# When does a resistor affect current and voltage?

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.

• 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". – Eugene Sh. Jan 21 '16 at 19:13

## 1 Answer

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.

• what happens if you dont use a constant voltage or amperage power supply? – Rhezner Jan 21 '16 at 19:27
• Then you don't have constant voltage or amperage. Many weird things might happen. you should be more specific. – Eugene Sh. Jan 21 '16 at 19:31
• 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. – user1582568 Jan 21 '16 at 19:32