Current source in parallel with voltage source

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philly

Senior Member
In light of some of the PV threads currently going on I need to refresh my memory on what happens when you put a current source in parallel with a voltage source.

For instance lets say we have a 10A current source in parallel with a 10V voltage source? Does the current soucre take on a voltage of 10V and the voltage source have a current of 10A running through it? If so can you refresh my memory of the numbers behind this.

I'm sure it will come right back to me when someone explains it, I just need a slight refresher and kick in the butt. :)
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110602-2055 EDT

philly:

See my discussion in the other thread in response to your question on the PV cell characteristics.

To this question you have the correct answer. See if the other answer helps you.

.
 

philly

Senior Member
110602-2055 EDT

philly:

See my discussion in the other thread in response to your question on the PV cell characteristics.

To this question you have the correct answer. See if the other answer helps you.

.

Yes your explanation in the other thread helped. Thank you.

However I'm confused as to the voltage across the current source. With an ideal current source the voltage across it is determined by the current passing through the load resistance V*I. In the case where a battery or constant voltage source is the load wouldn't the voltage across the current source be a function of its output current times the internal or source impedance of the voltage source?

Why does the current source just automatically take on the value of the voltage source?

Will the current source push its rated current into the voltage source regardless of any internal input characteristics of the voltage source?
 

philly

Senior Member
Also why is a charge controller required when between a PV module and a battery. It was mentioned somewhere else that without a charge controller the PV module or current source would take on the voltage of the battery and therefore would not output its maximum power capacity. Why is this?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110611-1538 EDT

philly:

However I'm confused as to the voltage across the current source. With an ideal current source the voltage across it is determined by the current passing through the load resistance V*I.

A pure resistance load on an ideal current source will result in a voltage across the resistance of R * I where R is said resistance and I is the ideal current source current.

In the case where a battery or constant voltage source is the load wouldn't the voltage across the current source be a function of its output current times the internal or source impedance of the voltage source?

If we have an ideal voltage source as the load, then the internal resistance of the voltage source is 0. Thus, the current source terminal voltage will be equal to the ideal voltage source and the voltage source's terminal voltage.

Next consider a non-ideal voltage source that consists of an ideal voltage source of V and an internal resistance of Rint, then the terminal voltage of the ideal current source is V + Rint*I.


Why does the current source just automatically take on the value of the voltage source?

Because of what I described just above.


Will the current source push its rated current into the voltage source regardless of any internal input characteristics of the voltage source?

Yes, unless that voltage source has an infinite internal resistance. The result is not defined for this single case.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110611-1958 EDT

philly:

Parallel two current sources, different current levels, and connect a resistor R to their output. What is the voltage across R?

.
 

tallgirl

Senior Member
Location
Great White North
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Controls Systems firmware engineer
Also why is a charge controller required when between a PV module and a battery. It was mentioned somewhere else that without a charge controller the PV module or current source would take on the voltage of the battery and therefore would not output its maximum power capacity. Why is this?

The maximum power point voltage doesn't match the battery voltage across the entire range of battery charging voltages.

Here's a real example. The MPP voltage for my array is something on the order of 93 volts and the nameplate rating is 2800 watts, or roughly 30 amps. The charging range is about 50 to 58 volts, and the inverters make power whenever the voltage is at or above about 53 volts.

If you take those 30 amps and you multiple them by all those different voltages you will come out with a number of different powers. 30 amps times 53 volts is 1,590 watts, or about half of what I have, and without the charge controller, I'd be wasting around 1,200 watts of solar panels (at phat dollars per watt).

But it's more complex than that because the 93 volts is at 25 degrees C at the solar panel. I could have designed the system with panels that had a Vmpp of 58 volts -- that would have wasted less power -- but what happens in the summer when the array is 60 degrees C (140F)? Solar cells lose voltage as temperature increases -- -0.37% / degree C, in the case of my panels -- so Vmpp in the summer is (60 - 25) * 0.37%, or 13% (12 volts) lower. Now instead of being a 93 volt array, it's an 81 volt array. And had I chosen 58 volts, the new Vmmp would be less than 52 volts -- not enough to recharge the batteries.

The Maximum Power Point Tracking serves two purposes -- the first is it converts the voltage to the battery's voltage, and the second is it adapts to changes in the array's performance. In the winter the array voltage is higher, and in the summer it is lower.

The charge controller performs another important job -- it controls the charging process itself. My batteries will be around 50 volts in the morning. They need to reach about 58 volts in order to be fully charged. Anything more than 58 volts and the batteries could be damaged. So the charge controller will start outputting power and the voltage will rise to 58 volts. At that point the charge controller reduces the current to hold the voltage at 58 volts. At some point in time the system will decide the batteries are fully and it will sell surplus power so long as the batteries are at or above about 53 volts.
 

philly

Senior Member
110611-1958 EDT

philly:

Parallel two current sources, different current levels, and connect a resistor R to their output. What is the voltage across R?

.

The voltage across the resistor would be the combined current from the two current sources times the resistor value.

For example if one source was 40A and the other was 10 A the voltage across a 10ohm resistor would be 500V
 

philly

Senior Member
110623-0729 EDT

philly:

Good.

Are you getting a better understanding on how to look at these sources?

.

Yes

So the voltage across the (2) parallel voltage sources will be that arcoss the load resistor.

Is this the same way a grid tied inverter works? Is it simply a current source that syncs up with the grid voltage and ouputs a current rather than a voltage. I know the inverters can operate themselves and thus dont ouputput a voltage but rather a current?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110625-0906 EDT

philly:

So the voltage across the (2) parallel voltage sources will be that across the load resistor.
Did you mean to say current sources?

The output of two ideal voltage sources of different voltages connected in parallel is not definable. Also two current sources in series is undefinable.

A simple grid-tied inverter is primarily a current source where the current is instantaneously made proportional to the terminal voltage.

A non-grid-tied inverter has to be both a voltage and frequency source. Ultimately there will be current limiting.

.
 
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