Hy guys. I am wondering if you could controll the curent from a DC PSU (12/15V 5/3A) just with a potenciometer (regulated resistor) or whould you need something else?
And what could you use to controll the voltage
I would prefere if the solution would not be a pre-build unit, but if it has to be one let mo know what should it be.
I'll give you a formula, and let you decide what you can and can't control.
Voltage = Current * Resistance
That DC PSU appears to be a constant voltage power source. No matter what you connect to it, it will always try to put out 12V or 5V from *it's output*.
If you connect a resistor or potentiometer in-line with it, then the resistor will drop some voltage across it based on the current passing through it... So then the voltage 'after' the resistor will be lower.
Special note here though... notice how I said the voltage depends on the current through the resistor? If you connect something that slightly changes how much current it draws, it will cause the resistor to change how much voltage is being dropped across it and your new output voltage will change! (Use the formula above to determine the relationship)
Here's the big kicker though...
If you have a 12V source, and a 50Ω light bulb that runs on 9V ... how do you provide 9V to it? Your method asks to use a resistor/potentiometer and it can be done... but lets look at the example with some math:
A 50Ω light at 9V draws (Current = Voltage / Resistance) 0.180 Amps ...
So we know the current through the circuit, and know that we need to drop 12V-9V = 3V across a resistor right? Resistance = Voltage / Current || 3V / 0.180A = 17Ω
There is just one more problem... this resistor might get hot... Power = Voltage * Current
Power = 3V * 0.180A = 0.54 Amps
It does not take much 'power' to make something as little as a resistor hot, and it takes FAR less to burn out a potentiometer... potentiometers are meant for signals, not for handling any kind of power. So although this is possible, you will find that you will need to buy specific high-power resistors instead of a potentiometer.
If you have a 100Ω Buzzer that runs at 6V, what value resistor do you need on your 12V PSU?
Bonus Question for you:
If you have a 50Ω Light bulb that needs 5V and 250mA, what value resistor do you need to use on your 12V PSU?
*This bonus question should help you understand the ability to control both current and voltage at the same time.
(Please no outside assistance on this one)