A voltage source that has very high internal impedance as compared with external load impedance is considered as a constant current source.
In such a case, the load current nearly remains the same when the output voltage changes. Fig(i) illustrates a constant current source. It is a dc source of 1000 V with internal resistance Ri = 900 kΩ. Here, load RL varies over 3: 1 range from 50kΩto 150 kΩ. Over this variation of load RL, the circuit current I is essentially constant at 1.05 to 0.95 mA or approximately 1 mA. It may be noted that output voltage V varies approximately in the same 3 : 1 range as RL, although load current essentially remains constant at 1mA. The beautiful example of a constant current source is found in vacuum tube circuits where the tube acts as a generator having internal resistance as high as 1 MΩ.
Fig (ii) shows the graph of a constant current source. It is clear that current remains constant even when the output voltage changes substantially. The following points may be noted regarding the constant current source:
- Due to high internal resistance of the source, the load current remains essentially constant as the load RL is varied.
- The output voltage varies approximately in the same range as RL, although current remains constant.
- The output voltage V is much less than the generated voltage Eg because of high I Ri drop.
Fig. shows the symbol of a constant current source.
Resistance in case of a dc source
Now [pmath]I = Eg/(Rl+Ri) Since Ri [/pmath]>>[pmath]RL, I = Eg/Ri [/pmath]
As both Eg and Ri are constants, I is constant
Example A dc source generating 500 V has an internal resistance of 1000 Ω. Find the load current if load resistance is (i) 10 Ω (ii) 50 Ω and (iii) 100 Ω
Generated voltage, Eg = 500 V
Internal resistance, Ri = 1000 Ω
(i) When RL = 10Ω
Load current, [pmath]I =Eg/(Rl+Ri) = 500/(10+1000) = 0.495 A [/pmath]
(ii) When RL = 50 Ω
Load current, [pmath]I = 500/(50 +1000) = 0.476 A [/pmath]
(iii) When RL = 100 Ω
Load current, [pmath]I = 500/(100 +1000) = 0.454 A [/pmath]
It is clear from the above example that load current is essentially constant since Ri >> RL.