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My 3D printer has a 240W power supply that takes 220V AC as input and gives 12VDC/20A as output. I'd like to use a relay, controlled by a raspberry, to turn the printer ON/OFF. My question is: should I put the relay between the 220V and the power supply, or after the 12V? What are there pro and cons between the two configurations?

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    \$\begingroup\$ What's the rated output current of your supply? Relays typically have a much lower rating for DC due to the absence of zero crossings that are present in AC. \$\endgroup\$ – calcium3000 Nov 21 '17 at 18:59
  • \$\begingroup\$ I edited my question, please see above \$\endgroup\$ – igng Nov 21 '17 at 19:03
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Switching mains is better because:

  • It eliminates vampire losses by the power-supply itself which will use some power in standby. This saves a little bit on the energy bill.
  • It eliminates risk of fire from PSU overheating (although it may introduce it somewhere else).
  • It avoids heating the room.

The disadvantages include:

  • Thermal cycling of the PSU, PCB and components. This may lead to component or board failure.
  • Delay before you can print.
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  • \$\begingroup\$ There's already a several-minutes delay before printing, a few seconds from the relay & PSU booting up isn't notable. however, the cheap Chinesium power supplies that come with most printers are going to die very quickly, and replacing those will dominate the savings from vampiric losses. Of course, they also catch fire, so the savings on buying a new house will outweigh buying new power supplies. \$\endgroup\$ – Bryan Boettcher Nov 21 '17 at 20:34
  • \$\begingroup\$ +1 I'd add a 240W PSU may actually not like having it's load removed and the sudden transients involved in breaking those kinds of currents may be rather detrimental to either the power supply, the load, or both. Same goes for the reconnect. Turning it off/on at the mains side should be no worse than unplugging the thing assuming the OP does not plan on doing it every minute. \$\endgroup\$ – Trevor_G Nov 21 '17 at 21:35

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