Help selecting TVS diode.

Thread Starter

Goofygutt

Joined Sep 15, 2012
3
Hi,

I work as an electrician and lately we've been installing door-magnets (Reed switches) in a nursing home to indicate when a door is open or not.
Some of the reed switches seem to get stuck every now and then so that even when the door is open the switch stays in closed position.
Giving it a couple of taps with your finger opens it.
The switch is powering the coil of a Releco 24VDC relay when door is closed, and goes off when door is open.
I´m guessing that since the coil is an inductive load it may cause high inrush current or arc-current when switching, causing the tiny reed-switch to get stuck.
This is just my theory though, and I guess a bi-directional TVS diode in parallell with the coil might help on the problem.
If I´m gonna try this, I´m wondering how to choose the right one?
We have 24DC thats going to power a Releco 24VDC relay coil through the magnetic contact (reed switch) , probably this one:
https://www.elfadistrelec.no/Web/Downloads/he/et/C7A2x_eng_datasheet.pdf

Looking through rs-online.com, I end up with 444 different types to choose from:
https://no.rs-online.com/web/c/semiconductors/discrete-semiconductors/tvs-diodes/?searchTerm=tvs diode&applied-dimensions=4294876280,4294821500
What should I look for?
 

RPLaJeunesse

Joined Jul 29, 2018
53
To make installation easy, pick a bi-directional TVS. If the TVS voltage is too low, the relay will not drop out quick enough and its contacts may burn. Pick a TVS voltage about twice the system operating voltage. Your relay takes about 1W of coil power, using a 1W rated TVS is probably overkill. I'd use a 48V 500mW bi-directional TVS, and place it across the reed switch. Why there? If a power surge comes along with the switch closed the TVS is obvoulsly protected. With the switch open the relay is in series with the TVS, now the relay protects the TVS. Put the TVS across the relay and a big enough power surge with the switch closed can destroy the TVS.
 

Thread Starter

Goofygutt

Joined Sep 15, 2012
3
Thanks for input, putting it over the reed might seem like a good idea, but was that to protect it from an external surge?
I thought the problem/surge is the kick-back voltage from the coil in the relay, thats why I thought putting it across the coil would prevent the kickback reaching the reed switch.

We also have a couple of reed-switches connected directly to the nurse-call system, these have never had problems with the switch not opening when the door opens.
They're probably just sending out a low monitoring voltage.

I first found this post about the problem and seemed like a thing to try, however, it suggested putting it in parallel with the coil.
https://forum.digikey.com/t/why-is-my-reed-switch-failing/889
 
Last edited:

RPLaJeunesse

Joined Jul 29, 2018
53
Yes, clamping the source of the kick-back works, and is fine if there is no other inductance in the system and the supply is close and limited well. (The DigiKey post presumes such a situation.) Once longish cables come in to play there is added inductance (admittedly, not much) but they do pose a chance for real-world induced noise to add a surge voltage to the system. The relay is inductive, it inherently resists voltage spikes and surges. The reed switch is a small gap, if it arcs over the contacts can get destroyed. Put the TVS across the reed switch and it cannot arc while open AND it clamps the inductive kick from the relay. Most people think "clamp the surge", I'd add "protect the switch" too.
 
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