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Electrical conductance in siemens (or mhos) is the inverse of electrical resistance in ohms.
- Conductance = 1/Resistance
- Resistance = 1/Conductance
Resistance Review:
The amount of current (I) in amps through a resistance is usually calculated by taking the voltage in volts across the resistor and dividing it by the resistance in ohms. The Ohms law for current is I = V/R
- Five volts across a 100 ohm resistor equals 5V/100Ω = 0.05A. Usually referred to as being 5mA of current.
- Five volts across a 1000 ohm resistor equals 5V/1000Ω = 0.005A. Usually referred to as being 0.5mA of current.
Converting Resistance value to Conductance:
Since components are usually given a rated value resistance in ohms instead of a rated conductance value in siemens, then in order to know the conductance, you will probably have to take the resistance and convert to conductance. Simply take the number 1, and divide it by the resistance. 1/Resistance gives you the inverse of resistance, which is the conductance.
- 1/100Ω = 0.01S
- 1/1,000Ω = 0.001S
To calculate the current that will flow through a component based on conductance, the current in amps is the voltage in volts across the component times the conductance in siemens.
- 5V x 0.01S = 0.05A
- 5V x 0.001S = 0.005A
Conductance is used for parallel resistors:
When you connect resistor in parallel, the equivalent resistance actually goes down. You end up with more current than if you just used any one of those resistors by itself.
2 or more parallel resistors have an equivalent (or total) resistance of…
- R =1/(1/R1) + (1/R2) + (1/R3) …
You take add up the inverse of each parallel resistor for the total conductance that they provide. Then you invert it again by taking one and dividing it by that total conductance to convert it to the total resistance.
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