Simple Diode Circuit Calculations 1. Ideal Diode

Summary

This educational video demonstrates a straightforward method for calculating current in a basic diode circuit using an ideal diode model, suitable for quick estimates without precise measurements. It explains the ideal diode's one-way current conduction and contrasts it with real diode characteristics. The video guides viewers through determining the diode's bias state and calculating current by substituting the diode with a short circuit in forward bias. It also addresses scenarios where the ideal diode model's accuracy varies with different applied voltages and covers the case when the diode is in reverse bias, emphasizing the importance of standard diode voltage and current notations.

Takeaways

• 🔬 The video demonstrates a method to calculate current in a simple diode circuit using an ideal diode model, suitable for rough estimates.
• 🌟 An ideal diode allows current flow only in one direction, acting as an open circuit in reverse bias and a short circuit in forward bias.
• 🔋 The video explains that in forward bias, the voltage across an ideal diode is zero, simplifying calculations.
• 🔍 The script shows how to determine if a diode is in forward or reverse bias by examining the circuit's connections.
• 📐 The calculation method involves replacing the diode with its equivalent circuit model and applying Kirchhoff's Voltage Law (KVL).
• 🧮 The video provides a practical example where a 5-volt battery with a 2-kiloohm resistor results in a 2.5 milliamp current.
• 📉 The ideal diode model is less accurate with small applied voltages, as it does not account for the real diode's forward voltage drop.
• 🔌 When the diode is in reverse bias, the current is zero, and the entire voltage appears across the diode.
• 📚 The video highlights the importance of standard notation for diode voltage and current, which can be negative in reverse bias.
• 💡 The ideal diode model is a useful tool for quick calculations when the applied voltage is significantly higher than the forward voltage of a real diode.

Q & A

What is an ideal diode?

An ideal diode is a component that conducts current only in one direction. It behaves like a short circuit when forward biased and like an open circuit when reverse biased.

How does an ideal diode behave in forward bias?

In forward bias, an ideal diode conducts current and behaves like a short circuit, with zero voltage across it.

What happens when an ideal diode is in reverse bias?

In reverse bias, an ideal diode does not conduct current, behaving like an open circuit with zero current flowing through it.

How is current calculated in a simple diode circuit using an ideal diode model?

To calculate the current in the circuit, we replace the ideal diode with its short-circuit model (in forward bias), and apply Kirchhoff’s Voltage Law (KVL) to find that the current is equal to the applied voltage divided by the resistance (I = V/R).

Why might we use the ideal diode model for rough calculations?

The ideal diode model is used for rough calculations when precision is not critical, as it simplifies the circuit analysis, especially when the applied voltage is much larger than the forward voltage drop of a real diode.

What is the key difference between a real diode and an ideal diode?

The key difference is that a real diode has a forward voltage drop (typically 0.6 to 0.7 volts), whereas an ideal diode has no voltage drop when forward biased (zero volts).

When should the ideal diode model not be used?

The ideal diode model should not be used when the applied voltage is very small, as it introduces significant errors in calculating the current, especially when the forward voltage drop of a real diode becomes comparable to the applied voltage.

What is the voltage across the diode when it is in reverse bias?

When in reverse bias, the voltage across the diode is equal to the negative of the applied voltage, as there is no current flowing through the resistor.

How accurate is the ideal diode model for large applied voltages?

The ideal diode model is quite accurate for large applied voltages. For example, with 100V applied, the error is less than 1%, providing a reasonable approximation.

How can we determine if a diode is in forward or reverse bias in a circuit?

A diode is in forward bias when the anode is connected to a higher voltage than the cathode. If the cathode is at a higher voltage than the anode, the diode is in reverse bias.

Outlines

00:00

🔌 Understanding the Ideal Diode Model

This paragraph introduces the concept of an ideal diode and its use in calculating current in a simple diode circuit. The ideal diode is a theoretical model that allows current to flow in only one direction, acting as an open circuit when reverse-biased and a short circuit when forward-biased. The video explains that this model is suitable for rough estimates where precision is not critical. The characteristics of an ideal diode are contrasted with those of a real diode, highlighting the negligible reverse saturation current and the zero voltage drop in forward bias. The video then demonstrates how to determine if the diode in the circuit is in forward or reverse bias, using the connection of the battery to the diode as a guide. The diode is identified as being in forward bias due to the direction of the current from anode to cathode. The calculation of the current through the diode is simplified by treating the diode as a short circuit in forward bias, leading to the formula I_D = V/R, where I_D is the diode current, V is the voltage, and R is the resistance. The example provided calculates a current of 2.5 milliamps for a 5-volt battery and a 2-kiloohm resistor.

05:03

🔋 Ideal Diode Model in Forward and Reverse Bias

This paragraph delves into the accuracy of the ideal diode model in different scenarios. It explains that the model can be quite accurate when the applied voltage is significantly higher than the forward voltage of a real diode, which is typically around 0.6 to 0.7 volts. The video provides examples of current calculations with different voltages, showing that the ideal diode model's accuracy decreases as the applied voltage gets closer to the forward voltage of a real diode. The paragraph also addresses the case when the diode is in reverse bias, stating that by definition, the current through an ideal diode in reverse bias is zero. The video then discusses the standard notation for diode voltage and current, emphasizing that these values are negative when the diode is in reverse bias. The equivalent circuit for a reverse-biased diode is described as an open circuit, confirming the zero current flow. The video concludes with a KVL (Kirchhoff's Voltage Law) equation for the reverse-biased scenario, which confirms that the voltage across the diode equals the negative of the applied voltage.

10:03

📉 Analyzing Diode Voltage in Reverse Bias

The final paragraph focuses on the voltage across the diode when it is in reverse bias. It begins by setting up a KVL equation for the circuit, considering the diode as an open circuit with zero current flow. The equation simplifies to V_D = -V, where V_D is the voltage across the diode and V is the applied voltage. The video confirms that with no current in the circuit and no voltage drop across the resistor, the entire applied voltage appears across the diode. This reinforces the concept that in reverse bias, the ideal diode acts as an open circuit, and all the applied voltage is reflected across it. The video ends with a thank you note to the viewers.

Keywords

💡Diode

A diode is an electronic component that allows current to flow in one direction only, acting as a one-way valve. In the video, the diode is central to the explanation of how circuits work, specifically how it behaves in forward and reverse bias. The ideal diode conducts current only in the forward direction and blocks it in the reverse direction.

💡Ideal Diode

An ideal diode is a theoretical model of a diode that conducts current perfectly in one direction with zero resistance and blocks all current in the opposite direction with infinite resistance. In the video, the ideal diode model is used for simplifying calculations, especially when precise current values are not necessary.

💡Forward Bias

Forward bias refers to the condition where the positive terminal of the battery is connected to the anode of the diode, allowing current to flow through the diode. In the video, this concept is important because it explains how the diode allows current to pass and how the voltage across it becomes zero in this state.

💡Reverse Bias

Reverse bias occurs when the positive terminal is connected to the cathode of the diode, preventing current from flowing through the diode. The video explains that in reverse bias, the diode can be modeled as an open circuit with zero current flow, illustrating how the ideal diode behaves in this scenario.

💡Current (I)

Current is the flow of electric charge in a circuit, measured in amperes. The video explains how to calculate the current in a diode circuit using the relationship I = V/R, where voltage is divided by resistance. In forward bias, the current flows through the diode, while in reverse bias, it is zero.

💡Voltage (V)

Voltage is the electric potential difference between two points in a circuit, measured in volts. The video shows how voltage behaves across a diode in different conditions, such as being zero when the diode is in forward bias and equal to the applied voltage when in reverse bias.

💡Resistance (R)

Resistance is the opposition to the flow of current in a circuit, measured in ohms. The video uses resistance in the calculation of current, where R plays a key role in determining the amount of current flowing through the circuit based on the applied voltage.

💡Kirchhoff's Voltage Law (KVL)

Kirchhoff's Voltage Law states that the sum of all voltages around a closed loop in a circuit must equal zero. The video applies this law to explain how the voltage across different components in the diode circuit can be calculated, helping to understand the overall voltage distribution.

💡Short Circuit

A short circuit is a low-resistance connection between two points in a circuit that allows a large amount of current to flow. In the video, the ideal diode in forward bias is modeled as a short circuit because the voltage across it is zero, which means it acts like a closed switch.

💡Open Circuit

An open circuit occurs when there is a break in the circuit, preventing current from flowing. The video explains that in reverse bias, the diode behaves as an open circuit, meaning no current passes through it, similar to an open switch.

Highlights

Introduction to calculating current in a simple diode circuit using the ideal diode model.

Explanation that the method is suitable for rough estimates and not for precise values.

Definition of an ideal diode and its one-way current conduction property.

Modeling an ideal diode as an open circuit in reverse bias and a short circuit in forward bias.

Comparison of ideal diode characteristics with those of a real diode, highlighting differences in forward bias voltage.

Procedure to determine if the diode is in forward or reverse bias by examining the battery connections.

Calculation of current in the circuit by replacing the diode with a short circuit model.

Application of Kirchhoff's Voltage Law (KVL) to find the current in the loop.

Mental calculation shortcut for current without KVL by using Ohm's Law.

Explanation of how to calculate the voltage across the diode when it's in forward bias.

Discussion on the accuracy of the ideal diode model for different applied voltages.

Example calculation showing a small error in current estimation with high applied voltage.

Example calculation demonstrating a significant error in current estimation with low applied voltage.

Advice against using the ideal diode model for circuits with very small applied voltages.

Procedure for calculating current when the diode is in reverse bias, resulting in zero current.

Explanation of diode voltage notation and its significance in reverse bias.

Final thoughts on the practicality of the ideal diode model for different circuit analysis scenarios.

Transcripts

hi everyone

in this video i will show how to

calculate the current

in a simple dial circuit using the ideal

diode

this is the simplest method and even can

be done without using pen and paper

it is used when we want to have a rough

estimate of the current

and the precise value is not needed

this is a simple diode circuit in fact

most diode circuits

can be simplified to this circuit let's

first see

what is the deflation of an ideal diode

and how

is its characteristics an ideal diode

can conduct current only in one

direction

and no current can go to the opposite

direction

if the voltage across the diode is

negative

no current goes through the diode or the

current

is zero it is said the diode is in

reverse bias

it can be modeled as an open circuit

or an open switch

when the current goes through the diode

the voltage across it is zero this is

like a short circuit or closed switch

and it is said the diode is in forward

bias

for comparison the characteristics of a

real diode are shown here

in reverse bias

the ideal diode has a very good match

with the real diode

because it has the reverse saturation

current

which is negligible and usually is

assumed

zero at forward waves you can see

more difference the real diode has a

voltage of about

0.6 or 0.7 volts but the voltage of the

ideal diode is defined

as zero i will talk about this

difference

later in this video now let's calculate

the current in this circuit

first we need to find out if the diode

is in forward waves or in reverse bias

the positive side of the battery is

connected to the anode of the diode

and the cathode is connected to the

negative side of the battery

through the resistor r therefore the

current

goes from the positive side to the

negative side

and it is in direction of anode to

cathode

and the diode is in forward bias

the arrow shape of the diode shows the

direction that the diode conducts the

current

to calculate the current we simply

replace the diode

with its model or equivalent circuit

in forward bias which is a short circuit

then we can write a kvl in this loop

we have v equals to

r times i

and equal to r times id

the current in the loop is the current

in the diode

from here we can calculate id

equal to v divided by r

and if we put the numerical values

5 volts divided by 2 kilo

equals to 2.5

milliamps

remember that if you use the value of

resistor with unit of kilo

the current is found in milliamps

here i have drawn the equivalent circuit

and

was writing a kvl equation only for

explanation and for clarification

we really don't need to do that by

looking

at the circuit

we can even have a mental calculations

5 volts divided by 2 kilo ohms is

2.5 milliamps what about the voltage

across the diode

it is a short circuit and short circuit

by definition

doesn't hold any voltage or as the ideal

diode is defined

when it is in forward west its voltage

is zero so we can write

vd or the voltage across the diode

equals to zero volts

when we use the ideal diode model

it is when we want to have a rough

calculation

and the precise value of the current is

not very important

the method can be quite accurate if the

if the applied voltage is very large

much larger than the forward voltage of

the real diode

which is usually 0.6 2.7 volts

for example if the voltage is 100 volts

the current is calculated as

50 milliamps

the precise value is around 49.7

milliamps

and the difference is about 0.6 percent

or less than 1 percent

for a voltage of 10 volts

the calculated current is

5 milliamps

and the precise value

the precise value is 4.7 milliamps

about 6 percent different still good

but what about if the applied voltage

is 1 volt

using the ideal diode we calculate

the current equal to 0.5 milliamp

the precise value is 0.2 milliamp

a massive error more than two times

so we should not use the ideal diode

model

when the applied voltage is very small

what about the current when the diode is

in reverse bias

in this figure i have changed the

direction of the diode

so the cathode voltage is more positive

than a node voltage

instead of changing the

diode direction we can change the

polarity of the battery

negative on top and positive at the

bottom

it would be the same

calculate the current actually we don't

need to

do anything to calculate the current

because by definition

the ideal diode in reverse bias has a

current of zero

so the current is zero

but if you want you can draw the

equivalent circuit of the diode

in reverse

which is an open circuit and hence

the current is zero

it is important that the standard

notation of the diode voltage is

the voltage of a node in respect of the

cathode

and the standard notation of the diode

current

is from anode to cathode

if the diode is in reverse bias

these values will be negative

so here i actually is

equal to minus id but because it's zero

it doesn't matter

what about the voltage across the diode

we can write

a kvl equation in the loop

if we start from the left

v equals to minus vd

plus r times i

and we know that the current is zero

so this is equal to minus vd

plus r times zero

and from that we can find vd

equal to minus v

and equal to minus 5 volts

this is very obvious because there is no

current in the circuit

there is no voltage drop across the

resistor

and all the applied voltage appears

across the

diode thanks for watching

you