Unregulated Supplies

Basic DC Power Inputs

At the most basic level, only a mechanical connector wired directly to the active devices (transistors, ICs) is needed to provide power to simple circuits. The connector can be a 9V battery snap, a 3.5mm phone jack, or the modern standard, the 2.1mm coax barrel jack (2.5mm is very common as well).

The above diagram adds a simple low pass filter between an external dc power supply and the circuit. The basic functions of the RC circuit are to smooth out any ripple that may exist in the incoming dc power, decouple this circuit from any other circuit sharing the power supply, and provide a local reservoir of supply voltage. If the cap is sufficiently large, are the circuit's power demands sufficiently light, the circuit can even stay functioning for many seconds after the power input is removed.

The resistors value is typically low to prevent any significant loss of voltage due to voltage division. A low value also reduces the current limiting effect of the resistor. There is a compromise between the filter's effectiveness and the current limiting factor. Typical values are 10-100Ω. If a short circuit develops in the main circuit, then the entire voltage supply will be dropped across this low value resistor. For a 9V supply, this is almost 100mA at 100Ω, and nearly a full amp at 10Ω. The 100Ω resistor will be close to 1W, and the 10Ω close to 10W. If a 1/4W resistor is being used, this will most likely result in the resistor failing open. In this way the resistor doubles as a pseudo-fuse, and many pedal repairs involve changing out a smoked up resistor in series with the main power connection.

Instead of fixed resistor, we can sub in a transistor and use it like a variable resistor to control the voltage to the main circuit. Those circuits are considered in the discrete transistor power supplies page.

Also see the page on reverse polarity and over voltage protection.