# KVL and finding circuit currents

• Engineering
• cjm181
In summary, the conversation discusses a bi-directional opto coupler input interface circuit and calculates the value of the LED current and current through a 3kΩ resistance when a 20V supply voltage is applied. The resulting values are 1.745mA for the LED current and 6mA for the current through the 3k resistor.
cjm181

## Homework Statement

The diagram shown below shows a bi-directional opto coupler input
interface circuit. When a supply voltage of 20 V is applied the LED
carries a current and 2 V is dropped across it. Calculate the value of
the LED current and the value of current through the 3 kΩ resistance.

## The Attempt at a Solution

[/B]
As the voltage in a parallel circuit is the same in each leg, for a voltage drop of 2V across the LED, the 470Ohm resistor must also have a voltage drop of 2V.

So the voltage drop across the 3kohm resistor using KVL is:
E - V1 - V2 = 0
V1 = E - V2
V1 = 20 - 2 = 18V

So I now have V and R for the 3k resistor, so using Ohms Law:
I = V / R
I = 18 / 3000 = 6x10^-3A or 6mA
6mA is the current flowing through the 3k resistor

As the 470 resistor has a voltage drop of 2V, I have V and I

I = V / R
I1 = 2 / 470 = 4.255x10^-3A = 4.255mA

Current in a parallel circuit is the sum of the current in each leg, so

I = I1 + I2

So
I2 = I - I1
I2 = 6 - 4.255 = 1.745mA

The answers are the current flowing through the LED is 1.745mA and the current flowing through the 3k resistor is 6mA

Is this looking somewhere close. I struggled with this

Kr
Craig

Your method and result values look good. You might want to round the final results to appropriate numbers of significant figures.

## 1. What is KVL?

KVL, or Kirchhoff's Voltage Law, states that the sum of all voltages in a closed loop in a circuit must equal zero.

## 2. How do you apply KVL to a circuit?

To apply KVL, you must first draw a closed loop in the circuit and assign polarities to all voltage sources. Then, write out the equation "sum of all voltages = 0" and substitute in the values for each voltage. Finally, solve for the unknown currents.

## 3. What is the purpose of using KVL to find circuit currents?

KVL allows us to determine the currents in a circuit without needing to know the individual resistances of each component. This is useful for analyzing more complex circuits.

## 4. Can KVL be applied to any type of circuit?

Yes, KVL can be applied to any type of circuit, including series, parallel, and combination circuits. However, it is most commonly used for series circuits.

## 5. Are there any limitations to using KVL?

KVL assumes that the circuit is in a steady state and that all components are linear. It also does not take into account the internal resistance of voltage sources. Additionally, KVL only applies to circuits with a single closed loop.

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