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Posted in: Developing Perspectives (October 2014)

Don’t Get Stung by a Wall Wart

By Bryan Bergeron

Until recently, a typical wall wart in my collection required at least two outlet spaces in my power strips: one for the prongs of the wall wart and at least one adjacent outlet partially obscured by the body of the wart. Given that new compact switching wall warts are so inexpensive, I recently upgraded my collection of conventional transformer and diode bridge warts to the switching variety. I’ve been happy with the upgrade — there’s more space in the outlets and less clutter around my workbench.

Unfortunately, I learned the hard way that the latest generation of “regulated output” switching wall warts can have at least one major shortcoming: the regulated output can be up to 100% over the stated output voltage with no load. For example, a 9 VDC wart can output up to 18 VDC with no load. This isn’t universal, but depends on the design of the switching supply.

I discovered this when I shipped out a dozen Arduino based animatronic systems for a research project. The systems left my shop — fully burned in — without a problem. However, the systems (which used 9 VDC switching wall warts for power) were DOA. I first thought of ESD, and modified the front end circuits of the animatronic systems to bleed off any electrostatic charges.

Luckily, before I sent the second batch of units off to the field, I ran across a thread in a forum about the no-load voltage levels in the same switching power supply warts I was using. It turned out that the users of my systems were plugging in the warts first, and then connecting the animatronic systems. This was guaranteed to generate a chipkilling spike if the no-load voltage was significantly greater than the load voltage. I solved the problem by ordering a dozen of the old-fashioned bulky wall warts with conventional non-switching circuitry. Problem solved — after quite a bit of expense repairing and reshipping the animatronic units.

Of course, not all switching wall warts suffer from this no-load voltage problem. The wall warts weren’t something I found on eBay. They were standard items from my favorite parts supplier. Bottom line: Verify that the wall wart’s output is what you expect before plugging it into that new system you’re designing. NV

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