What are all the different types of "load" and what are the key important facts to know about each?
According to Merriam Webster's dictionary (definition 8b), a "load" is:
a device to which power is delivered
This may be a circuit, a test device ("dummy" load), or a larger machine or device comprised of many sub-circuits.
The types of electrical loads are typically divided into three categories:
And of course there are devices which contain a combination of these.
Resistive loads: Resist current flow linearly and cause heat and light (potentially desirable or undesirable). For example, an incandescent light bulb produces light (desirable) but also heat (undesirable). A space heater's element produces heat (desirable) but may also glow. Resistance is measured in ohms for resistive loads, and power is measured in watts.
Inductive loads: Resist changes in current and as such, when you measure the current, it lags (is behind) the voltage. Electromagnetic fields are the key to inductive loads, and as such all motors (fans, pumps, etc), solenoids, and relays are inductive in nature. Inductance is measured in Henrys. The important thing to remember about inductive loads is that they have two types of power, real power and reactive power. The real power is based on the work done by the device (such as what a motor is spinning). The reactive power is that which is drawn from the source to produce magnetic fields. The total power consumed is real and reactive power combined, which is measured in VAR (volts-amps-reactive). It's rather a complicated topic, so check out terms like Power Factor and Real, Reactive and Apparent Power (under AC power).
Capacitive loads: Are for many purposes, the opposite of inductive loads. They resist changes in voltage, and as you'd expect, the voltage lags the current (or more commonly said "current leads voltage"). A capacitor is two conductive surfaces separated by a insulator, which store charge. When power is first applied, current is very high, but drops as the voltage of the charge reaches that of the applied voltage. Capacitance is measured in farads. Like inductive loads, capacitive loads also have reactive power, but it's opposite the polarity of an inductive load. Therefore, a capacitive load has a negative VAR. Capacitive loads are not very common, but things like a flashbulb or a heart defibrillator might be considered a capacitive load (probably a combination load, but you get the idea).
When we look at a system, such as an electronic device, it helps us to isolate that device from its environment, which is complex and full of details that are not all relevant.
If a device has an output, then it has behaviors which are dependent upon the device which that output drives. So we cannot remove the device which is driven.
What we do is we retain a simplified representation of the device which is driven by our output, and that is called the load.
Two important behaviors of a device with respect to a load are its basic reliability (can it drive that load while its performance parameters remain within range, and how robustly can it handle being overloaded?) and stability (can that load make the output of the device oscillate or otherwise misbehave).
The term load refers to
- the most relevant aspects of a device which is driven, from the perspective of an output. These aspects involve basic properties like capacitive and inductive reactance, and resistance, or more complex networks thereof. So we can talk about "resistive", "capacitive" or "inductive" loads.
- a model of the driven device constructed to match these most relevant aspects, while omitting the rest of the device.
Load also refers to power consumption: e.g. "heavy load on the power grid".
As a verb, "to load" means to present a load to an output, and to "load down" means to present a somewhat excessive load (such as too low an impedance to a device that acts as a voltage source, such that the voltage source is unable to maintain voltage). To "overload" is to load down extremely, ("the power supply couldn't handle being overloaded and burned out").
Sometimes "load" is used incorrectly to denote a signal or current, and the circuit or conductor to be a "vessel" that the load is poured into. Watch out for this in informal usage like "the amplifier's high output overloaded the sensitive microphone input it was plugged into". Or: "those wires are carrying a mighty big load for their gauge".