NiCad charger - based on a current source

based on a current source The 78XX/79XX series voltage regulators have become so typecast in their main application that we tend to forget that they can also be used as current sources. This sort of current source can be very useful in many applications, such as charging NiCad cells, as in the circuit here. We will start by taking a look at the principle of the circuit, which, as the diagram shows, is very simple. The circuit is based on a voltage regulator with a constant load be- tween the two points across which the fixed voltage is found. The result is obvious: voltage and load are both fixed so unless Mr. Ohm got it very wrong the current is also constant. The whole unit (voltage regulator and load) can then be connected in series with a (variable) load, in this case the nickel cadmium battery that is to be charged, and the current will still remain constant. This is, of course, always pre-supposing that the input voltage is high enough. The circuit has on subtle "extra", in ...