A power supply (PSU) is needed any time you want to use active components in a project.
What are active components? Active components are devices which are capable of controlling current by means of electrical signals. These include transistors (bipolar and FET), valves (tubes), op-amps, etc. Typically active components will contain some form of silicon circuitry. Beware however that silicon circuitry does not automatically mean a device is an active component as you will see below.
By contrast, passive components are those devices which cannot control current my means of electrical signals. These include resistors, capacitors, inductors (coils), transformers, diodes, LEDs, etc. Interestingly, both diodes and LEDs (light emitting diode) are considered passive devices, yet they contain silicon…
Most audio circuits will contain at least some active components. I say most because there are always exceptions to any rule. Examples include things like passive crossovers (as found in most loudspeakers) as well as passive pre-amplifier circuits (volume controls)
Because most audio circuits will contain some active components we need to implement a suitable PSU in order to use these components.
A PSU can be as simple as a battery to as complex as a precision regulated laboratory grade bench supply and everything in-between. There are many ways to implement PSUs and each has pros and cons related to it.
AC & DC Power supplies
If you are note too sure about the difference between AC and DC, please watch this excellent YouTube video by “AddOhms”:
Most of the electronic circuits you come across will need DC power in order to function. The most readily available source of DC power is probably a battery. While batteries are extremely easy to use and come in a variety of different voltages and sizes they do have certain restrictions: they will run flat. While this is not generally a problem for your wall clock (because it uses very little average power), it can become a problem with higher powered circuits.
There has always been some interest in battery powered audio circuits. Specifically things like pre-amplifiers and phono pre-amps seem to have a large fan-base of battery operated devices. While there is definitely some merit in this approach not much in this life comes without a downside. Can you imagine having to recharge the battery in your pre-amp so you can listen to it? What if you forget? To be honest it sounds like a tricky proposal to me at lest…
A more reliable method of generating DC is often needed:
The mains power available from the power sockets in your home is generally either 110V or 230V AC depending on the country you live in. The problem is that this voltage is too high for most circuits and equally importantly – it’s not DC.
So we need to find a way of achieving 2 things – reduce the voltage and convert AC into DC.
Changing the voltage of an AC power source
One of the most commonly used ways of changing the voltage (amplitude) of an AC waveform is by using a transformer. The transformer basically consists of 2 separate coils of wire wound around a special steel core. The two coils are called the primary and secondary. When used to convert AC mains power to a different voltage, the mains is connected to the transformer primary winding via a fuse. The AC waveform flowing through the transformer primary coil will induce a voltage into the secondary coil. The output voltage will be determined by the ratio of windings between the primary and secondary coils.
Example: If the primary winding has 1000 turns on it, and the secondary has 200 turns on it, then the ratio between primary and secondary is 1000:200 or 5:1. This means that if we apply 230V AC to the primary, we will see 230V AC / 5 = 46V AC on the secondary.
In real life the voltage on the secondary will be a bit less because of copper resistance in the coils as well as iron losses in the transformer core and stray magnetic flux, etc.
One of the inherent benefits of the transformer is that the output power is galvanically isolated from the input (AC Mains) power. This galvanic isolation is very important for safety.
Unfortunately not all manufacturers provide full specification for their transformers. The bare minimum they should always specify is the input voltage, input frequency, output voltage and power. Example:
230V input, 50Hz, 25V – 0V – 25V, 300VA
This transformer is specified for an input of 230V AC at 50Hz. Output is center tapped with dual 25V AC output and power is rated at 300VA. In order to calculate the AC current this transformer is capable of delivering, we divide the 300VA power rating by the output voltage of 25 which gives 12 Amps total. The transformer is center tapped, so each of the halves of the secondary is able to provide 50% of the total, thus 6 Amps AC each.
There should be no problems at all using a 50Hz transformer at 60Hz however be careful when using a 60Hz transformer at 50Hz frequencies as it may overheat or not provide the stated full power output.
NOTE: Transformers are specified for input voltage, output voltage and power. Be very careful to only use transformers designed for your specific needs and conditions. DO NOT connect a transformer rated for 110V AC onto a 230V AC mains line or vice-versa.
Changing AC power into DC power
The most commonly used method of converting AC into DC is by using 4 diodes in a configuration called a “bridge rectifier”, “bridge”, or simply “rectifier”. There are numerous types of rectifier but the one we are mostly interested in is called the “full wave rectifier”.
In conjunction with the bridge rectifier, we normally employ capacitors to smooth out ripple and provide storage.
If you want to understand how the full wave rectifier converts AC into DC as well as the role of the power supply capacitor, please watch this excellent YouTube video by “Electronics Circuit Collection”: