12V transformers in parallel to lower the voltage drop. ( Y /N )

davidro1November 11, 2010

On a new circuit, I've put enough 12V lights to make the last bulb shine yellow. The total wattage is below the Max. Watt. number shown on the transformer. Not close.


Is one possible solution to get another transformer and install it in parallel?

What might make it ill-advised to do this?

I have the space available. The total number of lights would be the same. I know about fan-out and voltage drop, or I think I do.

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That won't help. If I understand you correctly, the problem is the resistance of the wire causing excessive voltage drop. (This is one big problem with low voltage lights.)

There are three things you can do. You can replace the wire with larger gauge wire, shorten the wire, and/or reduce the load (smaller number of lights or lower wattage lights).

For example, you can replace the existing wire with wire of a larger gauge. Or you can put half the lights on a separate wire running back to the transformer. Or you can put half the lights on a second transformer, located so you can use a shorter run of low voltage wire. Or you can use some combination of these.

    Bookmark   November 11, 2010 at 9:55PM
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Agree 100% with davidr's advice but just wanted to put one other option on the table. Create a loop in the wiring system where the wiring at the last light returns back to the transformer (you do need to observe the "polarity" markings on the wiring when you do this). This will help balance out the voltage drop around the entire loop.

    Bookmark   November 11, 2010 at 10:51PM
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Interesting suggestion, Bill. FWIW, I've read that household circuits in the UK are often (always?) wired this way.

This would be fairly effective where the last light in the string is located somewhere that's reasonably close to the transformer. You'd only need to add a little wire to bring it back around.

If you had to run another wire out to the last lamp way in the distance, I guess that would still help - you'd halve the voltage drop there. But that's a lot of extra wire, and if you're going to buy that much, why not just parallel the new stuff with the existing wire, and run it to all the ilghts? (Or get a larger gauge and rewire altogether.)

I *think* paralleling low voltage conductors would be allowable under Code. IIRC, the NEC prohibits paralleled conductors in regular line voltage wiring, except in limited cases, but it doesn't seem to regulate low voltage wiring as much. However, I'm out of my depth here on code, and I don't have a copy of the code handy to check (it's not on this computer). Maybe someone else can comment on the code compliance of paralleled conductors in low voltage wiring.

    Bookmark   November 12, 2010 at 1:10AM
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You could put the transformer connection at the center of the light string instead of at one end. That would help also.
How many lights? How many watts each? What cable are you using now?

    Bookmark   November 12, 2010 at 8:13AM
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"You could put the transformer connection at the center of the light string instead of at one end. "

Making a loop effectively does this.

The effective length of the line is cut in half (though it real only works for equal loads uniformly distributed on the loop).

    Bookmark   November 12, 2010 at 8:52AM
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ok, i guess what I'm reading is: the length of cable is a big factor. In other words it's not likely that a transformer could produce less voltage when current draw is in the higher third of its rated range. I could remove two close lamps and see if the third and farthest lamp seems brighter or not. (Why didn't I think of that before?)

If length is the most significant factor, then the most efficient layout would be a starfish layout, which works when the cables are not exposed. Any other layout means accepting some yellowing as each light gets farther... totally or mostly because of the distance, not because of the power taken by the preceding lights in the "chain".

Does this explain the thinking?

Within a few days I can get up on a ladder to confirm all this. And measure voltage too.

    Bookmark   November 12, 2010 at 11:19AM
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"In other words it's not likely that a transformer could produce less voltage when current draw is in the higher third of its rated range."

No, tranformer voltage falls off with secondary (output) load.

If ALL the lights are uniformly dim you could have a transformer rating issue (not enough power).
You generally cannot simply parallel up transformers though.
The slight differences in their output voltages cause all sorts of circulating currents and can easily burn them out.

An even larger transformer may be capable, but the price goes up exponentially with power rating.

Dividing the light chain into two sections with separate transformers is likely to be a less expensive option.

    Bookmark   November 12, 2010 at 1:29PM
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OK. As it happens, I can divide the chain.

Thank you!

    Bookmark   November 12, 2010 at 2:57PM
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One other option that I used is go with LED units that plug in like the regular 12v bulbs to reduce load. I did this and went with the cool white and have a nice bluish hue on my outdoor lights. They also are brighter than my old bulbs because of the voltage drop issue is no longer there. I purchased mine from superbrightleds.com and will post a link. Oh yeah its been 3 years and I haven't replaced a bulb yet. I went with the WLED-xHP6 version.

They also do not look like dim solar lights.

Here is a link that might be useful: Led Landscape light link.

    Bookmark   November 14, 2010 at 10:40PM
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The 12V cable is steel. I guess this is for strength (because it's not the best conductor).

I'd like to minimize the voltage drop over a 4ft length because that is where I see the yellowing of successive bulbs. One idea is to run parallel lines for that segment, leaving the steel cable As Is for strength.

Are there alternatives? I've never seen or heard of a cable with a conductor in it. A central core in a sheath. Braided in. Whatever. I could replace the steel cable with another kind of cable.


    Bookmark   November 24, 2010 at 10:09AM
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What you're describing almost sounds somewhat similar to telephone "gopher resistant" underground wiring, except for the way you describe the inside conductors.

How did you even connect the lights to this in the first place?

What kind of lights are these? - I think most of us pictured landscape lighting with screw down piercing connectors.

More info please!

    Bookmark   November 24, 2010 at 7:36PM
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1/ My cable is from the Norrsken product kit of Ikea.
2/ Everything else is different: I got a different transformer, I had custom lights made, I made my own hangers to hold the cable, and so on.

goes with

    Bookmark   November 24, 2010 at 11:18PM
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The Ikea Norrsken cable IS copper, but coated so it looks like a tin color.

I'll open a new thread about where one might buy more cable of that kind. I'd like to make another series of low voltage lights on a cable, and not buy the Ikea kit.

    Bookmark   December 16, 2010 at 5:35PM
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You can parallel transformers. Its done all the time. Just make sure both have the same ratings. This web site explains:


This is not the problem you have however. You need bigger wire.

    Bookmark   December 23, 2010 at 5:53PM
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"You can parallel transformers. Its done all the time. Just make sure both have the same ratings."

Not with the type of transformers the OP is describing.

They are not built to the same level of match as power distribution transformers.

High ratios require multiple turns and they are rarely even wound the same way, resulting in impedance differences under varying loads.

    Bookmark   December 24, 2010 at 7:52AM
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I guess paralleling these low cost transformers could set up feedback instability of the kind described by higher order differential equations and since the transformers were not designed to receive much if any of that complex wave coming in the output side it might trigger a little burst that pops something in one of the two transformers. In layman's terms. After all, power being non-linear and impedance being an (i)squared kind of thing, having a greater-than-one power source array can only be modeled by complex math not obvious without a lot of study of the separate components. Or, perhaps the power sources don't hurt each other.

    Bookmark   December 24, 2010 at 8:45AM
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"I guess paralleling these low cost transformers could set up feedback instability..."

Transformers are passive devices with no feedback.

The problem that occurs is that they do NOT match well enough to load share (like powder distribution transformers).

    Bookmark   December 24, 2010 at 12:44PM
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Hi all,
I'm curious about this thread, I actually build 12V lighting arrays, but have not experienced "yellowing" except from poor contacts (or poorly tightened contact screws). Most of our problems come from inadequat transformers, most of the cheap ones have tricky heat protection, generaly badly calibrated, or simply tempermental. The recent (sic) "electronic" transformers are a bit better for low power needs. IKEA type modules are not too good, they will generaly melt/burn if you even slightly overload them.
Two comments:
- 12V linkage is misleading, heavy section is needed to overcome resistance at even short distances
- From experience, largely oversized transformers, well ventilated, seem to work best.

We have tried linking several (3) "electronic" transformers on small installations (3 x 75W) on 50% charge, with mixed success. One is stable, the other never functioned, even though the tranfos were (branded) identical.

If one of you had an idea on how to couple those little things, I would be glad to test it out and post the results;

    Bookmark   March 13, 2011 at 3:05PM
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"'electronic' transformers" are typically AC to DC switching power supplies.

They can be full rectifying supplies or 'buck' type switchers.

Paralleling them is a nightmare unless they have been designed to allow synchronization of the switching frequency (often in the 10s of kHz to allow for much smaller transformers and inductors) and fine adjustment of the output voltage (or they feed into each other and overheat their output stages).

A lightly loaded induction transformer has a lower peak field value in the core and should be well away from saturation of the iron core, improving efficiency and performance.
Once the core actually saturates performance drops very quickly and heat builds up quickly.

    Bookmark   March 14, 2011 at 9:46AM
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Ron Natalie

Besides, even if they would function in parallel...it would do SQUAT about the voltage drop caused by the resistance of the long line. About the only voltage drop it would correct would be if you somehow overloaded the transformer's capacity itself.

    Bookmark   March 14, 2011 at 12:43PM
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I have seen some ridiculous installation of LV lights lately.

Systems wit transformers rated over 400 V-A, meaning the 12 V output is capable of over 33 amps.

The huge wires required to limit voltage drop (and low voltage circuits are FAR more sensitive to voltage drop than 120 V circuits) is a PITA to work with, splice, and then feed to multiple loads.

I have installed a good number of 60 V-A power supplies that can each support 3 20 W 12 V bulbs.
Unless you go wild extending the leads on the lights you cannot tell if one light is shorter or longer than the others.

They have a common feed form the power supply and then are all fed from a small junction block.

You can sink a 120 V receptacle through the back of an upper cabinet, drill a hole for the plug, and put the small power supply, terminal block, and the three pucks under the cabinet easily.
The only complaint I have is the single mounint screw tab for the little power supply and the double sided tape they try to use to mount them.

A little bit of aluminum flashing makes a much better strap to hold the things up.

There are even double power supply units with two separate supply circuits on a common 120 V feed but with separate outputs.

Each can supply three 20 W lights for a total of six from a single 120 V feed.
They are useful for longer runs of cabinets.

Some manufacturers have listings that allow hard wiring the power supplies, others must keep their plug.
The units listed for hard wiring take very little room in an upper cabinet.
You can recess the box into the cabinet back (and wall if needed) just far enough to allow access to the bottom for the feed to the power supply from a hole in the bottom of the cabinet.

By having low voltage,low power (60 V-A) and EXPOSED wiring the wiring rules are not nearly as stringent.

    Bookmark   March 14, 2011 at 4:01PM
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