hi mike,

>>hurry to demonstrate something and plugged a laptop power supply to it
>>instead of a 5v power supply. the jacks were identical. i ended up burning
>>up the board.
>
>But was the polarity identical?  Did you end with +18V connected to
>the ground net and GND to +5V?  Or with 30V connected to something
>rated for 12V?  Reverse polarity protection might be more appropriate.

the polarity was identical. it was 26V connected to a 5V rated board. but i
do want to also add reverse polarity protection in my next design.

>See the recent thread for details.  IMHO, the best overvoltage
>protection is a label:  12V 250 mA.  If the user violates that label, it
>becomes his or her problem, not yours.

this is a fair statement. but i feel, and it's a subjective feeling of
course, that it'd be nicer to allow the user to make mistakes, at least to a
certain extent.

>
>>2. zeners. i'd put two opposing zeners in parallel with the +ve input. i
>>think this would be even simpler than option 1. will this one be more
>>expensive? i heard zeners are faster than varistors.
>
>Like this?
>----|>z-----
>----z<|-----
>
>If you connect them like that, you'll end up with one forward
>conducting, with a voltage drop of ~.7V, and the other wanting
>to reach its zener voltage but not able to.

i was thinking like this:

in_pwr -------------------- out_pwr
                 |
               -----/
              / /\
                ---
                 |
                 |
                ---
                \/
               -----/
              / |
                |
                |
in_gnd     -----
                --

i am sorry for my poor ascii art. i need to find a bmp-to-ascii art type
tool.
thus, the above would be a shunt to the power supply ground if a reverse or
overvoltage condition occured. i guess i would pick a 5+Volt zener for both.
i'm not sure what kind of wattage would be good to protect against stuff
like up to 36V. mostly i'm trying to protect against laptop power supplies
since they have the same jack.

>That depends.  Some power supplies (probably most) would have "foldback
>current limiting", where the current will increase to a certain point, and
>then
>the voltage and current fall off to prevent damage to the power supply.
>Whether a given supply has that feature is not up to you, so I wouldn't
>depend on it.

if i understand the above correctly, then using the zener's to shunt on
overvoltage or voltage reversal sounds like it may damage the power supply.
is that right? if so, what would be a good mechanism to protect the supply?

regards,
hirose

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