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Variable power supply

Discussion in 'Power Electronics' started by MrBret, Sep 23, 2013.

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  1. MrBret

    MrBret

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    Jul 26, 2013
    I have a phone charger that im using to make a hot wire cutter. its 12v 2.5A so I got 30W of power to this wire (load). I want to be able to make this supply variable by using a potentiometer to control the power to the load. I was thinking of using it in series with the load, but I'm sure there's another way to control power more effectively. any suggestions?
     
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
  3. MrBret

    MrBret

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    Jul 26, 2013
    I tried it with a 5V 2A (10W) phone charger and got 145F. It was able to cut foam but the process was a bit slow, I'm just trying to speed it up. Thanks for the suggestion though!
     
  4. MrBret

    MrBret

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    Jul 26, 2013
    also, why won't a potentiometer work?
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    cuz unless its a VERY high wattage ( and the price of them will knock your socks off)
    it will just burn out

    Dave
     
  6. MrBret

    MrBret

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    Jul 26, 2013
    could I use a voltage regulator like this: http://www.fairchildsemi.com/ds/LM/LM350.pdf ? And use the 12V charger as an input? I'm pretty sure the LM350 can handle up to 3A; well within my 2.5A (I think at 12V it can handle up to 4.5A).
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Have you considered PWM? It would be far more efficient. Heck, you don't even need DC to heat a wire. So a TRIAC controlled AC supply should work fine. They're simple too! ;)

    Chris
     
  8. MrBret

    MrBret

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    Jul 26, 2013
    But a variable voltage regulator could work right?
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Well there's more than one way of doing this. There's also the right way, wrong way, practical way, impractical way, and above all the inefficient way (power wasteful) versus efficient, where >90% of total power is dissipated by the load.

    If I were doing this linear, which I wouldn't, I'd be looking at constant current regulators not voltage regulators.

    Chris
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Have you read the thread that Fish linked to in post #2 on this thread?

    The important characteristic you need is high current. A few volts will be enough, unless your wire is very long and/or has a very high resistance.

    BTW if you can get different grades of nichrome wire, go for one with relatively high resistance (high ohms per foot). They will need less current to achieve a particular temperature. (But they will need more voltage. So they are more suited to your 12V/2.5A supply.) (Although I agree with Fish that a charger is probably not the most suitable power supply.)

    You could use a linear regulator but 2.5A may not be enough for your wire, and the regulator will need to dissipate a lot of power. For example if your wire needs 2.5A to get hot enough, and at 2.5A the voltage across the wire is only, say, 2V, the regulator will be dropping 10V with 2.5A current flowing through it, which means it will dissipate 25 watts (multiply voltage and current together.) At 25W you will need a biiiig heatsink on that regulator. A switching regulator is much more efficient.

    A tidy option would be a buck (aka step-down) switching power supply. These are available as assembled PCBs for prices around $10 on eBay. You can power it from your 12V/2.5A charger.

    Go for one with an ADJUSTABLE output voltage that can be set down to less than 2V (I've seen one that goes down to 1.2V) and has an output current rating of AT LEAST 5 amps. A lot of them are only rated for 3A and I think that's probably too low for your application.

    Here are some possibilities from eBay. Generally I don't recommend buying cheap products from China or Hong Kong and "your mileage may vary".

    Here's one that claims 12A maximum output current:
    http://www.ebay.com/itm/12A-100W-Ad...882?pt=LH_DefaultDomain_0&hash=item232e38aa32

    This one has a constant current output (this would simplify adjustment if you vary the length of your wire) and can supply up to 5A:
    http://www.ebay.com/itm/DC-DC-4-5-3...663?pt=LH_DefaultDomain_0&hash=item51b17531c7

    This one actually looks pretty darn good. It doesn't have constant current output, but claims up to 15A current and output voltage adjustable down to 1.2V.
    http://www.ebay.com/itm/Adjustable-...477?pt=LH_DefaultDomain_0&hash=item19e10b1d9d

    No doubt there are others. Those links should give you some idea what to look for.
     
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