I have a ups in my home.I want to install a fan in my room which run only when the mains power supply is cut and ups is supplying the power.How can I do this.
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1\$\begingroup\$ Find a product with such functionality and purchase it. Custom mains powered circuitry and switching is a bit complicated to do safely in a beginner project. In theory something like a relay with a mains rated coil also rated for switching mains might do it, but the actual implementation is more complex and full of dangerous pitfalls than the theoretical problem. \$\endgroup\$– Chris StrattonCommented Jun 18, 2020 at 15:14
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\$\begingroup\$ Get a relay that has a coil that runs on mains power. Connect the coil to the mains before the UPS. Connect the fan through the relay to the output of the UPS. (Use the normally closed contacts, so they are open when you have mains power.) Not the most elegant solution but simple and easy. \$\endgroup\$– John DCommented Jun 18, 2020 at 15:14
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\$\begingroup\$ @JohnD, that solution seems obvious, but watch out for heat build-up in that relay coil. It's going to be energized 24/7. I looked at some relays on Digi-Key, and coil currents range from 3.5 mA to almost 20 mA. That's anywhere from 0.4 Watts to 2.4 Watts. Continuous dissipation of 2.4 Watts in a small enclosed space...? Could get quite hot before long. I would check the data sheet to make sure it was rated for 100% duty cycle and, to check for any thermal management recommendations. \$\endgroup\$– Solomon SlowCommented Jun 18, 2020 at 15:51
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\$\begingroup\$ @SolomonSlow Excellent points, you do have to do the due diligence and engineering and select the right components. Like I said not the most elegant solution, but simple enough. \$\endgroup\$– John DCommented Jun 18, 2020 at 19:10
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I have a ups in my home.I want to install a fan in my room which run only when the mains power supply is cut and ups is supplying the power.How can I do this.
As you likely already found out a normally closed relay or contactor is hard to source. When they can be found they tend to be expensive, like this one: https://www.kele.com/product/sku?kid=LS7K-04-G
You can get this expensive contactor, get a proper enclosure, then wire it up. You can get the 24 VAC power supply for the coil from a number of sources, these are relatively common for things like HVAC controls and doorbells. Plug the power supply into the mains power and have the power for the fan come off the UPS.
One other way to trigger this, and perhaps avoid the use of an expensive normally closed contactor, is see if your UPS has an output to trigger a generator. I've seen UPSes with what is called a generator trigger to indicate a loss of mains power. Attach this somehow to the coil of a normally open contactor so that when power is lost the contactor is closed. Such output to trigger a generator will have a built in delay since short power glitches should not trigger a generator start. If such a delay is acceptable in turning on your fan then this could save a lot of trouble and expense.
Either way I'm seeing this as needing a contactor of some sort to close the circuit from the UPS to the fan. The coil will take power from mains supply or some output from the UPS. Whether you need a cheaper and more common normally open contactor or a harder to find and more expensive normally closed depends on how and where you get that control signal.
Here's an example of a far cheaper contactor: https://www.amazon.com/Packard-C140A-Pole-Contactor-Coil/dp/B003U81D2I/
With a proper selection of parts this likely could be constructed for less than $30. Adding other features like having the fan run based on temperature and/or humidity would add a few bucks more by using a pretty common household HVAC control. This makes me ask if there is a need to have the fan run on a power outage. It's likely easier and cheaper to have the fan run based on temperature.
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\$\begingroup\$ What I was thinking is to take 2 wires,one live wire which is directly connected to mains and doesn’t give output when mains is dead,and second also live wore which is connected to ups, which works at both time(mains and ups) and connec both wire to the two wires of fan.So when the mains will be on the net voltage will be zero as both will be live wire and fan will not operate.when mains is gone,the wire connected to mains will be dead which will make a potential difference and will act as a neutral wire and the fan gets working.When mains come back,potential difference zero,again off.What say \$\endgroup\$– saranshCommented Jun 18, 2020 at 18:29
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\$\begingroup\$ I say that if you attempt to use a live wire as a neutral that the power company will be very unhappy with you. First is that because this is AC we are talking about the phase and frequency will be important. The actual voltage across the wires when both are live may not be zero. It could be zero, and it could be 240 VAC. Another problem is that when the power goes out the wire doesn't become neutral, it floats. This is now potentially feeding a voltage to a line where utility workers expect it to be dead. This is dangerous and I highly recommend you not consider this tactic. \$\endgroup\$ Commented Jun 18, 2020 at 20:31