I had the 75 amp bosch relay in mine and it only lasted a few months.

 

You'll get twice the amperage with two relays in parallel

 

But wouldn't that mean half the voltage?...Mark Just thought about it some more and parallel means the same voltage ...I'm so confused. This is why I became a firefighter instead of an engineer. Just put the wet stuff on the red stuff and you have success ...Mark

 

No, it looks to me like he's just using two relays in parallel to split up the load- just giving the current two identical paths to follow. It's just the same idea as using one relay that's twice as large.

 

Thanks for the explanation, a little high school electronics doesn't go very far, I remember just enough to be dangerous...Mark

 

You're welcome! Most of my high school lessons are pretty hazy these days too. Getting old sucks.

 

The problem with 2 relays in parallel is that one of them will open later than the other, (maybe only microseconds) and take most of the inductive arc from the motors.

The reason the Bosch 75 amp toasted early was probably because it had an arc suppression diode across the coil. By suppressing the back voltage on the coil, the current is held up, and the relay releases slower, prolonging the arc on the contacts. With high currents like this, it's better to not use a diode on the coil. A resistor is better, and if the driving circuit (temp switch) can take it, nothing is best.

To mathematically describe what's happening here, I'd have to use imaginary numbers.

 

I finally understand something, this is how many more times that I am going to get married......Mark

 

Others have answered this (load handling) but another purpose is because they are cheap and available anywhere in case one goes bad.



On that note, I've been using the same relays for over 2 years now without any failures.