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Ive been looking for information on constant power, constant current and constant impedance/resistance loads, there's some information about the constant current loads, but very little that really explain constant power and constant impedance.

So please correct me if im wrong as ill try to define what I understand about this subject:

  • A constant current load is the one which varies its internal resistance to achieve a constant current regardless of the voltage which is being fed to it (within certain extent) and therefore the power will vary.

  • Im assuming a constant power load, also varies its resistance so the power (or voltage and current product) is always the same, regardless of the voltage or current that is being drawn.

And what about constant impedance/resistance loads? does it mean that both voltage, current will vary and therefore power will vary too? yet impedance or resistance will remain the same?

And if we are talking AC, im also assumming this valid for all frequencies in a certain range.

Now, on a more conventional scenario, when we are talking about regular every day loads, say a power supply inside a computer feeding the motherboard and peripherals, or a linear power supply inside a stereo feeding the inside components. Are we talking about a varying current, power and impedance loads?

How can it be determined if a load is constant current, power or impedance?

Thank you very much!

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2 Answers 2

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A constant power load varies it's impedance on change of input voltage to keep the power constant. A constant impedance load is simply a load that presents an unchanging impedance, like a resistor. An L-Pad is used to change speaker output level whilst maintaining a constant impedance load to the amplifier.

A good example of a constant power load is a switching regulator. Since this has to maintain it's power into it's load, it must draw the same power from it's source even if the source changes voltage.
This is also an example of a negative impedance because in order to maintain the output power, if the voltage in drops, the current must rise (opposite to a standard resistor where the current and voltage rise/fall with each other)

Here is an example circuit, made from a LT1377 boost switching regulator:

Constant Power Load

Here is the simulation:

Constant Power Simulation

The input voltage V(in) (blue trace) starts off at 4V, and gradually rises up to 10V. We can see the power (red trace) stays constant at ~1W over a change of 6V at the input (it's not perfect as it's meant to represent "real life" and not 100% efficient, but it's pretty close)
We can also see the dynamic negative resistance characteristic (green trace) which is due to the input current falling as the voltage rises. Tt falls from ~300mA to ~120mA over the voltage rise from 4V to 10V - don't be confused by the minus sign, that's just the direction of measurement in LTSpice.
The dynamic resistance slope can be roughly calculated by (4V - 10V) / (300mA - 120mA) = -33.3Ω. Looking it another way, 6V / -33.3Ω = -180mA.

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Thank you very much for that answer, it was really ilustrative. However I have just a question, when we are talking negative impedances, I can understand the fact that it raises the current when the voltage lowers, but how is this exactly, does it actually mean the impedance decreases as to allow a larger current flow of current, thus considered to be negative (because the impedance decreased)? –  Joe M Aug 27 '12 at 8:13
1  
@JoeM - see edit, I meant the trace to be the current, which it is now (sorry). A negative impedance has the opposite slope from a positive one, so we can measure the (dynamic, i.e. on change) impedance by taking the difference of two points on the slope. Have a read of the Wiki and look at NICs (Negative Impedance Converters) –  Oli Glaser Aug 27 '12 at 15:29
    
Thanks again Oli, im still trying to wrap my head around that Wiki link you provided. But I think I get the idea of negative impedances. A negative impedance converter looks pretty much like a non inverting amplifier but with an extra feedback resistor. –  Joe M Aug 27 '12 at 16:13
    
Yes, the NIC actually has the current flowing in the opposite direction too (flows from "load" to source), so it's a static negative resistance (e.g. you can measure at one point and get a negative value) The switching regulator input has the negative resistance slope, but the current is positive (e.g. flows from source to load) A true negative resistance implies a source of energy. Try plotting a few components I-V curves on the graph in the link.. –  Oli Glaser Aug 27 '12 at 17:04
    
..and compare to the negative versions (opposite slope and in the case of true negative resistance in opposite quadrant) –  Oli Glaser Aug 27 '12 at 17:06
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You are right in what constant current, power, and impedance loads are. However, most loads are not so nice and predictable. Some loads may look mostly resistive, like heating elements, but many loads have all kinds of screwy characteristics or vary dynamically. For example, the input to switching power supplies look largely like negative resistances because they are mostly constant power. Sometimes the power supply circuitry is specifically designed to present a certain load profile to the input. An AC to DC power supply with power factor correction is a good example of that.

To determine the characteristics of an unknown load, you have to measure it. Keep in mind though, that many loads don't fall into a nice neat category like constant current, power, or resistance. Think of a microcontroller, for example. It's characteristics can change dramatically and quickly depending on what it is doing inside and how it may be driving output pins.

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So basically, most loads could vary?, meaning will be at times a combination of constant current, power and impedance? –  Joe M Aug 27 '12 at 8:14
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@JoeM: Or more accurately none of the special cases. Constant xxx loads are useful for mathematical analysis, but don't occur much in the real world. A resistor makes a good constant resistance load, but that's not much use except as a heater. The input to switchers tend to be constant power, but there are significant deviations from that usually over the full load range. Constant current is pretty rare except when something is deliberately designed for that. –  Olin Lathrop Aug 27 '12 at 12:28
    
Thank you Olin, you were really helpful! –  Joe M Aug 27 '12 at 16:14
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