Confused with interpreting a relay current ratings

[floppy32]

Hello,

My question is about using a power relay model with link here: HF140FF 012 2ZT.

I use it with 12V DC load with "continuous" 4A DC current. But it is not clear form the datasheet what is the maximum DC current this relay can handle at 12V DC "in continuous operation". There is some data about 1 sec on 9 sec off. And on page 225 says max DC current is 8 amps, but I don't know whether this is for a very much shorter duration than "continuous". The maximum power switch plot shows 5A only for DC load. It is very confusing.

In my case I switch the relay ON or OFF when the load current is 0.5A but gradually increase the current to 4.1A and it remains constant for long hours. In the datasheet of this relay there is no carry current or continuous current mentioned but only switching current.

The datasheet has info about max. switching current, max. switching voltage, max. switching power ect. I guess "switching" means during the contacts are closed. But what I am looking for is what is the maximum current would the contacts handle during steady state.

How to estimate the maximum continuous DC current for many hours of operation(for 12VDC load) for this relay?

 

[KlausST]

Hi,

a relay consists of several parts. Each part is stressed differently and thus is specified differently.
There is:
* the coil
* the mechanics
* the contacts

I guess you are referring to the contacts.
* They get stressed at the event when the contact closes. Here especially the inrush current stressed the contact by burningthe contact when not yet the contact forci is fully applied. --> here the worst case is: switching capacitive loads ... multiplied by the switching events count
* they get stressed by the continous current. RMS current causes heat at the contact area due to contact resistance .. as long as the contact is closed.
Only the RMS current counts. Capacitove or indductive current does not matter, also the load voltage does not matter.
* they get stressed at the event when the contact just opens. Especially inductive load causes the contact voltage to rise and thus causing a spark. This spark destroys the contact surface. DC voltage is worse than AC voltage, because at AC the spark automatically gets extinguished because it crosses zero line.

Also - depending on contact materials - the contact surface may oxidize with time. This causes the contact resistance to rise. To "remove" this oxide a minimum voltage level and current level is of benefit.

Because all of this ... the answer is "it depends".

So if the contact is specified with 10A RMS .. it continously can withstand this current without getting too hot. True for AC and DC. You may keep the contact keep closed infinitely.
BUT .. the problem OPENING and CLOSING. Resistive load is the less stressing. Often there is - unwanted - inductance and/or capacitance. ... that may stress the contact.

There are solutions to some known problmes. Like inrush current limiter and snubber circuits.

Usually every relay manufacturer provides documents to explain these effect, problems, solutions in more detail.

Klaus