using a laptop PSU as a dc-dc

Discussion in 'General Electronics Chat' started by auraslip, Oct 20, 2011.

  1. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    People on the ebike forum have good success using standard SMPS as dc-dc convertors to run lights and other 12v and 5v accessories. I actually used to sell 36v-72v to 12v converters, but I can't find any that'll run off high voltages.

    So I got this 12v 5a smps charger off ebay.

    It works, but of course not as expected. It only outputs ~8v. The adjustment pot does nothing, but I assume it won't be too hard to figure this thing out. Something else I'd like to mod (remove really) is the rectifier so that it won't fail pre-maturely.

    [​IMG]
    [​IMG]

    The output dual op-amp ties into the feedback of the uc3843 through an optocoupler. Since this output has a voltage divider that is the pot, (and since the pot does nothing) I'm going to guess that nothing I do on the output stage will help. So this leaves the uc3843. I attached the data sheets to these devices.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Apart from making the power supply working again, it's not quite clear what do you want to do with it.
    You also didn't attach any datasheets. However, for the UC3843 you don't need to.

    Can I assume that you want a DCDC converter that works with 36 to 72V input voltages?

    You need to give us more information and you can also start drawing the diagram from the PCB, or the control/feedback part of it and then post it here.
     
  3. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    Thanks, sorry I forgot to mention what I'm using it for. I need it for a high voltage battery pack - 84v down to 74v

    I'll back trace the feedback section in mspaint or something.
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Somehow I doubt that you will get out 5A out of it if it was designed to work on 110AC. Is there something written on it, what is the minimum AC input voltage for full load? At the corresponding DC-bus voltage regulation would not work anymore because the duty cycle is on it's maximum.
    I believe that this threshold is more than 74V.

    When you get out 8V, what load did you put on it?
     
  5. auraslip

    Thread Starter New Member

    Oct 17, 2011
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  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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  7. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    Not to be contrarian, but later on someone else posted their testing

    Now that I think about it, maybe using a 15v unit would be better. It's far easier to tune one down rather than up.

    Maybe it'd be easier just to make my own dc-dc convertor?
     
  8. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Well, apparently his converter works down to 50VDC.
    However, 14.5V on 6Ohms is 35W, not 50W.

    You can try it with a 15V unit and if you are lucky it will work down to 74V...
     
  9. tom66

    Senior Member

    May 9, 2009
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    If you want to change the input range, you're going to need to mod the transformer by rewinding it.

    Have you considered building your own supplies? I've seen some NatSemi parts that can work with 100V inputs. For example, this: http://www.national.com/pf/LM/LM5020.html#Overview

    Also, these types of supplies aren't much different to a lower input voltage supply. The main difference is getting the SMPS controller to work well on ~330V DC. A resistor, zener and capacitor are used to form a basic supply to bootstrap the IC. Then, when operating the supply voltage is provided through another winding on the transformer. The transformer is key - as the output is higher than a couple volts the controller is switching, but it can't run the transformer at a high enough duty cycle to get the desired output.

    At higher voltages a computer power supply may work.
     
  10. auraslip

    Thread Starter New Member

    Oct 17, 2011
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    Thanks for posting that. I was looking at some of the ICs, but didn't see the 5020.

    The application circuit in the date sheet looks decently complicated, but not too bad. It's a lot of work to drive a simple 12v headlight.

    I don't really know what to do. I'm contacting the chinese manufacturer about higher voltage dc-dc. The 36v-72v to 12v models are actually pretty cheap in bulk, but it takes 1-2 months. And I want to ride my bike at night NOW :D

    I have an older 12v linear wall wart that puts out ~15v on 110va. I'll try that, and maybe I can build a regulator circuit.

    duh, I just realized linear wall-warts won't work on dc.
     
  11. tom66

    Senior Member

    May 9, 2009
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    Maybe a 19V laptop charger would still put out sufficient 12V at a low Vin?

    Some have potentiometers on the board to adjust the output so they could be regulated to 12V. If there is no potentiometer, you can add your own.
     
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