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Transient suppression

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Mosaic

Well-Known Member
Hi folks:

I have 4 Nfets switching to ground driving 4 relays.
I am using 4, 36V Transorbs to clamp transients, the NFET Vds is 60V.

Can i place a 1n4001 diode (reversed biased) between each NFET drain and the cathode of a single TVS to do the same job (+.6V on the transient clamp)?The diodes are facing cathode to cathode with the TVS.
 
Transorb reduces relay arcing and contact wear.

No, Transorbs are much harder on the contacts than just catch diodes would be (without Tranzorbs).

The advantage of Transorbs over catch diodes are two: a Transorb allows a little bit of arcing, which burns off some oxide, and the relay coil releases faster with a Transorb compared to a diode, possibly minimizing the tendency for the contact to weld itself. However, the contact life will be shorter with a Transzorb.
 
A 36V transorb is better than a catch diode...since any current below the 36V threshold is not recycled thru the relay coil. Thus the relay opens faster with less arcing/welding and contacts last longer. Classic snubbing doctrine.

Further, unidirectional transorbs are much faster than bidirectional units in surge suppression.

https://www.electro-tech-online.com/custompdfs/2011/04/13c3311.pdf
 
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... Thus the relay opens faster with less arcing/welding and contacts last longer. ...

faster:
Yes

less arching:
Less than using a diode? No.
Less than using nothing, yes.

Contacts last longer:
Longer than using a diode? No.
Longer than using nothing, yes.
 
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It seems like you should be able to do what you want to do. I think the other zener just acts like a diode in the oposite direction.
 
faster:
Yes

less arching:
Less than using a diode? No.
Less than using nothing, yes.

Contacts last longer:
Longer than using a diode? No.
Longer than using nothing, yes.

Please read the link I posted earlier. You are mistaken.
 
Please read the link I posted earlier. You are mistaken.

I read it a long time before you even knew it existed. I reread it just now. Nothing there contradicts what I said.
 
Can you explain why please Mike? I would have thought a coil diode gives the slowest field decay so the contacts get less and less pressure until they finally open (slowly). And a zener/transorb/snubber allows the field to decay very fast, causing a fast mechanical transition from contacts closed to contacts open.
 
You say that a transorb results in faster relay action, then u say that diodes cause less arcing. That's an oxymoron. The slower the relay contact's separate the more arcing and more erosion of the contacts.

The linked reference document uses the words "Devastating, tack welding" of relay contacts when using a plain diode.

If this is hard for you to understand then you need to think on it some more....
 
The contact erosion related to the speed of the contacts opening depends upon whether you are switching an AC or DC load. An AC load generally erodes the contacts less with slow opening, since it allows time for the voltage to go to zero and extinguish the arc across the contacts. You want fast opening with a DC load to extinguish the arc as soon as possible.

So my opinion is that for AC contact loads, diode suppression for the coil is better, and for DC loads, a Zener or transorb coil suppression is better.

Edit: Now if you are concerned about the erosion on the set of contacts that may drive another relay coil, then for that I would think diode suppression of the driven coil would be best.

Interestingly, old AC wall switches used to use snap-action contacts. Now they use non-snap-action type contacts.
 
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Well, I am not so sure about the ac component, but I am switching DC.
Anyway here is another reference to clarify things.

Snubber - Wikipedia, the free encyclopedia

Diode snubbers
Main article: Flyback diode

When the current flowing is DC, a simple rectifier diode is often employed as another form of snubber. The snubber diode is wired in parallel with an inductive load (such as a relay coil or electric motor). The diode is installed so that it does not conduct under normal conditions. When current to the inductive load is rapidly interrupted, a large voltage spike would be produced in the reverse direction (as the inductor attempts to keep current flowing in the circuit). This spike is known as an "inductive kick". Placing the snubber diode in inverse parallel with the inductive load allows the current from the inductor to flow through the diode rather than through the switching element, dissipating the energy stored in the inductive load over the series resistance of the inductor and the (usually much smaller) resistance of the diode (over-voltage protection). One disadvantage of simple rectifier diode used as a snubber is that the diode allows current to continue flowing, which may cause the relay to remain actuated for slightly longer; some circuit designs must account for this delay in the dropping-out of the relay. This delay often leads to greatly decreased life of the relay contacts due to arcing.
 
...
An AC load generally erodes the contacts less with slow opening, since it allows time for the voltage to go to zero and extinguish the arc across the contacts.
...

Thanks for that explanation Crutschow, it makes sense. :)
 
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