A very compact simple transformerless power supply circuit is explained here using the concept of capacitive reactance for stepping down the input AC mains voltage.
Advantages of using a Transformerless Power Supply Circuit
The idea is cheap yet very effective for applications that require low power for their operations.
Using a transformer in DC power supplies is probably quite common and we have heard a lot regarding it.
However one downside of using a transformer is that you cannot make the unit compact.
Even if the current requirement for your circuit application is low, you have to include a heavy and bulky transformer making things really cumbersome and messy.
The transformerless power supply circuit described here, very efficiently replaces a usual transformer for applications which require current below 100 mA.
Here a high voltage metalized capacitor is used at the input for the required stepping down of the mains power and the preceding circuit is nothing but just simple bridge configurations for converting the stepped down AC voltage to DC.
The circuit of a cheap transformerless power supply shown in the diagram above is a classic design of a transformer less power supply circuit and may be used as a 12 volts DC power supply source for most electronic circuits.
However having discussed the advantages of the above design, it will be worth focusing on a few serious drawback this concept may include.
Disadvantages of a Transformerless Power Supply Circuit
First, the circuit is unable to produce high current outputs, but that won’t make an issue for most of the applications.
Another drawback that certainly needs some consideration is that the concept does not isolate the circuit from dangerous AC mains potentials.
This drawback can have serious impacts for designs which have terminated outputs or metal cabinets, but won’t matter for units which have everything covered up in a non-conducting housing.
Therefore, new hobbyists must work with this circuit very carefully to avoid any electrical casualty. The last but not the least, the above circuit allows voltage surges to enter through it, which may cause serious damage to the powered circuit and to the supply circuitry itself.
However in the proposed simple transformerless power supply circuit design this drawback has been reasonably tackled by introducing a high voltage capacitor after the bridge.
This capacitor grounds instantaneous high voltage surges, thus efficiently safeguarding the associated electronics with it.
How the Circuit Works
The working of this transformless power supply can be understood with the following points:
- When mains AC mains input is switched ON, capacitor C1 blocks the entry of the mains current and restricts it to a lower level as determined by the reactance value of C1. Here it may be roughly assumed to be around 50mA.
- However, the voltage is not restricted, and therefore the full 220V or whatever may be at the input is allowed to reach the subsequent bridge rectifier stage.
- The bridge rectifier rectifies this 220V C to a higher 310V DC, due to the RMS to peak conversion of the AC waveform.
- This 310V DC is instantly reduced to a low level DC by the next zener diode stage, which shunts it to the zener value. If a 12V zener is used, this will become 12V and so on.
- C2 finally filters the 12V DC with ripples, into a relatively clean 12V DC.
PCB Layout for the above explained simple transformerless power supply is shown in the following image. Please note that I have included a space for an MOV also in the PCB, at the mains input side.
WARNING: THIS CIRCUIT IS NOT ISOLATED FROM MAINS AC, THEREFORE ALL ITS POINTS ARE EXTREMELY DANGEROUS TO TOUCH WHEN CONNECTED TO MAINS AC........