AC Adaptors in Parallel?

[toii]

I have some garden lights that are powered by mains power packs that output 12V AC. They are both rated at 15W. Can I use the two power packs together to provide more current to support more lights? I think the answer might be no because they are AC. Am i right that if I wire the two outputs in parallel (in phase with each other) I will have 24V output which is not what I want?

 

[audioguru]

If you connect them in parallel and with the same phase then the voltage is the same as one but their current is added.

If you connect them in series and in phase then their voltages are added but the current is the same as the lowest current one.

 

[toii]

Oh ok. Well that sounds like exactly what I am after - same voltage but double the current to support extra lights. If I only have a multimeter and I want to figure out how to get them in phase I wonder what is the easiest way to tell. If I just have a guess and connect them in parallel will the AC reading be 12V if they are in phase and 0V if they are out of phase? Is it safe to connect them out of phase (by mistake)?

 

[Boncuk]

If you connect the outputs parallel (different polarity) and they are exactly the same voltage the output will be zero.

If they have the same polarity the output voltage should be the same (doubling the output current)

Doing it for a short moment there shouldn't be a problem.

Keep in mind to always plug them the same direction into the wall outlet once you have determined the output polarity. It will change with the direction plugged into the wall outlet.

Check out the attached wiring diagram for better understanding.

Boncuk

 

[toii]

Thanks for your help - saves me having to buy another adaptor. Buying two four-packs of garden lights with two 15W power packs was much cheaper than just buying a power-pack of higher wattage by a long way (and I get some spare lights!). I dont know how they can manufacture and sell these at the retail price they do and make any money!

I live in New Zealand and our wall sockets are keyed so it is only possible to plug anything into a mains socket the same way each time.

 

[RCinFLA]

A less 'exciting' way to do it is connect one wire of each transformer together. Use the A.C. voltmeter to read the voltage between the two other leads.

If near zero then you got it right. If 24 vac you got it wrong, reconnect.

Also, if there is the possibility of flipping the A.C. input plug 180 degrees the secondary match will follow, meaning they will short out if A.C. input plug is reversed on one of the transformers.

 

[Hero999]

Hare these heavy low frequency transformers or lightweight high frequency electronic transformers?

It they're low frequency then fine, it will work.

If they're high frequency electronic transformers then it won't work because they won't be in phase and they'll work at slightly different frequencies.

 

[toii]

They are lightweight, the sort of adaptor you might get with a small electronic appliance only AC not DC

 

[Speakerguy]

Does it feel like it has a hunk of iron inside it?

 

[kchriste]

There is also a shock hazard here. If you parallel them up and unplug one of them you will get a 240VAC shock if you touch the progs! I highly recommend that you avoid paralleling them. One way to guard against this problem, and the phase problem, is to put bridge rectifiers on the outputs of each wall transformer and only parallel the rectified DC side.

 

[Hero999]

Assuming they're not electronic you could take the plugs of and permanently parallel them by connecting them to some sort of junction box so you can't only have one plugged in.