Unregulated Power Supply

Unregulated Power Supply

An Unregulated Power Supply is the simplest of power supplies to construct. Nearly all electronic devices and circuits require some form of a DC power source for their operation either from a battery, solar cell or as a mains power supply. While batteries have the advantage of being small, portable and ripple free, they need replacing (or recharging) frequently and are also expensive as compared to a conventional DC power supply.
unregulated power supply

Since in our homes, schools and workplaces we have a convenient, reliable and economical source of electrical power, it makes sense to use the domestic mains AC supply to power our circuits. However, the mains AC supply is a lot higher (usually 220-250 V rms) than the much smaller DC voltage provided by a battery. The process of converting this higher AC voltage into a much lower DC voltage is called Rectification.

Rectification is the process of converting AC power into DC power. In the Diodes tutorials we saw that a diode conducts current in just one direction (from Anode to Cathode) and not in the reverse direction. A diodes ability to switch current in one direction only makes it ideal for converting a two directional alternating current into a constant direct current or DC supply as shown.

Diode Rectifier

diode rectifier

We can see that the AC input to the diode is a sinusoid which alternates between the positive and negative half cycles, while the output from the diode is rectified DC having a waveform which goes only positive to zero volts, the negative half cycles being blocked. This type of output waveform is called “half wave pulsating DC”.

Half Wave Unregulated Power Supply

The purpose of a power supply is to provide the required amount of electrical power at a specified voltage and current level, for example +9 volts at 500mA. The electrical characteristics of any power supply will depend on the circuit or circuits being powered, but generally all unregulated power supplies consist of a transformer to step down the AC mains voltage to the required level as well as providing electrical isolation and a diode rectifier to provide an unstabilised output voltage.

Consider the half wave unregulated power supply circuit below.

Half Wave Unregulated Power Supply

half wave unregulated power supply

The mains input is applied to the primary winding of the mains transformer, T1 with the transformers secondary winding supplying low voltage AC to the rectifier diode D1. The resulting output waveform contains a DC voltage level which is approximately equal to 1/π or 0.318 of the peak voltage.

So for example, if the sinusoidal peak voltage is 10 volts, the equivalent DC output would therefore be: 0.318 x 10 = 3.18 volts. Then it is important that you choose the right voltage transformer for your unregulated power supply.

As we have seen above, the output waveform from the diode is pulsating DC. Obviously this pulsating DC voltage is not suitable to power most electronic circuits as not only does the supply voltage vary considerably and rapidly compared to an ideal DC battery supply, there is no supply voltage at all for 50% of the time during the negative half cycle.

Very often when rectifying an alternating voltage we want to produce a steady state direct voltage such as we would obtain from a battery supply and free from the waveform variations mentioned above. One way to overcome this problem is to add a smoothing capacitor across the output terminals effectively connecting it in parallel with the load.

We know that a capacitor has the ability to store an Electrical Charge on its plates and we can use this ability to help smooth out some of the pulsating waveform. The capacitor, C1, usually called a smoothing capacitor or reservoir capacitor, becomes charged up by the current flowing through the forward biased diode during the positive half cycle. The amount of charge on the capacitors plates depends upon the peak positive output voltage from the transformer, T1 and the value of the capacitor as charge, Q equals V x C (volts x capacitance).

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