My 2.1 speaker's transformer is consuming active power even when the PC is off. I know it because it is always warm. Thus, I want that only when my PC is operating the transformer will get the AC power from the gird. So I need to put a relay between the L1 from the grid to the correct pin in the transformer. I have managed to configure in my BIOS that when the PC is off the USB will not have output voltage, so I am planing to use a USB port (5V) as control pin for the relay.

I saw that there are relays which have a second level of isolation by using a LED. Is it too dangerous to use only 1 level of isolation between my PC (USB port) to the grid? I mean that the 5V from USB will output the current for the relay's coil.


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    \$\begingroup\$ you can buy a power bar that shuts off power outlets when the control outlet stops supplying power (PC turned off) \$\endgroup\$ – jsotola Aug 10 '18 at 16:57
  • \$\begingroup\$ Can you give a link to this device? I never heard about it \$\endgroup\$ – Daniel_ee571 Aug 10 '18 at 17:13
  • \$\begingroup\$ You can use a small 5V relay to trigger a larger 24V industrial relay if you really want that much isolation. I think the isolation that you are going to get from one small 5V relay will be plenty, relays are design for this purpose. Check digikey they have tons of relays like this. \$\endgroup\$ – Meozaa Aug 10 '18 at 17:43
  • \$\begingroup\$ Is it by any chance a traditional wire-wound transformer? In which case replacing it with SMPS adapter will reduce standby current to almost nothing. Having said that, the isolation provided by the relay should be sufficient. \$\endgroup\$ – Maple Aug 10 '18 at 18:43
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    \$\begingroup\$ Yes, I am not sure I can use SMPS because it transform from AC to AC not to DC \$\endgroup\$ – Daniel_ee571 Aug 10 '18 at 20:37

Almost any power relay with 5-V primary coil will do the job, providing up to 5 KV isolation between your PC and the AC mains. Example G2R-2-DC4:

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