I have a 3 phase zero fire SCR/SSR set that has 480V input. I put my Fluke 289 (which is a tRMS meter) across the different phases on the load side. Then adjusted the output. The voltage read ~470V from 0 to full output. I also switched my Meter to measure frequency and it read 59Hz regardless of the SCR output. Is this the result I should expect or is something wrong?

  • \$\begingroup\$ Do you actually have a load connected to the output of the SSR set? If not, the snubber capacitors that are usually connected across these devices will pass a small amount of AC current. This is enough for your meter (which has a high input impedance) to read the full input voltage. \$\endgroup\$ Feb 10, 2015 at 0:10
  • \$\begingroup\$ No I don't actually. I'll have to hook a load up to it tomorrow at try it. \$\endgroup\$ Feb 10, 2015 at 0:13
  • \$\begingroup\$ Is it blown? Semiconductors usually fail short-circuit. Also, as @DwayneReid said, SCR's need some extra parts around them to make sure that they actually stay off when they're supposed to (the snubber network, which includes a resistor and a capacitor, at minimum), and these parts will "leak" a bit. With no load, that leakage looks an awful lot like "on". \$\endgroup\$
    – AaronD
    Feb 10, 2015 at 0:14
  • \$\begingroup\$ The answer to this was it needed a load. However meter doesn't give me a steady voltage reading. \$\endgroup\$ Feb 11, 2015 at 0:27

1 Answer 1


The answer to this is, most SCRs have capacitors in them. They need a load to bring the voltage down.

After I added a load to the output I got a proper reading on my RMS voltmeter.


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