You can calculate the current through the diode when it is connected in the series to the resistor using Kirchhoff’s Voltage Law (KVL) and the Shockley diode equation. This article describes the step-by-step procedure and formula to find the current through the diode.
1. Kirchhoff’s Voltage Law (KVL):
According to KVL, in a closed electrical circuit, the sum of the voltage drop across electrical devices equals the sum of the supply voltage. The devices used in the circuit are a resistor and diode, therefore the sum of the voltage across the resistor and diode equals the supply voltage. In other words, input supply voltage Vin equls to the sum of the voltage drop across resistor VR and diode VD. For example, if the voltage across the diode is 0.7 volts and the supply voltage is 2 volts, then the voltage across the resistor will be 1.3 volts.
2. Voltage across the Resistor VR:
You can find the voltage across the resistor using Ohm’s law.
3. Voltage across the Diode VD:
You can find the voltage across the diode by the Shockley diode equation.
Putting it all together:
From KVL:
This is a nonlinear equation and we can solve it either numerically or using iterative methods.
Simplified Case (For Forward Biased Diode):
In the case of a forward-biased diode where I≫IS, the voltage drop across diode VD diode equation can be approximated as:
Where Von is the forward voltage drop of the diode.
The voltage drop across a silicon diode is 0.7 V and 0.3 V for a germanium diode when diodes are in a conduction state, therefore you can write the diode current formula as,
This is the basic and most practical formula for calculating the current through a diode in a series circuit with a resistor.
Solved Example
A diode with a forward voltage drop of VD=0.7 is connected in series with a 1kΩ resistor to a 5V power supply. Calculate the current flowing through the diode.
Given:
- Supply voltage, Vsupply=5 V
- Diode forward voltage, VD=0.7
- Resistor, R=1 kΩ=1000 Ω
Formula:
The current I in the circuit is calculated using Ohm’s Law, considering the voltage drop across the resistor.
Solution:
- Subtract the diode’s forward voltage drop from the supply voltage to get the voltage across the resistor:
- Now, apply Ohm’s Law to find the current through the diode:
The current flowing through the diode is 4.3 mA.