Sanity check on relay ratings

Hi all,

I have here a personal project involving switching Canadian mains power (nominally 120V @ 60hZ, probably operating on a 15A circuit).

I have some relays which are all marked with both "5A/250VAC" and "5A/30VDC". I want to control the power supply to a laser printer with a relay (solid state or not, I don't care; I just thought I'd see if the ones I have here would work).

I am thinking that the 5A current rating on the contacts holds regardless of the voltage. So while the thing could supply a maximum 1250W load at 250VAC, it could manage at best a 600W load at 120VAC. Is this correct?

Also, with electromechanical relays, should I derate the figures a bit for safety? Say, down to 300 or 400W max. for a laser printer, to account for startup current in the motors and fuser etc?

The particular printer in question is in my sleeping sister-in-law's room so I cannot check its wattage rating until tomorrow.


Torben

am thinking that the 5A current rating on the contacts holds regardless of the voltage. So while the thing could supply a maximum 1250W load at 250VAC, it could manage at best a 600W load at 120VAC. Is this correct?

Click to expand...

I don't think I am qualified to answer (Considering the importance) But that is the way I was taught. Power is the issue...

Also, with electromechanical relays, should I derate the figures a bit for safety? Say, down to 300 or 400W max. for a laser printer, to account for startup current in the motors and fuser etc?

Click to expand...

Always derate. Margin, Margin, Margin...

Mikebits said:

I don't think I am qualified to answer (Considering the importance) But that is the way I was taught. Power is the issue...

Click to expand...

That was the first thing I thought but then I figured that I want confirmation. Taking the power as the max rating would seem to indicate that the relay is rated to conduct 10.4A at 120V. If I assume that the contacts are limited to 5A regardless of voltage then at least I err on the side of not starting a fire. That's why I want to get a solid answer on this.

Always derate. Margin, Margin, Margin...

Click to expand...

I'm a big fan of overbuilding. I don't like rebuilding.


Torben

Torben said:

I am thinking that the 5A current rating on the contacts holds regardless of the voltage.

Click to expand...

Yes and No.

The 5A rating will hold for AC voltage up to 250V.

However, 5A rating holds for DC voltage up to 30V DC only. i.e. one should not use it to switch a DC load of 5A at 35V DC.

Hope it is clear.

eblc1388 said:

Yes and No.

The 5A rating will hold for AC voltage up to 250V.

However, 5A rating holds for DC voltage up to 30V DC only. i.e. one should not use it to switch a DC load of 5A at 35V DC.

Hope it is clear.

Click to expand...

Thanks for the reply! Yes, I understand that part. I'm just wondering about how to safely convert the amperage ratings for different AC voltages.

I mean, you say the 5A rating will hold up to 250VAC--but does that mean that I should still not exceed 5A at 120VAC, or am I OK up to 10.4A at 120VAC? I believe it's the former but I would like confirmation.


Torben

Your caution is advisable. I would wait for several responses to get a good sample then take the average mean of all input...

Mikebits said:

Your caution is advisable. I would wait for several responses to get a good sample then take the average mean of all input...

Click to expand...

I think I'd take a heavily weighted average.


Torben

The maximum AC rating is probably determined by the physical size of the contact area and connection wire.

The maximum DC rating is probably determined by the contact separation distance, speed of separation and whether there is any help to extinguish the arc from external means like using permanent magnet. In most cases, this will be less than the AC current rating.

So, one cannot switch 50A at 10V AC using a 5A 110V rated relay. The contact will simply melt.

A 5A rated relay can only used for 5A maximum from 0V to rated AC voltage.

I think if we stick to AC voltage, then Torben's thinking is on the right track. I also think for this case, we should discount DC as it does not seem apply to Torbens application.
That is not to say DC specs are not important, but if I understand correctly, Torben is concerned with AC.

hi Torben

All you'll ever need to know about relays is here:

http://pewa.panasonic.com/pcsd/tech_info/pdf/rti.pdf

Torben said:

I'm just wondering about how to safely convert the amperage ratings for different AC voltages.

I mean, you say the 5A rating will hold up to 250VAC--but does that mean that I should still not exceed 5A at 120VAC, or am I OK up to 10.4A at 120VAC? I believe it's the former but I would like confirmation.

Torben

Click to expand...

No, don't convert! As L.Chung states:

eblc1388 said:

So, one cannot switch 50A at 10V AC using a 5A 110V rated relay. The contact will simply melt.

A 5A rated relay can only used for 5A maximum from 0V to rated AC voltage.

Click to expand...

I think this is correct, and seems to be in accordance with the excellent pdf Eric posted. (Thanx! BTW)

It seems that higher voltages need smaller conductors to carry the same current, so it is logical that the contacts can still switch at 250V, the same current they switch at 120V.

If you need to switch more current, then change relays. The pdf points out that the power factor must be taken into consideration, so what kind of load is a switching supply? I think all of the motors, solenoids, etc. in a printer are run from the printer's psu, which will take care of their particular start up loads...

I like brick sh*t houses; build once, run forever.

OK, great! I mean, it would be even greater if my relays here could handle this printer, but no go. The ratings labels on the back are in Chinese (I think) but the ratings themselves state 110-127V, 6A, 600W print. I think I'll order some 8 or 10 A relays instead.

Thanks for all the replies (and for the awesome PDF, Eric!)


Torben

L.Chung posted the simplest, most accurate response possible. It's a 5A relay.

mneary said:

L.Chung posted the simplest, most accurate response possible. It's a 5A relay.

Click to expand...

Cool--that's what I got from what he said as well. My relays are not up to the task.

Luckily, I remembered this afternoon that I have the igniter circuit board from an RV furnace which includes an **broken link removed** relay, which at 15A/125VAC believe should be more than adequate for switching power to a 600W 6A laser printer, despite the fact that from what I gather it's nowhere near being a nice easy resistive load.

I intend to fit an RC network across the output lines to the load and a diode across the coil. The PDF that Eric posted gives the following hint for sizing the RC network components:

As a guide in selecting r and c,
r: 0.5 to 1Ω per 1V contact voltage
c: 0.5 to 1μF per 1A contact current
Values vary depending on the properties of the load and variations in relay characteristics.

Click to expand...

. . .so it seems like 100Ω and 4.7µF would be good starting points, but I think I may need to change them since the load isn't purely resistive.


Torben