Tek Tip 2sD - Electric Blanket Fix

Slim's Story - 

"So this [electric blanket] has been on my [impressively long] list for a while and I finally got around to fixing it.

"And I learned some things in the process.  Like there’s a term for the resistance of a capacitor in an AC circuit, called “Reactance.”  And since it is an inverse function of Capacitance and Frequency, as a capacitor loses its ability to store charges, it becomes a larger resistor.  I knew it had to be the two film capacitors, because they had mains voltages across them when I tested.

"Kids, don’t try this at home.  Never disassemble and power a device with exposed mains voltages.

"Having a second, functioning example helps.  I was able to probe and compare voltages in both for comparison.

"Desoldering and testing the bad capacitors confirmed my suspicion.  Testing capacitors was itself a lesson (somewhat in how much I’ve forgotten, or never learned because I never was in any ECE classes in school.)  But I did remember the RC Time Constant.  Then I remembered that I have a microcontroller which I used to tinker with various capacitors in the past.  Blunder or bumble may be a better word than tinker, because I was simply timing how long the capacitors took to charge and discharge.  If I just used a little tiny bit of brains, I could use it to calculate the capacitance, even if just a reasonable approximation for this case.

"I found a 1.6µF capacitor was down to 160NF.  That’s way off.

"Since you can’t really test while they are in-circuit, I ened up desoldering a “good” capacitor.  And since we live in a Technology Desert (akin to Lady Obama’s “Food Deserts”)  where you don’t have access to a good selection of quality components like you do in, say, Silicon Valley – I just grabbed what was close from Radio Shack.  And I bought all of the capacitors The Shack had in those sizes.  This means I only had a 1µF and a 0.1µF capacitor to replace the 1.6µF capacitor that went bad.

"So I ghettoed it up by reusing the old one along with a new one to supplement the 1.0µF up to 1.2µF, hoping that this would be “close enough.”

"Fortunately, the PCB had some room to spare in an area with no components so I hot snotted the capacitor assembly to the board and used some insulated wire so as not to put mains power back into places it should not go.



"Well, the display and indicator LEDs re super bright again, so I think it is a success.



"Using the data sheet for the integrated circuit (the Internet, again, is fantastic) I looked for the power and ground pins on the chip’s package.  It was originally only getting 1.8 V and it is spec’d at 3.3V to 5V so this should be good for a while."



[Another successful Internet-YouTube repair]