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Electrical 101 question

lndwarrior

Well Known Member
If you have two independent fused 12 volt power sources on separate switches, feeding a single device such as a light, what happens if both switches are inadvertently turned on at the same time?

This is just an example, not a specific issue on my plane.

I am trying to understand if this creates an electrical problem or not?

For whatever reasons my brain refuses to process anything to do with electrons, no matter how much I try...
 
Current will flow from higher voltage to lower voltage. For the light bulb current will flow from the positive side of the bulb to the negative side of the bulb. The amount of current to flow will be determined by the resistance of the bulb.

Connecting two power supplies to the positive side of the bulb means the bulb will still pass current, but current will also flow from the power supply at the higher voltage to the power supply with the lower voltage. This would create your problem depending on the capacity of your wiring. This is why there are warnings about common grounds so that your don?t end up trying to start the engine via your radio ground lead - or such.

Now, if you have two alternators feeding a battery for example, the current draw of the battery will be split between the two alternators, again more current on the alternator at the higher voltage (typically the larger alternator assuming the two alternator voltage regulators are set up the same).

This is for DC. AC works similar except power split between two alternators is more a function of alternator operating frequency.

Carl
 
Current will flow from higher voltage to lower voltage. For the light bulb current will flow from the positive side of the bulb to the negative side of the bulb. The amount of current to flow will be determined by the resistance of the bulb.

Connecting two power supplies to the positive side of the bulb means the bulb will still pass current, but current will also flow from the power supply at the higher voltage to the power supply with the lower voltage. This would create your problem depending on the capacity of your wiring. This is why there are warnings about common grounds so that your don?t end up trying to start the engine via your radio ground lead - or such.

Now, if you have two alternators feeding a battery for example, the current draw of the battery will be split between the two alternators, again more current on the alternator at the higher voltage (typically the larger alternator assuming the two alternator voltage regulators are set up the same).

This is for DC. AC works similar except power split between two alternators is more a function of alternator operating frequency.

Carl

Trying to understand - what if both power sources are 12 volt? (Your example talks about two different voltages)
 
Trying to understand - what if both power sources are 12 volt? (Your example talks about two different voltages)

If both supplies are the exact same voltage, current between them will not flow. In practice however there will always be some voltage difference. The current though the bulb will remain the same.

Carl
 
Current will flow from higher voltage to lower voltage. For the light bulb current will flow from the positive side of the bulb to the negative side of the bulb. This is for DC.
Carl

Actually, DC flows from negative to positive but it makes no difference in this example.

-Marc
 
If you're taking a graduate (or post-doc) course, you can debate it. If this is 101 (where we're operating), nothing happens.
 
In the 101 world, electricity is like weather....or water. Everything wants to flow from high to low.....and thus to equilibrium. As Carl said, in the DC world this is a good enough model. In the AC world, life is more hokes-pokus complicated, but its still a mostly good enough model.
 
What are the two sources of power? if say one is a ship power, 14.5ish VDC, and the other is a backup battery 12.6ish VDC, when you parallel the two power sources by flipping that switch, the ship power with the alternator will run current to that standby power and hopefully the wire size is adequate to handle this charge current into that battery.
If the alternator dies and the ship power drops to < 12.6 and you parallel the two, same thing but current going the other way. In the last example, with my setup, I flip on the 20AH backup battery, master off......and land somewhere now :rolleyes:
 
Please help me understand this.

The electrons in a complete direct current electrical circuit flow from negative to positive, same as cloud to ground lightning. Voltage is the (push/force) that is moving the electrons in the circuit. (Current/Amps) is the flow, or amount of electrons flowing. The light bulb is a dam in the stream, or something that is blocking the electron flow. If you have two direct current voltage sources, (always maintaining a relatively identical voltage), (in this circuit, 12 volts), and they are parallel wired to serve one resistance, (a light bulb), it makes relatively no difference if one, or the other, or both of the power sources are powering the light bulb circuit?

Do I understand what was being explained earlier?

The conductors (wires), and the physical connections in the circuit also create resistance to electron flow in the circuit. In general, a larger conductor (wire) in this circuit will offer less resistance to electron flow than a smaller conductor. (Dirty connections also offer more resistance to electron flow.)
Taking into consideration that 12 volts isn't a lot of push, and that current is proportional to voltage, and inversely proportional to resistance, then. In a parallel wired voltage supply circuit, serving multiple loads, if you have one load (downstream from the power supplies) that requires a high current, with all other loads in the circuit having a lower current demand, and all the loads are served by the same size wire with identical current carrying capacity, you'll create a circuit imbalance, and some negative results?

What happens if I size the wires throughout the downstream portion of the circuit appropriate to the current demand for each loads portion of the circuit? Identical parallel wired power supplies, (each capable of handling the total circuit load), still shouldn't be a problem, (no matter which combination of on and off, or both, that is used to power the circuit)?
 
It seems you are fishing for an answer to a problem or setup that you have not shared with us. What is it?

One side comment. It is factually correct to state current flows from negative to postive but by convention, circuit analysis assumes the opposite; current flow from positive to negative.

Carl
 
"Connecting two power supplies to the positive side of the bulb means the bulb will still pass current, but current will also flow from the power supply at the higher voltage to the power supply with the lower voltage. This would create your problem depending on the capacity of your wiring. This is why there are warnings about common grounds so that your don?t end up trying to start the engine via your radio ground lead - or such."

The quoted comment above is what sparked my interest, and thus my questions. To my knowledge, a system ground is a ground, is a ground, and unless my understanding is flawed, how can starting the engine through the radio ground occur if two power supplies of equal capacity are paralleled?
 
SNIP
The quoted comment above is what sparked my interest, and thus my questions. To my knowledge, a system ground is a ground, is a ground, and unless my understanding is flawed, how can starting the engine through the radio ground occur if two power supplies of equal capacity are paralleled?

The problem presents itself if something like your big fat greasy engine ground strap breaks or the engine bolt your are using comes loose. The starter will look for whatever ground path is present, regardless of wire size (e.g. sensor grounds and such). Now extend this to weird ways ground leads are somethings done and we have a plethora of lessons learned.

A ground is a ground. A ground path for a load is whatever you present as the path of least resistance. A nice thin wire might be the best ground path - until if goes up in smoke from over current.

There is a world of information on this in Nuckolls book.

Carl
 
There is a saying, "Electricity takes the path of least resistance". Supposing that there are two paths available. One path consists of a short 4 AWG cable. The other path consists of a much longer 22 AWG wire. If measured with an ohm meter, the short fat wire will have much less resistance than the long thin wire. So all of the current will flow though the short fat wire, right? No, wrong. Some electrons will find that long thin wire has less resistance and they will go that way. There will not be very many electrons flowing though the smaller wire. But there will always be some. An analogy might be cars on an expressway. The majority of cars take the wide 3 lane highway. But during rush hour, some cars will find side roads to be just as fast. Another way to look at is if the #4 wire is stranded. Each strand carries an equal amount of current. Now separate one of the strands from the cable, leaving it still connected at both ends. That strand is still carrying its share of current, just like the 22 AWG is carrying its share.
Back to the original poster's question. Current will flow through both current paths to the load. Nothing bad will happen. But since each fuse is only carrying part of the total current, the load could draw up to twice the normal current before blowing a fuse.
Short answer is, do not wire it that way. :D
 
Joe,

I really don't think that is an issue, since the fuse protects the wire; not the device. The device draws only what it needs, so no issues there. If the device fails shorted to ground, it will indeed take a lot more current before one fuse blows, but the other will quickly follow. No wires will be endangered, and the device is already dead. And if the device's mfgr did their job properly (admittedly, many don't), its *internal* protection would have activated and opened the circuit regardless of upstream fuses.

Multiple (redundant) power feeds are actually becoming more common on flight critical devices (IFR GPS, electronic engine controllers, etc). There's the original questioner's real world answer; nothing to worry about. Two supply paths will never be precisely equal (nor will two separate ground paths), so as you said, the load will be shared unequally. But it is of no practical consequence.

Charlie
 
Right you are Charlie. Some avionics have dual power inputs that are diode isolated. Some have dual inputs in parallel because a single D-Sub pin can not carry the full current.
 
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