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I want to use a relay to switch high power LEDs ON and OFF. Nothing out of the ordinary yet. My problem comes from the fact that my control voltage is not stable (it varies from 5V to 12V for example).

Is there a way to find a relay that will safely and reliably switch ON with any input voltage in the desired range? How will this be demonstrated in the datasheets?

I picked a random relay datasheet to try to understand better the concept.

From my understanding and looking at page 2 for the 1 form C configuration, the 12V version of this relay would switch on as long as 9V is provided to the inductor and would turn off at less than 0.6V. It would also be safe to use this relay at 20°C as long as I don't go above 18V on the inductor side.

Thank you for helping me understanding relays better. I haven't found any videos talking about these specifications. It seems like most of them only explain how to use a relay in basic Arduino circuits or something similar. If you got any good video on the subject, I would gladly watch it.

Edit:

Bonus question: What should the specifications for a flyback diode be? Should it's max voltage be the voltage at which the relay operates, the maximum of the voltage spike that occurs when the inductor is disconnected or something in between?

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  • \$\begingroup\$ Is this an "XY problem"? What technical requirement drives the choice of a relay as the switching device? \$\endgroup\$
    – John Doty
    Mar 21, 2022 at 12:38

2 Answers 2

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Mechanical relays are not designed to operate with a wide range of voltages like 2.5:1. Once you include temperature variations, they are probably reliable with +/-20% from nominal. At high temperatures (ambient + temperature rise from operation), the copper coil resistance increases and they will not pull in with a low voltage, and even if they do pull in you'll reduce the life because they are not pulling in smartly. Even nominal voltage may not be guaranteed to be sufficient if you have a special Class H high-temperature insulation relay operated near the limits!

Anything more would result in an expensive and inefficient product, and relays are a very competitive cost-sensitive market.

So, unless you intend to design your own relay you should modify your control voltage to fit the commercial offerings.

For example, you could use a 5V relay and an LDO regulator that can withstand 12V at the input. With 12V in you'll be wasting the majority of power in the regulator, but at least it's an off-the-shelf parts.

Or, if you have a stable supply somewhere (or can make one with a switching or linear regulator), use the 5-12V input in conjunction with a transistor to switch the relay.

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5v to 12v is a large range. Most relays have about a 25% up/down from nominal "pick up" voltage. If you want to support a larger range, then you need to add a regulator in front of it.

The datasheet shows you the nominal coil voltage, operating voltage, and max voltage. This specific one says it should really be +/- 5%. The 75% operating voltage may not work after a a few cycles. Notice the max voltage varies from 130% to 180% depending on its temperature. Same will apply to the initial pick up voltage over time and temperature (but not as extreme.)

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