0
\$\begingroup\$

When using ohms law on a residential A/C unit I got 7.3, But when using the multimeter it dropped to 5.5. What can cause this?

\$\endgroup\$
  • 4
    \$\begingroup\$ The A/C is not a resistor. A motor has very different impedances at DC, AC (startup) and AC (running). \$\endgroup\$ – Dave Tweed Jun 5 at 22:02
  • 1
    \$\begingroup\$ A DMM cannot measure real vs reactive power. But a True RMS power meter can measure real part of load impedance. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 5 at 22:17
  • 2
    \$\begingroup\$ 7.3 what, carrots? \$\endgroup\$ – jsotola Jun 6 at 0:53
1
\$\begingroup\$

Several possible reasons:

1) The resistance is different when more current flows. The ohmmeter will put a small current, measure the voltage, and calculate the resistance. The actual circuit may be using a much larger current that changes the resistance (heating element)

2) The impedance is inductive (motor) or capacitive.

3) There is a residual voltage from a capacitor. The ohmmeter requires a completely powered off circuit.

\$\endgroup\$

Not the answer you're looking for? Browse other questions tagged or ask your own question.