Practical current source with standard power supply

When we powered up any circuit using a standard power supply V+ and GND then power supply serve as voltage source and the current will be drawn according to the load.

What is the practical example of current source ? How to get or generate 1 mA current source ? Is it possible to connect a resistor in series to the power supply and consider it as current source ?

Hi,

The datasheet for the LM334 is an example of an IC current source. You can make current sources many ways, using op amps, or transistors, some circuits use a BJT, a diode or an LED and a few resistors, look at Wikipedia's "Current source" page, for example.

A resistor in series in a circuit draws current, so surely it is a (current) load, not a current source.

TO my understanding, a current source is a constant, reliable and known quantity of current. LED drivers use constant current sources if I'm not mistaken.

If you bias the base of a BJT to "1/10th" of the collector current required, e.g. collector current 1mA so bias base current to 100uA, more or less you get a constant current source, which will vary a little with temperature or if you alter the supply voltage.

Voltage reference ICs can be configured as current sources, so can some temperature sensors as they are all "basically" a diode.

One practical use for a current source is an Ohmmeter, and internally a lot of ICs have sections which depend on current sources, for example Op Amps.

A transistor makes the classic current source. As long as the bias is unchanging, then collector current is unchanging, whether the switch is open or closed.



The principle works if you keep values in a reasonable operating range.

If the load voltage is low and varies little, then a high
voltage supply and a high series resistance will appear
pretty current-source-like. Not "ideal current source"
but possibly close enough, depending.

Many power supplies have an adjustable current limit
and can be made to operate as current sources if you
raise the voltage setpoint / compliance above the max
load voltage, and throttle back the current limit (which
becomes the current setting).

A current source has an internal impedance much higher then the external load . Opposite to a convention voltage source. So a 1mA current generator might have an output impedance of 1M ohm to drive a 1mA into a 1K load. The ratio of the internal impedence and load, will give you a "goodness" factor which is for you to decide. i.e X 10 / X 100 ... .?
Frank