How to calculate overall voltage drop

Other Hardware General Electronics
1

Hello,

I found nothing to this issue. When I use Arduino 5V output and put a resistor into the circuit, I have an overall voltage drop. In my case voltage drops from 4.98 V to 4.93 when I put a 220 R resistor into the circuit.

First question for understanding: Is this because of heat production of the resistor?

Second question: Should this drop be constant per resistor of the same type? This I need to know because I also have a LM335 in the circuit and for temperatur calculation I have to know the exact voltage which is supplied to the LM335 and in my experiment the number of resistances is changing.

Thanks
Timmorn

2

http://www.raltron.com/cust/tools/voltage_divider.asp

here is simple voltage divider calculation . choose your respective voltage and resistor. to get output read in resistor network

3

timmorn:
In my case voltage drops from 4.98 V to 4.93 when I put a 220 R resistor into the circuit.

First question for understanding: Is this because of heat production of the resistor?

Yes and no. In a resistor all power is dissipated in the form of heat and a marginal amount in stuff we can't explain.

The voltage drop can be due to the power limit the power supply can deliver.

A solution can be: Read the voltage with Analog.read and use it in the calculations. Then, if there is another reason the voltage fluctuates your always safe.

4

5V/220 ohm = 22.72mA.
Atmega328P output is spec'ed to be >=4.2V at 20mA with Vcc of 5V.
0.05V drop when over 20mA is within limits.

5

The output transistors of the Arduino have an internal resistance of about 30--40 ohms
when the chip is operating at 5V supply, IIRC

External loads form a voltage divider with that internal resistance, hence the drop
in voltage at the pin. Your figures seem wrong, I'd expect about 0.7V,
I think you meant 4.9 to 4.3V drop.

6

C-F-K:
Yes and no. In a resistor all power is dissipated in the form of heat and a marginal amount in stuff we can't explain.

This is wrong, there is nothing that we "can't explain".

The thing is where exactly is this resistor?

In my case voltage drops from 4.98 V to 4.93 when I put a 220 R resistor into the circuit

So 4.98 - 4.93 is = 0.05V.
If you have that across a 220R resistor ohms law says the current through it is 0.05 / 220 = 0.227 mA
So that resistor is not powering the arduino so what is it powering?

First question for understanding: Is this because of heat production of the resistor?

Wrong way round, the heat produced is because you have current flowing through the resistor.

Should this drop be constant per resistor of the same type

No.
The drop is due to the resistance of what that resistor feeds into.