Hi. It is probably a very dumb question, but for me it is hard to understand.

I always read tha resistors reduce the current but not the voltage. I understand that they reduce the current, that is clear. But I also think the voltage across the load, provided that load works in acordance with ohms law. In that scenario, Can I say that putting a resistor in series is the same as reducing the voltage of the power supply?

Thank you

Best regards

 

If the circuit draws a constant current, yes, the resistor is equivalent to dropping the voltage just enough to get that current. The extra voltage appears across the resistor.

But if the current drawn is variable, the situation is different. At low current, the resistor will drop very little voltage, and at high current perhaps too much. Thus the voltage supplied to the load is varying. This is why we use voltage regulators, which basically change the resistance to keep the desired voltage on the load.

 

Hello,

The resistor will create a voltage drop depending on the current.
When the current varies, the voltage drop will vary.
An example of creating a voltage drop is the series resistor for a led.
The value of this resistor can be calculated as followed:
R = (Vsupply - Vled) / Iled.
In case of a supply voltage of 6 Volts and a led voltage of 3 Volts, with a led current of 20 mA, the result will be:
( 6 - 3 Volts ) / 0.02 Amp = 150 Ohms.

Bertus

 

I always read tha resistors reduce the current but not the voltage. I understand that they reduce the current, that is clear.

Where did you read this? There are cases where resistors don't limit current; such as a voltage divider giving a voltage reference.

But I also think the voltage across the load, provided that load works in acordance with ohms law. In that scenario, Can I say that putting a resistor in series is the same as reducing the voltage of the power supply?

Yes, but the reduced voltage will depend on load current. If that's not fixed, neither will the supplied voltage.

 

As stated in the previous replies, it depends on context.
If you wanted to create a constant current supply, you can select a high voltage, Vs, and a series resistor, Rs.

If the voltage across the load is small compared with the supply voltage, then the current is always approximately Vs / Rs.

As an example, suppose your supply voltage, Vs, is 24 V and the series resistance, Rs, is 10 kΩ.
The supply current Vs / Rs = 24 V / 10 kΩ = 2.4 mA.
You can drive any colour LED with this constant current supply and the LED current would be almost 2.4 mA, regardless of the forward voltage of the LED.

 

I always read ...

No, you don't, because text books and applications notes do not say that.

The value of a resistor affects the voltages and currents in a circuit. What those effects are depends entirely on the value of the resistor *in relation to the values of the other circuit components* and the overall circuit configuration.

ak

 

Hi. It is probably a very dumb question, but for me it is hard to understand.

I always read tha resistors reduce the current but not the voltage. I understand that they reduce the current, that is clear. But I also think the voltage across the load, provided that load works in acordance with ohms law. In that scenario, Can I say that putting a resistor in series is the same as reducing the voltage of the power supply?

Thank you

Best regards

In general, no you can't say that they are the same.

If you put one voltage source with a resistor in series with in a black box and then put another, lower voltage source without the resistor in another black box, I can tell which is which. So they are not equivalent.

If you have a single, unchanging load that is ohmic, then it might give the appearance of being the same. But as soon as you switch that load with a different load, even if it, too, is ohmic, you will not get the same result.