[SOLVED] Relay switching AC inductive load

I put together a simple automatic battery backup circuit to keep a 120V 18W aquarium air pump running through a power outage using a 12V battery, a DPDT 120V relay, a trickle charger and a 150W inverter. One pole of the relay is used to switch the 120V AC live feed to the air pump between mains and the inverter output, and the other pole is used to switch the +12V battery connection between the charger and the inverter supply. The circuit worked perfectly when testing with a light bulb, and switched properly with the air pump, but when power was restored so the relay switches back to mains power, the inverter blew and the "magic smoke" escaped. The air pump is a giant coil that uses an electromagnet to vibrate a diaphragm, so it is an inductive load. How can I protect the replacement inverter so that next time I disconnect one leg of the inductive load it does not blow up again?

Hi Tim,

Have you placed a diode across your relay to prevent back emf? See this article:

https://en.wikipedia.org/wiki/Flyback_diode

brain-dead said:

Hi Tim,

Have you placed a diode across your relay to prevent back emf? See this article:

https://en.wikipedia.org/wiki/Flyback_diode

Click to expand...

Both the relay coil and the air pump load use 120V AC. I see how a flyback diode would be important with a DC inductive load, but not AC. Perhaps a couple of 200V zener diodes connected back to back across the load?

Make some delay between switching power sources.

My thinking about this, You dont need all of this exibitions, you just need small steel tank with compressed air (few litres), and one electromagnetic valve with coil on 12V, timer circuit to release air in intervals.

Hello brain-dead,

flyback diodes only helps, if you have a DC supply and an inductive load. For AC it don't help, because you make a rectifier.

Regards

Rainer

tpetar said:

Make some delay between switching power sources.

I don't think a delay would help - I think the problem occurs when you break one leg of the inverter - load circuit,

My thinking about this, You dont need all of this exibitions, you just need small steel tank with compressed air (few litres), and one electromagnetic valve with coil on 12V, timer circuit to release air in intervals.

Click to expand...

Interesting idea - but the air pump does not create much pressure, so no matter how large an air tank, the pressure would quickly drop too low during a power outage.

Timwms said:

Interesting idea - but the air pump does not create much pressure, so no matter how large an air tank, the pressure would quickly drop too low during a power outage.

Click to expand...

Hi Tim,

This sentence is not mine "I don't think a delay would help - I think the problem occurs when you break one leg of the inverter - load circuit,"

You should have one adequate motor to compress air just as any air compressor device, but in your case you need smaller version.

In water there is enough oxygen for some time. What is power outage time ? Maybe you can use adequate air tank size, just calc how many air litres your present air pump makes for 24h.

;-)

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I think few litres is enough, its compressed air which expand, and your small air pump dont make compressed air. Make calc.

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I think few litres is enough, its compressed air which expand, and your small air pump dont make compressed air. Make calc.

rfredel said:

Hello brain-dead,

flyback diodes only helps, if you have a DC supply and an inductive load. For AC it don't help, because you make a rectifier.

Regards

Rainer

Click to expand...

What if I used a pair of 140V Zener diodes connected back-to-back across the load? I wonder what wattage zener would be needed?

Timwms said:

What if I used a pair of 140V Zener diodes connected back-to-back across the load? I wonder what wattage zener would be needed?

Click to expand...

Dont play with mains voltage and zeners, its ok if you want crispy fried fish.

I think few sec of delay time between switching will do the job, if you whant to use inverter.

Why you dont use standard UPS for PC ?

Better use modified sine wave or pure sine wave with inductive load (electromotors and transformers devices). Square wave is not good for inductive loads.

Hello Tim,

Zener diodes is not good, because if they break down, you have a short circuit. Better is to use a Varistor.

Also look for your relay. Is the distance between the contacts great enough, to open it without an electric arc. The arc must be off, before the other contact is close. else you get a connection between all contacts and your inverter will burn.

What type of relay you use?

Regards

Rainer

Connect some sort of snubbing circuit across the relay contacts, something like a 100 ohms in series with a .1 MF capacitor that is rated for 110V AC working, to soak up the magnetically induced spark from the solenoid pump.
Frank

tpetar said:

Dont play with mains voltage and zeners, its ok if you want crispy fried fish.

I think few sec of delay time between switching will do the job, if you whant to use inverter.

Why you dont use standard UPS for PC ?

Better use modified sine wave or pure sine wave with inductive load (electromotors and transformers devices). Square wave is not good for inductive loads.

Click to expand...

I was trying for a cheap solution that would last a full day. Once every couple of years we get a storm that nocks out power for a day or more. Looks like the PC UPS is probably the best solution after all.

Thanks all for your comments.

What is aquarium size, and what kind of fish you have?

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What you say on 12V DC Aquarium Air Pump ?

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http://www.aa-aquarium.com/catalog/product_categories.php?cPath=78&osCsid=5g5dp3cdr2g0rjstbs29i016q1

tpetar said:

What is aquarium size, and what kind of fish you have?

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What you say on 12V DC Aquarium Air Pump ?

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**broken link removed**

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I am starting an aquaponics farm - using fish as fertilizer factories for growing vegetables hydroponically. So far I have a small "family-sized" system in the basement with 64 square feet of hydroponic grow space, and 200 gallons of fish tank. I only have minnows and goldfish at the moment - in a couple of weeks I will be getting 500 small coppernose bluegill (a local fish that grows up to 2 pounds, and is good to eat and that can handle cold temperatures). 500 small fish will weigh around 12 pounds or so - I can use 20 or more pounds in my system. I will build (as time and budget allows) an outdoor system with 500 to 1000 gallons of fish tank. The system I am following is taught by successful commercial aquaponics farmers Tim Mann and Susanne Friend described at www.friendlyaquaponics.com. They live in Hawaii, and grow Tilapia, which is a tropical fish that needs warm water. I am also earning some money as a consultant designing and building aquaponics systems for folks in my area who are looking for an easier way to grow vegetables - no weeds, and vegetables grow twice as fast in half the space of conventional gardening! You can have up to a pound of fish for every 4 gallons of water, but only if you keep the air on!

I had heard that DC air pumps are expensive and hog power, but it does make sense for a backup unit, and I will compare power and price when I build my outdoor unit. Having a 12V air pump is also useful if you ever need to transport live fish.