Skip to main content

Let's Take a Trip Inside Low-Cost Power Bars, Part One

Sunbeam's Firestarter

From this angle, we can clearly see both of the MOV's leads going straight across metal strips on the switch; there's definitely no thermal cut-off here. Digging out that taped device reveals a lone orange 14D331K MOV overlaid on the top-left corner, with no brand or other apparent markings on it apart from UL and VDE indicators. Its other side is completely blank, save for the bulge from the other side's lead. We see no larger lump hinting at the presence of an integrated thermal protection under its coating. These all look like red flags.

I contacted Sunbeam about this and was told they're looking into it. Did I get a counterfeit power bar? Considering the pressure dollar-stores put on their supply chain to source the cheapest stuff possible, this is definitively a possibility. It certainly does not look like a product that should be on store shelves.

Let's see if this MOV-like device has MOV-like properties.

Testing The AMOV (Anonymous MOV)

Since I do not have a proper high-voltage power supply on-hand, I decided to dig out a 3.5kV, 40mA oil furnace ignition transformer from my miscellaneous parts bin. A real 14D331K MOV should have a varistor voltage rating in the 300 to 360V range at 1mA, so I would expect the un-branded device to be somewhere near the top of that range at 40mA.

What Were You Expecting?

Before plugging in my 1000V multimeter and risk frying it at 3.5kV on a defective or fake MOV, I did a MOV-only test run to see if the MOV warmed up as expected from dissipating somewhere in the neighborhood of 1.5W (or about 350V * 40mA). Once I was reasonably confident the MOV was doing something MOV-ish, I attached my meter and read 368V.

Those of you who may have looked up my multimeter's datasheet might be screaming: “but your old meter is not True RMS” and you are absolutely correct: it uses non-linear analog integration, which makes it quasi-RMS. Let's try another measurement method just to be sure.

Double-Checking

For this re-test, I added a 400V diode bridge and a 10nF 1kV cap for peak detection. I am reasonably confident in this 345V result and a little surprised the quasi-RMS result over-estimated by only 8%. If you are wondering why I am using RMS and DC voltages interchangeably here, this works because a square wave symmetrical about 0V (3.5kVAC clipped to ±345V) has nearly identical RMS and peak voltages. The difference would go down further after accounting for the diodes' forward voltage drop and reverse leakage current.

It looks like the MOV is real enough.

Our Second Contestant: Electro's EL-543

Coming out of a nearby flea-market, this $8 bar raises cord length to a slightly more useful 60cm (24”) and has a similar breaker-switch-indicator on the front. Its back cover features an UL holographic sticker with file number E302504. Warnings include not plugging this power bar into other power taps, use indoors in dry locations only and the “do not use within 10 meters or 30 feet from the electrical distribution panel” which I bet tons of people did not know existed until now – I honestly cannot remember noticing these warnings until now either. If you missed the explanation behind the 10 meters thing, revisit the Sunbeam Wall-of-Text slide.

Electro's Packaging

The packaging is the same style as Sunbeam's, claiming the same 90 joules of surge protection, but omitting its protection's voltage rating. You need to look at the back of the unit itself to find out its clamping is rated at 500V. Most of the rest is self-evident and already covered in the previous slide.

Inside The Electro

The overall construction is essentially the same with the live and neutral stamped metal strips connecting directly to the combination breaker-switch-indicator, and the MOV tucked behind the wiring, soldered directly to the switch. In this unit, the MOV does have a thermal protection device wired in series, as expected, and the two are held together by a small strip of heat-shrink tubing.

Electro Strips

Apart from the more jagged edges on the live and neutral strips, the ground and power strip designs are exactly the same as the Sunbeam model. Dimensions are different though: the power strips are 0.5mm x 4.6mm, while the ground strip is 0.4mm x 3mm after subtracting the holes. Still on the flimsy side, but better than Sunbeam's power strips.

Our Third Contestant: Noma's Nameless Unit

This is my first of two $15 entries, and I picked it up simply because Noma is one of those ubiquitous brands that seems to be everywhere. I own one old Noma bar, though it's welded or otherwise not easily opened: not so much as a squeak or a creak when I twist it or try to pry with a screwdriver. Our tear-down-friendly specimen has a 90cm (3') cord and not much else to distinguish it from the Electro.

Noma's Packaging

Yet another unit with that modest foldable packaging. Printing on the front is sparse. Warnings on the back only cover aquariums and indoor use. The surge rating goes to 13.5kA peak and 330V clamping voltage with a 312-joule capacity, so we can expect to see a 20D201K or equivalent (though those are only rated for surges up to 10kA).

Daniel Sauvageau is a Contributing Writer for Tom's Hardware US. He’s known for his feature tear-downs of components and peripherals.