I need a better definition of "load"

Thread Starter

fullNelson

Joined Nov 14, 2011
46
On the summary portion of the resistors page on this site, one of the summary bullets stated this:

"Any device that performs some useful task with electric power is generally known as a load. Sometimes resistor symbols are used in schematic diagrams to designate a non-specific load, rather than an actual resistor."

Is this applicable to only components that produce heat/light etc... as power or can I say that a (for example )transistor is a "load"?
 

Adjuster

Joined Dec 26, 2010
2,148
Clearly other power conversion devices or transducers can also be loads, such as motors, loudspeakers, earphones, transmitting antennas...
It is perhaps less common for active devices like transistors to be regarded as loads, but they can be. Some examples:


  • Any transistor circuit (any circuit at all) is the load of its power supply.
  • A transistor / FET / vacuum tube oscillator or RF power amplifier in a transmitter, or something like a RADAR magnetron may be the load for a modulator.
  • In any multi-stage amplifier, the input of a later stage constitutes a load for the stage preceding it ("driving" it).
  • In any circuit consisting of driving and driven stages at all (including logic) we can consider circuit inputs as loads on outputs.
  • Transistors or FETs operating as current sources may be used as high-impedance loads in amplifiers, avoiding large value resistors which can be inconvenient, particularly in integrated form.
  • High-dissipation variable load devices may be built using power transistors, for testing power supplies or rechargeable batteries.
At least that's my take on it. There are probably obvious categories missing.
 

Thread Starter

fullNelson

Joined Nov 14, 2011
46
It is perhaps less common for active devices like transistors to be regarded as loads, but they can be.
A few more questions:

What is an active device and what qualifies as one?

Wouldn't it be enough then to say that any component which draws current and has a voltage applied to it is a load? Or is the deciding factor that the power produced by the component must be useful?

Thanks for the response, btw.
 

Adjuster

Joined Dec 26, 2010
2,148
Active vs passive is a bit complicated. Electronics uses a definition which some general scientists would consider incorrect, but it has become conventional.

In essence, active devices can amplify, oscillate, perform controlled switching (most people would I think exclude electromechanical relays???) or in any situation permit a weaker controlling signal to influence a larger energy flow.

Passive devices more simply respond to currents or voltages applied to them. This Wikipedia reference may help: http://en.wikipedia.org/wiki/Electronic_component#Classification

It probably is true that passive devices are more commonly considered as loads, but the power dissipated does not necessarily have to be used for any particular useful purpose. One common use of a load is as a termination to a transmission line (a signal cable).
The load is required to prevent reflection effects which can be troublesome, but if the usual resistor load is used the power absorbed by it is simply wasted as heat.
 

GetDeviceInfo

Joined Jun 7, 2009
1,660
On the summary portion of the resistors page on this site, one of the summary bullets stated this:

"Any device that performs some useful task with electric power is generally known as a load. Sometimes resistor symbols are used in schematic diagrams to designate a non-specific load, rather than an actual resistor."

Is this applicable to only components that produce heat/light etc... as power or can I say that a (for example )transistor is a "load"?
Schematics are a language of communication. Much depends on what you are trying to convey, and to whom your addressing. If the black box, or all encompassing resistor confuses your audience, then you've errored.
 

studiot

Joined Nov 9, 2007
4,998
--------------------------------------------------------------------------------

Quote:
Originally Posted by Lundwall_Paul
Anything that draws current.

Thats what I thought too.

Current = Volts / Resistance, so a resistor can be used to symbolise a load and represent how much current it draws.
Life is a little more complicated than this.

'Draws current' is too vague a term for circuit engineering

What about a 'balanced load', an 'inductive load' and so on?
 

Thread Starter

fullNelson

Joined Nov 14, 2011
46
So it seems there is a division on what exactly a load is defined as. Anyone else care to chime in on what a load exactly is?
 

Wendy

Joined Mar 24, 2008
21,929
It is very simple. It is the device using current, no more, no less. Anything else is defining the circuit. A circuit can be a load for a battery, a terminating resistor or speaker is a load for the amplifier. A load is relative to something else.

The point has been made that schematics are a specific language. It is a point I am in 100% agreement with. Without a schematic, it is speculation.
 

thatoneguy

Joined Feb 19, 2009
6,359
So, would a purely inductive/capacitive load be an oxymoron?
Ok:

Any component, circuit, or combination thereof which requires a source to function. (I know, tautology)
-or-
Any component, circuit, or combination thereof which draws energy from a source.
 

studiot

Joined Nov 9, 2007
4,998
Any component, circuit, or combination thereof which draws energy from a source.
An earth plane for a radiating antenna is part of the load on the transmitter, but does it draw energy?

A voltmeter is a (unwanted) forms a load, but we try to minimise the energy draw. In fact in a true null measuring system the energy drawn is zero.
 
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joeyd999

Joined Jun 6, 2011
4,290
An earth plane for a radiating antenna is part of the load on the transmitter, but does it draw energy?
I think (and someone here is going to present a case that contradicts this!) that in the sense we are speaking, we are talking about electrical components and devices that are galvanically connected to a source of power. Therefore, in the case of an RF transmission system, the antenna itself would be considered the load, regardless of ultimately where the resulting RF energy was consumed.

Perhaps the answer can be Thevinized:

As we all know, an electrical circuit can be modeled as a voltage or current source and a series or parallel impedance. This works not only for large-signal cases (i.e. resistive/reactive networks), but also for small-signal cases (transistor, diode, other non-linear stuff).

So, perhaps a more generalized definition of a load would be:

"A load is that which is not a Thevinized source, and draws power (in the instantaneous sense) from the source"

in this context, anything can be a load or a source, depending on your point of view, and how you model it. For instance, a CE transistor amp is a load on the incoming signal, but becomes a source for the next amp stage.

Interesting, batteries can be both a source (while delivering power) and a load (on a charging circuit).
 

MrChips

Joined Oct 2, 2009
19,907
One cannot and should not attempt to find an absolute definition of a "load". It depends on the context and the specific focus.

Is the shirt on my back a load? Is an infant in a baby carrier a load? Is my backpack a load? Is the baby in the womb a load?

When does a load cease to be a burden and becomes the recipient of the energy being transferred?
 
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studiot

Joined Nov 9, 2007
4,998
Hello joey,

Well the earth plane is an integral part of the transmission system because without it, the same antenna would launch a different amount of RF power.
So the question remains, does it draw any power?

You didn't address my other point (about a wheatstone bridge) at all?

Mr Chips is right - we all know instinctively what a load is but have the Devil's own job pinning down a good definition.

go well
 

GetDeviceInfo

Joined Jun 7, 2009
1,660
Tell that to the power company! They charge industry for low PF (even though its *their* wires that are dissipating the power).
well, it's the resistive component that dissipates the power, and they already know, so telling them is mute.

The penalty isn't about giving it back, it's when you give it back. If you give it back on top of your regular load, there's implications in transmission capabilities. If you gave it back instead of your regular load, they might actually pay you for it.
 
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