View source for Voltage drop

{{short description|Decrease in electric potential along the flow of current in a circuit}}
{{confuse|Voltage droop}}

In [[electronics]], '''voltage drop''' is the decrease of [[electric potential]] along the path of a [[Electric current|current]] flowing in a [[electrical network|circuit]]. [[Voltage]] drops in the [[internal resistance]] of the [[Power supply|source]], across [[Electrical conductor|conductors]], across [[Electrical contact|contacts]], and across [[Electrical connector|connectors]] are undesirable because some of the energy supplied is [[Dissipation|dissipated]]. The voltage drop across the [[electrical load|load]] is proportional to the [[Electric power|power]] available to be converted in that load to some other useful form of energy. 

For example, an electric [[space heater]] may have a resistance of ten [[ohm]]s, and the wires that supply it may have a resistance of 0.2 ohms, about 2% of the total circuit resistance. This means that approximately 2% of the supplied voltage is lost in the wire itself. An excessive voltage drop may result in the unsatisfactory performance of a space heater and overheating of the wires and connections.

National and local [[electrical code]]s may set guidelines for the maximum voltage drop allowed in [[electrical wiring]] to ensure efficiency of distribution and proper operation of electrical equipment. The maximum permitted voltage drop varies from one country to another.<ref>{{Cite web |url=http://www.electrical-installation.schneider-electric.com/voltage-drop.htm |title=Voltage drop - maximum, determination, calculation of voltage drop |access-date=2010-03-06 |archive-url=https://web.archive.org/web/20100306031706/http://www.electrical-installation.schneider-electric.com/voltage-drop.htm |archive-date=2010-03-06 |url-status=live }}</ref> In electronic design and  [[power transmission]], various techniques are employed to compensate for the effect of voltage drop on long circuits or where voltage levels must be accurately maintained. The simplest way to reduce voltage drop is to increase the diameter of the conductor between the source and the load, which lowers the overall resistance. In [[power distribution]] systems, a given amount of power can be transmitted with less voltage drop if a higher voltage is used. More sophisticated techniques use active elements to compensate for excessive voltage drop.

== Resistive DC voltage drop ==

[[Ohm's law]] can be used to determine the DC voltage drop by multiplying current times resistance: {{math|1=''V'' = ''I''&thinsp;''R''}}. Also, [[Kirchhoff's circuit laws]] state that in any DC circuit, the sum of the voltage drops across each component of the circuit is equal to the supply voltage.

Consider a direct-current circuit with a nine-volt [[direct current|DC]] source; three [[resistor]]s of 67 [[ohm]]s, 100 ohms, and 470 ohms; and a light bulb—all connected in [[Series circuit|series]]. The DC source, the conductors (wires), the resistors, and the light bulb (the [[electrical load|load]]) all have [[Electrical resistance|resistance]]; all use and dissipate supplied energy to some degree. Their physical characteristics determine how much energy. For example, the DC resistance of a conductor depends upon the conductor's length, cross-sectional area, type of material, and temperature.

If the voltage between the DC source and the first resistor (67 ohms) is measured, the voltage potential at the first resistor will be slightly less than nine volts. The current passes through the conductor (wire) from the DC source to the first resistor; as this occurs, some of the supplied energy is "lost" (unavailable to the load), due to the resistance of the conductor. Voltage drop exists in both the supply and return wires of a circuit. If the voltage drop across each resistor is measured, the measurement will be a significant number. That represents the energy used by the resistor. The larger the resistor, the more energy used by that resistor, and the bigger the voltage drop across that resistor.

== Reactive AC voltage drop ==

[[alternating current|AC]] voltages additionally have a second kind of opposition to current flow: [[Electrical reactance|reactance]]. The sum of resistance and reactance is called [[Electrical impedance|impedance]].

Electrical impedance is commonly represented by the variable ''Z'' and measured in ohms at a specific frequency. Electrical impedance is computed as the [[Vector (geometric)|vector]] sum of [[electrical resistance]], [[capacitive reactance]], and [[inductive reactance]].

The amount of impedance in an alternating-current circuit depends on the frequency of the alternating current and the magnetic permeability of electrical conductors and electrically isolated elements (including surrounding elements), which varies with their size and spacing.

Analogous to [[Ohm's law]] for direct-current circuits, electrical impedance may be expressed by the formula {{math|1=''E'' = ''I''&thinsp;''Z''}}. So, the voltage drop in an AC circuit is the product of the current and the impedance of the circuit.

== Diode voltage drop ==
{{Main articles|Diode|Light-emitting diode}}
[[P–n junction|P–n junctions]] in [[Diode|diodes]] and [[Transistor|transistors]] experience a characteristic voltage drop when forward-biased (see {{Slink|Diode|Forward threshold voltage for various semiconductors}} for a list of values). The energy is dissipated through [[photons]], which for the case of [[Light-emitting diode|light-emitting diodes]] are emitted and visible.

==See also==
{{Portal|Electronics}}
{{div col}}
* [[Brownout (electricity)]]
* [[Capacitive dropper]]
* [[Electric power distribution]]
* [[Electrical resistivity and conductivity]]
* [[Ground loop (electricity)]]
* [[Kirchhoff's voltage law]]
* [[Mesh analysis]]
* [[Power cable]]
* [[Voltage divider]]
* [[Voltage droop]]
{{div col end}}

==References==
<references />
*Electrical Principles for the Electrical Trades (Jim Jennesson) 5th edition


[[Category:Electricity]]
[[Category:Electrical parameters]]
[[Category:Cables]]
[[Category:Voltage stability|Drop]]