Troubleshooting and repairing Switch Mode Power Supplies

Transformerless Power Supply Circuit

Transformerless Power Supply Circuit

Transformers are indispensable in the field of electronics. But the need for a bulky and expensive component like a transformer can be eliminated in circuits involving low current chips, by utilising the circuit presented here as a transformerless power supply.

In addition to being compact and versatile,this circuit is also low cost and capable of supplying almost any voltage ranging from 3 to 24 volts by selecting suitable component values.The Output is sufficiently regulated.

Fig.1. single polarity DC power supply

Fig. 1 shows a single polarity DC power utilises R1 as a series dropper.D1 rectifies the voltage obtained through R2, while D2  stabilises the output to the specified voltage.C1 filters the output.

R1 100K
R2 (see table) 10W
C1 (see table) 25V
C2 0.22µF
D1 IN4001
D2 (see table) 250mW

some ICs require a dual DC power supply.For this purpose, the circuit shown in Fig. 2 may be used.R1 drops the voltage . which in turn is rectified by D1. D2 stabilises the voltage and C2 filters it.The polarities of  D1, D2 and C1 are reversed to obtain a negative voltage.Neon lamp indicates the presence of the mains.

Fig.2. Dual polarity DC power supply

Values of R2, C1, and D2 Depend on the output voltage required (see table)

Both circuits are capable of  providing 25mA current, They continue to provide the specified voltage even if the mains voltage varies from 195 to 270 volts. Suitable component selection table given here.

Component Selection Table
V Out R2 D2 C1
3V 15K 3V 220µF
6V 12K 6.2V 220µF
9V 10K 9V 220µF
12V 10K 12V 470µF
15V 10K 15V 470µF
18V 10K 18V 1000µF
24V 8.2K 24V 1000µF

The circuit can be used with low current ICs.The circuit has been successfully tested with with ICs like UM3482, UM3484, UM66, UM3561, UM3562, NE555, LM741, LM748 and LM556.the circuit is capable of providing around 25 mA current.

The current supplying capability may be enhanced to about 40-50 mA by using a 5.6k, 2oW resistor as R2. However, this would require the use of at least a 1-watt zener diode.

Please send your ideas about transformerless power supplys, which are very important for our success…

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