Overvoltage Protection

[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?

[capt bullshot]

BAT54 is a schottky. Will do the job in most cases, except when you use high R values and have high ambient temperatures due to the higher leakage. This issue is more important when you use analog signals.
Most normal silicon diodes will be fine too, e.g. the BAV99 (two diodes in series, same configuration as the BAT54S)

[TheDane]

Single diodes can take up a lot of space, so if your layout space is limited I suggest taking a look at ESD Protection diodes - they come in a variety of packages, ranging from 1 pin protection and up.
Don't forget a high impedance input current limiting resistor, so your ESD protection doesn't blow up when connected to 12/24V.
If truly high power, use a dedicated tranzorber diode - they handle power well, and also protect against lightning surges.

http://www.onsemi.com/PowerSolutions/parametrics.do?id=248
The HDMI versions has up to 19 pins in a single package and are unidirectional and handle high-speed easily.
The USB versions has 4 pins, also unidirectional and low channel input capacitance, and may be easier to place close to the inputs.

The added bonus of using an external ESD protection diode solution, is extra ESD protection 

[Eka]

http://www.ti.com/lit/ds/symlink/txb0104.pdf[/url]). They won't survive +/- 12V.

There are level translator chips that will work with +12V signals.

This is one of many. It can handle up to +20V on all it's inputs. It's only unidirectional, so that may be limiting.
http://www.ti.com/lit/ds/symlink/cd4504b.pdf
A diode clamp to ground for under -0.5V voltages would be needed.

A lot depends on what signal type and voltage you are using. Some of the wide input range level translators may work if the logic signaling is compatible.  I personally would use standard ESD line protection chips as mentioned by others, and inline resistor if it will work, and if excess current is a possible issue maybe a MOV or similar.