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Current presence with no voltage?

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Menticol

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Hello guys!

I have a very basic question than haunted me since I watched the movie Apollo 13, years ago. When giving some battery readouts, the astronaut mentioned something like this (I don't remember the exact literal conversation):

Battery A: Eleven volts, two amps.
Battery B: Zero volts, two amps.
Battery C: Five volts, zero amps.

I understood A and C conditions, but not B. If the battery is dead and no voltage is present, how can circuit B draw current from it?

The question raised again today when I powered a small Flyback transformer using my LM317T based power supply. PSU output voltage indicator fell to zero, even the pilot LED light turned off, but the arc at the end of the flyback was present.


PS: The original question was for DC, but while searching for an answer I found this post https://www.electro-tech-online.com/threads/design-a-zero-to-20v-20a-lab-supply.112573/. If at some point AC voltage crosses 0, why current doesn't drop to zero too?
 
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First, the script writers for movies make lousy Electrical Engineers. The battery voltage could be zero, but it is being charged with two Amps. (yeah, I know it should have been -2A).
 
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First, the script writers for movies make lousy Electrical Engineers. The battery voltage could be zero, but it is being charged with two Amps. (yeah, I know it should have been -2A).

Or maybe the voltage was very close to zero but not zero, an analogous meter error.

But in the "Design a zero to 20v 20A lab supply" dougy83 mentions "~17.1A into a 0V load". How can it be Mike?

...Here's a simulation of ~17.1A into a 0V load. Peak current is ~17.2A, ripple is ~0.2A. Please ignore the fact the diodes are a little underrated...
 
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....................................

The question raised again today when I powered a small Flyback transformer using my LM317T based power supply. PSU output voltage indicator fell to zero, even the pilot LED light turned off, but the arc at the end of the flyback was present.

PS: The original question was for DC, but while searching for an answer I found this post https://www.electro-tech-online.com/threads/design-a-zero-to-20v-20a-lab-supply.112573/. If at some point AC voltage crosses 0, why current doesn't drop to zero too?
If the power supply current appeared to be zero then it either was too small to register or a filter capacitor was supplying the current.

But you can't have power without both current and voltage. A superconductor can carry current with no voltage drop, but there's also no power being dissipated.

Yes, in an AC circuit both voltage and current go through zero twice per cycle. So the power also fluctuates. You can notice this by the flicker of some AC lights.
 
OK guys. A lot of times, the sign of the current is ignored or even the sign of power. A 16 MW power plant is actually -16 MW. A solar cell with a Jsc of 100 ma/sqcm is actually -100 mA/sqcm. These make the equations work. Power generated is negative and power dissipated is positive.

That out of the way, there are two types of power supplies. 2 quadrant or 4 quadrant. These quadrants are the same in math with the X axis being V and the Y axis being I or J (current density).

So you have 4 quadrants:
V+, I+
V-, I-

and
V+,I-
V-, I+

The first two would be what's possible with an alkaline battery.

The latter is possible with re-chargeable battieries. It's possible, but not recomended to actually reverse the polarity of a NiCad battery.

if we were charging a battery in a car we might have a meter that shows -25 A to +25A. This is actually backwards from convention. When the alternator is delivering power, the current is positive, BUT this gage is relative to the battery charging or discharging. 10 A is 10 A being delivered by the alternator into the battery.

So, what I am trying to say is that the real number's sign is dependent on the meter's label.

When charging is involved, all quadrants are POSSIBLE, but V- and I+ is not recommended.

Purchased power supplies are normally 2 quadrant. They won't tolerate a battery discharging into them. A 4-quadrant power supply and/or an electronic load will.

Take a look here: https://www.youtube.com/watch?v=Gg6eHcfnsYs
Again, the signs are flipped. In reality, the graph should be flipped around the x-axis, then a negative current and positive voltage would be generated power SIGN(V*I)=-1. The X-axis is voltage.

One parameter of a solar cell is short-circuit current. This is when the voltage is zero and the current is negative.

But remember what I said, watch the signs. If this was a solar array charging a battery and the ammeter indicating like a car, you may well have zero voltage and positive current.

While I'm here, there is conventional current and electron flow because Ben Franklin got the signs wrong. Conventional current flows from positive to negative of a battery. Electrons go the other way. Electrons don't flow, but rather collide with each other like a chain reaction. Picture a row of bumping cars standing still and apart. Then a car rams them. There is an "appearance" of movement.
 
Current and volatge are part of the ohms law triangle, where theres one theres the other, or theres no smoke without somtheing burning.

One plausible explanation for your appollo film (it was more likey to be lousy electrical knowledge from the scriptwriters however) would be that the voltage was so low that the meter didnt read, often such meters can have expanded ranges, ie linearity drops off towards full and zero scale, so a voltage of a volt or 2 on such a meter might well show little or no deflection.
Even under a volt you can have 2 amps, if the batteries power electronic equipment then the volatge isnt likely to go below 0.6v.
As mentioned you could have a battery shorted and still read current if its connected series, however for the ammeter to read there will still be some voltage accross the meter at least.
 
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Purchased power supplies are normally 2 quadrant. They won't tolerate a battery discharging into them. A 4-quadrant power supply and/or an electronic load will.
In my experience, purchased power supplies are one quadrant. A positive supply can source current, but can't sink it. A negative supply can sink current, but can't source it.

Regarding battery #2, if this were a power supply, it would be a possible and acceptable result of a power supply with current limiting, and a shorted output.
 
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OK, details. You can manually change the polarity to the load by flipping the leads. Your correct, most supplies do not have polarity reversal relays.

Most supplies can do +V, +I and -I and -V by flipping the leads.

Then there are issues with or if, what side is connected to ground.
 
OK, details. You can manually change the polarity to the load by flipping the leads. Your correct, most supplies do not have polarity reversal relays.

Most supplies can do +V, +I and -I and -V by flipping the leads.

Then there are issues with or if, what side is connected to ground.
I guess I think of "2 quadrant" as implying 2 adjacent quadrants.
 
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