enter image description hereI have a very simple power supply circuit, consisting of a 220V:22V transformer, full bridge rectifier (1n4007 diodes) and a smoothing capacitor (electrolytic, 4700uF). The output voltage should sit around 28V. For some reason, the output voltage is often around 0.5V. This is the case even when there is no load attached. I’ve checked all my connections, checked that my diodes are working with a multimeter, checked that the peak forward current doesn’t go over the maximum using LTspice, replaced the capacitor (I replaced the 4700u with two 10000u in series to five around 4700u but this made no difference).

The circuit I’m ultimately trying to build is quite complex but I can’t even get the power supply to give a 28V output. Any ideas why this is the case?

  • \$\begingroup\$ Are you sure you are measuring it correctly? Are the multimeter leads in good condition so there are no internal breaks? And the meter is setup correctly for about 30V range, in DC measurement? What is the load and how much it draws current or power? How much current or power the transformer can provide? \$\endgroup\$ – Justme Jul 12 '20 at 15:20
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    \$\begingroup\$ A schematic is better than words. You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. A photo would be useful too. You haven't given us the secondary AC V measurement. \$\endgroup\$ – Transistor Jul 12 '20 at 15:20
  • \$\begingroup\$ Okay I updated the question to contain a schematic with some measurements \$\endgroup\$ – user16378 Jul 12 '20 at 15:44
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    \$\begingroup\$ Are you changing the meter settings to measure dc voltages when you measure across the capacitor? \$\endgroup\$ – Elliot Alderson Jul 12 '20 at 15:49
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    \$\begingroup\$ Because the peak forward surge current is limited to 2 amps for less than 1 us and they'll self destruct when charging up the output capacitor. Also, your words contradict the schematic re the diodes. Be clear. \$\endgroup\$ – Andy aka Jul 12 '20 at 16:02

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