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Bench Talk for Design Engineers | The Official Blog of Mouser Electronics


THAT Doesn’t Need Circuit Protection! Kelly Casey

How many times have I heard that line?  In future I’ll discuss the different classes of clients that circuit protection device manufacturers serve.  But right now I’d like to offer up a real-world example of an application that most folks would never dream would benefit from circuit protection:  Solar-powered walkway LED lights.  I purchased five of the lights for about $15 each.  



I didn’t buy any spare units because I expected them to last a while.  Well-constructed, they performed nicely.  But in the third year, two of them failed – they just quit working.  As you might imagine, losing two also makes the other three pretty useless, as my installation really needed five!  Of course there were no matching replacements to be found.  Time to break out the screwdriver and multi-meter!

 

Now it doesn’t take a genius to troubleshoot this design – the electronic bill of materials is pretty spare (see the PCB in the second photo):

2 x AAA Rechargeable Cells (not pictured)

3 x Solar Cells (not pictured)

1 x Photocell (not pictured)

1 x Resistor

1 x Inductor

1 x Diode

1 x Switcher IC

2 x LED

 

The batteries were fully charged, so I ruled out the batteries and the solar cells.  The LEDs were in parallel, so it was unlikely that both would fail.  I focused on the switcher IC.  After quite a bit of searching, I determined it was an oddball part and found an obscure distributor in China that would sell me 10 pcs.  About a month after ordering the ICs, they showed up.  I replaced the IC in the first light...


 ...and voila!- it works.  Pretty proud of myself, I replaced the IC in the second light and…… nothing.  Probably a bad IC, I told myself.  Replaced it again and….. nothing.  A close examination of the inductor revealed serious surface cracks.  I ordered a handful of J.W. Miller 542-77F820-RC (82uH) inductors from Mouser Electronics, they arrived quickly, and the second light was back in business.


 

I have to wonder how these failures occurred.  The PC board is inside a metal housing with no attached cables – how does a surge get in?  It must have been a surge – a pair of AAA batteries could never have damaged the inductor.  And it must have been an induced surge from nearby lightning.  While the metal housing offers electrostatic protection, it offers no protection from magnetic fields. 

 

From the oddball switcher IC’s datasheet, I could see that a trio of Littelfuse P0080EA sidactor devices connected from each of the three non-grounded leads of the IC to ground would have prevented any surge damage to the IC as well as all of the other electronic components in the solar light.  Maybe I’ll order some of those from Mouser and add them in.  That way my grandchildren will be able to enjoy these lights because I think they would be pretty much indestructible at that point. I just wish the OEMs had gone the extra step and added circuit protection. I’m certain that they could have charged a premium for a long-term warranty (on everything but the easily replaceable batteries) and never have to pay a warranty claim.  That would increase their profits, and with the right marketing, create a loyal customer base.

 



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Kelly Casey is VP of Engineering for FM Technical Consulting, and holds a Bachelor of Science Degree in Electrical Engineering from the University of Nebraska, as well as a Master of Science Degree in Electrical Engineering from the Georgia Institute of Technology. Previously, Mr. Casey has held various roles at Bourns, Littelfuse, and Teccor Electronics.  

 

Kelly Casey is VP of Engineering for FM Technical Consulting, and holds a Bachelor of Science Degree in Electrical Engineering from the University of Nebraska, as well as a Master of Science Degree in Electrical Engineering from the Georgia Institute of Technology. Previously, Mr. Casey has held various roles at Bourns, Littelfuse, and Teccor Electronics. 
Kelly Casey is VP of Engineering for FM Technical Consulting, and holds a Bachelor of Science Degree in Electrical Engineering from the University of Nebraska, as well as a Master of Science Degree in Electrical Engineering from the Georgia Institute of Technology. Previously, Mr. Casey has held various roles at Bourns, Littelfuse, and Teccor Electronics. 

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