I want to know why sometimes 2 diodes are used with relay, despite 1 is enough for safety purpose.... Also tell me the applications where 2 diodes are used ??

 

I only know that there is only one diode connected across the relay coil to prevent back emf that would destroy the driving transistor. Where is the second diode connected?

Allen

 

Using two would increase the amperage than can be handled without finding a new part with a higher rating. Just speculating

 

Two diodes in series can be placed across the coil for increased reliability. If one diode shorts, the other still works, allowing the relay to continue to function.

 

If so, it may be to speed up the release of the relay as described here: http://relays.te.com/appnotes/app_pdfs/13c3264.pdf

 

If so, it may be to speed up the release of the relay as described here: http://relays.te.com/appnotes/app_pdfs/13c3264.pdf

Great reference! That should go into AllAboutCircuits tutorials somewhere.

Ken

 

It's one of those things that seem so obvious once you see it. Cool.

 

For those 12 volt relays I found a diode that works great, it's 2 zeners back to back, it's like it was built to put across the primary of a coil,

1.5 KE24CA TVS Diode


http://www.newark.com/multicomp/1-5ke24ca/tvs-diode-45a-33-2v-bidirectional/dp/91R0634

 

Two diodes in series can be placed across the coil for increased reliability

 

If so, it may be to speed up the release of the relay as described here: http://relays.te.com/appnotes/app_pdfs/13c3264.pdf

John from the Great State of Texas nailed it!

I have the inside track on that: I just designed the coil suppression network for "a major relay manufacturer" (which may or may not be TE) and this describes it's function exactly. Basically it's just a zener and a diode on a .1x.22" board and two fly leads.

They put our part inside their relay, actually wrap it over the coil.

One thing to note is the zener voltage ADDS to the applied voltage during turn off, so don't be stingy with the controlling element's voltage rating.