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Chat_Ghosty's PSU

Discussion in 'Power Electronics' started by Chat_Ghosty, Jul 1, 2012.

  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No, you are probably looking at dual diodes (on heatsinks). A similar looking device will be a MOSFET (often on the other side of the transformer (one of the large lumpy things).

    Playing around inside these can kill. Be VERY careful. As well as mains voltage, there is 400VDC and a capacitor that will hold the charge for (often) a significant period. You may not get to touch them a second time. (with the power off, you're most likely to yell and throw the power supply across the room).
     
  2. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    I looked in my Component stash and I found the Fallowing:

    HD7400P - 2-Input NAND-Function Logic Gate
    7400PC - Quad 2-INPUT NAND Gate
    8 Segment Display W/ 7 Resisters
    8x 2N107 Transistors
    2x 121-Z9030 - NPN Transistor
    SK3003 - PNP Transistor.
    2 Pole Button.
    A Ton of Resisters.

    And all the Parts out of Two Computer PSU.

    And I wanted to say Thank you to all of you. But I found Kris for telling my Why my Diagram did not work. Anyway.

    Ill get to Desoldering one of the PSU and see what I can get. :)

    Note: Lol. My Teacher had some Rail Road Capacitors. He hooked them in Series. Charged them and discharged them by tapping the Leads. The Security Officer said somebody reported an Explosion.
    First thing I do is take a Screw Drive to all the Caps I would not like to tango with.
     
    Last edited: Jul 2, 2012
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, don't get complacent. A capacitor the size of your thumb can hold a deadly charge. Smaller capacitors can make you seriously wish you had never touched them.

    Once you've vaporised bits of a screwdriver a few times discharging capacitors...
     
  4. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    I see,

    I found another box:
    Two Transformers and a Switch.

    Ill keep looking on line for what I need.

    The 120W Soldering Gun I have, just wont cut the cheese for Desoldering anything.

    If I desoldered Two PSU. Would I have what I need?? Lets See,
     
    Last edited: Jul 2, 2012
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    hi ya
    sounds like you are having fun scrounging through some PSU's

    very basically.....
    A switching PSU is one that takes in 240VAC (or what ever you local mains voltage is)
    rectifies it and uses a transistor to switch the resultant DC very quickly anywhere from ~ 25 - 80kHz ( pretty much a manufacturing decision) and fed to a transformer, this allows the use of a much smaller transformer the output of which is rectified to produce your output(s). there is a feedback voltage from the low voltage DC output to the high voltage switching side that is used to control voltage regulation.
    do a google search of SMPS Switch Mode Power Supplies for more info

    A linear PSU is the older style and is still often used. It has a larger transformer, which gets much bigger and heavier as the current requirements increase. the output of the transformer is rectified, smoothed and regulated, if necessary
    The transformers are much bulkier because they are only working at the mains frequency 50/60 Hz and transformers are not very efficient at those frequencies, compared to much higher freq's

    Dave
     
  6. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    All this solder fumes is getting to my head, :p

    I found a 8A Rectifier W/ Heat Sink. The Diagram you had a 20A.

    Btw. I live in the USA. So we use 120v @ 60hz.

    I found that taking a Pair of Dikes and Needle Nose along with a Solder gun really speeds up the Process.
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    yup .... know where Springfield MO is been through there a couple of times.
    I have friends that live in Carrollton in nthrn MO :)


    Dave
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Dave, I understand. I made some general comments as well, and I thought maybe an analysis of that design might be interesting to the OP and useful to others who might browse the thread later.

    My main comment is that with a linear regulator there will be an issue with heat dissipation. Assuming say a 4V overhead for dropout, the input voltage needs to be ~16V. At the minimum output voltage, and maximum output current, the regulator will be dissipating 14.5V * 2.5A which is about 40W. To limit the case temperature to 100 degrees Celsius he will need a heatsink with a thermal resistance of about 2 degrees Celsius per watt to ambient, which is quite large.

    My recommendation would be to buy an adjustable switching regulator module with up to 12V output and rated for 3A, and modify the feedback network to make it switchable and adjustable. These are available pretty cheaply I think.

    Re the PC power supply, to the OP, trace out the secondary circuits. It will probably be possible to decouple the 5V and 12V circuits from each other and put them in series, to get a 17VDC output that can deliver 6A. That would be ideal to feed into the variable switching regulator I described.

    Adding an adjustable current limit to the switching regulator wouldn't be easy; he would probably have to stick with the limiting built into the switching regulator IC. Other current limiting could be done with a series resistor (not a proper current limit, but I think he wants to use it for charging batteries, so it might be workable).

    Regarding decoupling the 5V and 12V outputs. If you trace out the circuit of the secondary side of the PC power supply, you should find several independent circuits. One for 5V/18A, one for 12V/6A, and one (or maybe two) for the negative rails. These circuits will be tied together using a common ground rail and you'll have to isolate them from each other. There is a voltage detection and feedback circuit which is almost certainly connected across the 5V rail, so it's proably best to leave the 5V circuit as-is and isolate the 12V circuit. You really need to trace out the schematic of the whole secondary circuit, take a good photo of the tracks, and annotate that picture with the positions of the components in the secondary circuit. Then I, or someone else here, could work out how to isolate the 12V circuit and connect the outputs in series. This is not really a beginner's project but I'm sure it can be done, and would be a pretty interesting and rewarding project.

    To the OP, re the two trimpots you found, the second one that doesn't seem to do anything will be the current limit trimpot. If possible, return it to its original position.

    Oh, and be SURE to follow Steve's advice re safety! You may want to add a "bleeder" resistor across the main electrolytic on the primary side to make sure it discharges within a short time after power is removed. It will probably charge up to about 340V (mains voltage * 1.414 * 2 plus a bit). If you use, say, a 5W resistor and run it at 2W, the resistance is calculated as R = V^2 / P which is 340^2/2 which is about 56 kilohms. It will get hot while the power supply is running, so mount it away from the board, preferably with mechanical support. Assuming the input electrolytic is 200 uF (the value is not standardised but this is typical), the capacitor will discharge to a safe voltage after about two minutes from power being removed, and no screwdrivers need to be harmed. Another option would be to put a pushbutton in series with the bleeder resistor and only press it when you need to discharge the capacitor; that stops the bleeder resistor from getting hot during operation and you can use a lower value, e.g. 22K 5W which will bleed it quicker. In either case, use a multimeter across the electrolytic to check it's well discharged before touching the board.
     
    Last edited: Jul 2, 2012
  9. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    Wow. Kris. Every Time I read a Post from you, it's like eating at a all you can eat. (That's good.)

    I'v pretty much skipped rewiring a Whole PSU.

    I posted a pic of a Really basic PSU that does 5v and 12v Only. But I don't know of the Voltage Drop.

    I found a Project Box and Mounted a Button and Switch so far, Ill be looking at what Mouser and Radio Shack has for sale.

    I agree that this is a biggish Project for having almost no idea on what I'm doing. (Compared to you guys)

    I feel that making something simple and then add to it.
    Ill start with preset adjustable voltages.

    The way it's sapost to work. Is you set the Voltage and Fine Tune then turn on the PSU. Ill keep looking into the 5v and 12v. And see if I can't do some desoldering. :D
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Correction to my previous post.
    The input filter capacitor in the power supply is visible in your picture; it's 47 uF at 400V. This would take less than a minute to discharge to a safe voltage with a 56K 5W bleeder resistor across it.

    I think it's good that you've chosen not to get into modifying a PC power supply. There's lots of scope for electrocuting yourself and/or blowing up components when you play around inside them.

    Are you going to use the 5V and 12V outputs of the PC power supply directly? That sounds like a good idea. The 5V output can deliver 18A so always use thick wires to prevent melted insulation and burns if the load pulls too much current. A short-circuit load will cause the power supply to shut down or "hiccup" so there's no danger of overheated wires in that case.

    I'm not sure what you meant by "I don't know of the voltage drop".

    Yes I have far too much free time on my hands. Someone give me a job!

    Good luck!
     
  11. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    I was thinking more of using the 5V for Lower Voltage Regulation (1.5v - 3v). The 12v for Medium Regulation (4v - 10v.). And Bridge to get 17v for Higher Regulation (11v - 12v.)

    If the Higher the Voltage Difference from the Reference Voltage to the Output Voltage causes more heat. Then Using a Combination of Both Rails should lower the Heat.

    Or No?
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    The heat dissipation is equal to the output current multiplied by the voltage difference between the INPUT VOLTAGE to the regulator and the OUTPUT VOLTAGE of the regulator. The reference voltage is not a factor.

    Using different voltages as inputs to a linear regulator would require extra switching on the input, and the switch would have to carry the full current, which is not a good idea for a rotary switch (the contacts are not really designed to carry 2.5A).

    The connection you describe as "bridge" uses two separate power supplies, doesn't it? You'll also have to make sure that the grounds are isolated from each other; this might mean preventing the metalwork from touching, and lifting one of the earth wires at the mains plug. Do your power supplies have 2-pin or 3-pin plugs? In any case it's not ideal to have two separate PC power supplies.

    Do you really need to be able to deliver 2.5A at those low output voltages?

    BTW here's the sort of switching regulator module I'm suggesting. Brand is "Rantion", model number OT142:
    http://www.alibaba.com/product-gs/491127322/DC_DC_Step_Down_Adjustable_Power.html
    That one is rated for 2A continuous, 3A maximum.

    There's a "kitset" available on eBay: http://www.ebay.com/itm/2-Set-LM259...lectrical_Equipment_Tools&hash=item2c6676d581
    It's based on the LM2596 which is rated for 3A maximum. The picture doesn't show a PC board though. If you're interested, you'd better ask a question to confirm that there is one! The price is USD 6.50. The main IC is SMD and may be difficult to solder by hand because of the large heatsink area, but this at least gives you an idea of what you'd be looking at.

    Can I ask you again to review your requirements? What sorts of things do you plan to do with your power supply? It won't really be suitable for charging batteries unattended - you'd have to monitor the battery charge progress and temperature manually. NiMH batteries make a really nasty smell when overcharged - their magic smoke must be a special kind :)
    Do you really need 2.5A?
     
  13. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    I don't need 2.5A. I just wanted to be sure I could power anything that I might need, Ex. I was thinking of a Audio Amplifier next.

    The Bridging I did Two Nights ago and your right. Ill have to take a look at the Diagram.

    I would mainly be using to Power Computer Parts. That's why I was needing about 4 Channels. But Two Would be Plenty.

    Ex. The Nearest computer fan to me is:
    12v @ 0.17A
    But Iv seen fans run at 12v and 1A.

    If I wanted, A few USB Plugs down the road would be cool to have. My Kindle Fire Charger Broke and its. 5v @ 1.8A

    So I just like to keep my options Open. I seen Two Diagrams for the LM338 in a pdf that showed how to make a Thermal Shutdown and 2 Diagrams for Battery Chargers.
    Ill look at it again.

    All things are below 2.5A. But you never know what could Pop up.
    Maybe after 3 Year. I design my Own Computer.

    But then again. I might Drive you all Crazy by then,
     
    Last edited: Jul 2, 2012
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Those examples you give only need 5V and 12V. So all you really need is the PC power supply you already have! That and some USB sockets.
    Why did you think you needed the two fine adjustment potentiometers?
     
  15. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    Your Right. The two pots would be like Battery. As somebody said, NiMH run at 1.2v.

    Ill admit that I would be using 5v and 12v. More.

    But if I need to simulate AA, AAA Battery. Maybe a 6v and a 9v even.

    As of Funny Voltages. Maybe a LED or Laser Diode.

    Like I said. Keeping my Options Open.

    Better to have and not need than to need and not have.
     
  16. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Unless having stuff you don't need means a lot more work and you can actually get by with what you already have!
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    To me, the major problem with computer power supplies is that they are capable of a huge amount of current, and this is not limited (except to protect the power supply itself)

    They provide a range of useful voltages, and if you really need it, you can connect a small linear regulator the the 12V output to get other voltages.

    Alternatively, you can get one of the many switchmode regulators available for almost nothing from eBay and the like and get higher voltages (or more efficiently get lower ones)
     
  18. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    I see.

    Steve: You said keep the 12v and use a Regulator.
    I suggested that about 7 posts up.
    Only I made a Mistake on Wording.
    If the Rotatory Switch can not Handel Nice Current.
    What about a Transistor that can?

    Say I set about 12v with almost no Current in to the Switch and let it deliver Signal to about 5 Transistors that Activate a Regulator?
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No, keep the 12V, 5V, 3.3V (If it has it), etc. And do nothing else.

    Later, if you need to, you can use the 12V rail to power a regulator to get variable voltages up to 10V. If you want preset voltages, just use a switch to select different divider ratios for a variable regulator, No need for it to handle more than a couple of mA.

    You can get up to about an amp easily. If you need more, think about it when the time comes.

    Don't solve problems you don't have. Especially don't do it when you don't know what you're doing.
     
  20. Chat_Ghosty

    Chat_Ghosty

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    Jun 30, 2012
    I see.

    How can I test the Max Amps a PSU can pull?

    I found a PSU that was used as a External CD Drive. And it has only one 12v and one 5v Rail.

    From there, I would like to add the Regulator. I might go but Radio shack Later Today and see what they have, I have $20. Ill see what happens.
     
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