0
\$\begingroup\$

I need a little help with understanding the current I'm working with.

I have these LEDs, and trying to power them with an Arduino type board, a Wemos/Lolin D1 R1, based on an ESP8266.

I measured the (max?) current the LEDs can draw straight from a 5v USB adapter, measuring out to be 140mA-180mA for either blue or green, but not a constant reading like I'd expect. (Starts high and slowly draws less current) I powered both blue and green expecting to see ~300mA from 150+150, but only read 163mA through ground, and 105mA through either power lead on the blue or green LED.

Why are there different current measurements? Specifically why do I see 163mA going through ground, but 105mA in each power lead? 1. Where is the ~50mA going? [2*(105mA) - 163mA = 47mA]

Prepared for a chance to damage a different board, an Arduino Uno, I used the 3.3v and 5v sources to power the LED, but no matter 3.3v or 5v, I only see 8.5-9.8mA. I have no added resistor. 2. What's limiting the current?

According to the datasheet I linked, 12mA is the max current draw on the GPIO. I would hope for the PWM function to lower the current and not worry about the LEDs running with no added resistor. 3. Ultimately, am I O.K. running these LEDs on the PWM GPIO pins on the Wemos board?

Sidenote: My 5v source is 4.98v with no load, and 4.92v with the blue/green LED on, 4.89v both on. 4. Is this my voltage drop, 0.06v,0.09v?

I feel sorry for asking these questions as I'm just rusty on the basics right now. Thank you.

\$\endgroup\$
5
  • \$\begingroup\$ Measure the voltage across each LED while making current measurements of the LED. \$\endgroup\$
    – jonk
    May 7, 2022 at 1:13
  • \$\begingroup\$ How are you measuring current? I don't think you can safely draw 100+mA from a 5050 LED, so the decreasing current over time may be that you are damaging the diode. \$\endgroup\$ May 7, 2022 at 1:17
  • \$\begingroup\$ @jonk I measured 4.92v across a single LED. Now what? I can use Ohm's law, and I calculate resistance but I'm lost on what I'm calculating, do I need a resistor for these? \$\endgroup\$ May 7, 2022 at 1:31
  • \$\begingroup\$ If you plug them into a GPIO (which has a reasonable output resistance) then you don't need a resistor. If you plug them into a 5v supply with negligible resistance, then yes you must limit current somehow. They're probably only rated for ~20 mA or so per channel, give or take. \$\endgroup\$ May 7, 2022 at 1:34
  • \$\begingroup\$ @user1850479 That makes sense. Thank you. \$\endgroup\$ May 7, 2022 at 1:43

1 Answer 1

1
\$\begingroup\$

LEDs generally have a rather steep resistance curve, in other words a small change in voltage results in a large change in current. When you insert your ammeter in series with one LED it’s current will drop significantly as the meter has a small but significant resistance.

\$\endgroup\$
7
  • \$\begingroup\$ For most microcontrollers you won’t damage the device even by shorting a pin; if it will only deliver 12mA (although that’s for a specified output voltage, not at short circuit) then that’s all you can expect to get. \$\endgroup\$
    – Frog
    May 7, 2022 at 1:15
  • \$\begingroup\$ Sidenote: ‘voltage srop’ is rather a broad term, I think in this context it’s the difference between off-load and on-load for whatever load you’re using. \$\endgroup\$
    – Frog
    May 7, 2022 at 1:16
  • \$\begingroup\$ Thank you. I understand the multimeter resistance, voltage drop term not really the right word here, and I'll run the LED on my mcu with some pwm magic to help out anyways. You say "a small change in voltage results in large change in current", but I didn't see the current draw change with between 5v and 3.3v source. I thought the current would increase with the lower voltage, draw the same wattage, but it didn't. Am I not accounting for something here? \$\endgroup\$ May 7, 2022 at 1:26
  • \$\begingroup\$ @electricaln00b it appears that these devices are ‘raw’ LEDs with no built-in current limiting. So while a device would need to draw 200mA at 5V or 333mA at 3.3V to dissipate 1W, there’s nothing actually enforcing the power output, that’s the maximum rating for the device. I think you’re expecting the LEDs to be a lot smarter than they are. With the Arduino Uno the micro itself is limiting the current. \$\endgroup\$
    – Frog
    May 7, 2022 at 1:34
  • 1
    \$\begingroup\$ Okay that makes a lot of sense, I feel kinda silly now. Thank you again @Frog. \$\endgroup\$ May 7, 2022 at 1:44

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.