I recently purchased a used Hadco Lighting 12v 300W transformer (model TC354_15) at a thrift store.

This model has 2 common lines that are controlled by 2 corresponding switches in the box. I tested it with my multimeter with both switches on, and it's putting out 11.7v, which is perfect!

However, when I switch them off, I'm still getting ~5v across the line, which doesn't seem correct to me.

I've never dealt with low-voltage systems before, is this normal? I've tried looking for spec sheets for this model, but I've not been able to find anything.

  • Do you have any timers, led lights on the transformer thaat stay on when the switches are off?
    – JACK
    Jan 22, 2023 at 18:43
  • No, the transformer is just on my bench top with nothing plugged in. It does have a spot for a photodiode, but that is currently not connected
    – AOTA
    Jan 22, 2023 at 19:31
  • If you run 12v lights, led or halogen, the 5v will do nothing and the lights will stay off.
    – dandavis
    Jan 22, 2023 at 23:18
  • @dandavis I wouldn't rule out 12V LEDs flickering or lighting dimly on 5V, depending on their internal power supply, which may even be damaged. Halogen will glow dim orange. However if Ecnerwal's answer is right, both will be fine, because it's not really a source of 5V
    – Chris H
    Jan 23, 2023 at 9:38

1 Answer 1


You might be seeing phantom voltage due to using a high input impedance modern multimeter on an unloaded transformer output.

Try measuring the output voltage again, with at least one light connected to each output.

Of course, the easy solution to waste no power is to switch off the input power. Even with the outputs shut off, an idle transformer wastes some energy.

  • an idle transformer shouldn't waste very much energy that you are actually billed for (kW), it draws apparent power(kVA), which consumer accounts typically don't pay for, though I worry smart metering will let them start one day...
    – dandavis
    Jan 22, 2023 at 23:17
  • 3
    You pay for the fact that it's not ideal, so it sits there being warm, as there is real power being converted to heat in resistance in the coil, and eddy currents in the core laminations
    – Ecnerwal
    Jan 23, 2023 at 3:21
  • @dandavis: kWh is be the unit for energy, and kW for power. Also, you pay for kWh, not kW. Jan 23, 2023 at 9:22
  • @Ecnerwal i'm not saying it doesn't use power, i'm just saying we little people don't pay for it, except with commercial 3-phase accounts. It does seem like black magic that you could warm an equipment room w/o spinning the wheel on the meter...
    – dandavis
    Jan 23, 2023 at 20:59
  • @EricDuminil if you can get kW without accruing kWh, please share!
    – dandavis
    Jan 23, 2023 at 21:01

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