I understand that bidirectional TVS diodes have no polarity and can be used to limit positive and negative swings in an AC application.
Is there a down side to using bidirectional TVS diodes in 12 V DC applications, such as across the coil of the relay or solenoid, or to protect other sensitive equipment?
"Why?" you may well ask. Because I've got a bunch of them to the spec I need.
Usually the reason why not is reliability.
As its name suggests, a Transient Voltage Suppressor (TVS) is designed for suppressing the effects of infrequent and unusual transients into a board connector or cable. It's connected directly across that input, to dissipate ESD, lightning strikes etc.
It's not designed for dissipation of normal operating power in a circuit. Its body package typically cannot conduct power well compared to other components because it doesn't need to. It's datasheets reflect this, with discharges through it rated at a very low pulse width and repetition rate indeed to keep the peak power and average power dissipation right down.
So using a TVS outside of its rated behaviour will decrease its reliability and it will fail more quickly and unpredictably than its specified life.
A TVS is just an electronic component, like any other. And like any component, it has:
The manufacturer will list only some of its characteristics as specifications, in datasheets and reliability data etc. The manufacturer will qualify the part's behaviour and reliability against the specifications and guarantee against them, but not against all of its characteristics.
Other characteristics aren't specified. And if you go outside the specs, and rely on unspecified component characteristics, you're on your own in uncharted territory with a good chance of sinking. For one or a few circuits, you may choose to take your chances and it may work out OK. But it may not and for many/most mass production situations, that's unacceptable because the cost of failures in the field swamps the cost of properly spec'd parts.
So, with a TVS as an operating current clamp, there simply are better alternative components and circuits to use that do give specified reliability.
And, as said above, usually lower business costs in the long and short term.
Bidirectional 12 V TVS limit swings to +12 V in one direction and -12 V in the other direction.
Unidirectional 12 V TVS limit swings to +12 V in one direction and about 1 forward diode drop in the other direction.
If your application will not encounter or will not be damaged by negative transient voltages, then I guess bidirectional will be OK.
I am not sure I would put a TVS across a relay coil. I mean, it could be OK but you need to make sure the repetitive cycling is acceptable. I think that usually people let the coil energy freewheel through an ordinary diode.
But if you need to turn off the coil fast, then I guess you could use a TVS instead of a freewheel diode.
I am reading your questions somewhat like "Can I use a bidirectional 12V TVS across a solenoid coil as free-running diode?".
In this case: The point of a free running diode is to clamp the negative swing on the voltage across a coil to as close as possible too -0V. So by introducing the -12V bidirectional behavior you can harm your circuit.
In other cases like limiting the amplitude of a +/- analog signal they are just the easiest way to go. Feed the signal through a current limiting resistor and "clamp" it with the diode relative to GND on the full +/- amplitude band. If your resistance is big enough there will be no issues with power dissipation.
CAUTION: Using this circuit will introduce a RC-lowpass as TVS diodes (besides special types) can have significant capacitance!
This is the circuit I commonly use to limit external +/-5V analog signals and also provide ESD-protection. Even when used with two unipolar diodes and an extra cap I get f-3dBs at around ~130kHz which is good enough for my usual 100kHz uppper band-limit requirements.
