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What Is the Diode Forward Voltage?

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  • Written By: G. Robinson
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  • Last Modified Date: 28 September 2014
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A diode forward voltage refers to the voltage drop that occurs when an electrical current passes through a diode in an electrical circuit. It is generally independent of the amount of current passing through the diode, and will be the same for a given family of diodes. For silicon diodes, which are generally the most common, the diode forward voltage drop is about 0.7 volts (V).

Diodes are devices that pass electric current in one direction only, and therefore may be thought of as a kind of one-way valve for electrons. When the electrons are moving in the correct direction to pass through the diode, the diode is said to be forward biased. Thus, the diode forward voltage drop is also known as the forward bias voltage drop.

Diodes work by having a small bit of doped semiconductor material sandwiched between their electrodes, which pass electrons in one direction but not the other. This material has two zones, the n zone, which is rich in negatively-charged electrons, and the p-zone, which is poor in electrons. The p zone may be thought of having positive holes that can accept the electrons from the n zone. When the two zones are brought together to form a p-n junction, electrons transfer from the n-zone to the p-zone until all the n-zone electrons available for use as current carriers are used up, and the p-n junction becomes an insulator.

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If fresh electrons are injected into the depleted n-zone, while they are removed from the saturated p-zone, electric current will flow across the junction. This is what happens when the diode is forward biased. The electrons must be pushed by a voltage with enough force to cross the p-n junction, and this push is the source of the diode forward voltage, or forward bias voltage drop.

If the polarity of the circuit is reversed, and the attempt is made to inject electrons into the p-zone while they are removed from the n-zone, the diode is said to be reverse biased and no current will flow. If this reverse-bias voltage exceeds a certain value — the maximum reverse voltage — the diode may fail and pass large currents, in a process known as reverse voltage breakdown. The diode will then be permanently damaged.

As stated, different types of diodes have different forward voltage drops. The forward voltage drop of a simple light-emitting diode (LED) may range from 1.4 to 4 V, depending on the type. For the Schottkey diode, the forward voltage drop is usually only 0.2 V.

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anon933574
Post 4

Everetra, if you have this little experience with (LED) diodes, maybe you shouldn't be giving advice.

No, 0.7V is not standard for LED diodes, more common values range from 1.7V - 3,5V. Supply the rated voltage to the LED and it will light up, the current drawn will be about 20mA (for most LEDs), supply sufficiently lower voltage and the LED will not conduct any current (unlike an incandescent light bulb, for instance), supply sufficiently higher voltage and the current in the circuit will increase dramatically (unless limited by a series resistor) and the LED will burn out.

That's the point of the forward voltage or voltage drop listed on the datasheet. You should choose LEDs with a lower voltage drop lower than whatever your DC voltage source is (battery, adapter...) and use a current limiting resistor of an appropriate value (so that at the LED's rated current, the resistor will drop exactly the difference between the LED's forward voltage and the voltage of your DC supply).

nbrand86
Post 3

I am planning to use a Schottky diode which has specifications of:

1. Voltage - DC Reverse (Vr) (Max) = 400V

2. Current - Average Rectified (Io) = 20A

3. Voltage - Forward (Vf) (Max) @ If = 1.7V @ 20A

My application is run by 20V and tested maximum current draw in the full functional operation is 15 A.

Even though the specs 1 and 2 are fulfilling my requirement, I am little bit doubtful about the third spec.

Please explain its effects to me and is this diode still usable or not for my application?

everetra
Post 2

@Charred - You could, but I don’t think that it would make a difference.

That .7 voltage drop, which is standard for all diodes according to this article, is a fairly small amount of voltage.

I don’t think that if you saw two LED diodes, one with a voltage drop and one without, that you would notice any difference.

I would instead focus on the practical aspects of choosing your LEDs – like what color you want them to be, whether you want to buy them in strips or individually, etc. I wouldn’t worry about the voltage drop at all.

Charred
Post 1

I’ve been thinking about buying some LED diodes. I’m fairly new to the technology; all I know is that I like the LED lights better than I do the filament lamps. The quality of the light is more constant.

Anyway, I’ve been looking over the specifications and of course voltage is one of them. Since the article says that all diodes experience a voltage drop, should I try to find an LED diode with a voltage rating a little bit higher than what I want, so that when the forward voltage drops, it will still meet my needs?

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