TVS diodes

[stj]

i have to replace one,

the original is bi-directional.

i have some uni-directional ones,
can i use 2 in series or anti-parallel??

[helius]

Use them in series with their cathodes together.

[wraper]

i have some uni-directional ones,
can i use 2 in series or anti-parallel??

No, it would be just as 2 diodes anti parallel, therefore clamping at below 1V. Do as helius suggested.

[stj]

so -----|>|--|<|--------

hmm - gonna be fun working out the placement - probably have to be a "tent" style - it's all smd 

[wraper]

Depending on circuit you could just use one unidirectional TVS. I cannot think of that many circuits which would actually require bidirectional TVS.

[tszaboo]

Depending on circuit you could just use one unidirectional TVS. I cannot think of that many circuits which would actually require bidirectional TVS.

RS485. The common mode voltage could easily be negative.

[wraper]

Depending on circuit you could just use one unidirectional TVS. I cannot think of that many circuits which would actually require bidirectional TVS.

RS485. The common mode voltage could easily be negative.

Yep, pretty special cases like this. Usually when I see bidirectional TVS in a circuit, there is no good reason for ir being bidirectional.

[floobydust]

Many RS-485 bi-directional TVS are lop-sided.
Example SM712 has +7, -12V clamps. I know it is a tiny flea fart TVS, with fake 400W spec (SOT-23), but designed for max. according to RS-485 standard.

[SeanB]

Bidirectional is good from a buyer's side, you only need a single part for all applications, and putting it it the wrong way round does not cause any issues. If you need unidirectional you just use a regular diode in parallel as well. Yes you will need to build a tower to use the unidirectional ones to replace a bidirectional one though.