simplest way to detect 120VAC using a microcontroller

I haven't seen it happen, but the threat was real enough to require those fuses. Those power supplies are capable of a lot of current, though I'd think that the board trace would fry before the cable would catch fire. What was kind of bad is that people often blew the fuses by powering stuff of the +5V from keyboard and mouse connector since the picofuses were only 1/2 amp. At least with USB, the port shuts down only temporarily if there's an over-current condition.

What was surprising is that there were customers that didn't care about UL/CSA/TUV and we built boards with a zero ohm resistor instead of the fues for them. This could not have saved more than 4-5 cents (for two fuses), but the margins on motherboards are so terrible that they were watching the pennies. Ditto for the back-up batteries. The Tadiran UL approved batteries were more expensive than the non-UL batteries so the Tadiran batteries were used only on boards going to customers that cared.

We sold boards to a major computer store chain (now defunct) who had every store become a UL certified factory, and UL inspectors went to their stores to inspect their "factory." Once some boards got shipped to them from Taiwan with non-UL batteries and the UL inspectors caught it. I had to explain to the factory why they had to use the more costly batteries because they couldn't fathom it.

Ah, these days you just explain that they must be "UL marked" and the factory takes care of the rest.

Best regards, Spehro Pefhany

--
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speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Actually, what works very well and I do this at work to protect inputs is to use TVS diodes, the bipolar types in the 1.5k class. Just select the value you need for it to clamp at and they work very nicely.

Jamie

You have to remember that the keyboard connector was quite close to the power connector on the real IBM XT motherboard. All the + 5V current from the power supply was available trough a very wide internal trace.

A small Zero ohm will get quite hot if the current is high enough, and either open or melt the solder.

--
You can't have a sense of humor, if you have no sense.

Absolutely NOT. If anything, use a voltage divider.

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