Power Sources In Parallel

Discussion in 'General Electronics Chat' started by Thender21, Jun 13, 2015.

  1. Thender21

    Thread Starter New Member

    Jun 7, 2015
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    Lets say we have two power sources, a 12 volt battery and a 6 volt battery with their voltages applied to a light bulb.

    I dont know how to draw and share a simple schematic but just picture a light bulb with B+ of both batteries applied to one end and B- applied to the other.

    The question is how much current will flow from each battery assuming the bulb has a fixed resistance of one ohm and the battery voltages are fixed as well

    Some would say the current will be shared between the batteries. I think the one with higher voltage will provide All of the current to the bulb. And unless a diode is used in the six volt battery wiring, the 12 volt battery may actually force a charging current through the six volt battery.

    Depending on what its effective resistance is.

    In my work this is relevant when testing an alternator. Often the alternator is loaded with a carbon pile connected in parallel to the battery and alternator.

    If the alternator voltage falls below the battery voltage in theory there will be no alternator output to the load. Only battery current, which depends on the state of the battery.

    In fact the battery will supply current if the alternator doesnt raise its charging voltage.

    So a load test in this manner can be mistaken completely.

    If two voltages are applied in opposite directions then the net voltage should be the difference between the two?

    So if 12v is applied against 5v the net will be 7 volts?

    Kinda need to draw schematics to reduce ambiguity.

    New here,
    Thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Those "some" are wrong.
    The battery with the low voltage will short the battery with the high voltage with the high battery attempting to charge the low voltage battery. The current is limited only by the sum of the series resistance of the two batteries (which is typically quite low). Thus you need a diode, as you mentioned to prevent this.

    The gist of this is that you never want to connect two batteries in parallel if they are a different voltage.

    A alternator is different (as compared to a battery or a DC generator) since it has rectifying diodes at the output which prevent any reverse current through the alternator. It is thus okay to connect an alternator in parallel with a battery (and that is just the way all automobile alternators are connected).

    Don't understand your comment "So if 12v is applied against 5v the net will be 7 volts?". :confused:
    You need to draw a schematic of that.
     
  3. Thender21

    Thread Starter New Member

    Jun 7, 2015
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    Dunno that the second question makes any sense actually.

    The thing you mentioned about the resistance affecting the current flow is true but bothers me.

    I have had a point in a circuit that should be open when referenced to ground reading near battery voltage. It was a module connected on the same circuit/wire "bleeding" voltage onto the circuit. Only enough I would estimate for less than one mA.

    So then when that circuit is switched to ground, it reads at ground potential like it should, and it grounds a relay coil.

    So I have to scratch my head. If loaded, that voltage drops to nothing because of the resistance increasing under load.

    Open circuit it read a voltage.

    I would need to draw to explain this.

    But in a rest state two voltages were applied against each other with different resistances. One had resistance from the relay coil. The other from failed insulation in the module.

    So which way does current flow and how much? Probably toward the module because it will have less voltage if the current increases. And probably not very much because of the high resistance.


    Sometimes electrical work is head scratching. Particularly when things arent functioning as designed.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    You'll have to draw some circuit diagrams of what you are referring to if you want any further comment from here.
     
  5. Thender21

    Thread Starter New Member

    Jun 7, 2015
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    What is the recommended way to do that? Is there an online app we use with conventions?

    Sorry I am using my phone only, do not have a PC at home so it is harder to figure these things out.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,854
    767
    Using the ball pen to draw the block diagram or detail circuit and take a clear picture, it is used to let us know what you really want, you can download a photo compress app for you cellphone, compress it to 800x600 jpg file or similar size and attach it.
     
  7. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    We don't put unequal voltage sources in parallel. Ther 12 V battery would try to charge the 6V battery. Batteries have an ESR (effective series resistance). Car batteries like to call the parameter Cold cranking amps which is basically I = V/(ESR).

    We can put certain power supplies in parallel when they are designed (set options) to do so.
     
  8. Thender21

    Thread Starter New Member

    Jun 7, 2015
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    Hey KiSi,

    The alternator is an example of a power source, unlike the electrochemistry of a battery it uses magnetic fields and induced current/voltage.

    The stator windings are solid copper and should have negligible resistance but there may be some loss across the diodes of the rectifier. I dont know but I would have to guess that the diodes primarily determine the effective series resistance of the alternator. I have also been told all power sources have some inherent resistance to them.

    While we are on the topic, could anyone explain what happens if you try to induce current into an open circuit? Does it just develop a voltage?

    Like if I wind a fine enameled copper wire into a coil but leave the ends not connected to anything, what happens when I swipe a bar magnet past it?

    If I hook up a voltmeter to those ends I may see a mv reading and or some milli/microamps.

    This gets to be a big deal in an ignition transformer - if the secondary circuit is open it seems that the voltage builds to the point of dielectric breakdown of the insulator and creates shorted turns.
     
  9. Thender21

    Thread Starter New Member

    Jun 7, 2015
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    I will get on it.

    I know some schematic drawing software exists wasnt sure if we used one on the forum.
     
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    The alternator is a "regulated power source". It actually generates 3 phase AC which is rectified into DC. This is done in the stator and with the corresponding diodes.

    Residual magnetism will help generate some voltage, but the regulator turns on and off the rotating magnetic field such that the voltage remains constant. The automotive voltage is such that it's OK to apply to a 1 V battery forever without harm. When the engine is operating, power will be delivered by the alternator.

    By design, the losses are "negligible", but they are there. Just like 625 CCA at 12 V is 12/625 or 19.2 milliohms.

    This isn't a 6V and a 12 V battery in parallel. The alternator has some "smarts" to it.

    Power supplies that can operate in Parallel have some control connections that go to the "other" supply. That supply is actually configured as a current source with a compliance of the desired voltage, so that each supply delivers 1/2 the total current to the load. The "front panel controls" of the "other supply" are effectively "disconnected". The meters work, though.

    In "series" operation, one control controls each supply to 1/2 the desired voltage and the current limits are set to the same value. As in before, the "otther" power supply's controls are disabled.
     
  11. Thender21

    Thread Starter New Member

    Jun 7, 2015
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    One of my teachers described working with diesel generators on coastgaurd ships. There were multiple generators and a process for shifting the load from one to the other or sharing it according to what he said.

    I think he had to do that to maintain the ship's electrical power while taking one out of service for maintenance or repair.

    I can likewise imagine that the power companies have means for activating power sources and controlling load distribution according to demand.

    I still cant exactly imagine what happens when you induce current into a conductor with too much resistance to make any kind of circuit. Mind boggled!

    Thanks
     
  12. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Any man-made current source will have a compliance voltage associated with the current source, so that's what you get, the set compliance voltage and an indication that the current set could not be delivered.


    Now we have a varying magnetic field which will introduce a voltage. Lentz law: https://en.wikipedia.org/wiki/Lenz's_law

    Need to think on this one, but there is a test that can be done on a coil called the "ring test" which can check for shorted turns.
     
  13. Thender21

    Thread Starter New Member

    Jun 7, 2015
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    I am not sure what a compliance voltage or a current set is. Sounds way above my head.

    One way shorted turns are identified is by the ringing or oscillation in the current and voltage that is caused as the magnetic field collapses across layers of the windings.

    It is often clamped using a diode, a mechanism I still dont Fully understand, but the usual pattern is kind of a triangle that decays exponentially.

    I am told shorted windings show a change in the pattern.

    There are also advanced electrical test methods that "ring" a coil with some kind of frequency based signal and analyze the results.

    That stuff is kinda advanced AC EE I wouldnt understand.

    What I described relates to transformers. For plain solenoids I guess the current rise time is used to gauge shorting.

    But the ringing test method I me tioned can be used on any coil.

    I am not experienced with these because I have only recently gotten hands on some good scopes. And modern systems have better self diagnostics and less accessible components than past designs.

    Regards,

    :)
     
  14. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    A Keithley 6220/6221 current source can deliver from 100 Femtoamps to 100 mA. Additional specs are:

    ADDITIONAL SOURCE SPECIFICATIONS
    OUTPUT RESISTANCE: >1014W (2nA/20nA range). 1014W is actually greater than 1E14 Ohms
    OUTPUT CAPACITANCE: <10pF, <100pF Filter ON
    (2nA/20nA range).
    LOAD IMPEDANCE: Stable into 10μH typical, 100μH for 6220,
    or for 6221 with Output Response SLOW.
    VOLTAGE LIMIT (Compliance): Bipolar voltage limit set with
    single value. 0.1V to 105V in 0.01V programmable steps.

    MAX. OUTPUT POWER: 11W, four quadrant source or
    sink operation.
    GUARD OUTPUT Accuracy: ±1mV for output currents <2mA
    (excluding output lead voltage drop).
    PROGRAM MEMORY: Number of Locations: 64K. Offers
    point-by-point control and triggering, e.g. sweeps.
    Max. Trigger Rate: 1000/s.
    RMS Noise 10Hz–20MHz (2nA–20mA Range): Less than
    1mVrms, 5mVp-p (into 50W load).

    Note, the info in bold.

    The point is, if the current source can deliver the set current within the 100 V (or set compliance) it will.
    The other spec is the output impedance (Z) which is specified at 1E14 ohms. An ideal current source has an infinite output Z and infinite compliance (voltage to work with).

    There are two theorems called Thevinin and Norton's theorems which say that you can replace a voltage source with a current source in parallel with a resistor ad you can replace a current source with a voltage source in series with a resistor.

    In your area, you don't have to deal with very small currents (picoamps) being generated when wires move in the earth's magnetic field. I did,

    RING Testing


    I suggest you read this: http://www.mainelectronics.com/pdf/BlueRingTester.pdf
     
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