How do constant current power supplies work? For example I have a power supply that is constant voltage that can supply up to 5A and I have a 12V motor that takes ~500mA so I crank up the supply to 12v and my motor pulls ~500mA from the supply. But how does this work for a constant current power supply? Would it determine its own voltage output depending on the resistance of the load(ohms law)?
Electronic – How do constant current power supplies work
power supply
Related Solutions
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:
Here is the 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.
In general, the term "voltage supply" refers to a constant voltage supply, and "current supply" (or "current driver") refers to a constant current supply. Both are power supplies, they just try to regulate different things.
Typically* "supplying power to the load" means supplying the voltage that the load is rated for, and letting it draw whatever current it wishes or intrinsically does. It doesn't have to be a large voltage or current, but the point is to supply energy, instead of communicating by sending a signal, which usually involves only a tiny current.
*It's also a pretty vague saying that could refer to something different than what I wrote above.
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Best Answer
The simplest constant current supply is a large value resistor in series with a much smaller load resistance. Suppose we have a 12V supply to a 1k resistor in series with a 10R resistor: the current flowing through the load will be 12/1010 amps or 11.88 mA. If the load is 20R, the current will be 11.76 mA. It's almost the same although the load resistance has been doubled. The voltage across the load has been doubled, in the second case, keeping the current at about the same value.