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I am trying to install a video doorbell that requires an 18-24VAC supply voltage. The existing doorbell is supplying 28V.

How can I use a resistor to drop the voltage down to something below 24VAC so I can connect the doorbell without having to replace the transformer?

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    \$\begingroup\$ You can't. Replace the transformer. \$\endgroup\$ – JRE Apr 21 at 19:11
  • \$\begingroup\$ A simple resistor won't do because the video doorbell will draw a varying amount of current that would cause a varying voltage drop across a resistor. \$\endgroup\$ – Unimportant Apr 21 at 19:12
  • \$\begingroup\$ Gouda transformer probably rated for 24V. 28V is without load, may be 24 under load. \$\endgroup\$ – user263983 Apr 21 at 19:36
  • \$\begingroup\$ Can you see the nameplate of the transformer? Is 28V its nominal output, or is it what you measured? \$\endgroup\$ – Theodore Apr 21 at 19:42
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  1. Best solution - replace the power source to get the desired voltage.

  2. If you can't replace the power source for some reason, add another transformer in between.

  3. There is another theoretical "hard way" - remove some wire turns from the secondary winding of the existing transformer. However, this is not a good idea for many reasons:

  • may be unreliable or even dangerous if not done properly
  • knowledge needed of how to disassemble the transformer
  • even if done decently, will be most likely time consuming and thus expensive
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    \$\begingroup\$ There's also a possibility that the video doorbell immediately rectifies the AC into DC for it's internal electronics and simply takes AC to be compatible with existing doorbell wiring. In that case one could rectify the AC themselves and use a simple 24V regulator. \$\endgroup\$ – Unimportant Apr 21 at 19:42
  • \$\begingroup\$ That is very true.. Maybe the question should be asked for providing more information. I can't do that, too low reputation. \$\endgroup\$ – ethcz Apr 21 at 19:44
  • \$\begingroup\$ Thanks. Guess I will replace the xformer. \$\endgroup\$ – cpeddie Apr 21 at 19:45
  • \$\begingroup\$ Good, simplest and possibly (when counting your own time) cheapest. \$\endgroup\$ – ethcz Apr 21 at 19:46
  • \$\begingroup\$ a variant on #2 is a buck transformer before the doorbell transformer, reduce the supply voltage by about 20%. \$\endgroup\$ – Jasen Apr 22 at 4:48

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