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Can anyone help with UVC LED-resistor query?

  • 23-01-2017 1:31pm
    #1
    j4vier
    Registered Users, Registered Users 2 Posts: 1,893 ✭✭✭


    Hi

    I'm trying to develop an application which works on UVC leds for a life science application.

    The issue that I am having is the following.

    I have some basic knowledge on how led work, I've practiced with visible range LED a lot prior to using UVC ones (due to the cheaper price).

    My circuit is fairly simple, DC power in (12V), limiting resistor wired to the anode of the led and cathode connected to ground.

    With visible range LEDs, all is ok. They turn on fine, it obviously help that I can see them. I can measure the voltage drop across the resistor and the drop across the led as I would expect. However with the UVC led things are not working out in the same way.

    The spec of my UVC led are: forward voltage 7.1 V and the desired current is 20 mA.

    So I picked a resistor of 270 ohm for my purpose, connected between the +ve wire and the anode of the led.

    The first difficulty is first that I obviously cannot see such low wavelengths; the UVC led is inside a protecting enclosure which prevents leakage, hence I wired 2 extension leads on the anode and the cathode which come out of the enclosure.

    I used a tester to measure the voltage drop across the led and instead of 7.1V , I am reading 12 volts, so something is not right there.

    The resistor is working fine on its own, again i measured the resistance with a tester and reads 269 ohms with the circuit off, however it seems that once I turn on the circuit, there is no resistance at all.

    Would anyone have any insights on what is happening here?

    Thanks a lot for your help


Comments

  • #2
    Chet T16
    Registered Users, Registered Users 2 Posts: 2,320 ✭✭✭Chet T16


    Can you post a pic?


  • #3
    j4vier
    Registered Users, Registered Users 2 Posts: 1,893 ✭✭✭j4vier


    Chet T16 wrote: »
    Can you post a pic?

    Hi, apologies for the quality. With the tester , I am measuring the voltage between Ext 1 and Ext 2, extension leads for the anode and cathode part of the led.

    The reading is 12V


  • #4
    Pete67
    Registered Users, Registered Users 2 Posts: 1,257 ✭✭✭Pete67


    It sounds like the UVC LED is either open circuit, or the wrong way around. With no current flowing, the resistor will not drop any voltage so the full 12V will be seen at the LED.


  • #5
    j4vier
    Registered Users, Registered Users 2 Posts: 1,893 ✭✭✭j4vier


    Pete67 wrote: »
    It sounds like the UVC LED is either open circuit, or the wrong way around. With no current flowing, the resistor will not drop any voltage so the full 12V will be seen at the LED.

    Thanks for that.

    I have a feeling that the forward voltage information I was given might be wrong.

    In the spec sheet that came with it, it stated 7.1 V while online on the seller's site, the sheet for the same led states 10V.

    I chose my resistor to suit a forward voltage of 7.1 , therefore the led did not get enough voltage to turn on.

    Do you reckon that the led might still be ok?


  • #6
    ozmo
    Registered Users, Registered Users 2 Posts: 3,888 ✭✭✭ozmo


    Use a glow in the dark toy or sticker... or highlighter marker
    These will glow in UV light.

    Be careful with the light - Powerful UV light can be dangerous to eyes and skin.

    https://www.youtube.com/watch?v=cjFXKyMVsn8

    Also - probably no use to you - but at the other end of the spectrum - you can see IR light very clearly using most Androids (not IPhone) cameras.

    “Roll it back”



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  • #7
    whizbang
    Registered Users, Registered Users 2 Posts: 1,974 ✭✭✭whizbang


    But 10v is almost unheard of for any single LED, maybe the seller meant 10v power supply...
    Can you post the part number of the LED>?

    LED Power; 7v at 20mA is 140mWatt, safe enough to look briefly. A lot of these LEDS will give a visible purple light.

    otherwise, with your setup, if the LED is correctly wired, there is definitely be enough voltage to turn on the LED in some way.
    If its backwards, just swap the power wires. however check the Reverse voltage rating of the LED. Often this is very low (<5volts) and can be destroyed by wiring them backwards


  • #8
    ZENER
    Registered Users, Registered Users 2 Posts: 6,163 ✭✭✭ZENER


    My guess is that this is a 2mW UV-C device. Like the one at the bottom of this page.

    This one gives uses a V-fwd of 7.5 V @ 20mA, so from a 12V supply you would need a 225 ohm resistor in series with the LED. 7.5 Volts is the Max forward voltage though, typical value is 6.5 according to the RS data sheet for this particular device. It also shows a protection avalanche diode across the LED to protect it from too high a reverse voltage though a value isn't provided.

    It's possible you have the LED the wrong way round and it's behaving like any diode would when reverse biased. It's also possible you've overheated it ?

    Ken

    Edit: You say that there is 12 Volts across the Diode with the circuit on, is there any current flow in the circuit ?


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