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What the World
Needs Now --
AC to DC Power Supply with 40KV Isolation
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Previous Issues |
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This request came from a company who needed an unusual power supply.
They required a low power DC supply assigned to a high voltage power supply, which was
elevated above earth ground by 20KV. The power supply needed to be connected to the
20KV supply. They said that their circuit performed some logic function and
communicated to a computer through an optical fiber interface. The logic circuit
could operate using a voltage from 2 to 3 volts and only needed a couple milliamps of
average DC current. The kicker with this system was that to insure no arcing, they
wanted the supply to be able to withstand 40KV with respect to earth ground. Wow!
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The usual method to generate a low power supply with good isolation is with
an iron core transformer or a ferrite core AC to DC converter. But, those systems
usually only provide a few kilovolts of isolation. Getting isolation to 40KV would
almost be impossible with a magnetic approach. I think one way to do this job
is with an optical approach. The method I have in mind is illustrated below. |
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An array of infrared LEDs is positioned on one side of a thick piece of
clear 3/4 inch thick Lexan plastic sheet. Light from the LEDs would shine onto an
array of inexpensive solar cells or photodiodes on the other side of the plastic sheet.
Since the company said they only needed 2 to 3 volts with a few milliamps to perform their
logic function, then only 4 to 6 solar cells would be needed. Surprisingly, a good
photodiode has a light to electricity conversion approaching 50% if it is illuminated with
900nm infrared light. This infrared wavelength matches the peak response of the
typical silicon device. |
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The LEDs would be on one side of the thick plastic insulator and the solar
cells connected to the 20KV source would be on the other. I’m pretty sure that this
amount of plastic will hold off 40KV and maybe a lot more. At 40KV, any electrodes
would need to be at least 2 inches away to insure no arcing through air. |
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