Understanding Non-Isolated Power Supplies: Hazards and Applications

We often come across the concept of power supplies in electronic devices, but understanding the differentiation between isolated and non-isolated power supplies is crucial for safety and application.

What is a Non-Isolated Power Supply?

A non-isolated power supply, as the name suggests, does not provide an electrical isolation between the input and the output. This means the output ground is directly connected to the ground of the power plug, which can be either the safety third wire ground or the neutral side of the power.

Such configurations are simpler and less expensive to implement but come with significant risks. In contrast, isolated power supplies separate the input and output, making them safer but more complex and costly.

How Non-Isolated Power Supplies Work

When the input voltage, such as 110 VAC from the household power line, goes directly to a rectifier to convert it to DC, followed by a filter network, voltage regulators, and other components to derive the desired output voltage, a non-isolated power supply is formed. This direct connection poses a safety hazard under certain conditions.

Dangers of Non-Isolated Power Supplies

The primary danger with non-isolated power supplies lies in the potential exposure to high voltage. If the input voltage is not isolated from the output, external factors such as a poorly wired power plug, internal component failure, or even a momentary touch of the chassis while grounded, can result in exposure to full line voltage.

For instance, in early cheap televisions, the chassis was often connected to the neutral side of the A/C line. This setup, while simpler, was highly dangerous. A simple mistake by a service technician or a momentary touch could result in severe electrical shocks or even electrocution. This is why safety standards have evolved to require isolation.

Personal Experience with Non-Isolated Power Supplies

I once worked on a defective GE television that had this setup. At the time, I was working on the television while sitting on a metal stool on a concrete floor. Concrete can provide a decent ground. Unfortunately, my leg was in an awkward position, and when I accidentally touched the positive terminal of a filter capacitor with my test probe, I received a painful shock of around 130 volts. The shock was strong enough to knock me off the stool and resulted in small burns on my hand and leg.

This incident not only taught me a valuable lesson in safety but also emphasized the importance of using appropriate grounding and non-conductive furniture in work environments.

Conclusion

Non-isolated power supplies, while simpler and cheaper, pose significant risks. Understanding the differences between isolated and non-isolated power supplies is essential for both safety and effective application in electronic devices. It is crucial to follow safety guidelines and use appropriate equipment to prevent accidents and injuries.