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LED insertion

Discussion in 'LEDs and Optoelectronics' started by Chip, Dec 8, 2012.

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  1. Chip

    Chip

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    Nov 28, 2012
    I have this circuit with a 2n3055 Transistor and an LED. I have the LED soldered into the base terminal of the transistor. That terminal is fed by a 10,000 ohm resistor from the rectified 25 VDC positive side. The LED goes to ground. I am assuming the anode of the LED goes to the Base connector and conducts (EG light up). If I reverse the polarity, I am assuming I am blocking a path to ground and the LED will not light up, correct?
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    What are you trying to do?

    Chris
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    hi there chip
    welcome to the forums :)

    as chris said, "what are you trying to do ..."
    explain clearly :)

    the circuit description you have given isnt the normal configuration for a LED and a transistor

    Dave
     
  4. Chip

    Chip

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    Nov 28, 2012
    Circuit

    This is a DIY design I got from an elderly Radio Control enthusiast that had an electrical engineering degree. It is for charging, trickle charging NiCd and NiMh batteries at C-10 current rates. It is based on a 2N3055 transistor. It uses a simple filament transformer to bring down the AC to 25 volts, a full wave rectifier, one electrolytic (100 MF). The B plus (25 volts rectified) is fed to a 10,000 ohm resistor that connects to the base of the transistor. The LED is fed by the same resistor and goes to ground. I am sure it is there to provide a light indicating the circuit is active. The emitter is connect to a 25 ohm pot with wiper connected to the ground side of the pot. The positive out put has a rectifier in line to prevent battery back flow . In my set up I put in a jumper to insert Ammeter terminals to set the current flow with the pot. I use banana plugs to out put the voltage (color coded red positive, black ground). Once I have the current set, I insert the jumper, and hook up my battery.

    The circuit allows the voltage to float and regulate the current flow. Thus one can charge one cell to 30 cells in series. It does not matter if the battery is four cells or five cells or eight cells. One then charges for a period of time, say 8 to 12 hours at C-10 values of the battery/cell. Worked fine and then quit.

    I misplaced the schematic and have redrawn it from the actual circuit. However, since the LED is soldered in place, the leads have been clipped and I can not tell the polarity of the LED. The other rectifier in the charge line is easily identified by its band.

    It is simple and it worked fine. Then crapped out. Could be the transistor, LED, transfomer (I checked it, and it is okay). The transistor is T0-3 case and is mounted on to the aluminum box, so has plenty of heat sink. The LED is also mounted in a thru hole in the case as is the switch and jacks and power cord inlet (with protective grommet)

    And I need the schematic for record purposes anyway so need the polarity direction of the LED.
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Post the schematic less the LED and indicate where the LED nodes are connected..

    Chris
     
  6. Chip

    Chip

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    Nov 28, 2012
    I have drawn up a schematic but getting on to this board seems next to impossible. Is there a special way to import a bip map or a word doc? Until then, you will have to use the word picture.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    When you post a reply just click on the paper clip on the tool bar. It will bring up the attachment manager.

    Chris
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    and also... preferably convert it to a .gif file, they are much smaller kb size wise
    and keep the image pixel size to 800 x 600 or less

    Dave
     
  9. Chip

    Chip

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    Nov 28, 2012
    Well fellows, I can get my schematic down to 100 dots per BMP and it is still one kilo byte to big to be accepted by this forum. Will look to see if I can convert the bit map to GIF file.

    Cheers.

    Chip
     
  10. Chip

    Chip

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    Nov 28, 2012
    Schematic

    See attachment

    try again Chip

    there's just a blank image here ;)
     

    Attached Files:

    Last edited by a moderator: Dec 12, 2012
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    hey chip, OK can see that attachment test ok :)

    now draw the circuit and post again :)

    Dave
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    When I read the first lineI thought.. Oh sh!t my eyes are totally gone!!

    Chris
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009
    I just saw a blank white image no cct drawing ??

    Dave
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    That's a good thing. It means I'm not going blind! ;)

    Chris
     
  15. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    I may know that circuit:
    From the statement that this circuit was used in a battery charger I assume it is a simple constant current source.
    The base of the transistor is forward biased at a constant voltage. Lacking a low-voltage zener diode sometimes an LED is used. It's purpose is not lighting anything but holding the base voltage at Vled (e.g. 1.6 V for a red LED). The anode of the LED is to the base of the BJT, the cathode goes to GND. The desciption suggests that the LED was mounted visibly for indicating purposes (only ).

    What's lacking in the OPs original description is a resistor from the transistors's emitter to GND. With Vled=1.6V and Vbe=0.6V, the voltage across the resistor is a fairly constant 1V so the current is I=1V/R. R is the potentiometer (25 Ohm).

    To the OP: the base voltage of the NPN transistor should always be positive. Don't reverse the polarity.

    Harald
     
    Last edited: Dec 12, 2012
  16. Chip

    Chip

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    Nov 28, 2012
    And that is the circuit. Yes, the voltage rating of the LED is 1.7 volts, close enough. The emitter is connected to ground with a 25 ohm pot. No intention to change the bias positive polarity of the base. I assume that a positive source to the base is to make the transistor to conduct. Any way it worked until something happened, like it got kicked, stomped or otherwise has power spike or the like. It is just that all other circuits I looked at were voltage adjustable with a constant current. I wanted a constant voltage with adjustable current to trickle charge NiCds. I came to the conclusion on the LEDs polarity after reading up on them and thinking about it for a while. Thanks for the confidence. I gave up on trying to insert a schematic as the forum had a severe limit and all the drawings were either Bitmap or .GIF and all came in just above the limit for attachment.

    Again thanks. I also had my EE son take a look at it. He found it interesting and as yet has not give it an okay. But I know it works, it worked until . . .

    BTW, I spent almost six years between Bitburg and Spangdahlem. I quite fond of the Eifels, "Der snee Eifels, Ja?" Bitte en Bit, right?
     
    Last edited: Dec 13, 2012
  17. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    That is not what this circuit does. It produces a constant current and the voltage follows the charge state of the battery. The advantage is that this circuit is independent of the number of cells in series, as long as the voltage of the cell stack is less then the operating voltage of the current source.

    If you only want to trickle charge, a simple series resistor from the voltage source to the battery is enough, provided the trickle charge current is kept below ~1/20*capacity (e.g. capacity of the battery = 1AH, nominal charge current = 100mA, trickle charge current <=50mA). That goes for NiCd or NiMh. Other battery chemistries require different charging algorithms.

    That's the ad, not my preference.
     
  18. Chip

    Chip

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    Nov 28, 2012
    I reassembled the transformer into the circuit. Plugged it in and it works like a champ. I got the LED installed at the correct polarity. And yes, it is a infinitely adjustable current charger. The voltage is probably near 20 (I have not measured it yet). So I have my schematic correct and it is now in my electronic and hard files.

    Thanks for the help. Your forum is severly liimited by the less than a 100K byte upload. You need to fix that.
     
  19. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Usually we can live with that limit. Images can be resized!
     
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