Diode Voltage Drop: What, Why, How And Detailed Facts


In this article, we shall discuss the diode voltage drop, why it is caused and how we can calculate it. A diode is a semiconductor device that allows one directional current flow and restricts current flow on another side.

Diode voltage drop basically refers to the forward bias voltage drop. It occurs in a diode present in an electrical circuit when current passes through it. This forward bias voltage drop is the result of the action of the depletion region formed by the PN junction under the effect of an applied voltage.

What is diode voltage drop?

Diode voltage drop is the result of the current flow from anode to cathode. When the diode is conducting in forward bias, the potential drop across it is known as the diode voltage drop or  forward voltage drop. 

Ideally, there should not be any voltage drop across the diode when it is carrying current and operating to generate DC output voltage. In real life, a small voltage drop occurs due to the forward resistance and forward breakdown voltage. For silicon, the diode voltage drop is around 0.7 Volt. 

How much voltage does a diode drop?

Any diode drops a particular amount of voltage across its terminals. A diode voltage drop of 0.7 V means the voltage through the resistor or load present in the circuit is (supply voltage – 0.7) volt.

The voltage drop across different diodes are different. Typically it ranges from 0.6 to 0.7 volts for a small silicon diode. For schottky diodes, the value of the voltage drop is 0.2 Volt. For light emitting diodes or LEDs, the voltage drop ranges from 1.4-4 Volt. Germanium diodes have a voltage drop of 0.25-0.3 volt.

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Why does a diode have a voltage drop?

The diode, in forward bias, selects a suitable voltage level so that it can push the electronic charges towards the P-N junction. It can be said analogous to “lifting” each ball up to the top of the table from the floor.

The difference in energy level required to move the electronic charges across the PN junction causes the voltage drop. Also, there is some resistance in the diode responsible for a certain amount of voltage drop. The voltage drop due to resistance depends upon the allowable current flow rate at the PN junction.

How to calculate diode voltage drop?

The voltage drops of different diodes are different. For a silicon diode, it is approximately 0.7 Volt, for germanium diode it is 0.3 volt and for schottky diode it is around 0.2 Volt. LEDs have various voltage drop values. 

Now if we want to calculate the voltage drop across any other element in the circuit, we need to subtract the voltage drop of the diodes present between that element and the source from the source voltage. So the voltage drop of that element is (source voltage- sum of diode voltage drops).

How to drop voltage using diode?

Zener diodes are good for dropping voltage. However, a trivial method to drop voltage using diodes is to connect multiple diodes in series with the supply. Each diode causes a voltage drop of almost 0.7 Volt.

Diodes only allow one directional flow of electricity, but the diode will conduct electricity only, when the supply touches the threshold. The standard silicon diode threshold is 0.6 volts. … After each diode is joined in series, the voltage drops by 0.6 volts. Utilizing this technique, we can drop voltage in a circuit using diodes.

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FAQs

How to reduce voltage with zener diode?

Zener diode is a special case of diodes which permits current to flow reverse in a certain voltage, known as zener voltage. It can also reduce the reverse voltage and work as an efficient voltage regulator.

For using a zener diode to reduce voltage, we have to connect it in parallel with the load in the circuit. The supply voltage must be higher than the zener voltage and the diode should be in reverse bias. This connection helps to reduce the reverse voltage to a specific value and act as a voltage regulator.

Diode voltage drop formula

For the sake of simplicity, forward voltage drop across a diode is assumed as 0.7 V. Now, if there is only one diode in a circuit along with a load, the voltage drop across the load is (supply voltage – 0.7) Volt. 

In case of multiple diodes in series in a circuit, the voltage drop across the load is (supply voltage – the number of diodes * 0.7). For example, in image 1, the voltage drop across the diode D1= (5-0.7) = 4.3 V. The voltage drop across the diode D2= (5-2 * 0.7) = 3.6 V. The voltage drop across the diode D3= (5- 3 * 0.7) = 2.9 V. 

diode voltage drop
Image 1

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Diode voltage drop chart

Below table depicts the voltage drop limits of different kinds of diodes.

Type of the diodeVoltage drop
Silicon diode0.6-0.7 Volt
Germanium diode0.25-0.3 Volt
Schottky diode0.15-0.45 Volt
Red LED1.7-2.2 Volt
Blue LED3.5-4 Volt
Yellow LED2.1-2.3 Volt
Green LED2.1-4 Volt
White LED3.3-4 Volt
Orange LED2.03-2.20 Volt
Violet LED2.76-4 Volt

Diode voltage drop vs temperature

The diode voltage drop is the potential difference across the terminals of the operating diode. The voltage drop depends on the temperature coefficient of the diode, and the behavior of other elements in the circuit.

The positive or negative temperature coefficient will respectively increase or decrease the diode voltage drop. Most silicon diodes have a negative temperature coefficient which means the voltage drop decreases with an increase in temperature. Zener diode has a positive temperature coefficient which increases voltage drop.

Diode voltage drop vs current

The voltage drop across a diode increases with current in a non-linear fashion. But as the differential resistance is less, the increase is very slow. We can consider the forward voltage vs current characteristics. 

From the IV curve, we can see that a large amount of increase in current initially yields a negligibly small increase in voltage. Then more quickly the voltage goes up, and eventually very quickly escalates. The IV curve shows an exponential growth of voltage with current. By the time Vd crosses 0.6/0.7 V, it moves up rapidly.

When voltage drop across a PN junction diode?

When current passes through any component present in a circuit, a voltage drop occurs. Similarly, when current passes through the diode in forward bias, then there is a voltage drop, known as forward voltage drop.

The p-n junction diode can not send the current from the junction in the reverse bias for very high resistance. The p-n junction acts like an open circuit, so the voltage drop across that ideal p-n junction diode remains the same. It is equal to the battery voltage.

Also, please click to know about Organic Light-Emitting Diodes.

Kaushikee Banerjee

I am an electronics enthusiast and currently devoted towards the field of Electronics and Communications . My interest lies in exploring the cutting edge technologies. I'm an enthusiastic learner and I tinker around with open-source electronics. LinkedIn ID- https://www.linkedin.com/in/kaushikee-banerjee-538321175

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