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Playing with electronics I discovered a strange behaviour of a Graetz bridge. This is my scheme: enter image description here

The corresponding output looks like this: enter image description here

I don't understand why I have a additional peak once a period.

When I add a charge, in my case a resistor as shown in the following scheme, the output looks like learned at school: enter image description here enter image description here

Does anybody know, why this behaviour occurs?

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  • \$\begingroup\$ I think that two of those diodes (top left and bottom left) are not correctly orientated. \$\endgroup\$ – Roger C. Mar 18 '15 at 17:57
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Without a load on the bridge, the oscilloscope input capacitance charges to a peak voltage on one half cycle of the AC and barely discharges because the only resistive load is the o-scope input impedance and this is very high.

I can't explain why the waveform is the peculiar shape rather than being a flat line at some positive voltage but it's possibly due self capacitance of the diodes in the bridge.

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  • \$\begingroup\$ My guess about the particular shape is that it is due to the scope's power supply drawing more current on an alternation of the mains than on the other. \$\endgroup\$ – fgrieu Mar 18 '15 at 17:54
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I've no idea why you're getting anything at all, much less the results you are seeing. Please change your bridge - it is wrong. The two left-hand diodes are the wrong polarity.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ You're right, the scheme was wrong. I changed it. Thank you. \$\endgroup\$ – xaverbandi Mar 18 '15 at 19:44
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Together with the input capacity the scheme is a resonant circuit for about the third harmonic. The resistor absorbs the energy.

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  • \$\begingroup\$ This is pretty short for an answer. Can you elaborate? \$\endgroup\$ – Null Aug 3 '16 at 20:13
  • \$\begingroup\$ This does not answer the OP's question as it was asked. Please add details to your answers, such as how and why it works or is the correct answer. Single-line shallow answers are subject to downvotes or deletion. \$\endgroup\$ – Sparky256 Aug 3 '16 at 20:19

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