I was playing around with a computer power supply and some resistors and noticed that raising the ohmage decreased the amperage but the voltage was unaffected. Does this mean that voltage is a constant in V=IR. How can you change the voltage with resistors and leave current unaffected.

  • 6
    \$\begingroup\$ You have just discovered the difference between the "constant voltage power supply" and "constant current power supply". Or simpler put "voltage source" vs "current source". \$\endgroup\$
    – Eugene Sh.
    Jan 21, 2016 at 19:13

1 Answer 1


The voltage was constant because the power supply maintained the voltage (that's its job). In a similar way, if you make a constant current source and pass that current through a variable resistor, then varying the resistance value will change the voltage while the current remains the same.

  • \$\begingroup\$ what happens if you dont use a constant voltage or amperage power supply? \$\endgroup\$
    – Rhezner
    Jan 21, 2016 at 19:27
  • \$\begingroup\$ Then you don't have constant voltage or amperage. Many weird things might happen. you should be more specific. \$\endgroup\$
    – Eugene Sh.
    Jan 21, 2016 at 19:31
  • 1
    \$\begingroup\$ Then ohms law is followed.For example, if I have a power supply of 10V with a 2 ohm source impedance and I load it with an 8 ohm load, the output of the supply will fall to 8V and 1A will flow. \$\endgroup\$ Jan 21, 2016 at 19:32

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