One-shot surge protection
This moral of this story: any promises of protection and safety should be double-checked for validity.
Surge (over-voltage) protection is a rather frequent function, but implementing it robustly is not a very simple task. Recently, I stumbled upon a device called an “Extension Lead With Surge Protection“. I already owned several other gadgets from the same manufacturer, and they were by and large OK. “Why not?” was my thought, so the new gadget I also bought.
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My initial testing involved connecting the device to AC outlets both with and without ground. The gadget correctly identified both of these configurations, which was good, but I wasn’t yet done.
The gadget also promised protection from surges up to 2000 Volts (the normal AC voltage is 220V here where I live). This protection was its main merit, so of course I had to check this feature as well. I did so with the simple circuit shown in Figure 1.
Figure 1 This simple circuit supports confirmation of valid (or not) surge voltage protection.
The circuit produces a DC voltage of roughly double the input AC amplitude, approximately 600V in this case. The DC output shouldn’t be considered a shortcoming! The values of resistors R1 and R2, and diode Z1 (a 200V Zener diode in this case) need to be recalculated if your AC outlet voltage isn’t 220V.
The circuit also includes a LED which will illuminate only when this doubled voltage really is present on the gadget’s output. The LED should be bright enough for a current of ~ 1mA or less. Should I warn you here to beware of high voltage; to be cautious and not to connect any inappropriate load to the circuit? That said, I’ll continue the story.
I connected an AC/DC voltmeter to the output of the “Extension Lead with Surge Protection”, while its input was connected to the output of the circuit. The voltmeter showed 600+ volts! The gadget was simply translating its input to the output without any high voltage detection, far from protection.
To figure out what had gone wrong, I had to dismantle the gadget, which was not a simple task, as as it turned out. The screws required a very specific bit in order for them to be unscrewed. At this point, I prepared to see something interesting inside, and indeed there was!
The circuit within had several transistors to detect unconnected ground, which I’d already confirmed worked. It also had two varistor/thermal switch pairs, in thermal contact. Unfortunately, these thermal switches were only single-tasked! Being one-shot fuses, they could protect the load only once, leaving it permanently disconnected afterwards. “One-shot Surge Protection” would have been more accurate.
It seems that the designers realized this fault too late, so they instead connected the output of the gadget directly to its input, bypassing and completely disabling any surge protection in the process! My disappointing purchase had transformed into an interesting project, enabling me to re-enable the gadget’s protection again, albeit on a one-time-only basis.
—Peter Demchenko studied math at the University of Vilnius and has worked in software development.
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