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I have a 120V AV receiver (amplifier) connected to a step down transformer for converting from 230V mains. This is the only electronic equipment in my home to completely lose power and turn off/on when the inverter (UPS) switches to battery and back to mains when there is loss of mains power. The inverter is pure sine wave and has a transfer time of <= 15ms. As you can imagine, this is very annoying and I have been trying to find a solution short of buying a very expensive double conversion online UPS.

My knowledge in electronics is very lacking so I would be very grateful if someone could answer my questions here:

  1. Why does connecting an equipment (driven by an SMPS, same as everything else) to a step down transformer cause it to completely lose power when the inverter switches?
  2. What can be done to prevent this? Could a capacitor be used at the input/output of the step down transformer?

Thank you 🙂

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  • \$\begingroup\$ The problem isn't necessarily caused by the transformer; it could also be that your AV receiver doesn't like short interruptions, or has a nervous brown-out detector circuit that shuts it down. It's hard to tell what's going on without doing measurements, but may somebody will come up with an idea. Does the AV receiver also turn off/on when it is on stand-by or using little current (sitting there silently not amplifying music)? \$\endgroup\$ – ocrdu Sep 18 '20 at 10:51
  • \$\begingroup\$ @ocrdu The AVR doesn't turn off on standby. Do you have any advice on how to measure voltages on such short timescales (ms)? I have a normal multimeter which isn't much help. I am also familiar with Raspberry Pi and its GPIO extensions so maybe something along those lines is possible? \$\endgroup\$ – Joseph Sep 18 '20 at 14:54
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  1. Either your receiver is very sensible for 15ms getting not power supply or switching back to mains induces too high power pulses, and your amp is going into protection mode. Here a overvoltage protection could help.
  2. A capacitor is short circuit for AC. Don't you a capacitor as buffer for AC. Only for DC. I'd recommend an overvoltage protection or another UPS.
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  • \$\begingroup\$ I am connecting the AVR downstream of a surge protected outlet, but that doesn't seem to be much help. You're absolutely on point about the issue being when the mains comes back on, there is no issue when the power goes out, only when it comes back on. So any advise on how to smooth things out here? \$\endgroup\$ – Joseph Sep 18 '20 at 14:57
  • \$\begingroup\$ If the surge protected outlet is between the UPS and your AVR, it's either not protection good enough, or the the off-time on switching back seems just too long. You could get help from someone, who could measure your 120V line with an oscilloscope. I still think, you get distortion on your line. The AVR should have capacitors internal, which hold the voltage during a gap of short time. Or use another UPS, which continously generates 120V from the DC-Voltage of the batteries, and charges them simultanously. ==> 0ms gap \$\endgroup\$ – rundekugel Sep 25 '20 at 9:01
  • \$\begingroup\$ Thanks, I've decided to put a double conversion online UPS upstream of the AVR. \$\endgroup\$ – Joseph Sep 28 '20 at 2:35
  • \$\begingroup\$ Well, if anyone's seeing this in the future, I tried the double conversion online UPS method. Notwithstanding the huge amount of noise it makes (these are made for servers after all, 50 dB), the transfer time is zero only when switching from AC to battery. Battery to AC still involves some transfer time, around 4 ms from the specification. This is enough for my AVR to lose its video output for about 5 seconds. At least it doesn't turn off completely. So if you're going this route, make sure to check and verify both AC to battery and more importantly battery to AC transfer time. \$\endgroup\$ – Joseph Oct 16 '20 at 17:27

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