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Question on Relays

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I'm going through this Relay weld detection link.

I am unclear about the purpose and the working of the relay weld detection circuit. The link does not indicate or show any relay weld detection circuit. I tried to search for some other articles to understand the working of the circuit, but unable to find them.

Can someone help me to understand the working of the relay weld detection circuit and why it is required?
 
Somebody wants to know when the contacts have
"welded" so that the coil no longer controls the conduction.

You'd determine a clean signature of this fault, and make
a detection circuit.

One patent surely fails to show all of the ways this might
be done, as detection is intimately connected with the
source and load behaviors in normal and this particular
abnormal condition. And whatever the "backstop" is, for
such a loss of control.

Basically any solution would be a comparison between
"what it is" and "what it should be", and an action to
interrupt the conduction by other means for safety. The
details wil vary.
 
Hi,

here you are usually interested in, if the relay is really open, especially for high voltage applications. A basic approach is to perform a voltage measurement on both sides of the relay. If it is welded, and you have signaled an opening of your relay, the voltage readings (almost) identical.

An additional safety measure is, to monitor the coil current to exclude a stucked GPIO pin or an other faulty component involved in the closing/opening of the relay e.g. low-side MOSFET. So here, if you are getting a zero coil current in combination with two identical voltage readings (on both sides of the relay), your relay is welded.

BR
 

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