I have a lot of (new) LED light strips from a job I was on. These light strips were in 2x4 fixtures. As a project I would like to use these as undercabinet lights. But the CC driver that comes with the 2x4 seems to be insufficient.

I had to modify the length of most of them to fit under each cabinet. This was done successfully. I checked if my modification was good by using the old CC driver.

The problem: Once I got all the lights to the length I needed and plugged them all together (think LED ribbon) The old driver will not light the whole string. A problem I thought might happen.

The main question: What size CC driver is required?

The facts: CC driver in donor light. Osram optotronic OTi 30/120-277/1ao dim l. this driver has dimming capabilities. But I don't need dimming. Pout=30W, Vout=10-55 Iout=350-1050mA. Factory setting output current is set to 540mA.

LED's in donor lights. Unknown. (4) strips of 40 led's 3500k. Each LED measures about 25mmx45mm I could not find any typ. LED's online with these dimensions. I asked the factory for the data sheet but they were unresponsive.

My finished quantity of LED's = 320

How do I size a new driver based on these facts? Thanks

  • \$\begingroup\$ The old driver is designed to power one fixture? So if one fixture takes 540mA, multiply that by how many fixtures you have? \$\endgroup\$
    – hekete
    Jul 3, 2019 at 5:10
  • \$\begingroup\$ are the LEDs including series R’s ? What size are the LEDs per R in 4x4? Simplest solution is a 12V LED strip with 12 V source using 3S+R repeats in parallel in strip \$\endgroup\$ Jul 3, 2019 at 6:07
  • \$\begingroup\$ 25x45mm suggests an old 10W size. can you verify voltage to turn ON with a series R. Which is overkill for undercounter lights. You would only need 10 at most not 320 if they are 10W. A phot and some VI tests will tell me \$\endgroup\$ Jul 3, 2019 at 6:17

1 Answer 1



You may have connection issues.

Driving strings arranged 2S2P may work.

Drive ONE string of four at minimum acceptable brightness
Measure and advise Vstring and I string.



The old driver will not light the whole string.

You need to explain what this means.
If you means that it lights part of the string fully and part not at all then you have connection issues. If it lights part fully and part less brightly you have ??? issues.

IF you can drive one string of four to acceptable brightness with the desired driver at a voltage of half or less of the driver's maximum current, then you can drive two strings in series.

If you can drive two strings in series at acceptable brightness at less than 50% of the desired driver's max current then you can probably drive the 4 strings in 2S2P (2 parallel strings of 2 in series) - provided that max driver wattage is not exceeded.

  • \$\begingroup\$ The old driver will run 160 leds, 4 strings of 40 just fine. That is what it was designed for. After I modified the 40 led strips to my required lengths, lets say 32 leds instead of 40 (various lengths). Then I used the old driver to bench test my work. After all modified strips check out individually I connected them all together, 1 continuous string. 320 leds. They did not light, as I expected. I then started taking the modified strips off until the lights came on. This bench testing produced a result of anything over that original 160 leds was very dim. \$\endgroup\$
    – mud50
    Jul 4, 2019 at 2:29
  • \$\begingroup\$ Example. 320 leds=no light, 220 leds=no light, 164 leds=dim, 160 leds=works fine. All strips work individually. No mechanical issues. I would like to have one continuous sting 320 leds if possible. No parallel strings. \$\endgroup\$
    – mud50
    Jul 4, 2019 at 2:30
  • \$\begingroup\$ My feeling is I just need to buy a different CC driver. But I'm not sure how CC drivers are designed. If 540mA drives 160 leds that's 3.375mA per led. So 320 leds at 3.375mA = 1080mA. So I would need a driver that is set at that?? But I don't know if this is sound logic. Also because I'm not sure how led drivers are designed nor my math logic. I'm not sure about wattage. Data sheets on led's would help but alas. 3.375mA x 24v = .081w x 320= 25.92W total. But again I'm not sure if this is sound. \$\endgroup\$
    – mud50
    Jul 4, 2019 at 2:31
  • \$\begingroup\$ Do CC drivers need a cushion? Meaning all my math i based off 540mA. But if it's actually 3.2mA per led (160) then that would be 512mA calculated but 540maA shown. My understanding of CC drivers over CV is the beneficial efficiency. CV is not an option with these led strips. \$\endgroup\$
    – mud50
    Jul 4, 2019 at 2:32
  • \$\begingroup\$ @mud50 If you'd done what I asked we may be home by now. Your logic is OK but is working within a somewhat different "world" to what you envisage. Odds are the strings are arranged to operate with sets of 2 or 3 LEDs in series and all sets paralleld across a nominal 12V bus, or maybe 6 in series and expected 24V or so, or ... . ||Can you do what I asked above? viz "Drive ONE string of the four at minimum acceptable brightness. | Measure and advise Vstring and I string. If you KNOW what the driver I is set to then measuring I is less necessary. ... \$\endgroup\$
    – Russell McMahon
    Jul 4, 2019 at 5:55

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