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Is there any reason why I might choose an EM-relay rated at coil-voltage of 24V or 12V, over one rated at coil-voltage of 5V, all other specs like contact max. current, switching voltage, life etc being same. The only other difference in specs that I see are coil resistance, and contact material.

Most of the circuit schematics I found online, seem to use 12V relays, and very few use 5V relays. Given that low coil-voltage rated relays have lower coil-resistance, I'd only believe that one wastes lesser energy in switching and holding the relay, by using the 5V one, compared to the 12V one.

PS> By default I might order the 5V part, as I have quite a few 5V power supplies at hand, and none rated 12V. But if there's a good reason to use 12V EM-relay, I don't mind. Both parts (5V/12V) cost same.

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    \$\begingroup\$ Answer is correct, go with 5V in your case. Growing up I remember while 5V relays were convenient they were always more expensive and harder to source (especially compared to 12/24V auto relays) which is why I suspect a lot of Internet designs use them. \$\endgroup\$ – PeterJ Jan 24 '13 at 6:47
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Relays coils is a current-driven device. If you can apply the specified voltage across the coil, there will be sufficient current and the relay will click. Ohmic losses in the winding is about the only difference.

+5V, +12V, +24V are some of the common supply rail voltages. Of course, the makers of the relays want to make relays for various systems. That's the rationale behind having a family of similar relays with different coil voltages.

P.S. Don't forget the protection diode in parallel with the coil.

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  • \$\begingroup\$ Thanks Nick. The "...current-driven device" comment takes the cake. Ohmic losses aren't my #1 concern, as this is a wall-powered device. Wondering if you have any comments on the views of @user1988912 (other answer), specifically the part that higher coil-voltage may mean more reliable operation in noisy environments ? \$\endgroup\$ – icarus74 Jan 24 '13 at 7:28
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    \$\begingroup\$ Yes, relays are current-operated devices, but the thing that limits the current is the DC resistance of the coil. Since the coils for different voltages must all fit into the same physical space, it is necessary to use heavier-gauge wire for lower voltages, and this means that there must be fewer turns. This is what makes them constant-power devices, which means as the coil voltage goes down, the coil current requirement goes up proportionally. \$\endgroup\$ – Dave Tweed Jan 24 '13 at 12:50
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Your selection would be affected by:

  1. The availability of the digital outputs at a given voltage. For example, most industrial PLCs use 24V for control voltages by default, outputs at other levels are often optional add-ons. If you don't use a PLC, that's less of a concern.

  2. The availability of the relays. Go to your preferred distributor and check what are the stock levels of your relay depending on the coil voltage. Chances are, the 24V variant will be the most stocked one if it's of use for industrial applications.

  3. The availability of the coil voltage in your system. Sometimes if you have 5V already available it makes little sense to add a 12V or 24V supply just for the relays. As everything in the engineering, it's a tradeoff between simplicity, availability and cost.

  4. The availability of a suitable driver for your relay. Suppose that for simplicity and low cost you like to drive your relays using a bipolar driver like low-side ULN2003A or high-side A2982. Those have voltage drops around 1V, too high for a 5V coil to operate reliably.

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