Column: Circuit drill
Figure 1: A typical conventional power supply circuit
of 120Vac and 240Vac. The observed output voltages were then logged and tabulated.
Figure 2: A typical transformerless power supply circuit
Assumptions In the experiment we made the following assumptions: • A typical transformerless power supply circuit was used with a design output voltage of 6.5Vdc; however, the results of the experiment may vary for different output voltages.
• The experiment was conducted using two input voltages (120Vac and 240Vac), since they are the most applied in practice.
• Resistor failures were not simulated since they normally fail open. This mode of failure generally results in opening the circuit and diminishing the power supply output current.
Figure 3: Transformerless power supply circuit with 6.5Vdc output
• For simplicity, only one failure is considered at a time. The experiment’s findings are
shown in Table 1. Thus, it can be concluded that
transformerless power supply circuits are quite safe in many fail-open and fail-short scenarios, with these exceptions: • When C2 fails open, there’s a distorted AC voltage (between -0.25V and 6.2V). This occurs at both 120Vac and 240Vac input voltages.
Figure 4: Transformerless power supply circuit after connecting all the shunt switches
• When the Zener diode (X10) fails open, there is a gradually- rising voltage at the circuit’s output, reaching 120Vac and 240Vac, depending on the input voltage. This is a very dangerous scenario and could also damage the load. Our conclusion is: To protect
the circuit, it should be connected to the load via a fuse to prevent the connected load from drawing higher current than designed.
www.electronicsworld.co.uk November 2021 11
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