Using Parallel Resistors for more current Learning Electronics Lesson 0006

Resistors limit current based on the voltage across them. If you need more current for a given voltage, then adding resistors in parallel is an option.

Getting more current by using parallel resistors schematic diagram by electronzap
Getting more current by using parallel resistors schematic diagram by electronzap
  • Each volt across a 1,000Ω resistor will pass 1/1000th of an amp of current through it. That is 0.001A, or 1mA. Called “One milliamp”.
  • 10V across a 1,000Ω (1k) resistor, will pass 0.01A (10mA) of current through it.

A 1k 1/4W resistor gets pretty warm when it coverts 10V to 10mA of current.

  • 10V x 0.01A =  0.1W (1/10th of a watt).

If 20mA is desired from 10V, and a 500Ω 1/4W resistor is used, then it will get really hot. It is recommended to keep the wattage of a 1/4W (0.25W) rated resistor to below 1/8W (0.125W). Most resistors are rated for 1/4W.

  • 10V/500Ω = 0.02A (20mA)
  • 10V x 0.02A = 0.2W (Hotter than recommended for a 0.25W rated resistor). Always remember, it is recommended to limit component wattage to half of their maximum wattage rating. 

A better option than a single 500Ω resistor that is getting very hot, is to connect two 1k resistors in parallel.

  • Two parallel 1k resistors, each passing 10V/1,000Ω = 0.01A of current will combine those currents.
  • Two separate 0.01A current paths coming together = 0.02A total current.

Remember that some LEDs, such as red ones, typically drop about 2V of the power supply voltage from series components. 2V forward voltage LEDs will drop about 2 volts from any resistors in series with them. Even if multiple resistors are connected in parallel with each other, the series LED will only drop 2V from each of them.

  • 12V supply voltage minus 2V LEDdrop = 10V across resistor(s) in series with the LED.
  • Parallel resistors have the full voltage across them.

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