# Does voltage matter when using constant current PSU?

Let's say I have 90W RGB LED module/plate (10 Series and 9 in Parallel internally). It has Voltages: R: 20~22V; G: 32~34V; B: 32~34V; Can I power all colors with 3 identical 30W constant current driver that has voltage of 30~36V? To what extent does voltage matter/not matter when using constant current psu?

• That doesn't make a lot of sense. Picture of the specifications? Datasheet? Model number? Jan 15, 2014 at 23:47
• Yup, a [proposed] diagram of how the LEDs will be connected would make this question clearer. Jan 15, 2014 at 23:52
• See edit and included link to led module. LED module wil also be PWM controlled and dimmable (via led driver with arduino) in case it that matters. Jan 16, 2014 at 0:35
• Yes, it's the min/max voltage. The two values are examples of two different led drivers I am considering. Most drivers I see have similar voltage ranges, that's why I was hoping voltage didn't matter since then I could power the red led with same driver as blue and green. Jan 16, 2014 at 3:56
• @Phil, the overlapping range plainly means the minimum and maximum voltages it can work at. Jan 16, 2014 at 4:40

Yes, voltage matters. Just as a voltage source (as most power supplies are) have a maximum current and also a minimum current (usually assumed to be 0A, unless otherwise specified), a current source has a range of voltages over which it can work.

If required to work outside these ranges to supply the specified constant current, it may shut down, fail to supply the specified current, overheat, or spectacularly self-destruct.

I'm not sure what to make of the numbers you give. 26V-36V or 30V-36V doesn't make a lot of sense, because that's equivalent to just 26V-36V. Even so, if your red LEDs require 20V-22V, this is outside that range, which suggests this power supply will not work.

Furthermore, it sounds like you are thinking about driving multiple strings of LEDs in parallel with one supply. That won't work. Circuits in parallel have equal voltages. Circuits in series have equal currents. If you put multiple LEDs in parallel, and they don't each have some current regulation device (the simplest being a resistor), then the total current from the supply will be split between the parallel circuits. This is not what you want. See Why exactly can't a single resistor be used for many parallel LEDs?. Although that's about sharing a resistor, the question, and the answers, are just as applicable if you replace "resistor" with "constant current source".

Since your proposed LED modules are sold as a unit, we can probably assume they have some mechanism to equally share current among the internal parallel circuits. However, I see no datasheet, so we are taking it on faith. However, you will need a separate supply for each of those modules, or you can connect all the modules in series and find a current supply capable of supplying the sum of the voltages of each module.

• Yes I am assuming it has the required mechanism (plenty of youtube videos running similar modules with the led drivers I am considering). I would use one led driver for each color as it will be PWM controlled and dimmable. The voltages given are for 2 different drivers, but I will edit that out as it's confusing. Jan 16, 2014 at 4:06

Just a few thoughts.

1. The voltage of the LED should fall into the constant current region of the power supply. Some LED power supplies have a wide constant current region (e.g. 3V to 36V). Some have a narrower constant current region (e.g. 27V to 36V).
2. If you have an array of LEDs with 10 in series and 9 strings in parallel, you'll need a series resistor on each of the strings to facilitate current sharing.
• First point makes sense. What would happen if I use 30-36V driver with 20-22V LED? Would it light up, be damaged, light up dimmer than normal etc...? Second point doesn't apply (see edit). Jan 16, 2014 at 0:32
• @DominicM Revised my answer. I wouldn't use a 30-36V supply with a 20-22V LED. May be, the supply will detect that the LED voltage is "wrong" and shut off. May be, it will toast the LED. Probably, different models would handle this differently. Jan 16, 2014 at 1:09

Let's say I have 90W RGB LED module/plate (10 Series and 9 in Parallel internally). It has Voltages: R: 20~22V; G: 32~34V; B: 32~34V; Can I power all colors with 3 identical 30W constant current driver that has voltage of 30~36V? To what extent does voltage matter/not matter when using constant current psu?

To what extent does voltage matter/not matter when using constant current psu?

Perceived problem: 3 x psu, 3 x LED colours, one colour per psu. Vranges vary per colour ...

It is unusual for constant current drivers to have a high minimum voltage.
This may be done to limit dissipation and may assume a target load.

Your actual LED voltage at the specified current MUST lie within the CC driver voltage range for things to work as designed. If VLEd at CC is > Vmax you will probably just get Vmax and < CCspec. If VLED at CC is < Vmin you will be aty the driver makers pleasure. Driver may fry or shut down. Worst , it may deliver Vmin and CC will be exceeded and LED smoke may not be RGB>

Also - SOME drivers are badly behaved. At no load a CC driver SHOULD output Vmax. When you apply a step load the driver should instantly drop Vout until CC is reached. You MAY get a brief blip of V > VLED at CC. Most LED strings will tolerate this if Vmax is not too much > VLED at CC.

If you load the CC and then turn it on it SHOULD step up V until CC is reached. I have seen supplies that step to Vmax or some other voltage while they get their brains running. Magix smoke may happen. Most device made to drive LEDs should not do this.

When using a 'lab' psu for this sort of task I set Vout to just higher than required., then set Ilimit (short leads and adjust or use dial if available and accurate) then deshort and apply to load. Or apply load and turn on as in most cases the supply will not step above Vset. Most.

ie horses for courses, YMMV, caveat emptor, test carefully, ... but the above should apply well enough.

3 identical 30W constant current driver

Technically, NO. If all are identical they must be able to supply max I to red and max V to Blue & Green so you need 3 x ~= 50W supplies.

You can instead use 2 x ~= 35W supplies capable of about 1.2A for B & G and a ~= 35W supply capable of about 1.5A for Red. You need more than 30W per supply due to Vmin - Vmax range issues.

Detail:

You say 3 x identical 30W supplies.
Vmax x CC for highest string must be <= 30W with a 30W supply.
If you want 30W from red and have identical supplies then the B & G supplies must be capable of higher wattage than the red supply. The red supply is run at 20-22V and makes 30W. The other supplies run at higher V but lower I to make 30W. But the B&G supply are capable of higher V and red_i so are capable of higher wattage so the capability is unused. If you want equal wattage then you can use lower CC max supplies for B & G.

If so then lowest string wattage will be Red with Vstring MAY be 20V (worst case low) so Wred may be 30W x 20/34 = 17W !
If supplies are equal, to get red to 30W you need all psus able to give at least 30W x 34/17 =51W = say 50 Wsupplies.

• I don't understand the last section. If the drivers are identical then voltage range and CC are the same too, they are the same identical devices after all. From other answers I gather that I cannot use lets say 30-36V supply to power 20-20V red LED. This means as I understand it that no I cannot use identical supplies and since there are no supplies I can find that fit my needs my project is not possible at all. Jan 16, 2014 at 16:01
• @DominicM Your project is very doable. You can definitely buy or make or use components that exist. | My point about 3 identical supplies is that they must be able to supply the highest voltage and the highest current. Red is high I and lower V, G&B are high V and lower I for the same Watts. A psu that must supply the highest I and the highest V is CAPABLE of more Watts than is needed by any LED string. || Yes, driver Vmin MUST be below or equal to LED Vmin that you intend to use. But eg if a psu worked OK for G & B but Vmin was too high for Red an external resistor or zener would fix this. Jan 16, 2014 at 17:41
• I didn't consider a resistor because of high power requirements of the LED. What kink of resistor would I need to bring down the voltage by 10V? I would think it would be a very large resistor that would end up wasting a lot of power. Jan 16, 2014 at 17:51

It has Voltages: R: 20~22V; Can I power it with a 30W constant current driver that has voltage of 30~36V?

You could, but you'd might fry the red diodes. Since your constant current driver has a stated range of 30v to 36v, it immediately tells you that it is out of manufacturer spec to try to drive it lower. It might have failsafes and just shut down. It might blow a fuse or overheat a part. It could catch on fire, horrible horrible fire. Or it might just work as designed, and will hold the line to 30v, forcing a higher voltage through your red diodes, causing a higher current draw than they like, blowing them.

Or it could just be an overly cautious label and the supply can work at ~20v 900mA without problem and you luck out. Or you simply have to change some stuff on the board to make it work like that.

In the end, do you want to risk the cost of the panels and/or the supplies?

• Uncommented downvote... Mar 1, 2014 at 0:17