It should be -11V, but the oscilloscope shows something like this: -11V, with 100Hz frequency and high amplitude The frequency of this 'signal' is 100 Hz. And for the +11V it seems to be normal: +11V, quite normal

If I understand everything correctly, the input is 220V with 50Hz.

This shape can be obtained only when load is present.

What can be the cause of this behavior?


It was the capacitor - it was too old, so, replacing it solve the DC power supply problem.

  • \$\begingroup\$ What is the device producing these outputs? Even better, provide a schematic of the circuit producing the -11 V output. \$\endgroup\$ – The Photon Jan 30 '17 at 16:42
  • \$\begingroup\$ @ThePhoton I am afraid, schematics are unavailable, it is power supply for scientific equipment, quite old. Actually, at the moment, I am looking for schematics or any additional information. \$\endgroup\$ – XuMuK Jan 30 '17 at 16:44
  • \$\begingroup\$ You are picking up the mains hum. There can be thousands of reasons, without a schematic its impossible to pinpoint the part of the schematic that does this. \$\endgroup\$ – PlasmaHH Jan 30 '17 at 16:46
  • \$\begingroup\$ For an old power supply it very well may be the capacitors (electrolytic) that have dried out and no longer hold charge. (Or other-wise failed.) \$\endgroup\$ – George Herold Jan 30 '17 at 17:39
  • \$\begingroup\$ The best question is how do you measure negative terminal. In relation to what? \$\endgroup\$ – Gregory Kornblum Jan 30 '17 at 17:47

100 Hz is indicative of ripple caused by a faulty capacitor after the bridge rectifier. That capacitor would normally smooth the bridge voltage so either it has failed open circuit or has gone leaky (a lot leaky).

There could be other reasons but this is a strong candidate. Old electrolytic caps are somewhat known to do this.

  • \$\begingroup\$ Nailed it. Either faulty or totally non-existent or disconnected. \$\endgroup\$ – Brian Drummond Jan 30 '17 at 17:50
  • \$\begingroup\$ @Andy aka And this shape of DC is only on load. Without any load it is similar to normal DC. \$\endgroup\$ – XuMuK Jan 30 '17 at 21:33
  • \$\begingroup\$ @Andy aka This was exactly capacitor fault. After replacing it, all the ripples gone. \$\endgroup\$ – XuMuK Feb 7 '17 at 11:50

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