Ω
Known Quantities
Voltage
250 V
Current
2.5 mA
Resistance
100 kΩ
Power
625 mW
use ≥ 2 W resistor
power contours 250 mW, 500 mW, 1 W, 2 W, 5 WR = 100 kΩ
Voltage
V = I × R
Current
I = V / R
Resistance
R = V / I
Power
P = V²/R = I²R
Engineering Notes
How it works
Ohm's law is the foundation of every calculation in this workshop. The voltage across a resistor equals the current through it times its resistance; the power it turns into heat follows directly.
V = I·R P = V·I = I²·R = V²/R
In valve circuits the voltages are high and currents small, so it pays to think in kΩ and mA: 1 mA through 100 kΩ drops 100 V. That same 100 kΩ plate resistor dissipates 0.1 W. Use a ½ W part so it runs cool and quiet. Carbon-composition and metal-film resistors also have voltage ratings (typically 250–500 V), which can matter in B+ circuits before wattage does.
The plot shows the resistor's straight-line V–I characteristic, with dashed constant-power hyperbolas for orientation.