Anybody know about voltage reduction, DC?

[cmac2012]

One of my best clients, a personal training couple - trainers to the rich and famous in Silicon Valley - regularly entertain clients at their Woodside estate. I recently installed 4 outdoor type globe light thingies with an exposed copper wire with small LEDs every couple of inches wrapped around it. Each unit came with a 3.5 volt transformer, two of which have gone bad in the 6 months since purchase, one of these:



3.5 volts, .45A

You would think something like this, higher quality than the Ting Shen unit that came with it, could be purchased. I can't even find the identical unit. Most everything I find is 12v.

I did today find this item:

http://tinyurl.com/ycdd67cx

It's a 5v, 3.5A. My understanding is that the larger amp figure is not a problem, it's the voltage that will determine proper drive of the LEDs. The greater amps should allow me to power two, or even all 4 with no problem. I think.

The unit in the photo is different, stock company photo probably. The one advertised is APV-25-5. 25 watts, 5v. Electronics are hard to figure. They say 3.5A; 5 x 3.5 = 17.5, not 25.

I cut the lines between the lights and the jack to the dead transformer and wired the two together in parallel and hooked a 12v transformer to them, got real bright, unhooked quickly. I just wanted to see if there was any point in hunting for the right setup, if the LEDs were still good.

I've tried to learn about voltage reduction, 'buck converter' I gather they are called.

I found this one today, looks promising:

DC-DC Adjustable Buck Voltage Converter Stabilizer Step Down Voltage Reducer | eBay

I had earlier tried this item:

https://www.amazon.com/gp/product/B00C0KL1OM/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

with a 12v transformer, the same unit I had used to rewire some can lights to deal with dimmable LEDs for the same client. Absolutely no luck.

I don't quite get it, that transformer requires a resistor before it generates current. Perhaps that's why it didn't work.

I get that no one is going to be able to impart a degree in elec. engineering in this forum.

OTOH, I would sure like to figure how to make this work. These people pay me well because I make everything work. I really want to make this work. The lights do give a cool effect to a gazebo in their back yard, sort of the semi-formal back yard dining spot.

[engatwork]

My co worker has a masters in Electrical Engineering. I will run it by him Monday and let you know what I find out. I don't know how many lites you put out but would it be less headache if you replaced system with something of higher quality? I am of the opinion if you run the LEDs with the higher amp power supply they may not last as long as if correct amperage was used.

[Mxfrank]

Just get a 3.5v zener diode with an appropriate current rating from an electronics house (like digikey dot com). Put it in line on either side of the circuit, and use a transformer that supplies at least 5v.

[cmac2012]

I sort of thought that the right diode would do the trick. But whenever I researched it to find the right one to get I got confused. I'll look into that one.

[cmac2012]

Originally Posted by engatwork

My co worker has a masters in Electrical Engineering. I will run it by him Monday and let you know what I find out. I don't know how many lites you put out but would it be less headache if you replaced system with something of higher quality? I am of the opinion if you run the LEDs with the higher amp power supply they may not last as long as if correct amperage was used.

I could be wrong but I'm under the impression that a resistor will draw an amperage only as high as it has room for. For example, in 120 AC, a 20 watt bulb is dim, a 200 watt bulb is bright. Watts equals volts times amps, so the former is drawing .167 amps, the latter 1.67. If you plug a 13 amp skilsaw into the same outlet you'll draw, wait for it, 13 amps, if the voltage is 120, the wattage is 1560.

I can't tell you amps flow but if cut a lamp cord when it's plugged in, you'll see a bright light and cut a divot in your knife. Of course you know this, not trying to insult your intelligence.

But in all of this, the voltage is consistent. The way I learned, the best analogy I've heard, is the electrical current is slightly similar to water current. Volts is pressure, amps is volume.

These things came two to a box at Costco, one was about 14" in dia, the other 12", the smaller shipped inside the larger. The construction is kind of odd, but it's sort of lean, trimmed down, not a lot of stuff in the way of the lights. Then asked me to look for some more so they could move all four to the gazebo. I found some but they didn't like them, I didn't either really. Then I discovered Costco still had the same unit. The ones we have consist of a bunch of tiny LEDs strung on a braided copper wire. No actual bulb, no housing, just tiny LEDs seemingly floating in the air.

[Mxfrank]

I'll tell you more than you want to know. There is a voltage drop across every resistance. Let's say the load is 50 ohms, just for fun. You then add a 50 ohm resistor in series. What is the current draw if you use a 12v supply? I= V/R= 12/(50+50) = .12A

We know that .12A flows through the entire circuit, so what is the voltage drop across each resistance? V=IxR=.12x50=6V.

So to figure out what size resistor to use, you need to know the resistance of the LED strip and the voltage you will be using. If you add or subtract LED's or change the supply voltage, you will have to recalculate and swap resistors. It's more algebra than I care to step through.

A zener diode always passes the same voltage, regardless of load. It's the easy solution.

[cmac2012]

Good food for thought. I've read about voltage drop, it's not something I'm well versed in.

I decided to order a unit I put up a link for above.

DC-DC Adjustable Buck Voltage Converter Stabilizer Step Down Voltage Reducer | eBay

For $11 seems a reasonable gamble. The part looks more involved in what you'd think you could get for that sum.

My thought is to eventually have a higher quality transformer for all four. If the units themselves, the LEDs in total, are high enough quality to last significantly longer than the transformers that came with them could be a reasonable investment. Would be good to know about zener diodes however. I'll look into it. I'll see if I can determine the resistance of each globe. Would be a good learning process.

This client has a charming but old house and they give me a lot of work. Plus I really like them. He's 63, she's 58 and they are both gorgeous. More than skin deep in this case. They work at giving good party for their clients, the cool backyard is a big part of it.

[Mxfrank]

Another option is to wire all four lamps in series to a 14V power source. If you do that, you don't need any external components.

[cmac2012]

Originally Posted by Mxfrank

Another option is to wire all four lamps in series to a 14V power source. If you do that, you don't need any external components.

Hey now. I have that 12v transformer that I know works. I used two of the same brand in renewing their kitchen can lights. I suspect 3 volts would push them.

[Mxfrank]

Just wire them up in series, and done:

|-----------O--------O--------O--------O------|
|---------------Transformer---------------------|

[mytimeyet]

Here is a site that can figure out what resistors to add to a circuit to light up LEDs from any DC power supply.
Alan Parekh's Electronic Projects - LED Resistor Calculator

Actually bare LEDs with proper resisters in series will work with AC voltage as well, they will light on the FWD side of the AC sine wave. @ 60Hz this is hard for human eyes to detect. Not that I would recommend plugging a low voltage wire system into the wall... Just that a cheap landscape/AirCon/doorbell transformer without rectifier or filter will work fine.

[cmac2012]

So far I've avoided cutting up the wires on the two that are still hooked to working transformers. There are proprietary jacks connecting everything and the wires are pretty light. Just stripping a half-inch of insulation off is delicate work. But I'm thinking that I'm dreaming that these cheap ass transformers are going to last much longer. 2 down, 2 to go.

[cmac2012]

Originally Posted by mytimeyet

Here is a site that can figure out what resistors to add to a circuit to light up LEDs from any DC power supply.
Alan Parekh's Electronic Projects - LED Resistor Calculator

Actually bare LEDs with proper resisters in series will work with AC voltage as well, they will light on the FWD side of the AC sine wave. @ 60Hz this is hard for human eyes to detect. Not that I would recommend plugging a low voltage wire system into the wall... Just that a cheap landscape/AirCon/doorbell transformer without rectifier or filter will work fine.

Looks like a good resource. This stuff is tricky. Perusing that page leads me to wonder if LEDs react differently to more current (I mean amps, not volts) than is needed than do traditional resistors. And perhaps those rules are different for AC v. DC.

[ah-kay]

LED lights are driven by current, not so much by voltage. The LED is only interested in the voltage across itself, not the voltage across the supply plus the resister. The problem is that you cannot use a high voltage and try to drop the voltage with a resister. The heat generated across the resister would be impractical to dissipate or expensive high wattage resister. You can safely use 6V, 9V, 12V AC->DC transformer as long as you scale the resister. I probably would not use any transformer higher than 12V.