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Power supply volt/amp question? Answered


Hi,

My power supply is ac 110v to dc14V 700ma and I want to connect in parallel 2 strips of 4 led (350ma - 3.2v each) 

Total voltage per strip 12.8v with 350ma each.

I would like to power these leds at 2.5v each (total 10v)  and at around 250ma each.

I installed a buck converter at the output of the power supply to reduce voltage at 10v.

Will the amps also drop as voltage is reduced or will it stay at 350ma on each strip? 

If it does not drop, how could I reduce amps to about 250-275ma?

Thanks

13 Replies

user
Learning Exp (author)2014-10-18

Oh yea, do I need diodes to control flow or to pass through solar cells for in case there is no sun for that particular solar cell?

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Learning Exp (author)2014-10-18

I hope you figure it out.

I also need help with LEDs. (2 Part Question- Answer what you can)

I got a reel of 5 Meters


SMD 3528 Epistar LEDs

Power Source: 12 Volts DC

Wattage: 60 Watts

Power Consumption (W/m):18 Watts / Meter

LED Chip Model: SMD 5630

LEDs Number/M: 60 pcs / Meter

How many Amps needed in a charge controller?

Solar Cells

Average Power (Watts): 2.6 Wp

Average Current (Amps): 5.2 Imax

Average Voltage (Volts): 0.5 Vmax

Do I need to wire the solar cells 3 in series to reach 15.6 volts (Calculated loss so not 12v) and then parallel to reach 60 watts for the LEDs?(I plan to have around 70 watts just for incase and rely on the charge controller to lower voltage to 12V.)
(I am using a standard car battery. An old one that has lost a lot of juice, but I tested the LEDs on it and it still works.)
Is this okay? Thank you all for your help!!! :o)

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steveastrouk (author)2014-09-24

RIGHT. You need a constant current driver, not a constant voltage. The best way would be to use a circuit like this. Calculate the resistor needed, and calculate the power too, using P=I^2 x R. You need a supply that can provide about 2+IxR more than the worst case for your LEDs

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steveastrouk (author)2014-09-23

Do these strips have on-board resistors ?That's why I want to see the exact spec of what you have, to advise you on turning them down.

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Lion_Heart (author)steveastrouk2014-09-23

http://www.ebay.com/itm/191217602967?_trksid=p2059210.m2749.l2649&var=490359859358&ssPageName=STRK%3AMEBIDX%3AIT

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Lion_Heart (author)steveastrouk2014-09-23

These are just 1w power leds that I've soldered together to make 2 strips of 4.

I only have what I wrote for the specs. They are sold as is with no more info.

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Downunder35m (author)Lion_Heart2014-09-23

I assume you are talking about cree LED's?

Those flat star shaped one - or the standard type with two legs?

Those LED's need a constant current supply as otherwise they simply burn out as the amps go too high.

Check Ebay or your favourite electronics supply for a suitable LED driver.

Best option would be to use one for each LED and to connect all drivers to the power supply.

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user
Downunder35m (author)2014-09-22

Your power supply will be operating at the total limit anyway, better option is to get a suitable power supply.
There is a good chance your voltage and amps will break down even further than what you want, soon after the power supply will fail unless you only use the LED's for short periods of time.

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Lion_Heart (author)Downunder35m2014-09-23

Thanks for the reply so what would you suggest as power supply?

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Downunder35m (author)Lion_Heart2014-09-23

Get a 12V 1 or 1.5A power supply.
Often you can get them cheap as chargers for some tablets or other gaming equippment or old cordless phones...

It is simple math really, two times 350 is 700mA, so that the power supply is not constantly on the limit you go one or two sizes up for the supply.

Does not matter if you take a 3A one as long as it has 12V DC on the output.

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Lion_Heart (author)Downunder35m2014-09-23

Ok so how would you reduce the amps to the leds as I want to run them at 250ma not to their full capacity of 350ma?

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steveastrouk (author)2014-09-22

What's the exact spec of the LED strips ? Got a link?

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Lion_Heart (author)steveastrouk2014-09-23

Leds are 1w 3.2v 350ma each connected in serie so each strips are 12.8v and connected in parallel on the power supply.

The plan is to run both strips at 10v and +-250ma to extend the life of the leds as they will be on a 12hrs ON-OFF schedule.

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