Bench DC Power Supply

How is it that you can simply turn the current delivered to your circuit up or down on a bench DC power supply?

As of right now I think that it's the resistance of the load that determines the draw. What am I missing?

 

The bench power supply simply senses how much current is pulled by the load by measuring the voltage across a low value series resistor and either delivers a constant current or shuts down depending on how it's set.

 

That's true for a constant output voltage. But if the power supply is in the current limit (constant current) mode then it simply adjusts the output voltage to maintain the selected current. Thus the resistance of the load has no effect on the current since, if the load resistance changes, so does the power supply output voltage thus maintaining the same output current.

 

You are missing the idea that a power supply can be used as a constant voltage supply or a constant current supply. It's either a constant voltage with the capability to deliver up to a certain amount of current or a constant current supply that is capable of delivering up to a certain maximum voltage limit. Each of these has their advantages and weaknesses. It's up to you to pick which one will do the job best at this moment.

 

Bench supplies can operate in either constant voltage or constant current mode. There are two different control loops inside the PS.

 

You are getting the idea, but here is how I often explain it.

At a constant voltage, the current is determined by the resistance.

At a constant current, the voltage is determined by the resistance.

Often, we are taught about constant voltage supplies because that is most prominent in electronics. The battery, for instance, in it's bare form, is a constant voltage supply; You supply a load, and it "varies" it's current to match the rated voltage. There is another supply called a constant current supply. It will vary its voltage to provide its rated current.

On many bench power supplies (as pictured below), there are often two knobs, one for voltage limit and one for current limit. The supply will continue to increase voltage (and inherently current) until ONE of the conditions are met.


Examples. In all of these examples you have a 30V 5A Max power supply with 100 ohm resistor hooked between the positive and negative terminals.

Scenario1: You set the voltage dial to 10 V (remember, think of the 10V as a limit) and turn the current dial to 5A. When you turn on the supply, it will only provide 10V and inherently .1A (V=IR). Why? Because the voltage dial is the limiting factor. You are in Constant Voltage mode.

Scenario 2: You set the voltage dial to 20V and set the current dial to .15A. You turn the supply and get 15V and .15A. Why? Because the current knob is now the limiting factor keeping the supply at .15A. You are in Constant Current mode.


Scenario 3: You set the voltage dial to 20V and set the current dial to .1A. You turn the supply and guess what, you will get exactly the same output as scenario 1, 10V and .1A. Why again, the current is now the limiting factor, but we know that .1A through a 100ohm load is 10V. Hence why the output is at 10V, even though this time you are in Constant Current mode.

 

Current can only be limited and not controlled It is done by varying the voltage.

 

Don't know why you say that. You can control current to the same degree that you can control voltage. Both are bounded by the voltage and current limits of the power supply. Stay within those limits and you can have full control of either the current or the voltage, as desired (assuming the power supply has both current and voltage control circuits such as shown by Stuntman).