I need to validate the 200 mA @ 24 VAC claim on a Transformer/Power Supply. I have some resistors but I don't know how to proceed. Help anyone?

  • \$\begingroup\$ If the output is 24VAC then it would be more correct to call it a transformer. \$\endgroup\$ – JRE May 9 '16 at 15:27
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    \$\begingroup\$ R=V/I=24/0.2=120 Ohm. This is the resistance you need. Power on resistor will be hence P=V*I=24 *0.2=4.8W. So the resistor must be rated for 10W at least. That's it. Connect it to the output and check if the voltage is stable, if nothing gets overheated, etc. \$\endgroup\$ – Gregory Kornblum May 9 '16 at 15:30
  • \$\begingroup\$ @GregoryKornblum: That's an answer. \$\endgroup\$ – JRE May 9 '16 at 15:32
  • \$\begingroup\$ Mmm.. There is a mistake about power. Its visqrt2. I didn't see AC. The recommendation is the same though. Actually, if 24V is RMS, then everything was correct. \$\endgroup\$ – Gregory Kornblum May 9 '16 at 15:34
  • \$\begingroup\$ VAC would be RMS. \$\endgroup\$ – JRE May 9 '16 at 15:39

You basically want to create a graph that shows voltage vs. current measured at the output of the transformer. Assuming you know the values of the resistors, you can infer the current from the voltage.

The full-load rating of the transformer is 200 mA and 24 V, which represents 4.8 Watts of power, so your smaller resistors are going to need to be rated for at least this much power, but preferably about 2×.

You're going to want to test the transformer at low loads, through its rated load, and into mild overload. Create a chart or spreadsheet and start selecting resistors.

  • 25% load would be 24 V/50 mA = 480 Ω (24 V × 50 mA = 1.2 W)
  • 50% load would be 24 V/100 mA = 240 Ω (24 V × 100 mA = 2.4 W)
  • 75% load would be 24 V/150 mA = 160 Ω (24 V × 150 mA = 3.6 W)
  • Full load would be 24 V/200 mA = 120 Ω (24 V × 200 mA = 4.8 W)
  • 125% load would be 24 V/250 mA = 96 Ω (24 V × 250 mA = 6.0 W)

One way to attack this would be to just get a handful of 470 Ω, 2 W resistors and start connecting them in parallel, one at a time, to the output of the transformer. Measure the voltage for each combination and plot the results as voltage vs. current using your spreadsheet. There will probably be a fairly obvious "knee" in the curve when you start to overload it.

  • \$\begingroup\$ Thank you for the in-detail feedback. I'm an ME so my electrical background is limited. How do I go about the wiring? A step by step description will be very helpful. \$\endgroup\$ – Anthony May 10 '16 at 14:31

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