# Calculate voltage of Led driver required for a given amount of led's

I have built some LED lights for my fish tank using six of these LED lamps:

http://www.ebay.co.uk/itm/261813680886?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I have tried using this power supply:

http://www.ebay.co.uk/itm/291074673667?_trksid=p2057872.m2749.l2649&var=590234643632&ssPageName=STRK%3AMEBIDX%3AIT

When I connected the original six white LED lamps they flashed to indicate the 12v 60w driver was being overloaded. I changed it to four LED's two on each string so the circuit was well within the drivers current parameters; however They still flashed, at this point I took a voltage measurement of between 7 and 8 volts when the lights flashed. It did however work with only two LED's connected.

The driver seems to have a fixed voltage at 12v and from what I have read I need a new driver one that compensates for the drop in voltage.

I asked the person from who I bought the LED lamps what the power requirements if the chip were and he said it would be 12v per chip so around 66V? This left me a little confused because the 12v driver worked with two chips.

Is this the correct way to calculate the voltage of the driver required? Please note I found another source for these chips who states the reserve voltage if 5v; however I believe this may be a typo from the Chinese chap and he probably meant reverse voltage. Also; constant current driver so no resistor suggestions please.

Many Thanks,

Ross.

• Gotta agree with Andy. There's no part number on the E-bay site and no data sheet - not for the power supply, and not for the LEDs, either. Without accurate data, one guess is as good as another and just as likely to lead to a dead power supply or a dead LED. – JRE Jun 16 '15 at 14:58

Somewhere between 9V and 12V the LED will take a current of 900mA. If it does this at 9V and you apply 12V you might be getting a forward current of 3 amps thru the LED. There is no data sheet for the device (that I can find) so it's impossible to be exact.

But, realistically you probably need to put in a current limiting resistor for each LED. What makes you think it has a constant current circuit built into it?

The ebay link is really-really crappy at giving details - for example it states that the output power is 10 watts - this of course is rubbish!

• Hello very many thanks for the answers I appreciate it; so I do in fact have the right voltage but the current is too high? Here is a link to the other power supply I tried it with which states it is a constant current but is also dimmable. The original post only allowed me 2 links. ebay.co.uk/itm/… I have wired the whole thing up using terminal blocks so getting some sort of inline resistor might be the answer? I used similar but higher voltage chips before and they worked fine with a CC driver? – Ross Crowther Jun 17 '15 at 20:29
• No I think you might need current limiting resistors. – Andy aka Jun 17 '15 at 21:14
• I should add, the power supply of the higher voltage chips that worked had a variable voltage between 22-38v and it was definatley a constant current supply. Would the right constant current supply work though? I am reluctant to add more parts into the unit because I am running out of space too. – Ross Crowther Jun 17 '15 at 21:20
• My main message to you is good luck and next time, buy parts with full data sheets. – Andy aka Jun 18 '15 at 7:04
• Thanks Andy, I listened to some of your music. Your singing voice isn't that bad. – Ross Crowther Jun 18 '15 at 10:04

The LEDs need a constant current feed.

The power supply is a constant voltage supply.

You can PROBABLY get an OK result as follows.

Operate ONE LED from the supply with a resistor between supply V+ and LED Vin+.

The power supply is rated at 5A max.
For 6 LEDs the current per LED is 5A/6 ~= 830 mA
Let's work on 800 mA = 0.8A.

LED spec says 9 to 12V.
9V is worst case so start there.
PSU = 12V.
LED = 9V
Resistor = 12-9 = 3V R = V/I = 3V/.8A = 3.75 Ohms.
A standard value is 3.9 Ohms.
R dissipation = V x I = 3V x 0.8A = 2.4W
R needs to be 3.8 Ohm and say 5W (10 W even better).

Wire PSU+ve - Resistor - LED+ve - LED- - PSU -ve.
ie current flows through resistor and through LED.

Get voltmeter (eg in DMM)

Set to 20V range.
Turn on as above.
Measure voltage across resistor.
Target is 3V.
A bit more [tm] - say up to maybe 3.3V is OK
Lower is better than higher.
Say V across R was 2.9V. Iactual = V_across_R / R
= 2.9V / 3.9 Ohms ~= 745 mA = OK

If I is too high use a larger R - say 4.7 Ohms
If I is too low use a smaller R BUT be careful - say 3.3Ohms.

Once you have this working OK repeat for all LEDs in parallle.
One R per LED. R's are cheap, LEDS are not (although yours are a good price)

LEDs will vary in current somewhat. You can fine tine as above if desired.

## NBNBNBNB

LEDS MUST have heat sinks.
They will get very hot and die in no time with no heatsinks. This can be a simple large metal plate of available. Be sure not to short power supply when connecting LEDs to heat sink.

• Thanks for the in-depth answer. I have little graphics card heat sinks though I have only been testing at the moment so they are not wired up, but it would be a bad idea to wire them to the same parallel circuit? I provided a link in response to another post to a constant current power supply I used. I am prepared to solder if resistors are a must. However just to complicate things I would like to add four of these LED’s to add a bit of colour: kiwilighting.com/10w-red-blue-led-emitter. – Ross Crowther Jun 17 '15 at 21:10
• Sorry I ran out of characters: Given I will now be buying a new power supply; if I bought the right one would I get away without having to do all that soldering? I made a 10w x 10 parallel circuit with some similar lamps before but had the right power supply and it is still going strong without any resistors. – Ross Crowther Jun 17 '15 at 21:10
• @RossCrowther If you parallel LEDs at CV (constant voltage) resistors are needed EXCEPT if the supply hits its limit and starts to sag and approximates a CC (constant current) source. With a CC supply, if supply max oc voltage is > LED specified voltage then N LEDS will share the rated CC at what ever V it takes to force it down their throats. So eg LEDs that are rated at 1A each with a 10A supply - > 10 LEDS average 1A each, 5 LEDS average 2A each, 2 LEDS average 5A each !!! | BUT even if a CC supply is used and average current is OK, LEDS will not share exactly due to production .... – Russell McMahon Jun 18 '15 at 3:19
• @RossCrowther ... variations. How much it varies deep-ends on LED maker, whether there is any internal series R etc. AND mixing eg red-blue and white almost guarantees that Vf (forward voltage = operating voltage) will vary at rated current amongst LEDS If you parallel red-blue with white and white say 10-12V opn and red-blue say 9-11V then odds are the RB will get over driven and the Wh underdriven deep-ending (again) how many of each. Also the rated Vf may not be as said quite because it is a sellers rough guesstimate. | If your 10 x 10W worked well without resistors works well for .... – Russell McMahon Jun 18 '15 at 3:24
• .... you I'm (genuinely) pleased for you and many people have agood enough experience - IF all LEDs are run at less than full spec and cooled well then all should be well enough and current imbalance may be ok for your app. || From the RB picture you probably have 3 strings of 4r+3W + 4r. 3Wh x 3.3V ~= 9.9V. Tp balance this 4r would need to be 9.9/4 = 2.5V/red LED which is usually high BUT if LED were chosen for low Vf at 3V then 3 x 3V = 9V/4 = 2.25V red. They MAY tailor red & white Vfs to suit or may not care. Guess which is more likely.| Adding an R per LED is rather quick ... – Russell McMahon Jun 18 '15 at 3:29

simulate this circuit – Schematic created using CircuitLab

The bulb does light up. I took the measurements again and voltage fluctuated quite widely as the LED heated up. I don't have my heat sinks wired up at the moment; the plan is to use the same power source if there is any current to spare. I used the 10A setting on my multi-meter, swapping the leads over to the appropriate terminals. I also bypassed the resistor with a wire and measured the voltage across the led as 11.87v, not sure if that helps.

Been looking at this ohms law solver: http://www.ohmslawcalculator.com/led-resistor-calculator and it confirms I need a 3.9 ohm resistor. I think I will go buy a bunch of them, wire the whole thing up, then measure again hopefully that will take me in the right direction.

• $3.9\Omega \cdot {2.2\text{A}}^2 \approx 18.9\text{W}$ – Ignacio Vazquez-Abrams Jun 23 '15 at 17:47
• P.S. The lower voltage reading from the PSU came because I connected the LED this time. – Ross Crowther Jun 23 '15 at 17:54
• @Russell McMahon I should mention that the full circuit of six LEDs splits into two at one of the terminal blocks. With two on one string and four in the other. If this is the cause of the imbalance and why they won't all light up, might experimenting with stringing a few of the superfluous resistors together correct the imbalance on one side? – Ross Crowther Jun 24 '15 at 17:52