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    Need help with LED Power Supply Design

    Hello,

    I am trying to finish a small project...

    Here is the schematic:



    Here is the video of what the circuit does when I turn it on:

    You can see that the LEDs simply flicker off while they continue to draw current ... you can also see that I check components for overheat but nothing in the circuit is too hot to touch.

    I'm stumped, no idea whats going on here...

    Any clues?

    Thank you
    Attached Files

    #2
    Re: Need help with LED Power Supply Design

    You need to be clear about how many White LEDs you are driving.
    No caps' value shown. What is the output Voltage of the regulator? 10.25Vdc?
    45V input that means the diff V between input and out out is about 35V (40V TYP.) so when power is first applied, it will have about 45V Vdrops on the IC until te output cap is fully charged.
    Why use 32VAC as the supply? You can use lower Voltage and set the 317 as the constant current source instead. LED is current device so you will want to maintain constant current through it.
    I.E. http://www.bristolwatch.com/sr/cc_led.htm
    http://www.reuk.co.uk/wordpress/ligh...-led-lighting/
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

    Inverter testing using old CFL:
    http://www.badcaps.net/forum/showthr...er+testing+cfl

    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
    http://s807.photobucket.com/user/budm/library/

    TV Factory reset codes listing:
    http://www.badcaps.net/forum/showthread.php?t=24809

    Comment


      #3
      Re: Need help with LED Power Supply Design

      wow.

      well 32vac rectified would be as high as 46v
      that's too much for an lm317.
      you also want ceramic caps between the input & ground and the output & ground close to the regulator

      what's your led load??

      Comment


        #4
        Re: Need help with LED Power Supply Design

        DUDE! You are using a 25V filter capacitor ?!!! It's going to POP
        I see a 35V cap on the LM317 output.

        The flickering is what I see in LED's that are overdriven or overheating, with broken bonding wires.

        Comment


          #5
          Re: Need help with LED Power Supply Design

          Originally posted by budm View Post
          You need to be clear about how many White LEDs you are driving.
          Ideally, TEN, currently, eight.

          Originally posted by budm View Post
          - No caps' value shown.
          - What is the output Voltage of the regulator?
          - 45V input that means the diff V between input and out out is about 35V (40V TYP.) so when power is first applied, it will have about 45V Vdrops on the IC until te output cap is fully charged.
          - Why use 32VAC as the supply?
          Cap on input is 1000µf, Cap on output is 470µf (the source voltage is VERY noisy!)

          Regulator output is 12V

          32V AC Input, not 45. I am using 32 VAC as the input because I am retro-fitting these LEDs to light up an LCD display in my soldering iron, and the transformer in the Iron base only gives me 32VAC.


          Originally posted by budm View Post
          LED is current device so you will want to maintain constant current through it.
          I am aware of this.

          Thank you!

          Comment


            #6
            Re: Need help with LED Power Supply Design

            Originally posted by stj View Post
            wow.

            well 32vac rectified would be as high as 46v that's too much for an lm317.
            My scope showed (I think) 37 fully rectified, but when I cap it and load it, I'm pretty sure it went back to low 30's.

            Originally posted by stj View Post
            you also want ceramic caps between the input & ground and the output & ground close to the regulator.
            All of my ceramic caps are in pico farads ... nothing meatier than that ... got some you wanna donate?

            Originally posted by stj View Post
            what's your led load??
            Are you asking me for the measured R value of the LEDs when not in the circuit?

            Comment


              #7
              Re: Need help with LED Power Supply Design

              Originally posted by redwire View Post
              DUDE! You are using a 25V filter capacitor ?!!! It's going to POP I see a 35V cap on the LM317 output.
              Damnit, you're right! I missed that ... my 50V caps are lower farads but should do the job I'm sure.

              Originally posted by redwire View Post
              The flickering is what I see in LED's that are overdriven or overheating, with broken bonding wires.
              You know, now that I think about it ... I might have overdriven these when I was trying to see how bright I could get them ... I'll try a new batch properly choked and see if I get the same behavior.

              THANK YOU!


              Mike

              Comment


                #8
                Re: Need help with LED Power Supply Design

                Originally posted by EasyGoing1 View Post
                My scope showed (I think) 37 fully rectified, but when I cap it and load it, I'm pretty sure it went back to low 30's.
                All of my ceramic caps are in pico farads ... nothing meatier than that ... got some you wanna donate?
                Are you asking me for the measured R value of the LEDs when not in the circuit?
                32VAC RMS after rectification and filtering by the filter cap, the Voltage will be close to 45V due to the filter cap will be charged to peak Voltage. That input filter cap should be rated at least 50V or more since your AC line is not regulated so it can go higher.
                The value of the resistors you use will not give you 12V, it will be less but if you get way lot more than calculated value then you need to check your wiring.
                http://www.reuk.co.uk/wordpress/elec...ge-calculator/

                Are the 10 White LEDs connected in series or parallel? 10 LED in series will give the total Vf around 30V at rated current of the LED.
                You cannot check the resistance of the LED, it is not linear device.
                Last edited by budm; 08-17-2018, 12:12 AM.
                Never stop learning
                Basic LCD TV and Monitor troubleshooting guides.
                http://www.badcaps.net/forum/showthr...956#post305956

                Voltage Regulator (LDO) testing:
                http://www.badcaps.net/forum/showthr...999#post300999

                Inverter testing using old CFL:
                http://www.badcaps.net/forum/showthr...er+testing+cfl

                Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                http://s807.photobucket.com/user/budm/library/

                TV Factory reset codes listing:
                http://www.badcaps.net/forum/showthread.php?t=24809

                Comment


                  #9
                  Re: Need help with LED Power Supply Design

                  Originally posted by budm View Post
                  Are the 10 White LEDs connected in series or parallel? 10 LED in series will give the total Vf around 30V at rated current of the LED.
                  The LEDs are in parallel ...

                  Originally posted by budm View Post
                  You cannot check the resistance of the LED, it is not linear device.
                  That's why I asked, because I was unaware of a way to measure resistance of any diode short of looking at the datasheet, which I assume contains those values.

                  Comment


                    #10
                    Re: Need help with LED Power Supply Design

                    Can we info about that LED assembly? Are all 10 LEDs directly connected to each other in parallel without any balancing resistor for each LED?
                    Connecting LEDs directly in parallel is not good idea since Vf of the LEDs will not be the same.
                    Never stop learning
                    Basic LCD TV and Monitor troubleshooting guides.
                    http://www.badcaps.net/forum/showthr...956#post305956

                    Voltage Regulator (LDO) testing:
                    http://www.badcaps.net/forum/showthr...999#post300999

                    Inverter testing using old CFL:
                    http://www.badcaps.net/forum/showthr...er+testing+cfl

                    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                    http://s807.photobucket.com/user/budm/library/

                    TV Factory reset codes listing:
                    http://www.badcaps.net/forum/showthread.php?t=24809

                    Comment


                      #11
                      Re: Need help with LED Power Supply Design

                      Originally posted by EasyGoing1 View Post
                      I am trying to finish a small project...


                      Why are you adding the bridge, filter caps and regulator?

                      Just arrange the LEDs in a series string and feed them with a half-wave rectified source behind a suitable ballast (resistor).

                      E.g., if you have a 32VAC source, your peak voltage will be about 45V at nominal line voltage. Give another 10% for high line and call it 50V; ~40 for low line.

                      Assume you want to pass ~20mA through the LED string and don't want to dissipate too much heat in your ballast (resistive or electronic).

                      45 - ~2*N = voltage across ballast (at nominal line -- assumes ~2V for Vf)
                      0.02 * (45 - 2*N) = power dissipated in ballast

                      So, with 10 LEDs in series, you're looking at 0.5W in the ballast -- whether that is a semiconductor/active device or a passive. (as Vf increases, Pd will decrease)

                      Stick a few "extra" LEDs in the chain and you can move some of that power (heat) out of the ballast. E.g., with 15 LEDs, you're at 300mW (the other 200mW has been "relocated" into the extra 5 LEDs).

                      Omitting the filters means your LEDs will "blink" at 60Hz. <shrug>

                      Omitting the regulator means intensity will vary with line voltage (do you really think you will notice??)

                      Comment


                        #12
                        Re: Need help with LED Power Supply Design

                        OP said the 10 LEDs are connected in parallel. I have a feeling they are not in parallel.
                        Never stop learning
                        Basic LCD TV and Monitor troubleshooting guides.
                        http://www.badcaps.net/forum/showthr...956#post305956

                        Voltage Regulator (LDO) testing:
                        http://www.badcaps.net/forum/showthr...999#post300999

                        Inverter testing using old CFL:
                        http://www.badcaps.net/forum/showthr...er+testing+cfl

                        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                        http://s807.photobucket.com/user/budm/library/

                        TV Factory reset codes listing:
                        http://www.badcaps.net/forum/showthread.php?t=24809

                        Comment


                          #13
                          Re: Need help with LED Power Supply Design

                          My opinion of the circuit in pictures:

                          Comment


                            #14
                            Re: Need help with LED Power Supply Design

                            Yep, all of that just to light up LEDs.
                            Never stop learning
                            Basic LCD TV and Monitor troubleshooting guides.
                            http://www.badcaps.net/forum/showthr...956#post305956

                            Voltage Regulator (LDO) testing:
                            http://www.badcaps.net/forum/showthr...999#post300999

                            Inverter testing using old CFL:
                            http://www.badcaps.net/forum/showthr...er+testing+cfl

                            Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                            http://s807.photobucket.com/user/budm/library/

                            TV Factory reset codes listing:
                            http://www.badcaps.net/forum/showthread.php?t=24809

                            Comment


                              #15
                              Re: Need help with LED Power Supply Design

                              Originally posted by budm View Post
                              Yep, all of that just to light up LEDs.
                              I changed out the LEDs with 5730's and ended up using this circuit which works like a champ EXCEPT that there is some very slight but noticeable flickering in the light ... I figured maybe another cap would help ....

                              Comment


                                #16
                                Re: Need help with LED Power Supply Design

                                You should have a cap on the lm317 output, around 10-47uf should be ok, these regulators can go into oscillation without it.
                                Last edited by R_J; 09-17-2018, 11:33 AM.

                                Comment


                                  #17
                                  Re: Need help with LED Power Supply Design

                                  (i have a horrible keyboard as my preferred one just died recently and new one didn't arrive yet , until i replace it i may miss some letters or not use uppercase, the keys are truly awful, sometimes they don't trigger even when i press the keys ... my bad typing is not meant as disrespect, i apologize)

                                  The whole circuit is wrong.

                                  First of all, leds are current driven devices, they only have a minimum voltage (forward voltage) from where they are fully on and start producing light. once the supplied voltage is above the forward voltage, you have to limit the current going through the leds otherwise they can blow up due to overheating.

                                  in your circuit you're using the two resistors 250 ohm and 1.8 kohm to set the output voltage of the regulator. the formula used for that is:

                                  VO = VREF (1 + R2/R1) + IADJ R2 where Vref = 1.25v and I adj is practically nothing so you can ignore that.

                                  So basicaly in your case, Vout = 1.25 ( 1 + 1800/250) = ~ 10.25v DC.

                                  your output voltage must be at least number of leds x forward voltage of each led.

                                  For 10 white leds with an average forward voltage of 3v, I would configure the output voltage to around 32v (10 x 3v + ~ 5% tolerance)

                                  you have the formula voltage = current x resistance so you can use this to determine the current going through the leds.

                                  V = I x R => 32v (input voltage) - 10 x 3v (10 leds with forward voltage of 3v) = I x 260 ohm

                                  so current is I = (32-30) / 260 = 0.0076 A or around 7.6 mA

                                  if you want each led to be brighter you have to reduce the resistor value to allow more current to flow.
                                  let's say you'll want 10mA through the leds.... in that case 0.01A = 2 / R => R = ~ 200 ohm , so you'd either go with two 100 ohm in series or a standard 210 ohm

                                  the power dissipated by the resistor would be P = I x I x R where in your case current is 0.0076 A and your R = 260 ohm , so p = 0.0153 watts , so a simple 0.25w resistor would be suitable, but using a 0.5w resistor wouldn't be a bad idea.

                                  Now, another thing that's important t keep in mind is that if you check the regulator datasheet, the LM317 requires a minimum load in order to be stable. Most datasheets say the average minimum load required is 3.5 mA and maximum is 10mA.

                                  The datasheet recommends using a resistor in the 240 ohm range to adjust the output voltage because that causes the regulator to waste a few mA internally. I'm too lazy to do the math now but I suppose it's somewhere in the 5mA-ish range.

                                  Basically, your configured current for the leds at around 7.5 mA is high enough that together with the internal current consumption of the regulator, the total consumption should always be higher than 10mA so the regulator should be stable.

                                  However, if you think you're gonna reduce the current below let's say 5mA or you want to be absolutely sure the regulator will be stable, then you may want to change that 250 ohm resistor to a smaller value, like let's say 100 ohm and also change the other resistor to keep the output voltage where you want it.
                                  With smaller resistor there, you'll allow a bit more current to be wasted inside the regulator.

                                  Now... the regulator needs at least [output voltage + ~ 1.5v..2.5v ] at the input, in order to output a stable output voltage.
                                  The amount of voltage above the output varies with the current. For only 10mA or so, probably just 1.5v over the output voltage is enough. For 1A+, you'll probably need to use the 2v number.
                                  The minimum voltage also varies with heat, as the regulator heats up, the regulator may be happy with lower difference between input and output voltage but it's not good practice to use absolutely minimums ... hence it's safer to just use 2v when calculating things.

                                  So if you want to output 32v to the leds, then you should make sure to have at least 32v +2v = 34v DC at the input and that's the job of that input capacitor.

                                  You have a 32v AC transformer and you use a bridge rectifier to create Dc voltage ... this means the dc voltage will have a peak voltage of

                                  Vdc pea = V ac x 1.414 - 2 x diode voltage drop.

                                  for low currents like 10mA, the ac transformer will probably output more than 32v AC (probably +10% of that) and the diode voltage drop will probably be around 0.6v but let's assume you're making circuit for various currents and go with a more sane 1v per diode... in this case :

                                  Vdc peak = 32 x 1.414 - 2v = ~ 43v

                                  Note though that at low currents like 10mA like I said the transformer may output way more than 32v AC so your peak DC voltage could get very close to 50v, and that's why you should use input capacitors rated for 63v or higher.

                                  now you now the peak dc voltage is 43v, you need at least 34v at the input of the regulator, and you know the current you want the leds to use (~5mA inside the regulator and the current through the leds, in your case 7.5mA , so let's say 15mA in total or 0.015A ... but just to allow for adjustments later, let's calculate for maximum 50mA of current or 0.05A )

                                  So you can estimate the capacitor size with the formula :

                                  ac frequency i assume is 60 hz for us:

                                  C = current / [ 2 x ac frequency x ( Vdc peak - Vdc min) ] = 0.05A / 2 x 60 hz x (43-34) = 0.05 / 1080 = 4.6296296e-5 Farads, which I think works out to either 46.2 uF or 462 uF .... that's the minimum capacitor size you should use to guarantee voltage will be above 34v on the regulator.

                                  keep in mind also that the regulator really won't be designed to handle such high voltage, there are versions of these regulators designed to work with up to 60v but they're more expensive.

                                  There are LED driver chips designed for such purpose, which work much easier and better, here's some easy to use examples:

                                  (up to 150mA configurable current) 0.77$ each AL5812 : https://www.digikey.com/product-deta...DKR-ND/3678111

                                  https://cdn.badcaps-static.com/pdfs/...476041f3ab.pdf

                                  1$ each but easier to solder even by hand , up to 70mA of current NUD4011 : https://www.digikey.com/product-deta...SCT-ND/2122046 (check datasheet, esp. pages 4 and ... https://cdn.badcaps-static.com/pdfs/...b18cf442b0.pdf )

                                  more expensive but switching regulator design so less heat produced and can do up to 550mA current, you need an inductor and some resistors besides the chip but easy to find and datasheet explains well

                                  2$ each ZXLD1356 ET5TA https://www.digikey.com/product-deta...5TR-ND/1986324 datasheet : https://cdn.badcaps-static.com/pdfs/...533b95ad90.pdf
                                  Last edited by mariushm; 09-17-2018, 01:24 PM.

                                  Comment


                                    #18
                                    Re: Need help with LED Power Supply Design

                                    LEDs are current-operated devices. If parallel-connected, nothing forces the devices to share the current equally. An LM317 can be used as a constant current source as well as a voltage regulator.
                                    PeteS in CA

                                    Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
                                    ****************************
                                    To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
                                    ****************************

                                    Comment


                                      #19
                                      Re: Need help with LED Power Supply Design

                                      Originally posted by EasyGoing1 View Post
                                      I changed out the LEDs with 5730's and ended up using this circuit which works like a champ EXCEPT that there is some very slight but noticeable flickering in the light ... I figured maybe another cap would help ....

                                      That is only 16mA of constant current source.
                                      1.25Vref/80 Ohms = 16mA
                                      And Yes, it needs to have filter cap on the output.
                                      Last edited by budm; 09-18-2018, 04:19 PM.
                                      Never stop learning
                                      Basic LCD TV and Monitor troubleshooting guides.
                                      http://www.badcaps.net/forum/showthr...956#post305956

                                      Voltage Regulator (LDO) testing:
                                      http://www.badcaps.net/forum/showthr...999#post300999

                                      Inverter testing using old CFL:
                                      http://www.badcaps.net/forum/showthr...er+testing+cfl

                                      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                      http://s807.photobucket.com/user/budm/library/

                                      TV Factory reset codes listing:
                                      http://www.badcaps.net/forum/showthread.php?t=24809

                                      Comment


                                        #20
                                        Re: Need help with LED Power Supply Design

                                        Originally posted by budm View Post
                                        That is only 16mA of constant current source.
                                        1.25Vref/80 Ohms = 16mA
                                        And Yes, it needs to have filter cap on the output.
                                        The resistor I actually chose was not 80 ohms ... I just didn't change the details in the image, only the configuration.

                                        Comment

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