Reading +5V between negative output terminals on bench supply

Discussion in 'General Electronics Chat' started by spinnaker, Jan 4, 2016.

  1. spinnaker

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    I have a cheap dual bench supply from Circuit Specialists (CS13303S) which I believe is no longer manufactured.

    I have a switch Series / Independent/ Parallel. I have the switch set to parallel. There is a green ground connector and a red plus out and black negative out for each supply.

    Each supply works as expected if I use their own positive and negative terminals.

    I am getting the right supplies voltage when measuring between the two negative terminals. I thought I had read that tying the two grounds together will will result in a common ground and I assumed a common negative terminal.

    Am I doing something wrong? Or is there something wrong with my supply. If I am doing something wrong then how do I resolve this?

    I may have blown out a pin on a pic. :(
     
  2. Lestraveled

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    @spinnaker
    What is the model number of the power supply?
     
  3. spinnaker

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  4. Lestraveled

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    Spin, so in parallel mode you are getting 5 volts between the negative terminals? What does the manual say about the terminals to use when you are in parallel mode?
     
  5. spinnaker

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    So this is not sounding normal? I think I recall this working before.

    Parallel, I get the right supply's voltage any red to any black (any combo). Right control, controls both left and right display,

    Series right control controls both displays and I get the sum of the two supplies from the left black and right red.
     
  6. Lestraveled

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    This sounds right.

    Describe your problem again, maybe I am confused.
     
  7. spinnaker

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    I agree it works right in parallel and series but not independent.


    It is all in the first post. If I set the tight supply to 5V and set the left supply to %v, I read 5V between the negative terminals of the right and left supply. If I vary the right supply, I see that voltage changing.
     
  8. spinnaker

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    I aslo have a fixed 5 v out and I read 0 volts between the negative terminals of the 5V and the two variable supplies.
     
  9. spinnaker

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    On the variable supplies, I read the right supply voltage between the red terminals and the left supply voltage between the black terminals.
     
  10. Dodgydave

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    There are 3 separate transformer outputs, one for each of the outputs, so reading between each one will give a false reading.
     
  11. spinnaker

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    I don't know what this means. Are you saying I am no really reading 5 v between grounds?
     
  12. Dodgydave

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    You cant read between separate outputs, you need to read between the common black and positive supply.
     
    Last edited: Jan 5, 2016
  13. spinnaker

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    I get nothing. When I measure between the positive terminal (red) and ground (green)
     
  14. SLK001

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    The grounds are floating. It is possible that they are protected by reverse diodes. These might let a tiny current thru, and with a high impedance meter, could look like there is a voltage on the port. If you put an ammeter from ground to ground, you will probably see near zero amps. If you want two supplies for a circuit, tie the two grounds together.
     
  15. spinnaker

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    Ground or negative? Green is chassis ground black is negative out
     
  16. GopherT

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    On my dual supply, they are each floating and I measure between 2 to 5 volts between them on an given minute. Even wiping my feet on the carpet and then bringing my hand near the other will cause the difference to change. That is with my meter (10 M ohm resistance). If I use a JFET op amp follower to measure the difference (huge input resistance) the voltage swings are much bigger, 10 to 30 V. Maybe bigger because I have likely damaged that jFet op amp.

    There is no common reference for the two isolated/floating transformers so they can anywhere vs. each other. The transformer coils on the secondary literally float and do not touch anything else to give a common reference.

    On my old HP power supplies, If I want to different positive voltages, the Black leads are connected.

    If I want two different voltage for a neg and positive supply, then black goes to read (That becomes the 0V reference) and + voltage is adjusted on one and - voltage on the other.

    These are the benefit of dual floating supplies.
     
  17. GopherT

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    @spinnaker Actually, the voltage between one floating source may be thousands of volts different than other. Here is a good picture of it. The guy on the Helicopter (floating supply) is thousands of volts different than the power lines (different supply). He comes in close to inspect and slowly brings himself up (or down?) to the potential of the power lines. (54 second mark)

    The same thing happens as the two floating supplies are connected to each other to make a dual rail supply.

     
  18. SLK001

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    Sorry, you're correct. I meant that the NEGATIVE outputs are floating relative to each other. Putting an ammeter across the NEGATIVE ports will show near zero current.

    If you would measure from the red post with +5V on one supply and the other red post with another +5V on it, the results would be indeterminant, since there is no common reference.
     
  19. Lestraveled

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    +1 on what @GopherT said. It is probably just leakage. Make the same measurement with a 1K resistor across your meter leads.
     
  20. spinnaker

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    So tie the two negative poles together?
     
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