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Why is a simple PSU 25v trans putting out 41v ?

  1. Feb 4, 2010 #1
    my power unite consist of only

    a 25ohm 1amp rheo, 25.2v 1A transformer that goes to a small board that holds #1 a small black rectangle ?(with = - 2kbp06 printed on it) and#2 , 2 big *** caps 1000uf 200wv , and that goes to a filter reactor (.32HY-600MADC 10OHMS) and a 3 amp fuse

    so what is making it put out a 41v reading ?
  2. jcsd
  3. Feb 4, 2010 #2


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    The peak value of the input must be about 42 volts and the large capacitor is charging up to this value.

    The power supply would have a voltage vs current characteristic like this:

    http://dl.dropbox.com/u/4222062/power%20supply%20load%20curve.PNG [Broken]

    As you increase the load on the power supply, the voltage would drop as shown in the graph.

    I have shown the two extreme positions of the rheostat. Maximum resistance at the bottom and minimum (zero) resistance at the top.
    Last edited by a moderator: May 4, 2017
  4. Feb 4, 2010 #3
    i REALLY appreciate your help , only it isnt happening that way. I think i will make a video tonight so you can "SEE" . without load it reads an output of 40v when i attached to the machine set it at 2 v and turn it on , it reads 0.8 and if i turn it up the rheo to say 5v it will read at 0.9v and at 12v 1.02v

    would a video help ? i can do easy
    Last edited by a moderator: May 4, 2017
  5. Feb 4, 2010 #4


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    You can't believe the "voltage" settings because the output voltage depends on the load resistance. A series rheostat is not a very good way of controlling output voltage.

    It sounds like your "machine" is taking too much current for this supply.

    You need a different power source. Maybe a 12 volt battery?
  6. Feb 4, 2010 #5
    the machine is a tattoo machine so it is only requiring 5 volts to run, i can use a 9 volt battery maybe a car batter 12 v is maxing it out and i dont even know what 400 amps would do? the psu works it just seems to run a bit stronger than other psu's and the readings on a meter have me confused, the rheo works as far as increasing voltage ....waite i just saw this
    "A rheostat is a type of variable resistor that allows the users to adjust the length of material the circuit's electrical current passes through, which in turn varies the rheostat's total resistance. However, rheostats have three terminals, only two of which are necessary when wiring a series circuit. If the rheostat is incorrectly wired, the circuit will still technically "work" but you won't be able to adjust resistance."

    so maybe it is wired wrong.... hmm there used to be a small bulb 3w that i think ran between the rheostat and the switch i removed it because it went out easy, i disconected it and it still ran so i figured it wasnt needed (and im starting to feel stupid)
    could this be screwing up something ?
    Last edited: Feb 4, 2010
  7. Feb 5, 2010 #6


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    Do you know how much current your tattoo machine draws when it has 5 volts on it?
    It may be written on the side of it.

    Is your circuit like this? :

  8. Feb 5, 2010 #7
    yes basically,

    it is i made a video so you can see it https://www.youtube.com/watch?v=http://www.youtube.com/watch?v=Kh6GfPY3iX4

    Last edited by a moderator: Sep 25, 2014
  9. Feb 5, 2010 #8


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    Just a bit of theory.
    If there is no current in a resistor, there is no voltage across it.

    So, if you put a resistor in series with a power source and measure the open end for voltage, it will be the same as at the power source.

    This is apparently what is happening in your case. It is normal to measure the 41 volts at the output when there is no load.

    This power supply seems unsuitable for supplying 5 volts at several amps, which is what your tattoo machine needs.
    It could deliver 5 volts but only if the current was about 600 mA. If the machine had a lower resistance than this (less than about 10 ohms) then it would get less voltage.

    You may be able to measure the resistance of the machine with your digital multimeter on "Ohms".
  10. Feb 5, 2010 #9
    i allready know the resistance of the machine coils it is 2.0ohm (based on the length and gauge of the wire used to make the coils) and the meter verifies this .

    i only need or want up to 12v @ 2 amp my machine runs fine i just dont know why the meter reads 1v when the dial says differant and the machine feels and runs as if it is getting what the dial says , the meter reading is the only things i dont understand
  11. Feb 5, 2010 #10


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    Do you know if the tattoo machine is turning on and off all the time? If it was, the action of the circuit would become more complex.

    It could be getting short bursts from the 41 volts on the capacitors and then turning off before the capacitors discharge.

    The tattoo machine could have significant inductance as well as its 2 ohms resistance.

    If it works, probably that is all that matters. You would need to examine the actual waveform on the machine to work out why your meter readings are strange.
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