Power is to be supplied from a source whose resistance is 20 K ? to a load of 100 ?. Would you consider it as a constant voltage source or current source? Explain.

Ans.

Given source is considered as a constant current source because, a source that has a very high internal resistance (20 K?) as compared to the load resistance (100 ?) is a constant current source.

Explanation: In figure below, a load impedance Z_L is connected to a current source. Let I_t be the short-circuit current of source, and Z_s be its internal impedance. Current I_S is splitted into two parts I_L through Z_L and I₁ through Z_S. That is

I_S= I₁+ I_L

Or        I₁= I_S- I_L

Because impedance Zs and  Z_L are  in  parallel,  voltage  drop  across each must be equal, i.e.,

The above equation determines us that load current I_L would remain almost the same as current I_S, provided the ratio Z_L/Z_S= 100/20000 = 1/200 is small compared to unity. Source then behaves as a good current source.  Or we can say, the larger the value of internal impedance Z_S (20K?) compared to load impedance Z_L (100?), the smaller is the ratio Z_L/Z_S, and better it works as a constant current source.