Monday, May 16, 2011

Getting zapped isn't an option

Stupid me. Had already swapped a different value resistor into the breadboard before realizing the circuit was hot! With 220volts! No, I didn't get electrocuted. Didn't even feel a thing. That's because I was drawing the 220VAC from a homemade isolation transformer*. With such a transformer even grasping one of the lines (uninsulated of course) and standing barefoot on wet earth is absolutely safe. Getting grounded isn't an issue. Guess I was smart after all  :p

What would be a memorable (and no doubt traumatic) experience would be to touch both lines. 220 volts is 220 volts, transformer or no transformer. If the isolation transformer has sufficient power rating (greater than a couple of watts) then a nasty shock will be inevitable.

Years ago I was also working on a 5VDC + 220VAC circuit. I had the misfortune of accidentally grounding something. BANG! The acrid smell of toasted electronics. If I remember correctly all three ICs got fried. Luckily I wasn't part of any collateral damage. With this isolation transformer working with DC+AC circuits (either VDD or VSS and one of the AC power lines are part of the same net--they're electrically connected together) has become much much safer.

* It consists of two center-tapped 220V to 24V step down transformer connected back to back. So what happens is that mains voltage of 220VAC is stepped down to 24VAC by the first xformer then stepped up again to 220VAC by the second. 220VAC in 220VAC out, with the output completely electrically isolated from the power company's lines, making grounding an impossibility, as well as limiting the amount of current/power that can be supplied to the circuit--which is good when testing a new circuit you're not sure is bug-free!  The labels on the xformers say each has a rating of 1 ampere. Ergo, this isolation xformer has a maximum power rating of 24V x 1A = 24 watts

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