We had one fail in our group 2 weeks ago so I took it home and did some surgery on it to see why it was failing. The first test I performed was to measure the resistance between the common and the Normally-Closed contact (the one that is not used) - result was .2 ohms (as expected). Next step was to actuate the relay and measure the resistance between the Common and the Normally-Open contact (the one we care about) and the resistance was 230 ohms. This is the obvious issue so I opened it up to see what was going on.
In first picture, you can see that the Normally-Closed contact is clean and shows no sign of oxidation or carbon build-up. There is no current drawn through this contact so we should not expect to see anything unless there is moisture getting into the contact. Not definitive, but it gives some indication that it is not corrosion. Looking at other internal components in the relay, I also do not see any signs of moisture causing corrosion either.
In the next picture, you can see the Common (center pole of the switch) and there I see some signs of heating at this contact point. Which may indicate that the relay design is not compatible with the current load that is being drawn through the relay when it is actuated. So this could be a case of the wrong relay being used for this application.
In the last 2 pictures, I show the Normally-Open contact (the one we care about) before and after I scraped it. Just by scraping a small amount of the buildup off from the contact (see picture) the resistance dropped back to .2 ohms. By the way, this relay was heavily taped up to prevent moisture intrusion from the first day he purchased the sled. I think he pulled 6 feet of electrical tape off this relay before we could replace it.
So the failure mechanism is that there is a significant buildup of oxidation or carbon on the Normally-Open contact. As the contact oxidizes (or carbon builds up) the resistance increases and ultimately becomes significant with respect to the load and to much voltage will be dropped across this point.
There are several reasons that this can happen. There could be moisture getting into the relays, though I think I have ruled this out given that the other contacts do not show signs of corrosion. It can be that the load is too high for the relay, causing a significant arc when the relay opens which results in carbon buildup. Or, it could be that the materials used on the contacts are not suitable for the application. My first thoughts on solving this problem are to find a relay that has silver plated contacts - these appear to be copper, but I can't be sure.
The main reason for this post is to inform everyone that these relays are prone to failure and likely sealing them up will not help. So I want to encourage you to carry a few spares. I am considering setting up a test bench to simulate the current/voltage load on this relay and test some different relays to see if I can find one that will last. I would just test them in the sled but mother nature is not cooperating this year (at least out west). Also, with a microcontroller, I can switch the relay thousands of times/day for endurance testing.
Cheers,
Chris
In first picture, you can see that the Normally-Closed contact is clean and shows no sign of oxidation or carbon build-up. There is no current drawn through this contact so we should not expect to see anything unless there is moisture getting into the contact. Not definitive, but it gives some indication that it is not corrosion. Looking at other internal components in the relay, I also do not see any signs of moisture causing corrosion either.
In the next picture, you can see the Common (center pole of the switch) and there I see some signs of heating at this contact point. Which may indicate that the relay design is not compatible with the current load that is being drawn through the relay when it is actuated. So this could be a case of the wrong relay being used for this application.
In the last 2 pictures, I show the Normally-Open contact (the one we care about) before and after I scraped it. Just by scraping a small amount of the buildup off from the contact (see picture) the resistance dropped back to .2 ohms. By the way, this relay was heavily taped up to prevent moisture intrusion from the first day he purchased the sled. I think he pulled 6 feet of electrical tape off this relay before we could replace it.
So the failure mechanism is that there is a significant buildup of oxidation or carbon on the Normally-Open contact. As the contact oxidizes (or carbon builds up) the resistance increases and ultimately becomes significant with respect to the load and to much voltage will be dropped across this point.
There are several reasons that this can happen. There could be moisture getting into the relays, though I think I have ruled this out given that the other contacts do not show signs of corrosion. It can be that the load is too high for the relay, causing a significant arc when the relay opens which results in carbon buildup. Or, it could be that the materials used on the contacts are not suitable for the application. My first thoughts on solving this problem are to find a relay that has silver plated contacts - these appear to be copper, but I can't be sure.
The main reason for this post is to inform everyone that these relays are prone to failure and likely sealing them up will not help. So I want to encourage you to carry a few spares. I am considering setting up a test bench to simulate the current/voltage load on this relay and test some different relays to see if I can find one that will last. I would just test them in the sled but mother nature is not cooperating this year (at least out west). Also, with a microcontroller, I can switch the relay thousands of times/day for endurance testing.
Cheers,
Chris
