Drawbacks of Transformerless power supply


To reduce cost and size, Transformerless power supply is now widely used which uses an X-Rated Non-Polarized capacitor to reduce 230 volt AC to low volt AC. It is then rectified and used as low volt DC. But unlike the Transformer based power supply, the transformerless power supply has many disadvantages

1. There is no Galvanic isolation between the High volt AC and the DC load. So failure of the power supply destroys the device.
2. Mains voltage is not steady always so this will reflect in the output voltage which results in the erratic response of the circuit.
3. Transformerless power supply can give only minimum current up to 100 mA. So it cannot be used for inductive loads.
4. Voltage and current fluctuations are common in transformerless power supply unless both are regulated.
5. Shock hazard is high in transformerless power supply.



Fig.1 shows a typical transformerless power supply. It is not so much efficient because the output voltage is not regulated. It can fluctuate when the input voltage fluctuates. In less important circuits like LED lighting, it will not cause much problems. In the circuit, a 474, 400 volt X-rated capacitor is used so the output voltage from the circuit will be around 15-20 DC. Please note that, the range is given because, the output is not regulated. The capacitor C1 is connected to Phase line, while a 100 Ohms 1 Watt resistor ( R2) is connected to Neutral. So observe this polarity while connecting with AC lines. Resistor R1 is the “Bleeder resistor” that removes the voltage stored in the capacitor at switch off. Remember, without this resistor, there is great chance of shock since the capacitor stores high voltage for many hours. The remaining part is simple. A full wave Bridge rectifier comprising D1 through D4 converts the low volt AC from capacitor C1 to low volt DC. Capacitor C2 removes “Ripples” from DC to output a clean DC.
From this circuit, you can expect around 20 volts DC at 40-50 milli ampere current (tested value). As a rule, each uF capacitance can give 100 mA current. But this is only theoretical and will vary depending on the type, make etc of capacitor and input voltage.



Fig.2 shows the modified transformerless power supply with voltage regulation. Front part of the circuit is similar to Fig. 1, but a 7812 voltage regulator is added. So it gives well regulated 12 volts DC irrespective of input voltage fluctuations. Capacitors C3 and C4 on either side of the regulator IC, act as filters and removes transients from the output voltage. You can also use 7805, 7806, 7808, 7809,or 7815 voltage regulators depending on the requirement. Use this kind of power supply in circuits using ICs , Sensors and oscillators to avoid false indications.

Caution: Very important to note that, Transformerless power supply is lethal if handled carelessly. The circuit board is at mains lethal potential and can give a fatal shock. So do not test or troubleshoot the circuit when connected to mains. Build this kind of circuit only if you are experienced in handling high voltage circuits.

3 thoughts on “Drawbacks of Transformerless power supply

  1. Hi,

    Where Can I get X-Rated Non-Polarized capacitor in local market? Electronics salesman doesn’t understand difference :-(. They gave normal one, which didn’t worked.

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