# Parallel DC circuit parameters, constant current source and shunt circuit

1.The rules for calculating parallel circuit parameters
The rules for calculating parallel circuit parameters are summarized as follows:
(1) The total current (IT) is the sum of the currents of each branch.
IT =I1＋I2＋I3＋…＋In
(2) The voltage at both ends of the branch is equal to the voltage of the power supply for the branch.
Es =V1=V2=V3=…=Vn
(3) The total resistance (RT) is equal to the reciprocal of the sum of the 1/R term.
RT=1/(1/R1＋1/R2＋1/R3＋…＋1/Rn)
(4) The total power (PT) is the sum of the power of each branch.
PT=P1 +P2 +P3 +…+Pn

2.Constant current source
The constant current source can provide a constant current rather than a constant voltage to the load. For example, photovoltaic batteries are constant current sources, while batteries and mains generators are constant voltage sources that can provide a constant voltage to the load. For a constant current source, the power supply voltage will vary with the load, and for a constant voltage source, the power supply current will vary with the load. Photovoltaic systems require one or more loads to consume all the current output by the system. (Most wind turbines are constant voltage sources.)
In Figure 1, two current sources are connected in parallel to output current to the load. Note that the output current direction of the two power supplies is the same (the head direction is the same). When the arrow direction is the same, the current through the load will add up.
IR=0.5＋0.8=1.3A
In some applications, one power supply cannot provide enough current, and multiple power supplies need to be used in parallel at this time.

example 1
In Figure 2, two PV cells output current to the load RL. Calculate IRL, VRL and PRL (PV cell can be considered as a constant current source.)
IRL=I1＋I2=1.5A＋1.5A=3A
VRL=IRL×RL=3×40V=120V
PRL=VRL×IRL=120×3W=360W
Note that if RL = 50 ohms, then VRL = 3×50V = 150V. Here the current is constant and the voltage varies with the load.

Example 2
There are two PV arrays connected in parallel, as shown in Figure 3. The rated output voltage of each array is DC12V and the current is 10A. Calculate EL, IL, PL, and load polarity EL1 and EL2.
PV arrays A and B are connected in parallel, so
EL=DC12V
IL=IA＋IB=(10＋10)A=20A
Power（PL）=EL×IL=12×20W=240W
The load polarity EL1 is negative (-), and EL2 is positive (+).

3.Shunt circuit
Each branch of the parallel circuit can shunt current, so it can be used as a shunt. Figure 4 shows the method of calculating the branch current.
IX=(RT/RX)×IT
In the formula, IX is the branch current that needs to be calculated; IT is the total current; Rx is the branch resistance that needs to be calculated; and RT is the total circuit resistance.

Example 3
For the circuit shown in Figure 5, use the shunt calculation formula to calculate the current flowing through each branch.
(1) First calculate the total circuit resistance
RT=(R1×R2)/(R1＋R2)=(330×220)/(330＋220)Ω=132Ω
(2) Then apply the shunt formula
IR1=(RT/R1)×IT=132/330×0.6A =0.24A
IR2=(RT/R2)×IT=132/220×0.6A=0.36A
Note that the sum of the currents of each branch is equal to the total current.