# How do power sources work?

Summary:
In my study of electrical engineering I have been told that the voltage across a resistance is V = IR. If I connect a resistor R and a power supply of 5V 2 A then how does ohms law work here?
In my study of electrical engineering I have been told that the voltage across a resistance is V = IR. If I connect a resistor R and a power supply of 5V 2 A then how does ohms law work here? I could change R to anything I want but the source would stay at 5 V and 2 amps regardless of what I do because it is an independent source. Meaning that I could have a resistance of 500 Ohm while at the same time have a voltage of 5V and 2Amps which does not follow ohms law.

Wrichik Basu
Gold Member
As far as my knowledge goes, in electronics you have voltage sources and current sources. The former holds the voltage constant, while the latter holds the current constant.

You are right, one cannot hold both voltage and current constant simultaneously.

cnh1995
Homework Helper
Gold Member
power supply of 5V 2 A
I believe it is the "voltage and current rating" of the power source. It means the power source can produce maximum 5V and can supply maximum 2A.

davenn
I believe it is the "voltage and current rating" of the power source. It means the power source can produce maximum 5V and can supply maximum 2A.
Oh so the rating of the power source just tells you how much it can produce but now what I'd does produce? That makes way more sense! Thank you!

Janus
Staff Emeritus
Gold Member
Oh so the rating of the power source just tells you how much it can produce but now what I'd does produce? That makes way more sense! Thank you!
If it is a regulated voltage source, the amp rating tell you how many amps a load can draw without it effecting the output voltage.
So for example, with a 5V 2A source, you could connect up to a 2.5 ohm load and the source would be able to maintain the output voltage ( though in reality, you wouldn't want to push things this close to the limit). However, if you tried to connect a 1 ohm load, the output voltage would be drawn down to lower than 5V. You also risk damaging the power source.

davenn
If it is a regulated voltage source, the amp rating tell you how many amps a load can draw without it effecting the output voltage.
So for example, with a 5V 2A source, you could connect up to a 2.5 ohm load and the source would be able to maintain the output voltage ( though in reality, you wouldn't want to push things this close to the limit). However, if you tried to connect a 1 ohm load, the output voltage would be drawn down to lower than 5V. You also risk damaging the power source.
And that is because you would be drawing around 5 amps?

And that is because you would be drawing around 5 amps?
To add to this, voltage is the one being limited when a huge resistance is connected. Why can't the current be the one being limited?

anorlunda
Staff Emeritus
To add to this, voltage is the one being limited when a huge resistance is connected. Why can't the current be the one being limited?
You can make them either way. Constant voltage, or constant current, or switchable between the two. They can also have overcurrent protection, reverse current protection, over/under voltage protection, overheating protection, AC or DC, and more. It just depends on how fancy and how much money you want to spend.

davenn
Gold Member
2021 Award
And that is because you would be drawing around 5 amps?

ONLY if the PSU was capable of supplying 5A
When it isn't, the voltage out of the PSU will sag (drop)

To add to this, voltage is the one being limited when a huge resistance is connected. Why can't the current be the one being limited?

No, there's still the same voltage across the resistor, say 12V from the PSU, that doesnt change ...
You can easily measure that.
BUT the current is limited as the resistance increases....
That is easily measured as well

Baluncore
2021 Award
Why can't the current be the one being limited?
We distribute power over fixed voltage networks that have voltages close to ground. That is to protect the wiring insulation. We connect loads in parallel across the standard voltage supply. Each load is independent and draws the current it's resistance determines.

If the current was fixed we would have to connect loads in series. When anyone turned something off the terminals would need to be shorted to prevent other loads losing their power supply current. All devices plugged in would need to use the same current, but drop only sufficient voltage for their power requirement. The extremely high and variable voltage across the terminals at the generator end of the circuit would arc through the air and insulation.

Baluncore
2021 Award
The concept of constant electrical current distribution has an interesting practical analogy.

Machinery operated by hydraulic systems often have a fixed pump flow rate = current.
Power = the volumetric flow rate of fluid * pressure; just like the electrical; W = I * V.

When the freely circulating fluid flow is diverted to an actuator, the flow is obstructed until the pressure builds up to overcome the resistance of the obstructing actuator. Several actuators operated in series at the same time move with proportional speeds, but then share the available pressure.

There is an over-pressure relief valve at the pump that prevents hoses bursting, (equivalent to electrical insulation breakdown), when immovable objects obstruct irresistible forces.

Averagesupernova
Averagesupernova