regulator will be higher at lower load currents,)

Cheers,
Dale Y

Andy_2020
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Re: Power Supply for mains circuit _linear or switch mode

Post by Andy_2020 » Mon Nov 02, 2020 11:06 am

Yes, it I have input of 12 amps and 110 volts rms AC and going to 12 amps and 110 volts DC. This is a high volts DC, and this technology was in the 1900's , a very not a very efficient power conversion. Now everyone is using a switch mode power supply. The load is 15 ohms at 120 Volts and 12 amps DC. I have looked at this carefully, and every paper I see is in the 12 volts and 12 amps output, but it does not meet my needs.

That is why I went to Forums, to see how people are now converting 12 amps and 110 volts rms AC and going to an output 12 amps and 110 volts DC. I not found a vendor who produces power supply with these specs.

dyarker
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Re: Power Supply for mains circuit _linear or switch mode

Post by dyarker » Mon Nov 02, 2020 2:13 pm

I have never of anything needing 110VDC at 12 Amps. (probably why no vendor makes one)

And not safe unless isolated from mains.

What is the load?
Dale Y

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jwax
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Re: Power Supply for mains circuit _linear or switch mode

Post by jwax » Mon Nov 02, 2020 3:22 pm

Maybe a big-ass 90 VDC motor?
Would like to hear more about this 15 ohm, 110 VDC load, which BTW, is 7.3 Amps.
WA2RBA

dyarker
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Re: Power Supply for mains circuit _linear or switch mode

Post by dyarker » Mon Nov 02, 2020 8:34 pm

The load is 15 ohms at 120 Volts and 12 amps DC.
Does not compute!

120V / 12A = 10 Ohms

Or, if the 15 Ohms is correct, then 12Amps is wrong; like JWax said.
Dale Y

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Re: Power Supply for mains circuit _linear or switch mode

Post by gerty » Mon Nov 02, 2020 10:26 pm

Your AC currents will be different from your DC current. Here's some info to explain.... https://www.hammfg.com/electronics/tran ... /rectifier

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Lenp
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Re: Power Supply for mains circuit _linear or switch mode

Post by Lenp » Wed Nov 04, 2020 6:23 pm

Regarding a transformer
Unless a transformer is needed for power line isolation, don't use one. It would be a big bulky bugger to find in a rea$onable price range.
Millions of radio's were built and were direct line powered, If proper safety is employed, there is no hazard.
Most switch mode power supplies do not offet that high voltage, so a linear, or conventional, power supply is reasonable'
It might also be helpul to know what the load is. capacitive,Inductve...?

One idea that comes to mind is to use two like sized transformers, with the VA rating required wired 'back to back'
#1 transformer primary is conected to the line input. #1 transformer secondary connects to #2 transformer secondary and #2 primary becomes the isolated output. In effect it is a 1:1 ratio, double insulated transformer.

DO NOT confuse a transfomer with an autotransformer commonly used for 120/220 conversion. Yes, they can be wired for 120/120 or 220/220 by connecting the same as a two transformers but there is absolutely NO LINE ISOLATION! Each 'transformer' is just a single tapped inductor with no isolated windings. It is fine for it's purpose but it is not a true transformer.

Ideally,the transformers should match but as long as the transformer VA ratings are at least what is required, and the primary voltaged and secondary voltages are equal, all should be well. Common and cheap sources of these transformers (think scrap yards) are discarded microwave ovens (You must remove the HV windings!, large UPS power supplies and commercial battery chargers.

INPUT T1 T2 OUTPUT
-------P || S------- S || P-------
-------P || S------- S || P-------
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

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