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5VDC project, 110VDC overvoltage protection
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Zero999:

--- Quote from: Bouteille on January 17, 2019, 01:42:36 pm ---
--- Quote from: Zero999 on January 17, 2019, 01:25:20 pm ---PTC resistors are very slow. Nowhere quick enough to protect a semiconductor device. They're used for protecting cable, motors, transformers etc. which are big things with a large thermal time constant.

--- End quote ---

Hence the Zener for clamping. It dissipates power and waits for the PTC to save its life. Almost romantic   ;D
Not realistic though ?

--- End quote ---
A zener diode is a semiconductor device and would have to be very big to withstand the surge for the amount of time it would take for the PTC to trip.
Bouteille:

--- Quote from: Zero999 on January 17, 2019, 03:51:42 pm ---A zener diode is a semiconductor device and would have to be very big to withstand the surge for the amount of time it would take for the PTC to trip.

--- End quote ---

Yes, that was one of my initial concerns actually:


--- Quote ---2. If I understand correctly the Zener diode must handle the short-circuit current until the polyswitch 'trips'.
But how to estimate the correct 'trip-time' from polyswitch datasheet, and then select an appropriate Zener to use in this application ?

Must I use the polyswitch 'Typical Resistance' (datasheet page 238, ie. 8.0Ω), and 'Typical Time-to-Trip Curves at 20°C' (datasheet page 245) ?

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T3sl4co1l:
Yeah, TVS diodes are pretty robust actually, but the sheer amount of energy let through by a PTC is ridiculous in comparison.  Probably the diode would blow (fail shorted, most likely) in a few milliseconds, then the PTC opens in tens of milliseconds.

Not quite as bad as a transistor dying in a hundred microseconds, and quite tractable with the right component choice (large enough TVS, PTC with enough internal resistance -- or add a resistor in series to help out).  But such a solution will be more expensive and less reliable than a regulator or switch.

PTCs handling over 60V are hard to come by, and they will be quite large and therefore slow, making things worse.

The depletion MOS, resistor and zener is damn good for low current loads (under 100mA, say?), and a switch method is better for heavier loads.

Tim
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