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Protect the contact of one relay

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kkdelabaca

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Hello,

Time ago I designed one circuit that can close one contact Normal Open using the protocol RS485. The board has worked very well but now I have problems with an inductive load (coil of other relay) of 110Vdc from one battery. The problem is that in some operations the contact Normal Open is short (the relay is broken).

In the photo you can see the image of the simplifier schematic:


The caracteristics of the contact of the reed relay:


Now I am thinking about using a transil to put it in parallel with the contact NormalOpen, but I have never used a transil. What type of transil you recommend? The Bidirectional Transil solve my problem? The reference BZW04-128B (bidirectional transil) with one Vbr = 143V is Ok for my design? http://www.st.com/stonline/products/literature/ds/2971/bzw04.pdf

The datasheet of the reed relay: http://www.meder.com/fileadmin/meder/pdf/en/Products/Reed_Relays/Relay_DIP_Series_E.pdf

Thanks!!
 

Kral

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kkdelabaca,
The BZW04-128B should work as long as the 110V supply never reaches the BZW04-128B breakdown voltage when the inductive load is energized. If you can tolerate a slow current turn-off, you can simply use a forward diode across the load (anode connected to ground). I'm assuming that the 110V supply is positive. Otherwise, reverse the diode connection.
Regards,
Kral
 

dsPIC24F

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Have you put a diode on the control side?
 

kkdelabaca

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I can not put any element of protection outside of the PCB.

If I only put a diode in parallel with the contact, then the contact must be connected with polarity (the contact in parallel with the katode must be connected to the +). However, if you put a transil, the contact don't have polarity. Is right?

Thanks!
 

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