Relay question

[mstacho]

We bought this relay: http://garden.seeedstudio.com/index.php?title=Electronic_brick_-_5V_Relay_module_%28digital%29  It'll be used for a safety system eventually, but for the time being we're looking into using it as a motor controller relay.

For some reason I don't understand, the voltage for the direction signal required by the motor controller is 48V instead of the standard 5V (even though the actual motor control PWM signal is 5V...).  This relay appears to be rated for 24V DC, 10A, but we'll be using it at 48V DC with only about 50mA.  Further, it appears to be rated for 240V AC.

So my question is: Can I run it at 48V DC?  Is there anything different enough between AC and DC (besides the whole impedence thing...) that will make it only perform properly at a MUCH lower DC voltage than AC?

MIKE

[vinniewryan]

See the 'Instructables Tutorial' link on that page. It states:

The RELAY can handle up to a 1,000W device.

The coil requires 5V. The switched circuit can handle 1000W, no matter how you chop it up.

[mstacho]

Awesome, that's what I thought.  It didn't make sense to me to have such high wattage in AC but such low wattage in DC.  I know that AC and DC affect circuits differently, of course, but the ratings made no sense :-P

MIKE

[Billy]

Is there anything different enough between AC and DC (besides the whole impedence thing...) that will make it only perform properly at a MUCH lower DC voltage than AC?

Yes there is something different. AC current has a zero crossing 100-120 times a second. As the contacts open, an arc will start. As the contacts continue to open the arc allows current to continue flowing, which propagates the arc.

In an AC system, the arc will extinguish within 8 to 10 milliseconds as the current cycles through zero at double the frequency of the power system.

In a DC system, the current will continue to flow until the arc dies out all by itself. Depending on the contact gap and the load type, the arc will destroy the contacts much, much sooner than an equivalently rated AC circuit.

As for your specific relay, I didn't look at a spec sheet but can almost guarantee that the rating listed are for resistive loads only. Is see no way a relay that small can be rated for that power for an inductive load. If you use for inductive load like a motor, at double the voltage, you should expect reliability issues. It's also doesn't look suitable for use as a safety relay, but that is another discussion.

[Soeren]

Hi,

See the 'Instructables Tutorial' link on that page. It states:

The RELAY can handle up to a 1,000W device.

The coil requires 5V. The switched circuit can handle 1000W, no matter how you chop it up.

The statements continues with
  "It is not a good idea to run this RELAY at a full 1,000W demand.  It is a better idea to stop at about 800W to be on the safe side. "

Besides, a relay doesn't have a Watt rating, as the power depends on the voltage - you wouldn't try to run 1kA through it even if it was at 1V (I hope).

[Billy]

The switched circuit can handle 1000W, no matter how you chop it up.

That is not even a little bit true. Real life is much more complicated than that.

As Soeren already pointed out, 1000 amps at 1 volt wouldn't work, neither would 1000v volts at 1 amp.
As I already pointed out, there is a huge difference between AC and DC, and the type of load is also very important as inductive and capacitive loads are harder on contacts than resistive loads. Only a detailed reading of the application notes and spec sheets will tell you if a relay is appropriate for a given task.

Business is competitive. These guys know what they're doing and rate the devices in light of that competitive market. If the ratings don't make sense, it's probably because there are effects that aren't known to you.

[vinniewryan]

My statement was based on the known factors "we'll be using it at 48V DC with only about 50mA", not meant to be taken literally but more as a general way of saying yes, this relay will support your 2.4W DC application.

And by 'no matter how you chop it up', I meant no matter how they, the manufacturer of the relay, chop it up in the maximum ratings listed.