# Verification



## badnecros (Sep 30, 2008)

Hey guys! im building a home LED lighting, and using a power supply from a lap top.
the LED tape is rated at 12V, 2A, 14.5W/M

I am going to be using a power supply from a Lap top rated at 18.5V, 4.9A, 90Ws

I have some basics in electronics, so im trying to figure this out.

so i need to step down 6.5V, as well as 2.9 amps is there anything i can do with basic resistor work on a PCB?

##Edit##
Something seems off with my math. 
V=IR
6.5V=2A X R
R=6.5V/2A
R=3.25 Ohms?


ummm im thinking of using a stepdown. but that will lower a %. perhaps a 12V regulator? though then ill need to lower the amprage? 
or could my 12V LEDS just work on 18.5V?


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## badnecros (Sep 30, 2008)

or, i could split the Current, so i really just NEED to lower the voltage.


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## gcavan (Aug 13, 2009)

Using a voltage divider as you are proposing is a possible solution, but very inefficient. Fully a third of the power used, or about 15 watts, will be radiated as heat by the resistor R. The value for R is only valid when exactly 2 amps are being pulled through it. 

A much better (and still relatively inexpensive) method would be to use a DC-DC converter.


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## badnecros (Sep 30, 2008)

so this would be 18.5 converted to 12V?


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## Done_Fishin (Oct 10, 2006)

Current is not your problem .. unless the current required is more than is available. 

gcavan's circuit for DC to DC conversion is good but still wasteful. The bypass transistor will need to be mounted on a heatsink to dissipate the heat!
A switch-mode power supply would be more efficient. Why not try to pick up a laptop supply that gives out 12 Volts. Some old monitors use 12Volt external power supplies too.


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## badnecros (Sep 30, 2008)

id like to thank you all for your assistance. ive thrown in the towel, and just baught an AC/DC converter. that puts out 12 V. However, it also puts out 8.3A. the transformer has 2 12V outputs. 

ive linked up 4 stips of LEDs. (2 in parallel in each junction)

howeve, now the LED stips flash the light. the transformer itself constantly reads 12V, however, when i read the voltage over the nodes where it splits into two, it fluctuates between 0-12 volts Rapidly. (about 1.5sec intervals)


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## badnecros (Sep 30, 2008)




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## Done_Fishin (Oct 10, 2006)

1 strip of leds requires 12V @ 2A, 14.5W/M 

4 feet = 1.22m 

2A * 1.22 = 2.44A 

4 strips @ 2,44 A= 9.76 A

14.5 Watts * 1.22 = 17.69 watts

4 strips @ 17.69 = 70.76 watts

I would expect the power supply to be tripping.

Try doing just one strip at a time to see if all work independently correctly. Then add one at a time to see if at least 3 work OK together.


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## badnecros (Sep 30, 2008)

thanks for the advice. however, just one 4ft strip has the same issue.


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## badnecros (Sep 30, 2008)

ive recently switched it to 1X 4FT (2.44A)
2X 3FT (1.83A)
and 1X 2FT (1.22A)
total of 5.49 A

Which is fully within the threshhold of the power supply, and it is having the same issue.


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## Done_Fishin (Oct 10, 2006)

Are you saying that ANY single strip you are using goes into flash mode ?? Can you double check that (using any part numbers on the strips) that the strips are supposed to be permanently on and are not a type designed to flash ?? 

Do you have access to a 12V 60W car headlamp bulb (draws 5A) that you can use as a load for the power supply ?? Check Volts & Amps being drawn by the supply. If you have an oscilloscope you could also scope the voltage to see if either the LED strip or 60W bulb causes the power supply to trip.


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## badnecros (Sep 30, 2008)

Done_Fishin said:


> Are you saying that ANY single strip you are using goes into flash mode ?? Can you double check that (using any part numbers on the strips) that the strips are supposed to be permanently on and are not a type designed to flash ??
> 
> Do you have access to a 12V 60W car headlamp bulb (draws 5A) that you can use as a load for the power supply ?? Check Volts & Amps being drawn by the supply. If you have an oscilloscope you could also scope the voltage to see if either the LED strip or 60W bulb causes the power supply to trip.


ive got the probe, but my digital scope is busted up. so the oscilloscope is out of the question.

i used a 18W 12V power supply to test a 1M strip at 1600mA, and it stays solid.

i tested the voltage over the PS without the circuit hooked up, and it reads 12.1V

i tested the voltage over the PS with the cirvuit hooked up, it fluctuates.


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## badnecros (Sep 30, 2008)

now if im not mistaken, Kirchoffs law says that all 8.3A has to travel through my circuit. 

this means that the stips are taking as folows;
4FT 2.739A
3FT 2.075A
3FT 2.075A
2FT 1.383A

if this is correct, then could i need to add resistance in parallel to the circuit to drop out 2.81 A to make it the required 5. 49A?


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## badnecros (Sep 30, 2008)

This here is exacly the wiring ive done to it. if there is no issues with the wiring, and the voltage is right, is the anything i can do?

(connected + to + and - to -, incase people tried to suggest that)


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## Done_Fishin (Oct 10, 2006)

Since you say that even with just a single length connected the LEDs flash when connected to the 8 amp supply AND that using an 18W supply the LEDs are stable .. then there is a problem with the power supply .. was it a new supply or second hand ?? If it's second hand you may need to open it up and try to repair it by replacing the electrolytic capacitors (notorious for causing this type of problem in switch mode supplies)

Placing a resistor in line won't help when you have already told us that it fluctuates even with only one strip connected!


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## badnecros (Sep 30, 2008)

Done_Fishin said:


> Since you say that even with just a single length connected the LEDs flash when connected to the 8 amp supply AND that using an 18W supply the LEDs are stable .. then there is a problem with the power supply .. was it a new supply or second hand ?? If it's second hand you may need to open it up and try to repair it by replacing the electrolytic capacitors (notorious for causing this type of problem in switch mode supplies)
> 
> Placing a resistor in line won't help when you have already told us that it fluctuates even with only one strip connected!


Thank you for your help. this is the kind of assistance i needed. it was a new Power supply, however, i got it from SAYAL electronics, they build their own and sell them. i am going to try to hook them up to an XBOX 360 power supply, (8A, 12V PS) so if that works, then i am hoofing it an hour to SAYAL to replace it.

(Also, could you ensure me that my wiring is correct?)


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## Done_Fishin (Oct 10, 2006)

The diagram you show looks correct, however I would need to be on top of the circuit to know if you have wired it up the way you have drawn it, not to mention check out the power supply you are using to see if you can use any positive output with any negative output. I am assuming that the output pins are from a common source and not two separate 12volt supplies.


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## badnecros (Sep 30, 2008)

Done_Fishin said:


> The diagram you show looks correct, however I would need to be on top of the circuit to know if you have wired it up the way you have drawn it, not to mention check out the power supply you are using to see if you can use any positive output with any negative output. I am assuming that the output pins are from a common source and not two separate 12volt supplies.


Correct. the Voltage reads 12 Volts from any combination of positive and neutral leads. however, as soon as i attach a wire to the outputs, the voltage level becomes sparratic. :facepalm:


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## Vegassparky (Nov 24, 2013)

Can you post a picture of your LED strips, and power supply? I do a lot of LED lighting work. I've never seen the anomaly you describe.


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## Done_Fishin (Oct 10, 2006)

Unless something pretty weird is going on your power supply is faulty. It wouldn't be the first time that I have seen a brand new power supply (especially direct from the maker) that didn't function properly.

you have checked at least one of your lighting strips on an alternative good supply where it is stable.

The new power supply you are trying to use immediately trips when you attach any load. 

with the exception that possibly one of those pair of wires is for sensing and not for supply output, the power supply has to have a problem.

to prove as to whether there is a sense pair, wire the two positives together and the two negatives together then feed to one or all of your strips. 

sense pairs are used when voltage stability is critical at the load and send feedback at the end of the wire to the controlling circuitry to ensure that the voltage at the far end of the wiring is correct. This compensates for any voltage losses due to wire resistance and high currents. Remember that if you have a long wire run (speaking theoretically now) that has 0.5 ohm resistance from start to finish and your load wants 10Amps then your voltage loss along the wires will be 5 Volts! Sense wires ensure that the Voltage at the end of the wire is correct by giving enough voltage at the output that the voltage at the load is correct.


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