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This picture from another thread online relates to my situation:

schematic

(original)

I have a transformer from 240v ac to 18vac but need to step it further down to 12vac. What size resistor should I use in the above diagram?

Required output 12 VAC 5 W from current scrap transformer output of 18 VAC

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  • \$\begingroup\$ Why does your schematic say 22 V, 13.7 A? 5 W at 12 V --> < 0.5 A. \$\endgroup\$ – Transistor Nov 9 '19 at 14:15
  • \$\begingroup\$ You don't. No practical voltage divider can do what you want. \$\endgroup\$ – JRE Nov 9 '19 at 14:17
  • \$\begingroup\$ It's the closest schematic of what im after, found it on another thread. Since am posting question from my phone. Ignore the voltages written on the schematic. My setup is transformer output is 18vac and need it down to 12v ac \$\endgroup\$ – hihaho67 Nov 9 '19 at 14:18
  • \$\begingroup\$ @JRE would i have any luck finding a dc motor that rotates at around 8-10RPM? As dc transformers are so much easier to source \$\endgroup\$ – hihaho67 Nov 9 '19 at 14:19
  • \$\begingroup\$ If its output is DC, then it isn't a transformer. \$\endgroup\$ – JRE Nov 9 '19 at 14:20
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With voltage and power values given, not the voltage and current in the diagram, it is probably feasible to use a series resistor, not a voltage divider. A 15 ohm, 5 watt resistor will probably be close enough. The actual mechanical load on the motor and the motor design will determine what voltage you actually get.

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