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Old 16th August 2014, 03:22 AM   #1
Ripcord is offline Ripcord  United States
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I made a simple RCRC power supply, (full wave bridge), to go after a 24vac, 1.5A wall wart to rectify up to ~30vdc. The power supply is two 91 ohm resistors and two 4700uF caps. This then goes in to a regulated virtual ground supply (attached). When I test the ps with no load I get around 30vdc. When I connect the reg. virtual ground to the ps I get about 1.5vdc plus and minus. When I remove the first 91 ohm resistor and just go CRC it goes back up to around 15vdc plus and minus, which is what I want but I thought I would get lower ripple with the extra resistor. I tried it out in psud II first and it pretty much corresponded to the real values of the RCRC ps. Shouldn't I only be dropping about 2vdc per resistor? What am I missing?
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Old 16th August 2014, 12:17 PM   #2
DF96 is offline DF96  England
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Show us the circuit.

Remember that 91 ohms in the AC circuit from transformer to cap via rectifier will drop a lot more voltage than 91 ohms in the DC circuit after the cap.
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Old 16th August 2014, 02:40 PM   #3
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These virtual ground circuits are overcomplicated, never perform as well as a regular supply, and there's no conceivable reason for building one from scratch to run from the mains.

If you want to run dual-rail from a single wallwart, use half the AC voltage with a voltage doubler. Obviously smoothing is more difficult, but less obvious is that you may get some radiated interference. Some ferrites will help here to prevent the wallwart lead becoming an antenna.
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