A very compact simple transformerless power supply circuit is explained here can be built using the concept of capacitive reactance for stepping down the input AC mains voltage.
The Transformerless Power Supply Concept
A transformerless power supply works by using a high voltage capacitor to drop the mains AC current to the required lower level which may be suitable for the connected electronic circuit or load.
The voltage specification of this capacitor is selected such that it's RMS peak voltage rating is much higher than the peak of the AC mains voltage in order to ensure safe functioning of the capacitor. An example capacitor which is normally used transformerless power supply circuits is shown below:
This capacitor is applied in series with one of the mains inputs, preferably the phase line of the AC.
When the mains AC enters this capacitor, depending on the value of the capacitor, the reactance of the capacitor comes into action and restricts the mains AC current from exceeding the given level, as specified by the value of the capacitor.
However, although the current is restricted the voltage isn't, therefore if you measure the rectified output of a transformerless power supply you will find the voltage to be equal to the peak value of the mains AC, that's around 310V, and this could be alarming for any new hobbyist.
But since the current may be sufficiently dropped level by the capacitor, this high peak voltage could be easily tackled and stabilized by using a zener diode at the output of the bridge rectifier.
The zener diode wattage must be appropriately selected according to the permissible current level from the capacitor.
CAUTION:Please read the caution warning message at the end of the post
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 Transformerless Power Supply 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.
Example Circuit for LED Driver Application
The following transformerless or capacitive power supply circuit could be used as an LED driver circuit for illuminating minor LED circuits safely, such as small LED bulbs or LED string lights.
The idea was requested by Mr. Jayesh:
The string is made up of about 65 to 68 LED of 3 Volt in series approximately at a distance of let us say 2 feet ,,, such 6 strings are roped together to make one string so the bulb placement comes out to be at 4 inches in final rope . so over all 390 - 408 LED bulbs in final rope.
So please suggest me best possible driver circuit to operate
1) one string of 65-68 string.
2) complete rope of 6 strings together.
we have another rope of 3 strings.The string is made up of about 65 to 68 LED of 3 Volt in series approximately at a distance of let us say 2 feet , such 3 strings are roped together to make one string so the bulb placement comes out to be at 4 inches in final rope . so over all 195 - 204 LED bulbs in final rope.
So please suggest me best possible driver circuit to operate
1) one string of 65-68 string.
2) complete rope of 3 strings together.
Please suggest the best robust circuit with surge protector and advice any additional things to be connected to protect the circuits.
and please see that circuit diagrams are with values required for the same as we are not at all technical person in this field.
The driver circuit shown below is suitable for driving any LED string having less than 100 LEDs (for 220V input), each LED rated at 20mA, 3.3V 5mm LEDs:
Here the input capacitor 0.33uF/400V decides the amount of current supplied to the LED string. In this example it will be around 17mA which is just about right for the selected LED string.
If a single driver is used for more number of similar 60/70 LED strings in parallel, then simply the mentioned capacitor value could be proportionately increased for maintaining optimal illumination on the LEDs.
Therefore for 2 strings in parallel, the required value would be 0.68uF/400V, for 3 strings you could replace it with a 1uF/400V. Similarly for 4 strings this would need to be upgraded to 1.33uF/400V, and so on.
Important: Although I have not shown a limiting resistor in the design, it would be a good idea to include a 33 Ohm 2 watt resistor in series with each LED string for added safety. This could be inserted anywhere in series with the individual strings.
WARNING: ALL THE CIRCUITS MENTIONED IN THIS ARTICLE ARE NOT ISOLATED FROM MAINS AC, THEREFORE ALL THE SECTIONS IN THE CIRCUIT ARE EXTREMELY DANGEROUS TO TOUCH WHEN CONNECTED TO MAINS AC........
Upgrading to Voltage Stabilized Transformerless Power Supply
Now let's see how an ordinary capacitive power supply may be transformed into a surge free voltage stabilized or variable voltage transformerless power supply applicable for almost all standard electronic loads and circuits. The idea was requested by Mr. Chandan Maity.
If you remember, I communicated you sometime before with comments at your blog.
The Transformerless circuits are really good and I tested couple of those and running 20W, 30W LED.Now, I am trying to add some controller, FAN and LED all together , hence, I need a dual supply.
The rough specification is:
Current rating 300 mAP1= 3.3-5V 300mA ( for controller etc)P2= 12-40V (or higher range), 300mA (for LED)
I thought to use your 2nd circuit as mentionedhttps://homemade-circuits.com/2012/08/high-current-transformerless-power.html
But, I am not able to freeze the way how to get 3.3V without using extra capacitor. 1. Can, a second circuit may be placed from the output of first one? 2. Or, a second TRIAC, bridge to be placed in parallel with first one, after capacitor to get 3.3-5V
I shall be glad if you kindly help.
The function of the various components used across the various stages of the above shown voltage controlled transformerless power supply circuit may be understood from the following points:
The mains voltage is rectified by the four 1N4007 diodes and filtered by the 10uF/400V capacitor.
The output across the 10uF/400V now reaches around 330V which is the peak rectified voltage achieved from the mains.
The voltage divider network configured at the base of the TIP122 makes sure that this voltage is reduced to the expected level or as required across the power supply output.
If a 12V is required the 10K pot may be set to achieve this across the emitter/ground of the TIP122.
The 220uF/50V capacitor ensures that during switch ON the base is rendered a momentary zero voltage in order to keep it switched OFF and safe from the initial surge in-rush.
The inductor further ensures that during the switch ON period the coil offers a high resistance and stops any inrush current to get inside the circuit, preventing a possible damage to the circuit.
For achieving a 5V or any other attached stepped down voltage, a voltage regulator such as the shown 7805 IC may be used for achieving the same.