Odd voltages on home built PSU

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Discussion Overview

The discussion revolves around the construction of a power supply unit (PSU) converting 120VAC to approximately 12VDC. Participants explore issues related to voltage output after rectification and filtering, as well as suggestions for voltage regulation and circuit design.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that the transformer outputs 13.6VAC, but after rectification and filtering with a capacitor, the voltage spikes to 18VDC, leading to confusion about the expected output.
  • Another participant suggests using a bridge rectifier for full-wave rectification to achieve higher voltage output, while also mentioning the need for a DC load across the capacitor to improve voltage readings.
  • A suggestion is made to use a voltage regulator with capacitors on both sides to minimize ripple, indicating that a 7812 regulator would require at least 15V input.
  • Concerns are raised about the potential overheating of transistors when handling higher currents, with a request for a wiring diagram to clarify the proposed circuit design.
  • One participant mentions consulting an electrical engineer who indicated that the voltage output is unregulated and will vary based on load, potentially ranging from 18V to 9V depending on the current draw.

Areas of Agreement / Disagreement

Participants express differing views on the best approach to achieve stable voltage output, with some advocating for bridge rectification and regulation, while others highlight the variability of the output voltage based on load conditions. The discussion remains unresolved regarding the optimal design and expected performance of the PSU.

Contextual Notes

There are limitations in the discussion regarding assumptions about load conditions, the specific design of the circuit, and the effects of different components on voltage regulation. The exact relationship between load and voltage output is not fully explored.

relativespeed
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I'm working on building a power supply (120VAC to ~12VDC). The Transformer puts out 13.6VAC without any modifications. But once I put on a diode on each side (make into DC) then put on a 1000 microfarad capacitor to eliminate the cosine curve it bumps up to 18VDC. Without said capacitor it registers 6VDC (probably due to the cos curve still existing). I'm at a loss here, I tried to use a larger (4700) capacitor and the voltage was the same, so I don't believe it was that at fault. It's a typical 3 output transformer with one black and two yellow wires with the yellow ones being used. Thnx ahead of time!
 
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To get full voltage with a full wave rectification, you need a bridge (full wave) rectifier (four diodes). Full-voltage half-wave rectification requires only one diode. Put a dc load across the capacitor to drain charge off between every (half) cycle to get a better voltage reading.

To get better voltage regulation, put a large NPN transistor (e.g., 2N3055) in series. Use a resistor and zener to control base voltage. Connect collector to capacitor, and emitter to load. Or you could use a 2N7912 or other regulator.

The black wire is a center tap on the transformer secondary, useful for full wave rectification with two diodes. The black wire is ground. But you get only half the voltage out.
 
I'd use a regulator with caps on both sides, to minimize ripple. It's pretty much the standard thing.
For a 12V regulator like a 7812 you'll need at least 15V though, although you can probably find a low-drop version. And remember to attach it to some cooling flange.
 
alxm said:
I'd use a regulator with caps on both sides, to minimize ripple. It's pretty much the standard thing.
For a 12V regulator like a 7812 you'll need at least 15V though, although you can probably find a low-drop version. And remember to attach it to some cooling flange.
Well, I have 18V now, so that wouldn't a problem. Nor would a couple heat sinks and a small fan from an old CPU.

Bob S said:
To get full voltage with a full wave rectification, you need a bridge (full wave) rectifier (four diodes). Full-voltage half-wave rectification requires only one diode. Put a dc load across the capacitor to drain charge off between every (half) cycle to get a better voltage reading.

To get better voltage regulation, put a large NPN transistor (e.g., 2N3055) in series. Use a resistor and zener to control base voltage. Connect collector to capacitor, and emitter to load. Or you could use a 2N7912 or other regulator.

The black wire is a center tap on the transformer secondary, useful for full wave rectification with two diodes. The black wire is ground. But you get only half the voltage out.

I'd be a little nervous about running a couple amps thru a transistor though, wouldn't it burn out? I'm looking to build a 3AMP transformer box. And could you come up with a diagram or something? I'm not quite getting what you said on how to wire it all.
 
Well, asked an electrical engineer, and got a dur answer I should have known: it's not regulated, so the voltage will vary upon load, probably from 18-9 volts pending on ampheres going thru it.
 

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