1
\$\begingroup\$

I was working with some smd LED's trying to make their V-I curve. For that purpose, I connected two segments parallel to each other each containing two leds as shown, to a regulated dc power supply directly. connection of led Circuit assembled

The voltage from supply was increased gradually.LEDs started o glow as voltage reached around 4.8V with 0.07mA current in the circuit. As I increased the voltage, LEDs became brighter as expected. But after around 7.6 V, I wasn't able to increase the voltage any further. Current reading at this point was 189.8mA. The supply is capable to provide upto 30V, 2A. Why the voltage got stuck at 7.6V?

\$\endgroup\$
2
  • \$\begingroup\$ Were where you measuring? I guess you were measuring the voltage drop over the diodes. \$\endgroup\$
    – PlasmaHH
    Jun 15 '15 at 14:45
  • \$\begingroup\$ 1. Could you, possibly, try to turn the knob that says "coarse" in the current segment? 2. To do a V-I curve of LEDs one adjusts the current in the right range and record voltage, but these aren't LEDs, these are 12V LED modules, so I suppose that's okay then. 3. When parametrising a LED one certainly does not do parallel, but as with '2' - since these are modules, I suppose you could do that, though, with tolerances on Resistors and D-bridge... I doubt it being accurate enough to warrant the "statistical pool" of 2. \$\endgroup\$
    – Asmyldof
    Jun 15 '15 at 14:51
1
\$\begingroup\$

Your power supply can limit current - see the "coarse" and "fine" control knobs on the right side under "Current." Presumably, you had the current limit set to around 190mA.


As Wouter van Ooijen says, this also provides a safe, simple method of getting the data for your V-I chart.

  1. Set the voltage at a safe maximum (from the LED datasheet)
  2. Set the current down to 0.
  3. Read voltage,
  4. Increase current,
  5. Read voltage
  6. Repeat steps 3 to 5 until the voltage reaches the maximum you set or your current reaches the maximum for your LED.
  7. Finished.

It looks like you are measuring 1 LED from each module, with the two LEDs in parallel. You realize, that your readings may also be influenced by whatever else is in the circuit? That's the other LEDs, the resistors, and what looks like it might be a regulator or controller of some kind?

\$\endgroup\$
2
  • 1
    \$\begingroup\$ And it's easy to check. Just short the power supply leads and see what the current is. \$\endgroup\$ Jun 15 '15 at 15:18
  • 1
    \$\begingroup\$ And it provides a safer way to measure the V/I curve: set a current, and see what the resulting voltage is! \$\endgroup\$ Jun 15 '15 at 15:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.