Friday Free Board Quiz – the prize is RT5350F-OLinuXino – Is there anything wrong with this circuit?


RT5350F-OLinuXino is OSHW board which run openWRT Linux distribution and have WiFi.

You can win this board! To participate in the Quiz is enough to re-tweet the Twitter Quiz announcement message.

To double your chances you have to answer the Quiz question :)

The Quiz question is: Is there anything wrong with this circuit?


You have time to re-tweet and/or answer until Monday 1st of February.

In Monday we will post the correct answer and ask to generate random number in range then announce the winner and ship the board by post/airmail.

Good Luck!

14 Comments (+add yours?)

  1. sckstefStephane
    Jan 29, 2016 @ 14:50:32

    This type of resistor is only for 1/16 W ( 62 mW ) which is too low , we have to disipate at least 78 mW.


  2. stefanstefanov198
    Jan 29, 2016 @ 15:08:45

    LED Vf is around 2V. Resistor see voltage 3V and current is around 30mA.
    Dissipated power is around 90mW which is very close to the limit of 0402 package (0.1W). Schematics will work but in conditions close to the limits.


  3. jonsmirl
    Jan 29, 2016 @ 15:33:23

    The resistor is too small. Vf red LED is about 1.8V. 5% tolerance means you might get 95ohm. So about 34mA current. Typical red LED only wants about 20mA at most. Should be about 300ohm.


  4. SK
    Jan 29, 2016 @ 17:18:50

    Seeing the answers here made me wonder how they will fit in one tweet, but then I got one of these rare insight moments – the question is a simple yes/no one, there is no “If yes, what?” part 😀 😀


  5. R. E. Hunter
    Jan 29, 2016 @ 18:34:01

    Another trick question: without the specs for the LED (forward voltage and forward current), it’s impossible to know for sure. In a quick search, I found 0603 LEDs with voltages ranging from 1.7v to 2.9v and current from 5ma to 50ma.

    In all cases I found, the resistor value is too low and the LED will be overloaded, with resistor values from 150 to 470 ohms required. But with a bit more searching I might well find one that will work with 100 ohms.


  6. Lahcen
    Jan 29, 2016 @ 19:39:53

    yes, i think that the limiting resistor should be placed between the + of de power supply and the led to be able to limit the current that goes through the led.


  7. Malcolm
    Jan 29, 2016 @ 20:33:59

    Assuming a nominal 2v forward voltage drop on the red LED, the current flowing in the circuit will be 30ma.

    One problem is that most red LEDs have a maximum current of 20ma, and so this could damage the LED.

    Additionally the resistor will try to dissipate 90mw, however the 0402 package has a limit of approximately 62mw.


  8. Zoltan
    Jan 29, 2016 @ 22:17:49

    Value of resistor is too small. It disipate too much.


  9. bib
    Jan 30, 2016 @ 16:45:02

    Nothing wrong with this circuit. The LED forward voltage and current can’t be computed, only Spice simulation or graphical resolution (LED I(Vf) ) could give the static point I and Vf.
    Some Vishay Resistors 0402 could sustain 0.125W. Same on LED’s.

    On a Spice simulation point of view GND reference is missing.



  10. Alphonsos
    Feb 01, 2016 @ 15:41:15

    Searched in mouser for the components. There are led 0603 LEDs that can go up to 60 mA *e.g. which is more than enough – around 30 mA will pass the LED assuming a Vf of 2V. The resistor, however seems to be a problem. 0402 100 Ohm resistors in mouser go up to 30mW – in this cirtcuit we would need at least 90mW, more to have some headroom.


  11. Alphonsos
    Feb 01, 2016 @ 16:03:17

    Using a vishay RCS e3 100 Ohm 5% resistor ( and an Osram LS L296 LED ( it can be safely done. Voltage drop across the LED (Vf) is at least 1.8V, therefore voltage across the resistor is max 3.2V at 95 Ohm (100 Ohm-5%), this gives us a 33,68mA current which is well within the specs of the LED (60mA max) and a power dissipation at the resistor of 107.8 mW, which is again within specs (0.2W).


  12. Federico Battaglin
    Feb 07, 2016 @ 20:53:03

    100 Ohm is too low!


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