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Voltge/ current sources

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Electronman

New Member
Hi,

Just 2 questions:

1: Can I kill the voltage of a real power supply while I am drawing it in a paper (I mean my power supply is not an Ideal one).? Or It is not an independent (Ideal) Power supply?

2: Is my regulated power supply a voltage supply? if so What is current source in real life?

Thanks
 

MikeMl

Well-Known Member
Most Helpful Member
Several of my power supplies have two modes: constant voltage and/or constant current. They have two knobs, one which sets the maximum voltage that the supply will deliver (e.g. open-circuit); the other knob which set the maximum current that the supply will deliver (e.g. into a short circuit).

For example, I can use such a supply to charge a lead-acid battery. Suppose you want to limit the battery charging voltage to 14.2V, so set the supply to 14.2V open-circuit. Further suppose, that you want to limit the maximum charging current into the battery to 3A, so short the supply terminals (before connecting it to the battery), and set the current limit knob so that the supply puts 3A into the short circuit. Then connect the supply to the battery.

If the battery is mostly discharged (~11.5V), the supply starts out in constant-current mode and puts 3A into the battery, whose terminal voltage slowly rises as the battery accumulates charge. Eventually (after several hours) the supply/battery voltage rises to 14.2V, where the constant-voltage mode takes over, now the voltage stays at 14.2V, and for the next several hours the current flowing into the battery slowly decreases to a few 10s of mA as the battery finishes charging.


An example of a constant-current source is your Ohmmeter, where depending on the range it is set to, it puts a constant-current into the resistor being measured, and reads the voltage drop across the resistor being measured. Typical currents that an Ohmmeter puts out range from 10uA to about 10mA.
 

Electronman

New Member
Thanks,

I get confused a bit.

My power supply has 2 modes too, but I never called those modes a Constant voltage/current source because When you tell about word CONSTANT then Imagine an IDEAL voltage/current source?!

besides what did you want to transfer to me when you are using the phrases 'open circuit' and 'short circuit'?

Now that you are telling me that my ohm meter is a current source then I guess any voltage source connected to a load is current source too because a current is generated and passes through load?!

P.s If I tune my power supply so that it gives me a constant current when is shorted the do I have a constant current source?
 

audioguru

Well-Known Member
Most Helpful Member
A regulated voltage is a constant voltage source. The voltage stays the same when the resistance of the load causes the current to change.

A regulated current is a constant current source. The current stays the same when the resistance of the load causes the voltage to change.
 

Electronman

New Member
A regulated voltage is a constant voltage source. The voltage stays the same when the resistance of the load causes the current to change.

So is It an ideal voltage (or power?? I do not know what to call it?) supply!!!

A regulated current is a constant current source. The current stays the same when the resistance of the load causes the voltage to change.[/QUOTE]

What is a regulated current source? Are you talking about a POWER supply( like what I have and is a 0 to 30V, 5A regulated POWER supply??)

what do you call a Battery? A constant (idea??) voltage (power??) supply?
Can I kill Battery voltage while I am dealing with analysing a circuit? Can I consider a battery or my regulated supply as a supplyer which can be killed when I want to make an Analysis to a circuit connected to it????

Thanks
 

audioguru

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Most Helpful Member
A constant current is a regulated current. Changes in the load do not change the current.

I don't know if your supply has a regulated current (constant current) or just a simple current-limiter that is not regulated.
Its voltage is regulated so it does not drop much if the current is less than the current setting.

A huge truck battery is constant voltage. You can load it with a current as high as 100A and its voltage does not drop much.
A little AAA cell is also a constant voltage if it is new and if its load is 100mA and less.
 

Electronman

New Member
A constant current is a regulated current. Changes in the load do not change the current.

What do you mean by that?? my regulated power supply is able to give me a constant current when is Shorted (I can tune its MAX output current). your Above sentence means that if I put any resistance to a constant current supply, its current will remain constant?! It should vary till it reach to its MAX I defined it by tuning, right?

I don't know if your supply has a regulated current (constant current) or just a simple current-limiter that is not regulated.
Its voltage is regulated so it does not drop much if the current is less than the current setting.

Can regulated chips like 78xx convert the incoming voltage to them to current so that they could give a constant output Current?

A huge truck battery is constant voltage. You can load it with a current as high as 100A and its voltage does not drop much.
A little AAA cell is also a constant voltage if it is new and if its load is 100mA and less.

What Can I call the said AAA battery if I draw more that 100mA from it? Is it a dependent voltage source when It is connected to a load which causes it to give more than 100mA to it?
 

audioguru

Well-Known Member
Most Helpful Member
What do you mean by that?? my regulated power supply is able to give me a constant current when is Shorted (I can tune its MAX output current). your Above sentence means that if I put any resistance to a constant current supply, its current will remain constant?! It should vary till it reach to its MAX I defined it by tuning, right?
1) Set its voltage to 30.0V and short its output with a piece of wire then set its current to something, maybe 1.0A. Its voltage is 0.0V.
2) Then remove the short and put a 1.0 ohm resistor. The current will still be 1.0A but the voltage will be 1.0V.
3) Then change the load resistor to 10.0 ohms. The current will still be 1.0A but the voltage will be 10.0V.
4) Then change the load resistor to 30.0 ohms. The current will still be 1.0A but the voltage will be 30.0V.
5) Then change the load resistor to some value higher than 30.0 ohms. The current will be less than 1.0A because the power supply was set for a max voltage of 30.0V. The current is not regulated anymore because the voltage cannot go high enough.

Can regulated chips like 78xx convert the incoming voltage to them to current so that they could give a constant output Current?
Yes, but they need an input voltage that is much higher than if an LM317 regulator is used.

What Can I call the said AAA battery if I draw more that 100mA from it? Is it a dependent voltage source when It is connected to a load which causes it to give more than 100mA to it?
No.
It has an internal resistance that causes its output voltage to drop when its current is more than about 100mA. So its output voltage is regulated (constant) up to 100mA.
 
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