'Bidirectional peak limiting diodes' for contactor coil

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tortuga

tortuga

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Electrical Design
How are 'Bidirectional peak limiting diodes' or Transorbs sized for a given contactor coil?
I get that Transorbs are supposed to help bleed off inductance from contactor coils but that's about it.
I recently tracked down a dead short in a complex 24VDC control circuit to one of these and it made me curious how they are sized and how necessary they are?
 
I know in low voltage (<24 volts) DC, a diode is wired across the coil of a relay with reverse polarity- so that when the coil is energized, the diode does not conduct. When the coil is de-energized, the collapsing magnetic field returns energy to the circuit. The diode is there to conduct energy from Neg to Pos--saving the rest of the circuit from a sudden surge of current.
In the model train world, for a 15v DC circuit a diode with PIV of 150-200 volts was usually used-- mainly because Radio Snack had a 5-pack for $1.98. Theory is that the voltage should be at least 4X the activation voltage. And it was hard to find power diodes for lower voltages.
We didn't use Bi-directional peak limiting diodes.
 
In my early years (no ELECTRONIC duties since about 1985), we used reverse biased diodes as mentioned by @PaulMmn; "blue suppressors", a series RC circuit; and Transzorbs (a Vishay trademark). The simple diode worked well properly applied for DC solenoid coils and was then BY FAR the least expensive. The RC circuit was preferred by some of the guys; almost a religious preference. It is important to install them very close to the coil; sometimes just an inch or 2 was too far.

Transzorb technology was higher priced and used on circuit boards. I don;'t remember the technology, but thought of them as a pair of zener diodes.

Automation Direct has excellent videos on surge protection.
for part 1, https://www.automationdirect.com/videos/video?videoToPlay=Ij0B-2gxH7w for part 2.
 
230426-0612 EDT

I don't like to call a Transorb a diode. I prefer to call devices with very high conduction in one direction at very low forward voltage drop, and a high reverse voltage with low current a diode. All diodes in the reverse direction have a breakdown voltage. In normal application you want this breakdown voltage high enough that it is not reached.

There is a special device where this reverse breakdown is well controlled and is called a Zener diode. It has a normal diode's characteristic in the forward direction, but a well controlled breakdown voltage in the reverse direction.

There are also some other diode devices with special characteristics.

I prefer to call a Transorb or equivalent device a bidirectional nonlinear device.

For low voltages, + or -, a Transorb is a high impedance. When some controlled threshold voltage, + or -, is reached the Transorb becomes a much lower impedance above this level. It does not collapse to a very small voltage, like a volt, as a SCR does.

The selection of a Transorb's threshold voltage should be somewhat higher than the maximum expected norminally applied voltage.

At this point I won't go into further detail.

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Just a comment on terminology and the intended result: You are not bleeding off inductance, you are bleeding off energy stored in the inductance which is proportional to current flow. A diode will prevent contact damaging high voltages when the circuit opens by giving the current somewhere to flow when the driving voltage is removed. But it will not absorb much energy and so will not reduce the current very rapidly. This can lead to, for example, the armature of a relay staying attracted for some time as current continues to flow in the inductor and the magnetic field does not decrease rapidly.
When it is important to reduce the magnetic field in the inductor quickly, the "shun", in whatever form, needs to absorb energy. To do that it must have a non-zero voltage across it in phase with the current. A capacitor alone will not do this, producing ringing. But an RC circuit, a zener diode or a Transorb can controllably dissipate energy.
 
When designing boards with inductive external devices (relays, contactors, solenoids, etc) it is typical to use a SMBJ series transient voltage suppressor (TVS) diode as the standard inexpensive default for surface mount boards.

It has a 600 watt peak rating. It's trivial to do a sanity check of coil current times clamp voltage and skip detailed simulation if it's not close. There is a 5 watt average rating that might apply if the relay will be frequently switched. The peak surge current is 100 amps, but that parameter rarely needs to be considered.

Conventional wisdom is that it's better to put the voltage clamp on the coil, but it's usually (only) on the control board because a clamp needs to be there anyway for reliability.
 
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