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Overvoltage Protection
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NivagSwerdna:
I would like to build a gadget that interfaces to old 5V TTL logic and to make it robust from over-voltage.  Specifically it should be able to cope with having +12 or -12 volts 'accidentally' applied to one of the lines.

I envisage approximately 8+16+8 i.e. 32 individual connections.

On the gadget side I will probably be operating at 3.3 volts so I also need voltage translation, bi-directional on the majority of lines.

Any suggestions for components?

Thanks in advance
capt bullshot:
Series resistors and clamping diodes do a good job for OVP on inputs. Series resistance may be inappropriate for outputs, though.

One can get bi-directional voltage translators like the TXB0104 (http://www.ti.com/lit/ds/symlink/txb0104.pdf). They won't survive +/- 12V.

For bidirectional lines, depletion mode MOSFETs (two of them in "anti-series" connection) in conjunction with clamping diodes might work (the clamping diodes to protect the IC, the FETs to limit the fault current).

Small signal MOSFETs in cascode connection should also do a limited job of overvoltage protection.

Always remember: If you create something foolproof, a new fool is born who destroys your foolproof device.
NivagSwerdna:
How would I select the clamping diodes?

With a forward voltage at my limits?
capt bullshot:
The usual way is to use diodes clamping to GND / VCC. 1N4148 or similar will be fine, for some more protection one may add one (common to all inputs) zener or TVS across the supply.
NivagSwerdna:
BAT54S x 32?
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