Building a Constant Current Source for Linear Charging

In a previous post (Charging a capacitor exponentially), we explored a simple circuit for charging a capacitor over a period of 1 millisecond. As we saw, the voltage against time characteristics of this circuit were exponential in nature. However, what if you want to charge the capacitor in a straight or linear line? Thankfully, you can achieve this by switching from a constant voltage source to a constant current source.

If you supply a constant current to a capacitor, you can calculate the time it will take to charge using the formula Vc = (I x t) / C, where Vc is the voltage across the capacitor, I is the current supplied, t is the charging time, and C is the capacitance of the capacitor.

 To achieve a linear charge, we can use the following circuit:

In this circuit, the capacitor C is charged through a constant current source, which ensures that the current flowing into the capacitor remains constant. As a result, the voltage across the capacitor increases linearly with time.

If you want to charge a capacitor with a constant current, you can use a constant current source. One way to build a constant current source is to use a PNP transistor.

The circuit shown above can produce a constant current of 1mA through R4. To calculate the value of the constant current, we need to determine the base voltage, Vb, which is set by the potential divider R1 and R2. Once we know the base voltage, we can easily calculate the constant current using the formula (V1 - Vb - 0.7) / R3, where V1 is the voltage supplied to the circuit, and 0.7V is the voltage drop across the emitter of the transistor.

In conclusion, a PNP transistor-based constant current source is a simple and effective way to generate a constant current for charging capacitors or driving LEDs. With some basic knowledge of transistor operation and circuit theory, you can easily design and build a constant current source that meets your requirements.