hi guys, what is the purpose of the diode inside the relays?thanx
It's only purpose is to absorb voltage spike when current is removed from relay coil. Thats all I know about them.
I'm not certain, but I think you may be referring to a resistor inside the relay. It's common to connect a resistor between the work terminal (terminal that relay sends voltage to, like to a coil or something), and the ground terminal of relay circuit. That way, when you switch relay from on to off, the resistor removes residual voltage on the component that relay was sending voltage to.
The coil windings creates a magnetic field that when turned off, it instantly collapses and causes a brief high voltage spike. The use of a Diode is to slow down the collapse of the field and so the voltage spike will not be as high. Cat use them on almost all relays due to the amount of electronics. Electronics are susceptible to damage from the spikes.
The ones that I have seen used on equipment are called Zener diodes. They do not slow down the collapse of the field, rather they allow part of voltage spike, above system voltage, to return to ground side and not allow spike to go to computer or other voltage sensitive equipment.
Komatsu is famous for them as they usually install about twenty five of them on all their excavators. The issue has to do with the spike as everyone is saying but it is what that spike causes that is the reason they are in many machines. I saw the issue big time on Sumitomo excavators in the early to mid nineties. We would install an open / close valve for a thumb function. Hitting the function quickly five or six times in a row and the computer would go to sleep. Digital circuits send information in packets of ones and zeros. A zero is no voltage and a one is system voltage. Equipment computers work on the ground side of all the circuits and sudden spikes in voltage can be felt by the computer in all kinds of different places. If that spike hits just right it will turn one of those zeros in a packet into a one and the computer doesn't recognize what that packet is, so it just says I quit and your machine either doesn't work at all or some particular function starts working real odd. You generally have to shut off the machine until the master disconnect drops out and then restart. The computer starts again on fresh code and you go back to work. At worst the voltage spike can blow out one of the components in the circuit board and you are done. I've seen this by taking the boards out and looking for the black spots in the lacquer or a little cylinder looking thing on the board turned into a miniature version of the back yard burn barrel. The diodes being talked about here are regular ones with two legs that will let current pass through going one way but not the other. A Zener diode is a three legged diode that has a logic circuit function. The third leg controls when current can pass through the other two legs or it might control which way current can pass. They are used a lot in alternators. I haven't seen a lot of them in Cats, maybe two or three. Hitachi machines used to have five or six, Sumitomo machine didn't have any until we started putting them in. If you have a Komatsu find out the part number of one from Cat. The Cat ones were about three dollars each while the Komatsu ones ran over fifteen bucks for the same thing.
Nice explanation John C.
Jhon c, thanks for the explaination