Relay Minimum Contact Load?

[veryevil]

Hey, I'm looking at a Relay to use in a 24 VDC loop powered from mains (230VAC) coil

I've found the following relay http://docs-europe.electrocomponents.com/webdocs/05c8/0900766b805c8c02.pdf (PDF link) which specifies the Minimum Contact Load as 5V / 100ma which is 0.5W.

Since I want to use this relay at 24VDC does that mean as long as I put half a watt through it it will be reliable and work? e.g. 0.5/24 = 0.0208A = 21mA or do I need the 100mA current?

Thanks Steven

[Simon]

you need 100 mA current and will only achieve that at the correct voltage. The relay follows ohms law, like a resistor. It is more about the current thsan the voltage but you need enough voltage to make the required current flow

[jucole]

I'm not very good with analogue stuff but surely if you stick a 190 Ohm resistor in you should be good as that will drop 19V leaving 5v @ 100mA  across the relay coil ?

[David_AVD]

I've used relays with almost no current flowing through the contacts before and never had an issue.  It's quite a common occurrence in audio gear.

Maybe a relay expert will chime in with the science behind these minimum ratings.

[Simon]

ARE WE TALKING ABOUT THE POWER TO ACTIVATE IT OR THE CURRENT FLOWING THROUGH IT ?

[David_AVD]

Well, the "Minimum Contact Load" part of his question pretty much says it all doesn't it?      

[Rerouter]

generally the contacts can have no minimum load, what that would appear to be is, it needs either 5V across the coil or 100mA flowing through it for the contracts to make reliable contact,

[David_AVD]

A quick Google found this:

... but some caution needs to be exercised on high current relays. These relays typically have a minimum operating current/voltage that is needed to overcome surface films that can accumulate on the contacts. The contacts have to designed for their robustness when hot switching signals, so this limits the material choices available to the relay manufacturer. If the relay has not been used for a while, or if it has been used for hot switching loads, the contacts at low current/voltage may exhibit variable or even open circuit values.

There's probably more to be found.

[Psi]

I think it's to do with keeping the contact terminals clean, if you want to switch ultra low current the contacts need to be "wetted"

With normal contacts the energy/arc has to be enough to burn away any oxide when the contacts touch.
If the current is too low you don't get that spark and the oxide can build up until it don't work any more.  Same thing applies to regular switches.

In reality it takes a while to become an issue. But if you need the relay to meet its stated lifespan you should follow the min requirements.

[peter.mitchell]

Also the inverse applies, high inrush currents can cause the contacts on a relay to burn/wear out prematurely.

[Simon]

yea i completely missed that, The current capability will stay the same as no matter the voltage the current determines the voltage drop and the heat in the contacts

[robrenz]

Dry circuit resistance modes on a micro Ohmeter are related to this. It limits the drive voltage to 50mV or 20mV depending on the spec. so that the oxide layer is not punctured when measuring the contact resistance. This is a concern on switching of low voltages where the intended milivolt level signals would never have the energy to rupture the oxide film but the micro Ohm meter does if not in dry circuit mode.

[GK]

Sealed relays with gold plated contacts (to avoid oxidation) are mandatory for small signal stuff and audio (where there is no appreciable contact current).