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The danger of transformerless power supplies on the connected load

Series

By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions

Transformers 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: they are smaller and cheaper, they are lighter, and, most importantly, they work with a wide range of input voltages – from 100Vac to 240Vac. On the negative side, they do have one rather critical drawback: they connect directly to the mains, potentially making them 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 could happen to a typical transformerless power supply circuit if one of its components was to fail.

Figure 1 above: A typical conventional power supply circuit

Figure 2: A typical transformerless power supply circuit

Methodology

We built a transformerless power supply circuit with an output voltage of 6.5Vdc; see Figure 3.

Figure 3: Transformerless power supply circuit with 6.5V dc output

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 simply done by disconnecting the tested component pin from the circuit).

Figure 4: Transformerless power supply circuit after connecting all the shunt switches

The experiment was repeated twice with every shunt-switch closure, for input voltages of 120Vac and 240Vac. All observed output voltages were logged and tabulated.

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, but 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.
  • Failures of resistors were not simulated since they normally fail open. This mode of failure generally results in opening the circuit and diminishing the power supply output circuit.
  • For simplicity, only one failure is considered at a time.

The experiment’s findings are shown in Table 1.

Failed ComponentFailure ModeInput Voltage V acOutput Voltage
C1Fail-short1206.3 V dc
2406.5 V dc
Fail-open1206.1 V dc
2406.1 V dc
D1Fail-short1206.1 V dc
2406.1 V dc
Fail-open1206.1 V dc
2406.1 V dc
D2Fail-short1206.1 V dc
2406.1 V dc
Fail-open1206.1 V dc
2406.1 V dc
D3Fail-short1206.1 V dc
2406.1 V dc
Fail-open1206.1 V dc
2406.1 V dc
D4Fail-short1206.1 V dc
2406.1 V dc
Fail-open1206.1 V dc
2406.1 V dc
C2Fail-short1200 V dc
2400 V dc
Fail-open120Negative 0.25 to Positive 6.2 V ac
240Negative 0.25 to Positive 6.2 V ac
Zener Diode
X10
Fail-short1200 V dc
2400 V dc
Fail-open120Voltage rise gradually to reach 240 V dc
240Voltage rise gradually to reach 120 V dc

Table 1: Experiment results

Thus, it can be concluded that transformerless power supply circuits are quite safe in many of the components’ fail-open and fail-short scenarios, with exception of the following:

  • When C2 fails open, there’s a distorted AC voltage (between -0.25V and 6.2V). This is noticed at both 120Vac and 240Vac input voltages.
  • When the Zener diode (X10) fails open, there is a gradually-rising voltage at the circuit’s output, reaching 120Vac and 240Vac when using 120 and 240Vac input voltages, respectively. This is a very dangerous scenario and could damage the load. To protect the circuit, it should be connected to a fuse to prevent the connected load from drawing higher current than designed.

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