Parallel leds driver problems

Edgierprawn

Joined May 14, 2014
6
Hi all

I have done extensive previous research into this but before I go ahead with my project I want to clear a few things up. I have attached a picture of my proposed design.

My questions relate to the driver itself, I am going to be running 28 5w red leds. They are ledengin and the specs as follows;

2.5v forward voltage
1A current

So based on them running in 6 series of 4 leds. I have calculated that I will need 2.2ohm resistors and a 12vdc 6amp 140watt driver

But I'm unsure. Does the amps of the driver need to be increased by a factor of one for each parallel line added to the circuit? And also how do I know what wattage driver to use?

My final cicuit is going to include other leds too, but I was thinking to run these from a separate driver as their forward voyage and amps are different?

Any help is very much appreciated. Other than physics goes I know nothing! And that was long time ago so please if possible keep it to lamens!

Thanks guys!

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DickCappels

Joined Aug 21, 2008
8,831
Yes, the currents for all seven branches are in parallel so you add the currents together.

If you run 1 am thought each branch, you will need a power supply rated at 7 amps + some extra for margin.
Yes, you might want to use a different driver for the other LEDs; it depends on the voltages and currents involved, of course.

A note about using a 12V power supply and a 2.2 volt current limiting resistor: A change of 5% of the applied voltage (typical tolerance for a monolithic 12 volt regulator will result in a change of about 270 ma current through the LEDs. And if the LEDs were not sorted by voltage, you might not know how much current you will have in the string. For those reasons and the fact that you would wind up wasting a lot of power in the resistors, LEDs with running over a watt each are usually powered by switching type constant current LED drivers.

Here are some examples by way of Google search:

http://bit.ly/1lycHfX

Edgierprawn

Joined May 14, 2014
6
Has sorry just noticed I said six branches, I did mean 7! If the amp of the driver exceeds the required from the branches would his cause any problems?

I was planning on using a constant current driver, I was just wondering about what specs it would require? So one with a constant current of 7A or above would be fine? The reason I have the resistors in there is because the additive voltage of a single string would be 10v but the supply is 12vdc or would the driver with a constant current compensate for this, making the resistors unecessary?

I was also wondering on what wattage driver I require? I know the leds added up would equate to 140w total. With 20w on each string. So would this mean a 20w is needed or a 140w?

And finally with the driver would having one with a higher rated wattage than require cause a problem?

Thank you for the help it really is appreciated! I know I am asking a lot but this is my second attempt as my first failed miserably with the blowing of LEDS!

OldSkoolEffects

Joined Nov 18, 2009
68
If I'm not mistaken, you'd also want to match LEDs in each string by their Vf.

wayneh

Joined Sep 9, 2010
17,167
I was planning on using a constant current driver, I was just wondering about what specs it would require? So one with a constant current of 7A or above would be fine?
No, not for 6 legs summing to 6A. The driver would keep increasing the applied voltage until 7A was passing. This would exceed what the LEDs can tolerate.
The reason I have the resistors in there is because the additive voltage of a single string would be 10v but the supply is 12vdc or would the driver with a constant current compensate for this, making the resistors unecessary?
The resistors are needed to help balance the current amongst the strings. Otherwise, one might hog too much, burn out, and then the next would burn out even faster and so on. With a controlled current and a single string, the resistor would not be needed. I believe you want to choose the resistor to produce a voltage drop of at least ~0.5V at the expected current flow.
I was also wondering on what wattage driver I require? I know the leds added up would equate to 140w total. With 20w on each string. So would this mean a 20w is needed or a 140w? And finally with the driver would having one with a higher rated wattage than require cause a problem?
A higher wattage rating is not a problem, so long as the current to the LEDs is proper. A higher wattage, at a constant current, means that the controller can supply a higher voltage. This would allow more LEDs per string and thus more wattage per string. This is fine.

Edgierprawn

Joined May 14, 2014
6
Thankyou so to conclude with running 7 branches of LEDs with 4 LEDs on each branch I would only require a driver that had a 1amp rating and this would provide a constant 1amp current across all the strings? The wattage does not matter, so I will make sure the wattage is a higher rate than the sum of my 7 strings?

If I was to use this, then am I right in thinking I would still require the resistor to drop the voltage of each string. 12v going in 10v used by LEDs and then 2v drop from a resistor?

Thanksyouuuu

Edgierprawn

Joined May 14, 2014
6
If I'm not mistaken, you'd also want to match LEDs in each string by their Vf.
Yes Thankyou! I was going to run the same LEDs on each string and keep different LEDs seperate powered by another driver that meets the requirements of those LEDs.

BobTPH

Joined Jun 5, 2013
5,248
Thankyou so to conclude with running 7 branches of LEDs with 4 LEDs on each branch I would only require a driver that had a 1amp rating and this would provide a constant 1amp current across all the strings? The wattage does not matter, so I will make sure the wattage is a higher rate than the sum of my 7 strings?

If I was to use this, then am I right in thinking I would still require the resistor to drop the voltage of each string. 12v going in 10v used by LEDs and then 2v drop from a resistor?

Thanksyouuuu
No, you have almost everything wrong.

If you have 7 parallel strings each wanting 1A, then you need a 7 Amp driver.

If you use a constant current driver the voltage is irrelevant, the way a constant current driver works is by adjusting the voltage to get the set current. (However, it will have a min and max voltage, and the combined Vf of the string must be in that range)

Driving parallel strings with a constant current driver defeats its purpose. The current will not divide equally across all the strings.

You should use a 1A constant current driver for each string. If the input is 12V and you use a boost type driver, you can put more than 4 LEDs in a string. These often go up to over 30V so maybe 12 in each string.

Also, a 2.5V 1A LED is 2.5W not 5W.

Bob

Edited:

Here is an example of the type of driver I am referring to. It will boost to 35V so you could theoretically have 14 in a string. I suspect it would not work well at that extreme though, so I would use 3 of them to run 28 of your LEDs

http://www.ebay.com/itm/DC-DC-100W-...943?pt=LH_DefaultDomain_0&hash=item461090eca7

Bob

Last edited:

Edgierprawn

Joined May 14, 2014
6
No, you have almost everything wrong.

If you have 7 parallel strings each wanting 1A, then you need a 7 Amp driver.

If you use a constant current driver the voltage is irrelevant, the way a constant current driver works is by adjusting the voltage to get the set current. (However, it will have a min and max voltage, and the combined Vf of the string must be in that range)

Driving parallel strings with a constant current driver defeats its purpose. The current will not divide equally across all the strings.

You should use a 1A constant current driver for each string. If the input is 12V and you use a boost type driver, you can put more than 4 LEDs in a string. These often go up to over 30V so maybe 12 in each string.

Also, a 2.5V 1A LED is 2.5W not 5W.

Bob

Edited:

Here is an example of the type of driver I am referring to. It will boost to 35V so you could theoretically have 14 in a string. I suspect it would not work well at that extreme though, so I would use 3 of them to run 28 of your LEDs

http://www.ebay.com/itm/DC-DC-100W-...943?pt=LH_DefaultDomain_0&hash=item461090eca7

Bob
Hi bob! If anything I am now more confused than when I first posted in the forum. I have had different answers from different members regarding the driver and it's amp rating. However if the amps needs to be 7amp + I cannot find a driver of this spec anywhere.

I am wondering bob whether you could provide me sensible advice through my project? I won't be hassling all the time but I'd just like some consistent feedback on my project. As I'm getting different answers from people.

I am going to re draw up my circuit and research further into running my LEDs serial rather than parallel.

Also is there anyway to private message a member on the forum

Thanks

Matthew

Edgierprawn

Joined May 14, 2014
6
Okay so I have re-assessed and decided to go with what Bob has said. By using a boost driver to provide the correct voltage to power 9 LEDs in series. Once again I have attached a pic of my design for criticism.

I have a few more questions now though! Haha! I have looked at the driver you linked to me (bob) and it states the input voltage between 11-35. I will be using a 12vdc power supply. How can the driver alter that voltage to provide the 22.5v I will need for 9 LEDs? And also the output amps states it is 10a. I would only require 1amp of current. Would the driver allow me to adjust this or will the 12vdc supply to it need to be regulated to 1amp before entering the boost driver?

I think I am getting there now!

All the advice and help is very much appreciated thanks

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BobTPH

Joined Jun 5, 2013
5,248
That diagram is correct. I will try to answer your questions.

A boost driver can output a voltage higher than the input voltage, that is what "boost" means.

The 10A is the maximum current it can output. It will be quite happy to output only 1A.

I believe these drivers are voltage drivers with a current limit. So to drive a string of 9 LEDs with 2.5 Vfd it would need to output 22.5V. To use these drivers you would set the max voltage at something above 22.5V (say 25V) and set the current limit to 1A. This would cause the driver to output whatever voltage it needs to get 1A.

Note that one string will draw about 2A from your 12V supply to supply 1A at 22.5V. This is because you cannot create power, and power is V * I. So the power at 1A and 22.5V is 22.5W. At 12V that amount of power would be 22.5 / 12 = 1.875A. But boost converters are not 100% efficient. Assuming 85% efficiency (which is pretty normal) it would have to draw 1.875 / 0.85 = 2.2A from your 12V supply to supply the needed power.

Since you have 3 strings, the total current drawn from your 12V supply will be something like 6.6A.

Bob

wayneh

Joined Sep 9, 2010
17,167
As I'm getting different answers from people.
Nothing Bob has provided you is inconsistent with what I posted. You're on the right track.

You could shop around a bit for your DC-DC converter. You don't need to pay for one that can handle up to 10A if you really only need 1A. A converter rated to 2A or more would be reasonable for your 1A string, and might be cheaper. More amperage is better, but you don't need to pay for a lot of excess you don't need.

Also, a higher output voltage works in your favor. Increasing the LEDs per string will, at some point, reduce the number of strings you need and this can also help reduce cost and complexity. So a converter that's capable of, say, 30V is worth more than one at 20V. It's a balancing act to optimize it all, and it just depends on what you can find out there.

You probably won't find converters at ~50V or more, but you'd want to avoid those anyway, for safety reasons.