Capacitive and Resistive Transformerless Power Supplies




Warning: 
Don’t Try these circuits if you don’t have much experience with electronics. Don’t touch any points of the circuits since some points of this circuit are at Mains Potential. After constructing and testing enclose the circuits in a metal casing without touching PCB and metal case. The metal case should be properly earthed to avoid shock hazards. 


DC voltage is an essential voltage type used in Embedded System devices. It is the type that powers Microcontrollers and Processors to run (The heart of Embedded systems). There are several ways to convert an AC voltage into a DC voltage. The most common way is to use a transformer and rectifier circuit to build it. Also, many high-quality circuits (as I have seen) use another type called Switching power supply, but for a low-cost application transformer-based or switcher-based power supplies are not cost-effective, because transformers, inductors, MOSFETs, in Switch based and transformer-based are expensive and take an amount of space in PCBs. So Transformerless power supplies provide a low-cost alternative to transformer-based and switcher-based power supplies. But even this, there is no isolation from the AC line voltage in these power supplies, so this introduces safety issues, Be careful when developing this type of circuit. 


The common two types of transformerless power supply are capacitive and resistive. The capacitive type is more efficient since its heat dissipation and power loss are very low. The Resistive type dissipates more heat, so the power loss is quite high.

I choose a resistive type circuit to explain how a transformerless power supply works. The transformerless resistive power supply circuit is: 



R1 (with C1 for capacitive circuit) here is to limit inrush current, so it is chosen so that does not dissipate too much power and chosen large enough to limit inrush current. D1 is chosen depending on the application, it is responsible for voltage limitation. D2 is used to convert AC to DC so the V+ and V- of AC will move to the same line. C2 is used to store Current for release to the load ( that creates a ripple on Vout) because D1 can change only V+ to Vz that is chosen for the application (for example 5.1V) and V- is changed to Vd1 which is related to the diode type (for example 0.7v), so we need to store and release the current when the voltage tries to drop to Vd1.   

Zero-crossing feature

At node N3, we can capture zero-crossing of input AC signal, and use it as input to the microcontroller, but for capacitive supplies, some delay is present due to the in series capacitor (C1).

* Transformerless Capacitive Power Supplies Calculation

- The Circuit

- The calculation 


* Transformerless Resistive Power Supplies Calculation

- The Circuit



- The calculation 


Resistive vs Capacitive

Resistive Transformerless power supply is lower cost than a capacitive power supply, but it is less energy efficient than a capacitive power supply. Also, The loss of energy is dissipated as heat in R1.

Improve the safety of Capacitive transformerless power supplies

To improve the safety of capacitive transformerless power supplies, we can:

- Added a fuse to protect the circuit during an over-current.

- Added resistance R2 in parallel with C1 to create a filter that will attenuate EMI from traveling back to the line (1M ohm resistance is a good fit, but change it depending on the application).

- A varistor or MOV will provide transient protection


Improve the safety of Resistive transformerless power supplies

To improve the safety of Resistive transformerless power supplies, we can:

- Added a fuse to protect the circuit during an over-current.

- A varistor or MOV will provide transient protection

- Split R1 into two resistors (R11, R12) in series so that a high voltage transient will not bypass the resistor, also lowers the potential across the resistors and reduces the possibility of arcing. 

- Added a filter to prevent EMI created by the circuit from migrating into the line or Neutral busses. 




Can transformerless power supplies certify (UL, EN, etc)?

The first thing, we should check is whether the certification requirement fits a power supply without isolation, then check all the components if it is Safety-Critical Component depending on the certification requirements, using your calculation, test results, technical documents, etc to fit those. I have seen many products, especially home appliances products using this type of power supply. 

The testers of the certificate will focus on Single-point failure, so they might be considering what happens if R1 and C1 fail short. 


Improve output current, and make Vout more stable

Instead of providing current during only one half of the AC waveform period, current is supplied by the source during both halves

But This circuit should improve its safety as we do on the other circuits. 


Is Transformerless Power Supply safe?


Transformerless Power Supply is not safe because when it is on no-load then no current flows through the circuit, therefore, no voltage drop in the capacitor. So when we touch the circuit we will get an electric shock.

Simulations using LTspice software

I have used values of the Transformerless capacitive circuit from AN954 Microchip application note. 


LED as a load:




Change C2 from 470uF to 6800uF:




Change load to the resistance of 10kohm:





Electromula files

Any files related to the calculations or simulations of this type of power supply will be here: https://github.com/walidamriou/Electromula/tree/main/Power-Supplies/Capacitive-and-Resistive-Transformerless


Note: Any information or files are provided "as is" and the author disclaims all warranties with regard to these informations including all implied warranties of merchantability and fitness. in no event shall the author be liable for any special, direct, indirect, or consequential damages or any damages whatsoever resulting from loss of use, data or profits, whether in an action of contract, negligence, or other tortious action, arising out of or in connection with the use or performance of these informations or files.


References

- Transformerless Power Supply by Electronics Foru

- Transformerless Power Supplies: Resistive and Capacitive AN954 Microchip application note

- Can transformerless power supplies be UL/CSA certified and is there any specific standard they must meet? question on Electronics Stackexchange

- Transformerless Power Supply. Why Transformerless Power Supply not Safe? by etechnog

- ATtiny13a capacitive power supply question on Electronics Stackexchange

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