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I have a dead outdoor lighting transformer. Not realizing it was VAC I looked on Amazon for a replacement. The bulbs are what I thought to be 12vdc. I received the new outdoor lighting transformer and it didn't appear to work, nothing lit up. Thinking it was a 12vdc supply that died, I hooked up my battery charger to the various view feeds and voila, the lights came on. So I continued to think DC. So my conclusion was that the unit I purchased was DOA. Digging a little deeper I found that outdoor lighting power supplies are 120vac to 12vac, not 12vdc. I get the advantage of AC over DC for range. So now I'm confused, why are the lights working with a 12vdc power supply? Perhaps the new unit I purchased was DOA. Lastly, dumb question, is it possible to power 12vdc bulbs with a 12vac power supply? Thanks

  • You had the bulbs working with 12VDC, so what does that tell you? – Solar Mike Jul 9 at 16:56
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    (1) Incandescent bulbs don't care whether the power source is 12VAC or 12 VDC. What kind of bulbs to you have? (2) The advantage of AC over DC does not apply to this situation, in which runs are quite short and no step-up/step-down is being used. (3) Some supplies require a load to be meaningfully measured. – Phil Freedenberg Jul 9 at 16:58
  • @PhilFreedenberg - I had thought that to be the case. But didn't realize that LED would. I know the original transformer is/was bad, it tested no output on either AC or DC. And the replacement I got had no output on either v. But I did replace "some" of the bulbs with LED and of course worked w/ DC. – hackerkatt Jul 9 at 18:17
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Are they incandescent bulbs? (IE: Not LED). Incandescent bulbs don't care about A/C or D/C. So yes, you can power the bulbs with a DC power supply.

Regarding A/C being better than D/C for range: again, doesn't matter. Wire size, distance and power draw determine voltage drop. A/C and D/C voltage drop would be about the same, all else being equal.

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  • Both. Originally incan, but I replaced some of the burnt out bulbs with LED. So the 12vdc worked. The supply I got from Amazon didn't have any output AC or DC so I wasn't sure what I was looking at – hackerkatt Jul 9 at 18:20
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Usually landscaping lights (incandescent and retrofit LED bulbs) work with AC or DC. If they were limited to DC, then polarity could matter (if the manufacturer didn't bother to put a 2 cent bridge rectifier in each lamp).

So either your replacement power unit is dead or your lamps require a certain polarity and the polarity is swapped. If you have a multimeter you can quickly check the output voltage, if you don't just swap + and - at the output and see what happens.

AC is not really any better for "range" (okay, if you're talking EHT lines running hundreds of miles/km then yes you can step it up/down easily), but AC transformers are very simple and almost never fail unless they are physically damaged or allowed to get wet. It's more likely the electromechanical timer has failed. Electronic power supplies have a limited life and will conk out eventually. They are light to ship and can be made fairly cheaply, despite their much greater complexity, at least they can be in Asia.

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  • Interesting... If I had had a working 12vac supply, I might have answered my own question. No idea if the manufacturer included a bridge rectifier, any markings are worn off. I know my replacement unit from Amazon was DOA, had great ratings, but it can happen. Not fully versed in current, I've worked with AC a lot. I knew to test the old supply for AC and DC. I did check polarity when I had the 12v battery charger connected to test the circuits. And I just had in my mind that the old transformer was DC ( i.e. low voltage) and not 12vac. – hackerkatt Jul 9 at 18:31
  • Never heard of 12vac for landscaping. But I'm not in the business. Runs are 100ft at max. – hackerkatt Jul 9 at 18:31
  • Here is an example of low voltage transformers like the "Pro series" one we have. Although it is not specified, all voltages are AC RMS volts. 600W would be a huge rating for LED lights, not so much with old-fashioned incandescents. – Spehro Pefhany Jul 9 at 18:36
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The War of the Currents and AC/DC

The whole point of the “War of the Currents” was that AC could be run through a Transformer to easily step it up to a higher voltage. The higher voltage could be transmitted with far less transmission loss.

  • The thing that steps it up/down is called a transformer, and it only works on AC. Therefore, that DC power supply you got is not a transformer. Also, transformers are made of copper and iron, and are quite heavy. You probably have a switching power supply. Different thing.

If you’re at a specific voltage, AC doesn’t help at all. You have exactly the same voltage drop. We don’t even bother calculating it differently, honestly.

The landscape lights do not care if they are AC vs DC.

Your battery charger test reveals the problem

Since the lights work on the battery charger (and it may be overloading the battery charger but most of them tolerate it for short times), we know the issue is the old transformer and new power supply.

The power supply is either too small for the job, or is defective.

Read below, and if you were ripped off with non-UL-Listed cheap junk, make them pay. Return it for sure! If you’ve never done Amazon returns, there are several easy ways, including any UPS Store will take them even unboxed - just the loose item and the return slip off your phone, which they scan.

Don’t buy mains electrical gear off Amazon

Well, if it says “Ships from and sold by Amazon.com” it’s probably alright, but there have been some counterfeiting problems on popular items.

If it says “Ships from and sold by <not Amazon>“, then don’t attach it to AC mains power!

If it says “Sold by <not Amazon> and Fulfilled by Amazon” [Prime], then don’t attach it to AC mains power!

That stuff is a sea of cheap Cheese junk, and it’s not about American jobs, it’s the fact that direct mail bypasses the product-quality and safety laws that make it illegal for Home Depot to sell that crud at retail. It’s never been inspected by an independent testing lab like Underwriter’s Laboratories (UL), and it shortcuts all the design standards that keep us safe (e.g. the UL White Book). A million things, like using the wrong plastic in the case, so it won’t self-extinguish and will accelerate a fire... or fill the house with toxic smoke so you’re dead before you even wake up.

If you see “CE” as the major marking, that means they can’t qualify for a proper UL listing or other NRTL. CCC, RoHS, FCC, these are all junk markings they put on stuff to confuse you. There’s a list of trusted NRTLs that is readily available.

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  • RoHS, FCC, these are all junk markings they put on stuff to confuse you. I generally agree. Though, for example, FCC if real does mean something important (on a device likely to cause interference if not designed properly). It just isn't the most important thing as radio frequency interference, in most cases, is not a fatal error. – manassehkatz-Moving 2 Codidact Jul 10 at 14:02
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If your bulbs cared about polarity, they would only be working half the time on AC. Which isn't impossible in a very cheap lighting system, but is rather unlikely.

If you replace with DC, then you need to make very sure everything is moisture proof, as with DC the direction of ion flow for electrolytic corrosion is always in one direction, so one pin of a damp connector will erode, whereas for AC the ions flow away then back again 50 times a second, so the effect is much less. Hence AC is preferred in outdoor applications that have to be out in the weather for many years.

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    If the output of a DC supply is ground isolated, I think that attaching the positive wire of the supply to ground would help reduce corrosion, while attaching the negative wire would substantially increase it. Telephone wires are biased below ground voltage for that reason. – supercat Jul 10 at 13:57

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