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This is my first post. I have been coming here for years though.

I am designing a relay board for my motorcycle to control power to various accessories - lights, heated gear, horn, GPS, etc. It's a pretty straightforward design... some terminals, fuses, and relays. Nothing really new except the goal is to make it small because my bike doesn't have much space and it's a good excuse to practice PCB layout.

Right now I have a few channels and relays that can be individually controlled by various switches. I figured the maximum current any channel will realistically see is 10-12A so I went on digikey and searched for 12VDC automotive relays rated for 20A and higher.

I ordered this little guy which is rated at 30A: https://www.digikey.com/en/products/detail/omron-electronics-inc-emc-div/G8N-1H-AS-DC12-BY-OMR1/6817243

When I got it in the mail I honestly don't believe this is rated for 30A. It's so tiny. Every other automotive relay I've dealt with is HUGE. I've also used 10A relays that are 3X this size.

Everything I'm reading says this is fine, and that if the switching current rating is 30A then the continuous current rating is probably even higher. I still don't believe it though, because it's so small. Even the power pins are small, they look equivalent to 20 or 22awg wire.

I kept looking around and found some other relays (similar form factor) that specifically say in the datasheet they're made for lights and heaters. The one linked above says it's used for window motors and such (things that get used for maybe 5-10 seconds at a time). But just looking at the numbers, there's nothing that says it can't be used continuously.

Am I missing something here? Would this thing be fine with a 30A resistive load running through it for 6 hours or would it just melt? Please enlighten me. There doesn't seem to be any link between size and current carrying capacity. And there also doesn't seem to be any reasoning for the datasheet numbers and the "typical applications" either.

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  • \$\begingroup\$ I would trust Omron, they're a very reputable brand and you got it from a reputable seller. In the datasheet they do advertise these as the "smallest class in the industry", which makes me think they were deliberately designed to be small. It does specify that its rated load is only 25 amps, but that's a motor load; loads with better power factor may be less stress on the part. \$\endgroup\$
    – Hearth
    Dec 26, 2020 at 19:21
  • \$\begingroup\$ The datasheet says: "Even at a small size, high-wattage switching is possible due to the contacts and heat-release design. 100,000 times at 14VDC/25A". So I guess they used their magic and stand behind it. Altough it clearly says 25A for "rated load" in the datasheet and 30A for "max. switching current". \$\endgroup\$ Dec 26, 2020 at 19:40
  • \$\begingroup\$ Wow. That's small af \$\endgroup\$
    – DKNguyen
    Dec 26, 2020 at 19:42

1 Answer 1

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Omron is a reliable company. If they say it's rated at 30 switching resistive, it will give you the stated 100,000 operations life switching 30A resistive under the stated test conditions.

Take care that the life ratings of relays (in general, and including this one) are with no diode across the coil so adding a diode (rather than something that allows the coil voltage to go much higher) may reduce the relay life.

As far as continuous current rating, the overcurrent failure curves from this datasheet indicate that it might be a bit close at 85°C Ta and 25A continuous.

So maybe you're onto something there. Looks like if the ambient is hot you might want to keep the current a lot lower than 25A to be conservative.

enter image description here

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  • \$\begingroup\$ I like to use a diode in series with a zener, to prevent huge spikes but still allow for a faster turn-off than you'd get just using a 1N4004. \$\endgroup\$
    – Hearth
    Dec 26, 2020 at 20:51
  • \$\begingroup\$ @Hearth Or a diode plus a resistor. \$\endgroup\$ Dec 26, 2020 at 20:52
  • \$\begingroup\$ Thank you so much! Where did you find those graphs? I don't see them in the datasheet. Am I correct in interpreting this graph that the current can be applied for an infinite amount of time if kept below ~25A? Is 85C junction temperature, ambient, or coil? You have me intrigued by the diode comment. I planned to put a 1N4007 freewheeling diode on each relay. Are you saying this is a bad thing? \$\endgroup\$
    – kobra20
    Dec 26, 2020 at 21:25
  • \$\begingroup\$ @kobra20 Ah, I guess I went looking for a better datasheet when I saw the Digikey one. Link added. Sorry about that. \$\endgroup\$ Dec 26, 2020 at 21:31
  • \$\begingroup\$ The diode slows the opening of the relay and can reduce the lifetime somewhat in terms of switching operations. A lot of designers don't care and use it because it saves a part. You can use a diode + Zener or a diode + resistor to protect the transistor. \$\endgroup\$ Dec 26, 2020 at 21:33

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