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DC float switch controlling a 110v ac pump

strider887

Nov 11, 2014
3
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Nov 11, 2014
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ok so this is the only float switch small enough for the project im working on but it is a dc switch and I need it to control a water pump that runs on a standard 110v ac circuit. I need assistance as this is most certainly not my specialty so feel free to talk to me as if I know nothing about this subject at all. I need to know what additional components I will need to built this circuit and if possible where I can find them as cheap as possible. The switch itself can be found at this url from amazon.com
http://www.amazon.com/gp/product/B0...pf_rd_t=101&pf_rd_p=1688200382&pf_rd_i=507846
If any other info is needed please let me know, these are the specs for the switch.

Product Name : Water Level Sensor;Max Contact Rating : Max Contact Rating;10W : 10W;Max Switching Voltage : 100V DC
  • Max Switching Current : 0.5A;Max Breakdown Voltage : 220V DC;Max Carry Current : 1.0A;Max Contact Resistance : 100mΩ
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
Hi there and welcome to Electronics Point :)

Your simplest option is a relay. This is an electromechanical device that is activated by a low DC voltage (12V and 24V are common) and can switch a circuit with higher voltage and current. Here's the general arrangement.

271280.001.GIF
Your 110VAC comes in at the top left, and goes out to the pump at the top right. One side (preferably the Phase, not the Neutral side) is switched by the contact of the relay. The relay closes its contact when its coil is energised by a DC voltage. This DC voltage comes from the DC power supply through the float switch.

A diode must be connected across the relay's coil as shown because of a phenomenon called "back EMF" which causes the relay coil to generate a large voltage spike when the float switch turns OFF and the current into the coil is suddenly interrupted; this voltage spike will eventually burn out the contacts of the float switch unless the diode is present to absorb it. The 1N4004 diode I've suggested is available from Radio Shack etc.

The DC supply voltage can be anything from 5V to 48V but 12V and 24V are common choices. You may already have a suitable DC supply available; if so, you can use that, provided that it can supply about 100 mA for the relay coil (most relays draw less than that).

Choose a relay wtih a rated coil voltage equal to the DC voltage you have available, and a contact rating comfortably higher than the current you need to switch. Radio Shack have a limited range of relays from NTE, but they are overpriced. Digi-Key have a much wider and better selection. See this filter for examples of suitable relays with 12V and 24V coils: http://www.digikey.com/product-search/en?FV=fff40010,fff80367,1140006,1200002,1200003,a700002,a700008,a8c0007,16040014,16040015,1604001a,1604001b,16040020,16040022,16080002,16080004,16080005,16080007,16080013,16080017,1608001a,1608001e,1608001f,16080022,16080024,16080029,16080041,16080042&ColumnSort=1000011&stock=1&quantity=1&pageSize=250

Many of those relays have more than two contact terminals. Double-pole relays can switch both Phase and Neutral simultaneously; this is a good idea. Double-throw relays have a "normally closed" contact as well as a "normally open" contact on each pole; the NC contact is not used.

When you build this circuit you must take great care with the mains voltage wiring. Make sure all connections are reliably insulated - a good smothering of hot melt glue can be good for this - and make sure that the components are all firmly mounted and cannot move and short onto anything. If there is any metal nearby (e.g. if you put them in a metal case), that metal must be grounded. Please post a photo of your construction so we can check it.
 

strider887

Nov 11, 2014
3
Joined
Nov 11, 2014
Messages
3
wow, that line diagram is very simple to follow thank you. it will take me a little time to acquire all the components but I will for sure post pics when ive completed it. thank you again and for such a quick response.
 

Kiwi

Jan 28, 2013
471
Joined
Jan 28, 2013
Messages
471
That power supply would be fine. The power supply just has to be the same voltage, and have a current rating higher than the coil current of the relay you choose.

A relay similar to this could be a good choice.
http://www.amazon.com/Power-HH62P-L...TF8&qid=1415779739&sr=8-11&keywords=relay+12v

From memory, the coil current for 12v DC Omron LY2N relays is about 80mA(0.08A).

As Chris advised, be careful with mains voltage.
 
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