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I'm using a RadioShack 12V DPDT relay, model 275-249.

enter image description here

I applied 6 V across the coil and the switch closed. Why is this? I didn't have any resistors in series with the 6 V power supply (the only limiting resistance was that of the coil). I'm wondering why the switch closed when the relay is rated with a 12 V nominal coil voltage.

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    \$\begingroup\$ I think David is a competent engineer and his answer is a good one, but I think you accepted his answer too quickly. Questions with an accepted answer get fewer new answers, which otherwise might be interesting as well. I usually advise to wait for a day with accepting, so that the question can go around the world, or at least until you have 3 or 4 answers you can choose from. \$\endgroup\$ – stevenvh Aug 18 '12 at 6:39
  • \$\begingroup\$ Okay! I unaccepted it. Will reaccept tomorrow (unless there's a better answer) \$\endgroup\$ – Mason Aug 18 '12 at 15:45
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The simple answer is this: It is guaranteed to switch with 12v on the coil. It is guaranteed to not switch with 0v on the coil. Between those two voltages, there are no guarantees about what happens.

Or, to put it a different way: 0v=Off, 12v=On. Somewhere between 0v and 12v it must switch between off and on. It's not like it will stay off from 0v to 11.999999v and then switch on at only exactly 12.0v. Some relays might switch on at 4v. Some at 6v. Some at 8v. You might even buy a bunch of the exact same relay and some will switch on at only 4v, while others of the same relay might need 8v to switch on. But you know that it will be off at 0v, and on at 12v!

Just to confuse matters, there is the voltage at which a relay switches, and a different voltage at which it will stay on. For example, if you slowly raise the voltage the relay might turn on at 6v. But if you then reduce the voltage, the relay might not turn back off until it drops below 4v. 6v would be the switching voltage, but 4v is the holding voltage.

Of course the switching voltage and the holding voltage will vary from relay to relay and are not usually specified in the datasheet. Of course, with Radio Shack you almost never get good datasheets anyway and this is no exception.

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    \$\begingroup\$ Manufacturers almost always specify a voltage above which the relay is guaranteed to turn on and a voltage below which it is guaranteed to turn off. They may also specify a voltage at which the relay will stay on if on. In every case, there will be a range of voltages in which behavior is unspecified, but the size of that range may vary considerably from one manufacturer or product line to another. \$\endgroup\$ – supercat Aug 18 '12 at 5:47
  • \$\begingroup\$ Adding to this, it's possible that if you apply the "hold voltage" to an open relay, it won't trigger, but if you flick the case it will move the armature enough to allow the smaller hold voltage to pull the relay in. "Flick the relay" isn't a sustainable operation mode, so don't rely on the hold voltage to do anything but hold an already triggered relay in :) \$\endgroup\$ – Bryan Boettcher Aug 20 '12 at 18:26
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One reason not to buy from RadioShack is that you get a product but no information (unless you find "supported languages: English" for a relay useful), or wrong information: for your relay the scarce data given doesn't match the relay on the photo.

A 12 V relay is designed to operate without any problems at that voltage. But you can't expect that the user's 12 V supply will be 12.00 V, so you still want the relay to operate at 11.8 V, or 11 V. There is production variation as well. Therefore relays have a must operate or pickup voltage. That's the minimum voltage at which the relay will activate guaranteed. According to RadioShack that's 9.6 V for this relay, while the relay on the photo specifies this at 9 V, which is a typical value; for many relays this pickup voltage will be 75 % of nominal.

If you guarantee pickup at that voltage there still has to be some slack. A relay with a 9.6 V pickup voltage will almost certainly pickup at 9.5 V, in many cases even at 9 V and for some 6 V will be sufficient. You just can't rely on that.

Note that there are also parameters like must hold and must release. Must hold is the minimum voltage the relay needs to remain activated guaranteed after it's switched on. That can be pretty low, like 3.5 V for a 12 V relay. You can test this with a Li-ion or LiPo battery. The battery's voltage may not be high enough to activate the relay, but if you activate it manually, but pushing the contacts together with the tip of a pencil you'll see that it remains on.

Must release voltage is the voltage at which the relay will switch off guaranteed. Because of the variation in production they need some margin here as well, and therefore that value can be very low, 1.2 V for a 12 V relay is no exception.

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  • \$\begingroup\$ They don't even give the current rating! You have to read it off the image - ridiculous. \$\endgroup\$ – Chris Laplante Aug 19 '12 at 2:43
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Just as a follow on from the original question which also essential in understanding relays. I have two relays, both rated at 24Vdc. One is an OKS2C-SDC24V max 10Amps DC, the other OKS2C-SDC24V-S 15A 28VDC.

The coil resistance of the first is ~500 ohm and of the second ~1.6kohm, can one be substituted for the other given, ideally, they both switch on at 24Vdc (or close to), should the coil current be taken in account because a higher coil resistance for the same voltage means less current which would force the relay to operate close to its nominated Vdc value or is it just the voltage that counts.

Ultimately, it is the magnetic field created that operates the relay which in turn depends on the current. For a particular value of magnetism to be established by both coils, the voltage in the first relay, OKS2C-SDC24V, would require more current and therefore more voltage than in the second relay. While in the second coil, the higher resistance means less current, however, more turns and therefore more magnetic field.

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