Need help with voltage reg for a Power Supply.

Discussion in 'The Projects Forum' started by Ranger71d, Apr 8, 2011.

  1. Ranger71d

    Thread Starter New Member

    Apr 8, 2011
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    I am not very familiar with electronics, but I arrived here via tutorials on converting a computer PSU to make a power supply for LiPo battery chargers.
    I have an old laptop pwr supply I would like to use for the same purpose and I though it would be fun to build this myself and it would allow me to learn something new.
    My charger accepts up to 18v DC, and charges up to 5A.

    My power supply is rated at 20v and 9A.

    I would like to drop the voltage down between 16-18v. From what I have been reading (if I have been understanding it correctly) my problem would be the a large amount of heat generated from dropping down 2-4 volts.

    I think that the LM 338 may work for my application, but I am at a loss as to the rest of the components for the circuit.

    Any help would be appreciated and these forums have been giving me ideas for other projects.

    Thanks
    R71d
     
  2. mbxs3

    Active Member

    Oct 14, 2009
    141
    3
    If you take a look at the LM338 datasheet,

    http://www.national.com/ds/LM/LM138.pdf

    The last few pages will show you some ideal set-ups depending on your application. The people who can give your more information on this forum are probably going to want to know more info about your specific battery charger as well as what you are going to be charging.
     
    Last edited: Apr 8, 2011
  3. Ranger71d

    Thread Starter New Member

    Apr 8, 2011
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    The charger's specs are:
    Operating voltage: 10 - 18.0 Volt
    Circuit pwr: max 50w charging
    max 5w for discharging
    Charge current: 0.1 ~ 6.0A
    Discharge current: 0.1 ~ 1.0A
    Current drain for balancing Li-po: 300mAh/cell
    Lithium battery cell count: 1 - 6 Series

    I will be charging 2S1P 20C 5000mAh 7.4v LiPo batteries
    Continuous discharge: 20C (100A)
    Charge rate: 1C (5A)
    Max volts per cell: 4.2v
    Max volts per pack: 8.4v
    Watt hours: 37.00

    I will also be charging 3S1P 25C 2200mAh 11.1v LiPo batteries later, I don't have these yet as I don't currently need them.

    The power supply I would like to use is a Li Shin Intl Enterprise Corp AC Adapter.
    Mdl: 0415B20180
    Input: 100-240V~, 50-60Hz 2.9A
    Output: 20V 9.0A

    This power supply came with an Alienware laptop that has since died. It currently has a 4pin adapter that plugged into the back of the laptop. I plan on replacing that with banana jacks to fit my application.

    I have looked at the data sheet and have done some reading here, but as I am new to this sort of thing I don't really trust my math. I went to radio shack today and they didn't have any books in store that could help me out.

    Hoping that I can get some help here to help me better understand how to read the schematics and understand how to figure out what components I will need to build a safe power supply.

    Thanks
    R71d
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    You probably don't need anything.
    18V to 20V...pretty close. The way I see it, I think you can just connect it.
    But to be sure I have to see your charger inputs circuitry....can you post a picture of the charger
     
  5. Ranger71d

    Thread Starter New Member

    Apr 8, 2011
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    The charger I bought from Hobbytown. I clipped the pwr supply's connector off and used the chargers alligator clips to see if it would work. No luck. The charger shows input voltage error on the screen.
    Do I need a regulator or will a set up of resistors work?
    R71d
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Do U have a DMM.

    Show me the PSU clipped end wire termination. I like to see how many wires the adapter uses
     
  7. mbxs3

    Active Member

    Oct 14, 2009
    141
    3
    You should probably get a voltage reading on your power supply. Even though it says it is rated at a 20 volt output, it might be a little higher. But either way, get a reading so you know for sure. That way, when you build a regulator circuit, you will know exactly what your input voltage is so you know you won't be exceeding any tolerances and what not and can properly calculate power dissipation.
     
  8. Ranger71d

    Thread Starter New Member

    Apr 8, 2011
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    I used the very old multimeter that I have and got a reading of 18v. I probably need a newer digital multimeter for better accuracy. I tested another pwr supply that I have and got a reading of 9.5v, it was rated at 12vDC. So I don't feel confident in the readings I am getting from it. Looks like a trip to the store.

    I attached a pic of the adapter's plug. It is a four pin connector. Two of the pins were connected to the Neg wire and two pins were connected to the Pos wire.

    Thanks for all the help guys.
    R71d
     
  9. Ranger71d

    Thread Starter New Member

    Apr 8, 2011
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    This charger is built for the vendor by another company. I found a chinese company online that builds these for several companies. Their website lists these chargers at 10 - 18 volts and 6 amps for input. It seems that their circuits have over voltage and over amperage protection. If it sees more volts or amps than it's specs state it will refuse to charge.
    I will need to do some more reading on Amps for a better understanding and find out what components regulate that.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    When attaching pic, re size them as it makes easier for us to load.
    Some will ignore if the pics are too large and if takes longer to download, just do ur part and you'll get more response.

    What I like to see is how much wires ur adopter has. from that I can find what wires you have to use for ur charger.
    Can u see inside of the adapter
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Rifaa, I can see where you're going with this - but keep in mind that our OP is at a novice level with electronics.

    If our OP tries to open the switching supply's case, they may damage it beyond repair. These supplies are frequently cast into a block of epoxy for ruggedness, and to hamper people's efforts to reverse-engineer them.

    If it is not cast in epoxy, our OP will have mains power exposed when they plug it in.

    I can't think of a good way to reduce the supply voltage without changing something inside the regulator itself, but I suggest that would require greater skills than a novice would have.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Thanks Big daddy.....
    I'm always careful..
     
  13. Ranger71d

    Thread Starter New Member

    Apr 8, 2011
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    I was able to open the pwr supply's case rather easily. 4 screws located under the rubber foot pads on the bottom of the case. What I found was a very large aluminum heat sink with thermal paste placed between it and a component and several smaller heat sinks placed vertically on the board.
    I removed the larger heat sink (3 screws) so that I could take a pic of the wires leading out of the board.
    The cable also has 2 items covered in a rubber molding placed inline with the wiring. Not sure what these are.

    Pic 1: Opened pwr supply w/ large heat sink removed.
    Pic 2: 2 wires leading away from board. Its not a high res pic, but the board is labled V+ for the white wire and V- for the Black.
    Pic 3: is of the items placed inline on the cabling.

    Hope the pics are a better size. If a specific pixel size would be better let me know.
    Thanks
    R71d
     
  14. soda

    Active Member

    Dec 7, 2008
    174
    13
    Hi,

    You can use 4x 8Amp silicon diodes in series, forward bias which will give you a 2,1v drop. If you don't know what i mean, the anode of the first diode go to the output of your supply +v then he's cathode went to the next diode anode and so on. Each diode will give you 0,7v drop.
     
  15. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    It's just two wires. I thought u had multiple wires.
    It's just a standard laptop adapter.

    If you can measure the rated output voltage at ± terminals, then ur adapter is OK.

    We need to see ur charger input and it's specs. I think you may be doing something wrong
     
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