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Help making a 21.5V adaptor

  1. Jul 28, 2009 #1
    I have a device that requires an AC adaptor rated at 120V AC input, with 21.5V, 300mA DC output.

    Seems this is a bit unconventional as I can't find anything like this anywhere, and I'm wondering if there's a way to make one. I was thinking that I might somehow combine a couple 12V adaptors, it doesn't have to be pretty at all.

    I'm good with a soldering iron, but bad with the physics. Can anyone help me?
     
  2. jcsd
  3. Jul 28, 2009 #2

    berkeman

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    Welcome to the PF. What is the device you are powering? Can you post a link to its datasheet/info?
     
  4. Jul 28, 2009 #3

    negitron

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    Get one of these:

    http://www.national.com/mpf/LM/LM317.html [Broken]

    And feed it with any DC source with sufficient current capacity up to 40 volts over the required output voltage (in this case 21.5 + 40 = 61.5 VDC). The PDF datasheet will guide you on selecting the appropriate biasing resistors to set the output voltage.
     
    Last edited by a moderator: May 4, 2017
  5. Jul 28, 2009 #4

    vk6kro

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    This input is possibly reduced to 12 volts or so inside the device, so it probably doesn't need to be be regulated.

    A simple brute force power supply is just a transformer giving about 16 volts, a bridge rectifier followed by a large electrolytic capacitor. This would give about 21 volts after the rectifier. The capacitor should preferably be a 40 volt type and maybe 3000 uF or more.

    A ready made solution would be a laptop power supply which you may be able to find cheaply. These give 18 to 20 volts DC and often get thrown away when the batteries in the laptop fail. There are a lot of these on Ebay at present.
     
  6. Jul 28, 2009 #5

    negitron

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    Yeah, well, these kinds of assumptions can be expensive.
     
  7. Jul 28, 2009 #6

    berkeman

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    That's why I asked for a link to the datasheet/application. Hopefully we won't let any smoke out of the device... :redface:
     
  8. Jul 28, 2009 #7

    vk6kro

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    Not assuming anything.

    If it needs to be regulated, that has been covered quite well, and if it doesn't, he can be a bit more brutal with a simple power supply.
     
  9. Jul 29, 2009 #8
    Thanks for the welcome.

    The device is a Boss BX-8 stereo mixer, but I can't seem to locate a data sheet for you anywhere online and don't have the manual.

    I actually tried the laptop power supply idea, but all I had around was a 16v and it worked for a second, then petered out, I suppose as the transformer decharged or something. I gave up on that, but maybe I should revisit the idea, it seems like the most simple.

    I don't see anything with the exact specs on eBay, but if the original was 21.5V at 300mA, is there a range that will suffice, and perhaps something I shouldn't exceed?
     
  10. Jul 29, 2009 #9

    vk6kro

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    Does it drive speakers or does it use an amplifier to drive the speakers?


    Are you sure you got the polarity right last time you tried the laptop supply?
     
  11. Jul 29, 2009 #10

    negitron

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    http://www.boss.manuals-in-pdf.com/BX8-p-1143615.html [Broken]

    You may be able to find it online for free someplace, but it's here if you want it.
     
    Last edited by a moderator: May 4, 2017
  12. Jul 29, 2009 #11

    vk6kro

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    Last edited by a moderator: Apr 24, 2017
  13. Jul 29, 2009 #12
    Yeah, I saw the pdf thing, but not the replacement. But I really don't want to spend any money on a manual that probably doesn't have the info I need, it isn't the service manual.

    And the replacement, if they can even get it is gonna be at least 50 bucks. I doubt the mixer is worth much more than that so I think I'll try something else.

    Yeah, I had the polarity correct, I'm certain of that. And when I turned it on, it worked for a few seconds, then fizzled out. I could turn it off for a few seconds and it would do the same thing, again and again. I think there just wasn't enough power at 16V to keep it going with a requirement of 21.5V. Now, I may be wrong about that because electronics and power ratings are topics over my head.

    Still hoping for some advice about what to get on eBay from my previous post...anyone?
     
  14. Jul 29, 2009 #13

    berkeman

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    I'd look for a 24V, 500mA brick power supply. Should be some available via e-Bay.
     
  15. Jul 29, 2009 #14

    vk6kro

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    Yes, mixers like that would draw fairly constant current so the power supply doesn't need to be anything special. It would have a regulator built into it.


    Ideally, you could use a variable voltage power supply to get a proper idea of the voltage range it would work over.

    It does seem odd, though, that it didn't work at all on 16 volts. You might expect it to give distorted output but not stop altogether if the voltage was just a bit too low.
    Maybe it has some internal fault.

    It looks like it was an expensive unit when new.
     
  16. Jul 29, 2009 #15
    Well, it sorta worked. It has an on-off switch. If it was plugged in to the 16v while switched off, I could then turn it on, and it would work fine for a second or 2, then start to distort for a second or two and then would seem to turn off or be dead. I could turn it off with its switch, and then turn it on again, and the same thing would repeat.

    I tried it again today, using a different laptop power supply with slightly higher specs and I didn't seem to be able to get it to do much of anything, so I hope I didn't fry something along the way. I probably did, but I don't know what would do that, if anything.

    I notice the original power socket is 3 prong, and all the laptop power supplies I tried are 2 prong. I don't know if this matters.
     
  17. Jul 29, 2009 #16

    vk6kro

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    Do you know the history of the mixer? Was it working when you got it?
    It sounds like it is faulty now.

    There is probably an internal regulator in the mixer. One possibility is that this has failed and is letting the 16 volts input voltage through to the lower voltage circuitry.

    The way it works for a while may mean that it needs an new regulator and it will then work. It might not, too.

    Take the lid off the mixer and have a look at where the + input goes after it enters the box. Look for what looks like a transistor but is really a regulator.

    If this is there, try measuring the output voltage of it when you apply power from the 16 volt laptop adaptor.
    If it is still 16 volts or so, switch off and repair the regulator circuit.
    You may need to ask for help from someone familiar with this sort of repair.
     
  18. Jul 30, 2009 #17
    Hmmm. I'll give that a try. I think it was working, though it had been set aside for a long time.
     
  19. Aug 26, 2009 #18
    What you all are missing here is that this takes what is called a split or dual rail power supply (I could have the name wrong). Look at the picture- there is a +21.5 pin, a -21.5 pin and a ground. This will take more than a wall wart to power it. Bonus points if you can guess why I am taking a picture of it with the power supply plug removed- it's because I am in the same boat as the original poster. I immediately recognized that the supply was something different and have not tried to hook it up to anything. I hope the original poster didn't cook anything inside- I don't know how they tried to hook it up.

    A power supply could be built with a center trap transformer or you could use a circuit like this:

    http://discovercircuits.com/DJ-Circuits/splitsupply1.htm

    I found this after a few seconds of searching. Change your supply voltage and regulator chip voltage (if there is such a thing as a 21 volt regulator) and you might have a supply that will run it. I have not tried this, although I do operate a TV satellite truck and work in the maintenance shop when not out in the field so I do have the skills to build such a device. Compared to the cost of the original one, you would have to get the supply transformer for next to zero to make this cost effective and even then there might be hum issues unless the supply was very well regulated. Perhaps that was the reasoning to use a + and - type supply- to have any hum cancel itself out but I wouldn't want to take the time to build such a supply and then have hum problems with the mixer- it would kind of negate any reason to use the mixer.

    I will be saving up a few pennies to buy the overpriced factory supply in a month or so. If the original poster successfully builds a supply, I'd like to know however.

    Val
     

    Attached Files:

  20. Aug 26, 2009 #19
    Well what you could do is pull a transformer out of an old audio amplifier, these are typically centertapped 35v or so units in low power equipment, like around 30W or so. From there you can place a bridge rectifier across the output and use the center tap as your ground. *Bonus* the amplifier or stereo you pull apart should have everything you need to build a supply inside it!
     
  21. Aug 26, 2009 #20
    HA! No wonder I couldn't get this thing going. I just figured it was toasted. I looked at the board and saw something that looked like some damage on the printed copper itself, soldered a couple bridges in there. Hooked it up to a laptop power supply I found and BINGO! Nothing. I figured the thing was junk and set it aside.

    Seems like a lot of trouble to get this thing going and the only factory supply I found is stupidly expensive and non-refundable, so not worth the risk. I would love to hear if someone figures this out, I guess I'll hold onto it for a bit and see what turnes up.

    Good luck to you Val, let me know what you discover.
     
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