Adding and subtracting voltages

Discussion in 'General Electronics Chat' started by Copey84, Aug 24, 2015.

  1. Copey84

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    Jul 27, 2015
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    14404577291281861155424.jpg Hi all, was hoping someone could explain how a power supply with +-12v and 5v output could be combined to create multiple voltages.
    I have included the wiring diagram of both circuits. By combining +12v and 0v of 5v circuit a 29v supply is available across the 5v and -12v, how does this circuit work? As I see it link between +12v and 0v will cause caps to charge, can't see how voltages add together especially with regulator in circuit. Also if 5v and +12v are combined a 7v supply is available across both 0v terminals, this is as described in book that I got wiring diagram from, but again can't see how it works
    Appreciate if anyone could explain how voltages and currents work in these circuits, also if there is any other voltages available.
    Thanks
     
  2. MikeML

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    By connecting the two independent supplies in series, I can see the following single voltage outputs: 5V, 12V, 24V, 29V, 17V, and 7V.
     
  3. Copey84

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    Jul 27, 2015
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    Thanks mikeml, can see how you get 17v not sure about 24v. Obvious is 12 and 12, but one is a negative voltage so how could it be? Also when creating 29v, the +12v has to be linked to 0v of 5v circuit. Would this link cause the caps in 5v circuit to charge, and how then does it combine with other voltages to create 29v. How does regulator allow voltages to add and subtract?
     
  4. MikeML

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    Oops, you are right. The positive regulators can only source current, and the negative one can only sink current, so you can get 5, 12, 17, 24, or 29V by adding. You cannot subtract 5 from 12, or 5 from 24...

    Once you connect the supplies in series, you can define any of the taps as 0V, to get the +- combinations.
     
  5. AnalogKid

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    Aug 1, 2013
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    The + and - 12 supples share a common ground, but they act like two 12 V supplies stacked one on top of the other. Starting at the -12 V terminal, GND is 12 V above it, and +12 is 12 V above GND. Works every tie, all the time.

    OTOH, while combinng the +12 output with the +5 output to make +17 V works the same way (because the 5V outut is fully isolated), combining them to make +7 V is problematic. This because both output terminals are regulator outputs, designed to source current; neither has any significant current sinking capability. So while the output may look ok on a scope or DVM, those are not significant loads. I doubt that the arrangement will work with a 1 A load, for example.

    ak
     
  6. Copey84

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    Jul 27, 2015
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    Still not sure how you get 24v, and as regulators only output set voltage then how do they add up. For 29v to add up the +12 has to cross link down to 0v of 5v output, then cross diodes and through regulator, i think?,but how can they combine when regulator only outputs 5v. Obviously something I'm not getting, understand how battery voltages simply add up positive to negative to increase voltage, don't get how voltage can add when regs are in circuit, and also how voltage is available from to 0v points. Could you help clarify, please?
     
  7. Copey84

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    Jul 27, 2015
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    Hi analogkid, just got your reply thanks. All three outputs will have 500ma each, so should be ok.
    Is it possible to get 24v between both 12v outputs. 12 to 0 then to-12
     
  8. AnalogKid

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    Yes. But a lot depends on what you are connecting to the supplies. Can you provide a sketch of the power supply connections?

    ak
     
  9. Copey84

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    Jul 27, 2015
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    Hi ak, each voltage has 500ma available, connections will be same as wiring diagram. The power supply will only be used on bench circuits using up to maximum 500ma.
    Think i understand how current will flow in circuit with voltages combined. Basically current must flow from one of the 78 regulators and return through 79 regulator.
    With the 7v output there is a link between +12v and 5v, so isn't it possible to source the current from 0v 5v terminal and return through 0v on 12v side, current would go through 7912 from 7805.
    Is the 7v created simply because two + voltages subtract +12 and +5, and is 17v created by adding -12 and +5.
    How is +12v added to 5v when linked to 0v of 5v terminal, get how current flows but can't see how voltages combine, to give 29v. Please could you explain, thanks
     
  10. AnalogKid

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    There is no common connection between the 5 V circuit and the +/-12 circuits. in other words, the 5 V GND and the +/- 12 V GND are electrically separate. This is because each is isolated by separate transformer windings. Because of that isolation, it is ok to stack the +5 V supply on top of the +12 V supply by connecting the 5 V GND to the + 12 V output. The isolation means the two supplies are just like two batteries; the relationship between them depends on how they are connected together. In this case, current could flow out of the 5 and into its own GND (connected to +12), into the +/- 12 GND, or into the -12. In conventional current flow, current always flows from a more positive terminal to a more negative terminal *** as long as the more negative terminal can sink the current.

    It can be a bit confusing. Try this. On paper (not with real parts), add a diode to each supply's output. For the +5 and +12, the anode goes to the supply output and the cathode now is the new output. For the +12 the cathode connects to the s-12 output and the anode is the new output. Now when you connect the outputs in drawings, you can see which ones have a DC path and which ones have a diode blocking the current. You can connect either of the 5V supply pins (but not both) to any of the three +/-12 V supply pins and trace the current paths for various combinations.

    ak
     
  11. Copey84

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    Jul 27, 2015
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    Thanks for reply ak, still not sure though. If diodes are poled the same on both +5 and +12 outputs how can current flow between both circuits. Can see how it would flow between +voltages an -12v as the diode is opisit.
    Also when the +12v and 0v are linked how is both voltages combined, they must combine at +5v to = 17v and then there is a potential difference of 29v between both points. I know voltage is like a pressure that creates a pd between two points, but struggling to see how it is added when there is a regulator , why doesn't reg always want to pull voltage down to output level?
     
  12. KL7AJ

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    Since this particular supply seems to have floating DC grounds, this should be quite simple. To get +17 V, for instance, you can connect the +5 line to the center tap of the +12 volt supply. You will then have 17 volts between the center tap of the 5 volt supply and the positive lead of the +12 V supply.
     
  13. AnalogKid

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    It can't. That was my point in post #5. While the arithmetic makes it look like you can combine 12 V and 5 V in a way to get 7 V, the parts don't work that way because you have two sources and no return. Visualizing all sources as having output diodes makes clear which combinations will work and which ones won't.

    ak
     
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  14. Copey84

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    Jul 27, 2015
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    Ak, how does one of the 0v terminals increase to 17v with a link between 5v and -12v. Get how current will flow between opisit regulators but how does voltage appear across the 0v terminals?
     
  15. Copey84

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    Jul 27, 2015
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    Hi all, still some confusion with voltages, will try and explain.
    When creating the 29v output between 5v and -12v a link is placed from +12v to 0v of 5v circuit, why doesn't the regulator try and pull the 12v down to its rated 5v output? It's in series so why does it allow the 12v to pass through and add to 5v?
    Obviously getting something mixed up here as voltages add to give 17v which gives 29v when measured to -12v terminal.
    Also when current is flowing back to source does the 7912 regulator sink the current from its output pin to ground thus completing the circuit?
    Hope this made some sense appreciate any replys.
     
  16. MikeML

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    Stare at this: Note the direction, and magnitude of the currents. Note that the regulators you showed can only "regulate" current in one direction. Just so happens that this connection is legit because that happens to be the direction that the regulators work. If any of the supply currents were positive (meaning they are sinking; not sourcing, current) then the output wouldn't be as predicted by simple addition...
    109.gif
     
  17. Copey84

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    Jul 27, 2015
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    So current will flow out of 7805 through load and return through output then ground of 7912. Basically any current must flow from + reg to - reg. Think that's ok.
    Does the 7805 regulator have no effect on 12 v because it has came from negative side of circuit? So it passes through reg and combines with 5v.
     
  18. MikeML

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    This is also a legit connection. Note the direction and magnitude of the currents through the three sources... As pointed out by AK, this can be done because there is total isolation between the +-12V and 5V supplies.

    109a.gif
     
  19. MikeML

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    True voltage sources can do this (because they can both source and sink current), but your regulators cannot:

    109b.gif
     
  20. Copey84

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    Jul 27, 2015
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    Thanks for those attachments mikeML, have helped understand things better.
    Do you know of any free to download circuit simulators that I could use to see circuit in action?
    Is it possible to build a simulated circuit with 78 and 79 regs input in circuit?
     
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