I have built a power supply with the following order. Variac-Full Wave Bridge Rectifier-Capacitor.

I get the input from the wall. I connected neutral and line to the rectifier. Not using the ground.

The multimeter shows a steady DC output. However, I'd like to see the output with more details.

I build a really simple circuit with a resistor and an LED to test the power supply. Here is the problem! Everytime I connect the circuit to the scope the LED dies. I don't go beyond 40V out.

I tried to sketch what I have and what I am doing so far.

Any suggestions would help. Thanks in advance. enter image description here


1 Answer 1


A variac is an autotransformer and does not provide isolation. When you connect the ground clip of your scope you are shorting the diode bridge.

(The scope ground is tied to earth ground which connects to the neutral conductor back at your service panel.)

It's much safer and better practice to put an isolation transformer in series before the diode bridge.

  • \$\begingroup\$ Thank you! So, does it make sense to use an isolation transformer between the variac and the rectifier. It might sound funny but I can vary the voltage with the variac while isolating the rest of the components? \$\endgroup\$
    – CMarine
    Dec 30, 2016 at 2:32
  • 1
    \$\begingroup\$ Yes, you can vary the voltage with the Variac and maintain the isolation. If you put the isolation transformer after the Variac, make sure it's rated for the max voltage you will have going into it, and best practice would be to include a fuse and maybe a thermal cutout. It might be a better idea to put the isolation transformer in front of the Variac and make sure it's rated for the input voltage and max load it will see. \$\endgroup\$
    – John D
    Dec 30, 2016 at 2:57
  • \$\begingroup\$ Thanks!!! So, Variac has a 10 Amp fuse on it. Would that be enough, or, you recommend me to add another one? \$\endgroup\$
    – CMarine
    Dec 30, 2016 at 4:53
  • \$\begingroup\$ Well, if you put the isolation after the Variac you should put a fuse before the isolation transformer that will blow if the transformer exceeds its rated primary current. If you put the iso transformer before the Variac and it's not rated for as much power as the Variac then you should still add another fuse in front of the iso transformer. \$\endgroup\$
    – John D
    Dec 30, 2016 at 4:59
  • \$\begingroup\$ I have made the connections and now I can connect the scope in a way that I wanted. Now I have another problem with the voltage regulation. Can I add the following configuration directly to the tail of the capacitor that I have on the diagram? Do I need something in between? Thank you! \$\endgroup\$
    – CMarine
    Jan 12, 2017 at 3:17

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