I have a seemingly simple question: I have a full wave bridge rectifier that is connected to 220 VAC. enter image description here

When I stick a phase probe on it's (-) negative pole, I see it's light: enter image description here

And that's something I would not intuitively expect, so question is why is that? Does the diode in the bridge has some reverse voltage current leakage or what's happening?

Thank you for explanation.

  • \$\begingroup\$ When Vac is negative the diode between 3 and 4 is forward biased, because the touch-point on probe is Gnd. \$\endgroup\$ Jul 3 at 16:19
  • \$\begingroup\$ The voltage detector will respond to any AC voltage of large enough amplitude and a somewhat wide range of frequencies. If you simulate your circuit, you will see a significant AC waveform from bridge rectifier minus to ground in your configuration. \$\endgroup\$
    – winny
    Jul 3 at 16:20

2 Answers 2


Your input supply is 220 volts AC and that means it has a positive peak of 311 volts followed by a negative peak of 311 volts with respect to your GNDPWR node. This happens cyclically at 50 or 60 Hz (country dependent).

I'm also assuming that GNDPWR is basically at earth potential.

So, the positive voltages from the AC supply will be transferred to the terminal of the bridge rectifier marked with a "+" symbol and, the negative voltages of the input supply will transfer to the bridge rectifier terminal marked with a "-".

Your voltage tester will respond to positive and negative voltages thus, it illuminates on either output of the bridge rectifier.

  • \$\begingroup\$ okey, but like... i imagine diodes put waves to the positive side while - being 0; when you say positive to the positive side and negative to the negative it seems like nothing has changed ;D \$\endgroup\$
    – greengold
    Jul 3 at 18:56
  • \$\begingroup\$ Something important has changed; the negative and positive parts of the AC waveform have been segregated into two different wires. \$\endgroup\$
    – Andy aka
    Jul 3 at 19:38
  • \$\begingroup\$ if I would connect (-) negative from such bright on PEM..will it trip the fuse and blow up to my face? \$\endgroup\$
    – greengold
    Jul 3 at 21:08
  • \$\begingroup\$ I don't understand the question. \$\endgroup\$
    – Andy aka
    Jul 3 at 22:37
  • \$\begingroup\$ Seems to me that you can discriminate between +ve and -ve with a neon, since one terminal inside the bulb is cathode while the other is anode. And AC lights up both. You just have to look closely. \$\endgroup\$
    – glen_geek
    Jul 4 at 0:01

If the AC input is 0V neutral and 220VAC live phase, the bridge negative output will vary approximately between 0V and -311V, which will light up a neon lamp probe.


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