printer power supply

Discussion in 'General Electronics Chat' started by timelessbeing, Oct 12, 2011.

  1. timelessbeing

    Thread Starter New Member

    Jul 5, 2011
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    I kept a circuit board that I pulled out of an old Lexmark laser printer, because it was full of juicy components. I was able to determine that it is a low-voltage power supply (LVPS, part 56P1097).

    From what I know, a power supply (such as wall warts) usually consists of a step-down transformer, a rectifier, and perhaps some smoothing capacitors. As expected, this board has all of these, but it also has some huge honking transistors, power resistors, inductors, choke and 200V electrolytic capacitor. What are these for?

    These components could come in handy later when I build my own bench power supply, but maybe this board is more useful in its entirety. What do you think?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    This not your simple AC to DC wall wart. This is a switching supply that provides multiple voltages such as +5V, -5V, +12V, -12V. Sometimes the voltages are printed with the white silk screening on the board. From the size of that thing I would guess it delivers about 200W.
     
  3. #12

    Expert

    Nov 30, 2010
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    I say, keep it the way it is. Zero hours labor for voltages you will certainly find uses for.

    I kept a power supply out of an ancient computer...how ancient? It has 70 VDC for the monitor! Very handy with a 12 12 here and a 5 5 there, here a 12 there a 5 everywhere a volt volt.

    Old McDonald had a volt, E I E I R
     
    Last edited: Oct 12, 2011
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  4. timelessbeing

    Thread Starter New Member

    Jul 5, 2011
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    Aha, a SMPS. Presumably chosen to reduce weight and size. Perhaps better regulation? This is something I've wanted to learn to build. The board does indeed have 2x +5V and 2x +24V outputs. Certainly useful, but is it more useful than a universal adapter? Looks it it can provide more power. The outputs are discrete, and I'd rather have a 0-30V variable power supply with current knob as well.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    If the output connector is labelled +5 +5 and +24 +24 then my guess is that you only have one +5V and one +24V supply. If you look at the bottom side of the board you may see that the two +5V pins are connected and same also with the +24V. They twin the pins to give you higher current carrying capability. You can also check this with an ohm-meter across the twinned pins.

    I would be very careful about powering this up since it is open framed and there are lethal voltages in there. If you plan to use this, make sure it is put into a proper enclosure.
     
  6. #12

    Expert

    Nov 30, 2010
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    After you find a proper enclosure, you can still use it "as is" and add on a regulator circuit or two so you can dial in zero to maybe 22 volts and/or zero to whatever milliamps.

    or...you can hold out for 30 volts and start from scratch.
     
  7. timelessbeing

    Thread Starter New Member

    Jul 5, 2011
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    Not to worry gents. I've been jolted enough times in my life to have a healthy respect for electricity and take the necessary precautions.

    Given the superior efficiency of a SMPS, wouldn't using a regulator (such as LM317) be counterproductive? Isn't it smarter to modulate the duty cycle?

    Also (probably not a simple answer) how does one control the current output of a SMPS?
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    the same way you control voltage output - with a series pass transistor, except in this case you measure the current through a current sense resistor in series with the supply and the load.
     
  9. #12

    Expert

    Nov 30, 2010
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    The switcher is doing an efficient job of stepping the voltage down from 120 VAC to 24 VDC. You won't lose that part of the efficiency equasion if you use a linear regulator after the 24 V output.

    It is possible to change the output of a switcher by changing resistors on the switching board, but I fear it won't turn out well because of the multiple outputs. I stepped a 5 volt switcher up to 6 volts to run some vacuum tube filaments, but it was a single output model and the tubes used way less than the rated amps.

    You do have the option of using a switcher type of supply to regulate the 24 VDC to lower levels. You could even use a buck/boost switching regulator to get to 30 volts, but the linear regulator is just so much simpler to design and build.

    Your choice.
     
  10. timelessbeing

    Thread Starter New Member

    Jul 5, 2011
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    Hello again guys. Thanks for all the info so far. I have another question related to this power supply. As well as providing the 5V and 24V DC outputs that were talked about, this board also drives a 600W 110VAC quartz infrared heater lamp for the fuser. This output isn't connected directly to the input though. It goes through the huge inductors you see in the photo above, and some caps and fuses. I'm just wondering what it's doing. Regulation?
     
    Last edited: Jun 29, 2012
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    No, it's typically an offline flyback converter with multiple outputs of various voltages and currents.
     
  12. timelessbeing

    Thread Starter New Member

    Jul 5, 2011
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    I'm asking about the single 110VAC output.
     
  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    ?????????????

    There is no 110 VAC output, that is the input voltage source to the converter.
     
  14. timelessbeing

    Thread Starter New Member

    Jul 5, 2011
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    Maybe you missed my last post on page 1?

    I can take photos if it helps.
     
  15. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Sounds like an EMI noise filter to reduce switching noise coming out the AC line. It can't be regulating, it would be a passive filter.
     
  16. timelessbeing

    Thread Starter New Member

    Jul 5, 2011
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    The big coupled inductor does also branch off to the SMPS portion of the board. The other big plain inductor is only connected to the lamp, which is a resistive load and doesn't make noise, right.
     
  17. timelessbeing

    Thread Starter New Member

    Jul 5, 2011
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    A series inductor on an AC circuit produces an inductive reactance right? So the component will limit current to the lamp, like a magnetic ballast. But the lamp has a positive resistance, so it should be self-limiting. Still not sure what it's there for :confused:
     
  18. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    It is current limiting. A cold filament will draw a large amount of current till it heats up. Which takes just a handful of milliseconds.
     
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