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LED resistor value/voltage doesn't make sense

Discussion in 'LEDs and Optoelectronics' started by TTL, Jun 7, 2021.

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  1. TTL


    Oct 24, 2013
    I'm trying to replace an old (late 80's/early 90's) "power on indicator" 5mm LED in a vintage computer with a recently purchased 3mm LED (the old LED had the wrong colour), but the new one appears to light up too much IMHO.

    I know modern LEDs are more efficient than the ones back then, but I fear the voltage might be too high for this, so I've quickly disconnected it and tried various resistors in series to get what looks acceptable, but none of the resistors I've tried make sense compared to the suggestions that come up with the various online "LED resistor calculators". According to the new 3mm LED specs (20mA/2V) I'm told to use a 150 Ohm resistor, but that makes just about no difference in the light intensity. I've ended up with a 47 K resistor (!!!) and measure around 2.3V so I should probably use even a higher resistance to get it down to 2V. How can this be?

    According to the schematic the original LED connects to +5V (Vcc) through an on-board 120 Ohm resistor and the other pin to GND. If I remove the LED and measure across the LED connector (J290) I measure around 5V which seems very strange to me. Isn't R290 (120 Ohms) supposed to lower the voltage to around 2V or so?
  2. bertus

    bertus Moderator

    Nov 8, 2019

    The voltage will only be lowered when a led is connected.
    Measuring the voltage on an open connector will give the 5 Volts.
    It depend a bit on the input resistance of the meter.
    A digital meter is usualy about 10 MegaOhms.
    An analog meter is given in KiloOhms per volt.

    The newer led will have a higher efficiency than the old one and shine much brighter.

  3. TTL


    Oct 24, 2013
    I have a digital multimeter (and its impedance is 10 MOhms as you say, according to the user manual).
    So to check if the voltage is correct for the LED I need to measure across the LED while the LED is attached, or will the reading be wrong in any case with a digital multimeter?
  4. bertus

    bertus Moderator

    Nov 8, 2019


    Dec 19, 2019
    The digital multimeter is OK. Yes you must measure the voltage with the LED connected. I'm sure your LED will not light with a 47K resistor feeding from 5V. We have extremely bright LED's at work and to make the brightness "normal" we use a 2.2K resistor.
  6. Alec_t


    Jul 7, 2015
    The LED will set its own voltage (2V is just a nominal value) depending on the current through it. If you want, say, 5mA, then the series resistor value will be (5V-2V)/5mA = 600Ω. So 560Ω or 680Ω would be convenient close standard values to use, but you might find a higher value more suitable.
    dave9 likes this.
  7. TTL


    Oct 24, 2013
    Thanks for all your comments!
    I'm still not quite "getting it" but I noticed something interesting:
    This gave me something to think about when I conducted a test by measuring the voltage across the LED while swapping different resistors between GND and the LED's cathode pin:

    direct connection (no resistor): 2.461V
    120 Ohms: 2.447V
    150 Ohms: 2.445V
    180 Ohms: 2.442V
    220 Ohms: 2.438V
    270 Ohms: 2.107V
    390 Ohms: 2.422V
    510 Ohms: 2.414V
    2.2 KOhms: 2.34V
    3.2 KOhms: 2.29V
    3.9 KOhms: 2.309V
    4.7 KOhms: 2.299V
    5.1 KOhms: 2.29V
    47 KOhms: 2.185V
    56 KOhms: 2.173V

    What I found strange here were the much much higher values at the end (47K and then 51K to check if indeed I had gotten it right nor not) didn't change the voltage much -at least not in consistency with the voltage drop relative to the previous resistor values. Does this have something to do with the LED settings its own voltage, and also the reason I don't appear to exceed the limit of the LED's max. voltage (2.5V according to the datasheet, typically 2.2V), or is this just by chance in this particular setting, or does the 120 Ohm resistor (between the computer's +5V line and the LED's anode -see the schematic in my earlier posting for R290) limit that voltage?
    By the way, voltages around 2.3V or so seems to visually be the best solution in my opinion (as a power indicator). Voltages above this made it appear more like a flashlight or something. I haven't tried it through the computer casing's diffuser though, so that may help even it out yet some more.
    The important thing is that I don't end up with a voltage that would fry the LED, but that doesn't seem to be the case here, regardless of omitting the resistor in its entirety.
    Last edited: Jun 7, 2021
  8. Ylli


    Jun 19, 2018
    LEDs are *current* driven devices. Forget about the voltage across them (unless your circuit is marginal on headroom). Just let the LED drop whatever it likes. The brightness is set by the current through the LED, not by the voltage across it. Put that voltmeter away and just select a resistor that gives you the brightness you desire.
    Martaine2005 likes this.
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Have a look at the I-V characteristics of the LED. Here's an example (from this website):
    See how little the voltage changes when the current changes drastically? a change in voltage (ΔV) of 0.25 V changes the current (ΔI) by 20 mA.
    Note that this is only an example, exact values will depend on the specific LED you use.
    From your measurements you get:

    120 Ω2.447 V21.28 mA
    2.2 kΩ2.340 V1.21 mA
    ΔV = 2.447 V - 2.340 V = 0.107 V
    ΔI = 21.28 mA - 1.21 mA = 20.07 mA
    Your LED has much less change in voltage for a 20 mA change in current, therefore the characteristic curve of your LED would be much steeper compared to above graph.

    Don't t do that. LEDs will require a current limiting resistor. Connecting them to a power supply without any current limiting will destroy them. See our resource.
  10. Martaine2005


    May 12, 2015
    Just to add to this, I had a very similar situation not long ago.
    Putting LEDs in a car to indicate door status was more like a disco in the dark. IIRC, I was down to less than 1mA for the desired brightness.
    Do your calculations as a safe ball park figure then do as @Ylli suggested.

  11. TTL


    Oct 24, 2013
    Now that I did understand! :)
    I gather from all this that a current limiting resistor is mandatory, but the value isn't critical (though I assume there's a minimum safe value -but that's where the online LED resistor calculators can help, right?).

    Ah! So again, the voltage isn't the main thing we're concerned about, but the current.
    I'm glad that my quote above was slightly inaccurate, because there is still the 120 Ohm current limiting resistor between the LED and +5V (see the schematic in the first post), but what I meant was that I didn't insert an additional resistor in series with the LED connector on the computer's motherboard. So this was how the original LED was attached, but with the new LED the light just got too intense.
    Following the suggestion by Ylli (and keeping the resistance higher than 150 Ohms which the LED calculators told me to use) I found 2.2 K to be a suitable value to get the right intensity.

    Thanks for all the info resources about the subject. I've taken a quick look but will read closer whan I get the time.
    Last edited: Jun 8, 2021 at 10:07 AM
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