The configuration shown in figure bellow is used to maintain constant the current flowing through a load. The value of R1 is limited by the output voltage. The following formulas can be used to calculate the components and voltages in this circuits.
Formula 1
Resistor R:
R = Vref / Iout
Where:
R is the resistance in ohm (Ω)
Vref is the reference voltage in volts (V)
Iout is the constant current in the load in amperes (A)
Application example:
Determine R for a 100 mA constant current source using a 7805 Integrated Circuit (Vref = 5 V) as shown in figure 2.
Data:
Iout = 100 mA = 0.1 A
Vref = 5 V
R = ?
a) Calculating R from formula 1:
R = (Vref / Iout)
R = 5/0.1
R = 50 Ω
