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Thread: LED Novice seeking some friendly help

  1. #1
    Unenlightened
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    Smile LED Novice seeking some friendly help

    Hello everyone!I have been a browser on here for years and have finally registered, I have to say i'm completely blown away by the work you guys do and now have an unhealthy interest in torches!I am an electrician by trade but my main day to day job is engineering in which I design and build control panels. I pretty much walked into this and have built up my knowledge along the way, sometimes just copying what I have seen others do but mostly researching and doing things properly. The reason for the post today is to get some advice on making one particular area of my production go a lot quicker. I build control handsets for my panels which house individual LED's for each channel, these are Bi-Colour Green/Red 5mm.Inside the panel I use a 24v AC Transformer, the reason for AC is the fact the cable runs for the systems we build can sometimes be in excess of 100 mtrs and the volt drop from DC is too much.Currently I purchase the LED's loose from RS link below.....https://uk.rs-online.com/web/p/visib...3537363426With these resistors....https://uk.rs-online.com/web/p/throu...2353131373626I then solder a Brown cable to the Green leg, a Yellow cable for the Down leg and a Blue to the centre pin. Inside the handsets I use TPDT Toggle switches to which I solder a resistor to each common for the LED side of the switch. I then solder a Red cable in series from each resistor back to the incoming 24v AC feed. All the Blue cables from the LED's are connected to the incoming 0 volts from the panel.What I have a problem with is this obviously takes a long time to do for each job! I'm actually pretty fast at doing this now and to build a handset like you will see in the pictures below only really takes me a couple of hours from scratch. I have attempted to purchase pre made LED's in the spec I need but each time they have either been too bright...too dark or simply blown within a few days.My main question is.... am I doing something completely wrong here?? Is there a simpler way to wire this in to save me some time?? I have never had any issues with the LED's in the way they are wired now.I was going to post some pictures but realised I'm not allowed to yet!! Ha I can email if needed!

  2. #2
    Flashaholic* DIWdiver's Avatar
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    Default Re: LED Novice seeking some friendly help

    Wow. So much ground to cover.

    First, I suspect the reason you are blowing LEDs is that you are ignoring the maximum reverse voltage specification that RS didn't publish. LEDs typically have a max reverse voltage of around 5V. Running them on 24VAC with just a resistor is pretty dicey. To protect them, you need to put a series diode that can block the ~35 volts peak reverse voltage, or put one in reverse-parallel that can limit the reverse voltage to less than a volt. Some lamps designed for 5, 12, or 24V have this protection built in. Some do not.

    24V and 1.2K gives 20 mA, which is generally a good current to run 5mm LEDs at, but the LED causes half-wave rectification, so you really only have 10 mA RMS in the LED.

    If you are wiring the same circuit time after time, I would consider making a PCB. Believe it or not, it is now possible to get PCBs made and delivered for under $2 each in low quantities, using free on-line design software that doesn't take a degree to use.

    Nobody is allowed to post pictures on the forums. But AFAIK anyone, yes even you, can post links to pictures that show up as pictures when people view your post. CPF is run on a shoestring budget, and there's not room for the massive data storage that pictures represent. So you have to put your pictures on a separate site -picasa, shutterfly, dropbox, google drive, etc. Then when writing a post you click the 'insert an image' icon and type in the URL for your picture. When people view your post, the picture shows up even though the data is not on CPF servers. The advantage is CPF doesn't need massive servers. The disadvantage is that if you change your picture hosting site, even years later, the pictures disappear from CPF. I know how to do if from Dropbox (which includes editing the URL that Dropbox gives you). Others could advise you about other sites if necessary.

    I assume you are running line voltage down your 'long cable runs', then stepping down to 24V at the far end where the 24V doesn't have to travel along long wires. If you are stepping down to 24V then running your long cable runs, we need to have a conversation about wire losses.

    Oh, and welcome to the forums!
    Last edited by DIWdiver; 12-20-2017 at 10:40 PM.

  3. #3
    Unenlightened
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    Default Re: LED Novice seeking some friendly help

    Hi DIWDiver,

    Firstly thank you for the reply! Much appreciated.

    I'm unable to update my profile yet so I can't display that i'm in the UK if that makes a difference!

    Regarding the LED's I use at the moment, these work perfectly in the set up i'm using. I've never had one blow or do dim etc.

    The problem has come when I have asked another company to make them for me, from what you have said they must be using sub standard equipment. Regarding the transformer I use a 160VA Schneider/Telemech unit, moving to a 250VA on my bigger panels. I do indeed step down to 24V in the panel (From 400v 3 phase) and have this running down long cable runs. As said though I've not had issues even after 100 mtrs?

    Regarding the PCB this would be tricky! I will get some pictures together to show you my work and just how little space I have to work with

    Thanks again

  4. #4
    Flashaholic
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    Default Re: LED Novice seeking some friendly help

    Quote Originally Posted by DIWdiver View Post
    Wow. So much ground to cover.

    First, I suspect the reason you are blowing LEDs is that you are ignoring the maximum reverse voltage specification that RS didn't publish. LEDs typically have a max reverse voltage of around 5V.
    5V max reverse is in the data sheet.

  5. #5
    Flashaholic* DIWdiver's Avatar
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    Default Re: LED Novice seeking some friendly help

    Quote Originally Posted by ssanasisredna View Post
    5V max reverse is in the data sheet.
    Oops! I misspoke. I didn't actually read the datasheet, just the marketing page in the link.

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