8' LED Star, 750 total diodes, series - parallel 12v. Power supply/driver issue

Dustin

Firstly your 12v strip LED's are designed to work with a 12v supply, and as such has the necessary resistor to limit the current to each 3 LED group.

A LED driver circuit performs the same as the resistor, but adjusts itself according to the voltage.

In your application, it would do nothing.

Batteries

Batteries are supplied in various voltages and have a capacity that is in Amps per Hour.

Your 12v 2AH batteries will supply 1 Amp for 2 hours, 0.5 amps for 4 hours or 4 Amps for 0.5 hours while remaing close to the nominal 12v.

Dimmer

Your dimmer simply reduces the 120v down to something less.

Reducing the 120v to 90-100v should not affect the output voltage of 12v. (otherwise it is a poor design)

The power supply converts the 120v into 12v at 2Amps which is 24watts.

There are always losses, hence the 43w observed.

I understand these devices are somewhat inaccurate measuring low currents, and 43w at 120v is approx 0.36Amps.

How much current

The led strip should state what the current is and could be per metre or similar.

This information might be on the suppliers site or maybe there are some other specs ...

Lack of meaningful information is the price you pay for cheap suppliers.

It seems that it needs more than 2Amps and is happy with a 12v 4A power supply.

If the supply is not getting warm and it runs happily, I say enjoy it ...

but I didn't want to "over current".

Common thought by many people ...

The circuit will draw whatever it needs, much like a sprinkler on the end of a hose will use less than the water mains can supply.

You only get into trouble when you try to supply more voltage than it was designed for.

Mark

Thanks for the great reply Mark

You are correct about the 'cheap' product ($4.99 each including shipping) and yes I forgot to mention the built in resistors.

Could you describe what would likely occur I there were no resistors with this same scenario, using a 12V 2ah battery and a 12V 4A power supply.

I am still trying to understand this over current issue in a common scenario.

A somewhat related question is about the 12v power supplys. I understand the concept to 'transform' 120AC to 12VDC it uses coils of wire and am not aware of any other way. I believe the two I have are likely built the same way, but from the weight and size they seem vastly different The 2amp one was a typical small and very light wall type unit, and the 4amp is an laptop power supply much larger and heavier requiring the standard 3 prong 'computer' cord.

Since they both used the exact same watts 43 (According to Kill-a-watt) and checking with Multi meter they both put out almost the exact same voltage (which surprised me) the 2a was around 12.15v and the 4a was  closer to12.5v. Where does the extra 'Current' come from? And could I use two of the 2amp 12v power supplys in parallel to achieve the same 4a the large one puts out?

Thanks again

Dustin

Thanks for the great reply Mark

You are correct about the 'cheap' product ($4.99 each including shipping) and yes I forgot to mention the built in resistors.

Could you describe what would likely occur I there were no resistors with this same scenario, using a 12V 2ah battery and a 12V 4A power supply.

I am still trying to understand this over current issue in a common scenario.

A somewhat related question is about the 12v power supplys. I understand the concept to 'transform' 120AC to 12VDC it uses coils of wire and am not aware of any other way. I believe the two I have are likely built the same way, but from the weight and size they seem vastly different The 2amp one was a typical small and very light wall type unit, and the 4amp is an laptop power supply much larger and heavier requiring the standard 3 prong 'computer' cord.

Since they both used the exact same watts 43 (According to Kill-a-watt) and checking with Multi meter they both put out almost the exact same voltage (which surprised me) the 2a was around 12.15v and the 4a was  closer to12.5v. Where does the extra 'Current' come from? And could I use two of the 2amp 12v power supplys in parallel to achieve the same 4a the large one puts out?

Thanks again

Dustin

Dustin

what would likely occur I there were no resistors with this same scenario

The LED's would go really bright for a millisecond or less and possibly smoke might rise.

You would need to have some form of current limiting BUT since you have 1-200 of these in parallel it is impossible.

'Overcurrent' is a term often used by people who don't understand.

In reality you are operating the device outside of it's design, rather like overloading a car or bike.

LED's are designed to operate at a certain current and at this current they emit x amount of light.

Assume a LED has the following specs x light at 20mA. The voltage drop across the LED at 20mA is 3.7v.

If you connect three in series, it would be 3 x 3.7 = 11.1v.

If you want to use a 12v supply then you need a restor that will drop 12 -11.1 = 0.9v at 20mA.

Ohms law ( http://www.rmcybernetics.com/science/cybernetics/electronics_volts_amps_watts.htm  ) is R= E/I or 0.9/ .02 = 45 ohms.

Just to check the resistor power rating W= E x I or 0.9 x 0.02 = 0.018 watts

I understand the concept to 'transform' 120AC to 12VDC it uses coils of wire and am not aware of any other way

The use of a transformer inside power supplies is old and not really used in modern supplies.

They are heavy, costly to make and limited to a single mains voltage (120/230).

Many devices simply rectify the incoming mains and then the DC voltage (200-400v)  is converted down to 12v or any other voltage using switchmode technology.

It is easy for you to think of them as a switching power supply.

they both put out almost the exact same voltage

They are both designed to be a 12v supply ....

If you had the LED's connected, I think you will find very different readings.

I suspect the 2A one is much less than 12v.

could I use two of the 2amp 12v power supplys in parallel

A $64 question.

It depends on the design, since they might try to fight each other if the outputs were simply wired together.

You would need to match them exactly so that one wasn't supplying 2.1 Amps and the other 1.9 Amps, and then there is the issue of if the voltage is the same when they are on load.

You could use a 2Amp diode in series at the output, but then you still have the issue of both supplies being exactly the same as they age, get warm, etc.

It is far easier to use the 4A one and be sure it works correctly.

Where does the extra 'Current' come from

In the internal design

It is designed to provide 4A and the components choosen to alow this.

Sit back and enjoy your star, you have it working on a 4A supply that by now you will have measured with the LED's on, and it will be at 12-12.5v.

Mark

Mark

Once again, your detailed information is very appreciated. My home is a former church, it is 3 stories and over 4,400sq.ft.  From my first 150w replacement Lights of America circle lights I purchased in 1996, I have been trying to conserve energy and way possible throughout my home. With lighting always being something I was very interested in. I realize now I should have been asking these types of questions years ago when I first started purchasing LED's, Instead I just kept buying.

The left is mostly 12v and right mostly 110v, the bag is all no longer working units both 12v and 110.

With few exceptions, my 16 exterior lamps and 87 interior fixtures & lamps are all now quality 120v LED's (Phillips,sylvania,etc) spending roughly $500.

So my last question. The small flat faced bulbs above with the exposed domed top led's are all replacements that I have never even used. The manufacturer sent me replacements (8 total) for identical ones that burned out quickly (1-3months). They are rated at 110V, which is common I believe even though 120v supply is normal for the U.S. With everything you have said,

Would you lean more toward the failure was due to inferior product or you think my 122-123v actual power at socket is to blame for the short life? The company does make a 120v(rated) version of the same product, they indicated though they could not issue a different (the 120v vs the 110v I bought) product as a replacement, even if I paid the small difference in price (+$0.17).

You are very helpful (& correct)

Dustin\Sorry I didn't get this in my inbox , and I've been out of touch last couple of days.

I'm glad we've been able to help with some information, and some guidance.

Would you lean more toward the failure was due to inferior product

A very good observation ... they are cheaply made and quite likely to be a factor.

you think my 122-123v actual power at socket is to blame for the short life

The nominal voltage is 120v but they have a tolerance which is usually to allow for voltage drops in the feed system

123v is actually 11.8% greater than the design voltage of 110, so yes it might have bearing as well.

I would talk with your energy supplier and ask what is the voltage you should be getting at your supply.

If you are getting 123 at the socket and assumming your meter is accurate, then it seems a little higher than I would expect.

You might want to purchase 120v versions in future.

Poor design

Many of the LED modules rely on each part being exactly the same, and unless they test indivual items, they won't be.

If one LED is slightly higher in internal resistance then it will have a larger voltage drop, and conversely if several LED's are lower resistance, the resistor and others need to drop a higher voltage.

if this balance isn;t maintained then the LED string will draw a higher current and could have a shorter life.

I suggest pulling apart the broken ones and identify how they are constructed ie how many in series and parallel, and it might give you an idea of the design.

IMO LED lighting is not there yet.

The costs and the life expectancy aren't enough to make me want to change.

I have dicroic lamps with electronic transformers so they soft start and are always the right voltage.

I used a good quality brand direct from the electical supplier at NZ$6 each, and our total lighting load for a room is generally 90-150w in a 2100sq ft house.

In the 12 years I've probably replaced 12-15 lamps due to exceeding their hours, rather than a failure.

So in my case I find it hard to justify the expense with LED's.

Mark