Column: Circuit drill


The danger of transformerless power supplies to the connected load


By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions T


ransformers are vital to all conventional power supply circuits. However, more recently, transformerless power supplies have started making their way into appliances; see


Figures 1 and 2. These type power supplies offer many advantages, like being smaller and cheaper, but also lighter and, most importantly, they work with a wide range of input voltages – from 100Vac to 240Vac. But, they also have drawbacks, one of which is rather critical, and that is they connect directly to the mains, making them potentially dangerous. In Figures 1 and 2 you will notice that in


the conventional power supply circuit the transformer provides good isolation between its primary coil high-voltage side (120Vac, 240Vac) and the low-voltage side of its secondary coil. This isolation is absent in transformerless power supply circuits. Our experiment here will unearth what


happens to a typical transformerless power supply circuit when one of its components fails.


Methodology We built a transformerless power supply circuit with an output voltage of 6.5Vdc; see Figure 3. To examine the failure of the various


components in a transformerless circuit, we placed normally-open shunt switches across all capacitors and diodes. By closing one shunt switch at a time, the circuit’s output voltage is monitored to check how much voltage exists at the power supply’s output when shorting. Figure 4 shows our circuit after connecting all shunt switches for the experiment. We also examine the fail-open scenarios of


all components (not shown in the diagram, since it could be done simply by disconnecting a tested-component pin from the circuit). The experiment was repeated twice with every shunt-switch closure, for input voltages


10 November 2021 www.electronicsworld.co.uk


Table 1: Experiment results


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