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I have a ESP8266 which has 3V logic, and a heating system which takes 24V logic. I want to switch the heating system with the ESP8266. A relais looks to me like the best solution to do this.

However on the relais I've found like:

enter image description here

But I am a kind of unsure how to read the values of the relay. This one says:

  • 10A 250VAC
  • 10A 30VAC
  • 10A 125AC
  • 10A 28VDC

Does 10A 30V AC means it can also switch for 24V or is the 30V a minimal value for the output? Or do I need another relay?

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  • \$\begingroup\$ Contact and coil rating are two completely different things. You need both values. Few have 3 V coils. \$\endgroup\$ – winny Jun 25 '17 at 17:56
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    \$\begingroup\$ But... it says 10A 30VDC not 30VAC! \$\endgroup\$ – nickagian Jun 25 '17 at 18:00
  • \$\begingroup\$ This relay appears to have a 3V coil, that is the 03VDC in the part number. ghielectronics.com/downloads/man/20084141716341001RelayX1.pdf \$\endgroup\$ – Evan Jun 25 '17 at 18:05
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UL has tested the relay and it is capable of safely switching up to 10A at up to 28VDC (resistive load only), or 10A at 125VAC (resistive).

According to TUV it is capable of up to 10A at 30VDC (resistive load only) or 10A at 250VAC (resistive).

That does not necessarily mean that UL thinks the relay is unsafe at 250VAC, but it doesn't mean they think it's safe either.

If your load is a relay or contactor (inductive load) you had best use some kind of suppression across the coil such as a diode to prolong the life, assuming DC. If your 24V switching is AC rather than DC (as typical with a home thermostat type of system), a diode will not work and you would have to use an RC snubber or a TVS or MOV. A TVS would probably be the best choice and it should be rated to block about 35V and be of the bipolar type.

If your micro is powered via a regulator from a 5V supply it's much better to use a 5V (or 4.5V) coil relay and drive it from a 3V signal via a transistor. Generally putting big noisy loads across your MCU power supply (such as a relay and the coupling to the load it is switching) is not a great idea.

Also keep in mind that lifetime is only ~100,000 operations at full load. You can look at the increased life (again, for resistive load only- that's what cos(theta) = 1 means) at decreased current in the datasheet. At 1A you've got more like 700,000 operations.

enter image description here

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Those ratings are maximums, you can run a 30 VAC relay at 24 VAC no problem. There are minimum ratings needed to ensure you break through the oxide layer that can coat the electrodes, but that is usually not relevant for power switching applictions. This is only an issue when switching low-level signal currents such as audio -- for instance the relays that were/are used to switch phone lines.

Relays have different ratings for AC or DC with DC being considerably lower. The reason for this is that the reversing nature of AC stops arcs faster reducing damage to the contacts. That is why the relay is rated for 250 or 125 VAC, but only 28/30 VDC.

The reason it has two separate AC and DC ratings appears to be that those are different approval agencies, UL vs TUV. They are different because UL mostly lists stuff to switch 120 VAC circuits as found in North America, while other organizations certify for switching the 230 VAC found in Europe, and likewise other organizations certify for use on their local power standards. That would matter if you were making a product for sale, but not for hobby use.

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