PeachParts Mercedes-Benz Forum - View Single Post - Is a relay just a device that turns things on and off?

JimF · #4 04-20-2001, 01:38 PM

I'd like to expand on Steve's statement" They are each current carrying switches that cause an action."

Were talking now about the "current carrying portion" of the relay. What goes on here?

Relays, in theory, are the best way to switch two signals into one source. They have better than 10 Meg ohms isolation to the non-switched signal and 0 ohms loss to the needed signal. The best of all worlds!

BTW, practically speaking, "0 ohms contact resistance" is more like 0.01 to 0.1 ohms resistance during the relay's life expectancy.

But the realities of a relay make it not-so-perfect! Among the many things that they are rated for, is 'contact life'. In other words, it's good for a certain number of switches. That means that after those number of switches, its contact resistance now starts to rise from 0 ohms to as much as 10MEG ohms. Of course, at 10 Meg ohms, it's not a relay any more, it's a resistor! Under high contact resistance, the relay is said to be "OHMIC".

The ohmic condition is what affects a car's operation. When the relay contacts become omhic, electrical current is dropped across the relay's contacts 'robbing' the power from the intended device. That is why the car won't start or fuel pump won't run or runs intermittently. Remember, I'm only talking about the contact portion of the relay. In relays like the OVP, there are other functions going on but these can NOT work if the relay becomes ohmic!

Most military grade relays are rated for 100,000 operations. Commercial ones are rated for typically 10,000. But this does not mean that it will last for the rated number of operations. This data is gathered statistically so any given relay could fail immediately whereas the next one could last for 1,000,000 operations. It's kind of a crap shoot!

So not only can the relay fail by the zener shorting, using the OVP as an example, but can also fail due to becoming OHMIC. And this is the most common method of relay failure!

Things that you NEED to know. Hope this helps.

[Edited by JimF on 04-22-2001 at 11:26 AM]