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I have some questions about relays. I'm putting together a board that will use 5 V as the trigger for a SPDT relay. The device I'm connecting to the load side of the relay consumes 24 VAC up to .75 Amps.

I've seen several relays and have some questions about the writings that are on the relays.

For example:

5v relay

For the above relay, obviously the trigger voltage is 5VDC.

  1. It states "15A 125VAC". Does that mean it will support any voltage up to 125VAC, including 24VAC? Is there a formula I can use to calculate the maximum amperage that can be drawn on voltages below 125VAC? It makes sense that it would support 24VAC at .75Amps, but I'm interested in knowing the support logic.

  2. Why does it list "10A 250VAC" twice?

  3. I connected 24VDC (not 24VAC) to drive an LED as a test and it worked. So, even though the writing doesn't show DC support, it worked. Any rules on using DC voltage on a relay that supports AC voltage?

  4. I'm curious about the support for 15A @ 125VAC. How can the relay support that much current going through those tiny pins? I would think it would burn up somehow.

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7 Answers 7

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  1. Yes, it can support lower voltages (e.g., 24VAC), so long as the current is less than 15A.

  2. Different regional standards, different ratings. The first "10A/250VAC" is the TÜV rating (EU countries.) The second "10A/250VAC" is the CCCC rating (China.) The "15A/125VAC" is UL (North America.)

  3. For that low voltage, no: 24VDC is fine. Higher DC voltages can require voltage de-rating because when switching DC the contacts need to separate farther to extinguish a DC arc (this applies for switches and breakers, too.) You'll need to contact the manufacturer for that rating.

  4. The 15A rating is for UL (North American) markets. More generally, the contact resistance is low enough that even at 15A the contacts won't heat enough to be a problem.

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    \$\begingroup\$ Thank you for the response. The responses by you and Julien answered by questions. A follow up, though, on Q4. Lets say the current flow is 15A at 125V. Why does house wire require thicker gauge to support 15A at 125V while this relay pin diameter is much smaller? It is because the relay is a short distance haul compared to house wire? \$\endgroup\$
    – LoganRoy
    Jul 27, 2023 at 16:20
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    \$\begingroup\$ Yes, because the contacts and leads are short compared to wire, and thus have less I2R loss even though they're not as thick as #12 ga wire. \$\endgroup\$ Jul 27, 2023 at 16:24
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    \$\begingroup\$ As well as being a short distance, the relay housing may also be a different type of plastic that can withstand higher temperatures than your house wiring. (Unlike house wiring, relay housings do not need to be flexible) \$\endgroup\$
    – abb
    Jul 28, 2023 at 0:48
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It states "15A 125VAC". Does that mean it will support any voltage up to 125VAC, including 24VAC?

Yes.

Is there a formula I can use to calculate the maximum amperage that can be drawn on voltages below 125VAC?

15A. General rule: don't go over any of the numbers you find in a rating, unless you have an unimpeachable source that says you should.

Why does it list "10A 250VAC" twice?

The three different ratings have three different logos next to them, which correspond to different standards organizations and different methods of testing. The first one is TüV, the second one is UL "recognized component", the third one looks like CCC (China Compulsory Certificate). Which set of numbers you use depends on whose jurisdiction you want to sell the doodad that contains this relay in (but if you're under 10A and under 125VAC then you satisfy all three).

I'm curious about the support for 15A @ 125VAC. How can the relay support that much current going through those tiny pins? I would think it would burn up somehow.

They're not that tiny. As long as you have a good conductor it doesn't take that much cross-sectional area to carry several amps with minimal loss. And minimal loss means minimal heating.

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  • \$\begingroup\$ Thank you for the response! Much appreciate it! \$\endgroup\$
    – LoganRoy
    Jul 27, 2023 at 16:24
  • \$\begingroup\$ Nobody is stopping you going over the numbers if you like, but you're taking the risk the part won't work as intended. I want to emphasize this so people don't get the impression that the relay police will come and take them to jail. That won't happen. You might break the relay, though. Especially if you are just a little bit over the numbers, the relay might work, but only for a week or a month or a year. That's why we don't do it in reliable designs. But by all means, you can do it at home and see what happens if you want to. \$\endgroup\$
    – user253751
    Jul 27, 2023 at 18:15
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    \$\begingroup\$ @user253751 I don't think the general concern with component ratings is "will the [component] police take me away" so much as "will the [component] melt into my breadboard", "will I start a fire", "will I risk electrocution" etc. \$\endgroup\$ Jul 28, 2023 at 0:28
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    \$\begingroup\$ Following on from @KarlKnechtel apart from the relay itself overheating in a failure, you have to consider the knock-on effects if it fails - for example if it fails shorted and a heater that should only have been on briefly is on continuously, will it do some damage. Of course in that case you should have another level of protection, but if you use a (non-resetting) thermal fuse, then you might have 2 parts to replace, and a non-obvious failure \$\endgroup\$
    – Chris H
    Jul 28, 2023 at 9:47
  • \$\begingroup\$ @ChrisH and you should absolutely start a small fire, in a safe way, at least once in your life, so you'll have an idea of how fire behaves. Melt plastic, too. I remember reading about a professor who would blow up some capacitors and resistors in front of the class at the start of each semester to de-scarify them. Watching how aeroplanes behave in real emergencies makes flying less scary to a lot of people (they don't just fall out of the sky). \$\endgroup\$
    – user253751
    Jul 28, 2023 at 15:55
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  1. Yes. And, both the voltage and current ratings are maximums that are independent of each other. For a 1 VAC signal, the max current still is 10 A.

  2. The symbols to the left are the logos for safety agencies in different countries. They do not all agree on things like what the contact materials can support, how far apart is enough for a certain voltage, etc.

  3. Much more complex question. If you spend a lot of time reading datasheets from over the decades (and, really who doesn't?), you will see that in the relatively rare cases where both AC and DC specs are given for a relay or switch, the DC ratings are considerably less. Less current, lower voltage, both, whatever - less. I know of no rules or equations for this.

  4. The rating is for the contact material and pressure (current), and the contact spacing (voltage). Part of the way it works is that while the pins might not be very fat (for low resistance), they are short. Most of the electrical structure within the relay is made of fatter conductors. And while the pins might be relatively high in resistance per inch pr per millimeter, they still are very good conductors and the distance the current goes through the pins is very short.

Chassis mount relays have Faston connections that act as heatsinks to remove heat from the contacts. For pc mount, the board is a poor heat conductor but the copper traces are pretty good. So there are two reasons to run fat pc traces to the pins, electrical and thermal.

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  • \$\begingroup\$ Thank you for answering Q4 in detail. Very helpful in my understanding! \$\endgroup\$
    – LoganRoy
    Jul 27, 2023 at 16:23
  • \$\begingroup\$ Do relay contacts produce significant heat? I've never thought of needing to heatsink relay contacts. The coil may or may not need one (usually not) \$\endgroup\$
    – user253751
    Jul 28, 2023 at 16:12
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You can't rely on the data on the component

Like Peter Green says, you need to locate the data sheet for the device, and read its data thoroughly. It will have very different figures for resistive, inductive, motor, ballast, tungsten and other applications. And different ratings for DC, if any. You need to follow the rating which most applies to your application.

You need to use the data sheet. The numbers on the device are a summary. You need the data sheet to know what they're a summary of. I.E.

This particular component is rated only for a straight resistive load, and is rated 0 amps for inductive etc. That was a bit of laziness on the manufacturer's part - they didn't pay UL or TUV for testing the component on other load types. See how they shave cost and quality?

You will need to assure that you only use this for a pure resistive load, or that your design compensates for that. For instance if you have a tungsten load whose starting impulse is 12 amps, and you can prove that, you are fine since the device is rated 15 amps at 24 volts AC.

Note that you can't compensate for an AC ballast, motor or inductive load, because the issue is inductive kick when the load is interrupted. There are no freewheeling diodes in AC.

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What are the markings on relays?

The markings on any device are generally a summary, NOT a full specification. Unfortunately I was unable to find a data-sheet for your relay, or even a website for the manufacturer. I consider this to be something of a red flag.

I did find a data-sheet for a relay with the same model number but a different brand. https://denkovi.com/Documents/JQC-3FF-S-Z.pdf How similar they are is anyones guess.

It states "15A 125VAC". Does that mean it will support any voltage up to 125VAC, including 24VAC? Is there a formula I can use to calculate the maximum amperage that can be drawn on voltages below 125VAC?

Generally you should treat the max voltage and max current as separate specifications. That is you should be safe to pass up to 15 amps at any voltage up to 125VAC.

You should be aware that the headline current ratings for relays relate to resistive loads. Other types of loads may put more stress on the relay contacts and may require significant de-rating. Capacitive loads may cause high inrush currents when the relay closes. Inductive loads increase the risk of arcing when the relay opens.

Why does it list "10A 250VAC" twice?

When something is rated by multiple approvals bodies sometimes the ratings from each approval body (which may be different due to different standards/test conditions) are printed next to the approval marking.

It's also possible that the relay has the same ratings under all approvals and someone just screwed up and printed the same rating twice rather than printing the DC rating.

To be sure you would need actual documentation from the manufacturer. Sadly though even the good manufacturers are sometimes less than stellar at documenting their products and once you go to off-brand Chinese manufacturers it's even worse.

I connected 24VDC (not 24VAC) to drive an LED as a test and it worked. So, even though the writing doesn't show DC support, it worked. Any rules on using DC voltage on a relay that supports AC voltage?

DC is generally more prone to arcing than AC. In my experience this generally means relays have a much lower voltage rating for DC than for AC.

The relay I found with a different brand but the same model number lists ratings of 10A 250vac, 15A 125vac and 10A 28vdc. The tables in said data sheet don't imply any relationship between the approvals body and the ratings.

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  • \$\begingroup\$ That is you should be safe to pass up to 15 amps at any voltage up to 125VAC. - A different phrasing would better emphasize why those ratings are independent: It can safely pass up to 15 amps, and safely switch any voltage up to 125VAC with a load that allows currents that high. (From other answers, only guaranteed for resistive loads). This phrasing steers away from the idea of power being 125V x 15A. You never have 125VAC across the relay while it's carrying current; almost all of that voltage is across the load when on, unless you're switching very low voltages. \$\endgroup\$ Jul 29, 2023 at 7:29
  • \$\begingroup\$ If someone is trying to show whether an assembly employs standard-rated components in a manner consisten twith the ratings, having the agency's ratings printed on the components can assist such verification. It's likely that a relay contact which is designed to interrupt 10A at 125VAC would have no problem interrupting 10mA at 5VDC, but components used for such voltages and currents wouldn't require any rating agency approval. \$\endgroup\$
    – supercat
    Jul 30, 2023 at 18:36
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The relays are manufactured by Donghai County TONGLING Electrical Appliance Co., Ltd. of Jiangsu China. The company uses the Tongling and Tengfei names. Publicly available data for the component in question is here

As others have said, the three ratings are with reference to the three standards. The UL rating is with reference to the ANSI/UL 508 standard. CQC is the Chinese safety standard.

Note that the manufacturer provides life curves up to 15A at 250VAC resistive, however the life is only 30,000 operations under those conditions.

The pins are intended to be soldered into a PCB which will provide a certain amount of heat sinking, so they are probably adequate to carry 15A RMS at least if the ambient temperature and coil power is not too excessive.

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Q1: It states "15A 125VAC". Does that mean it will support any voltage up to 125VAC, including 24VAC? Is there a formula I can use to calculate the maximum amperage that can be drawn on voltages below 125VAC? It makes sense that it would support 24VAC at .75Amps, but I'm interested in knowing the support logic.

Yes it will support any voltage up to 125VAC. The formula to calculate the current is Maxcurrent=max_current written! Lowering the voltage don't improve the current rating.

Q2: Why does it list "10A 250VAC" twice?

Probably because it's its rating from different norm. I would need to see the datasheet to confirm this.

Q3: I connected 24VDC (not 24VAC) to drive an LED as a test and it worked. So, even though the writing doesn't show DC support, it worked. Any rules on using DC voltage on a relay that supports AC voltage?

Usually, DC current or AC doesn't make difference, but their are less requirements for DC systems therefore, they are often only rated for AC. That being sais, if you plan on using it in dc, leave some margin and contact the manufacturer for some specs.

Q4: I'm curious about the support for 15A @ 125VAC. How can the relay support that much current going through those tiny pins? I would think it would burn up somehow.

Once again, different certification means different rating.

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  • \$\begingroup\$ Thank you for your comment. It is very informative! \$\endgroup\$
    – LoganRoy
    Jul 27, 2023 at 16:15
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    \$\begingroup\$ Switching DC is different from switching AC. The same set of switch contacts cannot safely interrupt as much DC current as AC current at the same voltage. It's because DC arcs are not as quickly extinguished. \$\endgroup\$ Jul 27, 2023 at 16:47
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    \$\begingroup\$ AC arcs automatically extinguish every time the AC cycle reverses polarity, but a DC arc can theoretically continue forever, so the design limits are fundamentally very different. The ratings on the cover show that the relay will support any voltage up to 250v, not merely 125v \$\endgroup\$
    – MikeB
    Jul 28, 2023 at 9:04
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    \$\begingroup\$ @MikeB well, occasionally an AC arc can continue for a while :-) \$\endgroup\$
    – uhoh
    Jul 29, 2023 at 8:46
  • \$\begingroup\$ Apologies for my generalization on ec specs. I add in mind the current and not voltage since i rarely work with 120V dc. Usually was ment in most dc case, but obviously, if you are working with hazardous dc voltafe, you should use a relay specified for dc. I didn't tought op needed that much details for his application. \$\endgroup\$
    – Julien
    Jul 30, 2023 at 15:03

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