Resiter question

Sorry for asking such a basic question but I’m stuck and don’t know why I’m not getting the expected or even hardly any, voltage drop with my resisters.

The problem is I’m trying to drop the voltage form a 5v pin to approx 2.2 volts using a 180 Ohm resistor. But I’m barely getting any drop. I tested the resistor and that can be seen in the attached photo and another one with a with the resistor infant of my multi meter so you can see that there was no voltage drop.

This is what I’m building

It is not very clear from the information provided, but if these are current limiting resistors for say a led or part of an opto-coupler, and you are measuring the voltage on open circuit, you will get a value very close to 5 volts (depending on the internal resistance of your multimeter).

If you want to measure the voltage drop across the resistor, do it when the resistor is in circuit with the led etc.

Read up on Ohms Law

V=IR

You will only get a volt drop if there is current flowing. To get 2.2v drop across a 180Ohm resistor you will need to draw 12mA through that circuit leg.

Hi,
Welcome to the forum.

Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html

If you are trying to get a lower voltage than 5V from the micro to supply the sensors, then you are not going to succeed.

What are the sensors, what voltage do they operate off?

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Label ALL components and pins and explain what you are trying to do?

Thanks… Tom… :slight_smile:

If the current draw is going to be changing, you need something more than a resistor to drop the voltage and keep it consistent. As others have said, voltage drop across a resistor is dependent on the current being drawn through it.

Something like this may work for you, depending on what it is you are building.

LM2937-2.5 - 2.5v Voltage Regulator

If the current draw is going to remain constant, then 1st determine what the current will be, then use OHM's law to determine what value resistor to use to get from 5v down to 2.2v.

I’m not getting the expected or even hardly any, voltage drop with my resisters.

That is correct you will not see any voltage drop from one resistor.

You need two resistors to drop a voltage.

Ok, you guys are awesome. Grumpy_Mike I think answered why I’m not seeing a drop.

so in the original diagram that I built my border from (see attached) how would I test to see the voltage drop with the ITR9608 in place? to be clear the both of the two ITR9608 would be connecting to tx0 and rx1 respectively

Hi,
PLEASE
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Label all pins and components, including part numbers.

You need to be more descriptive of your project components.

ITR9608 are photo interrupters.

The 180R I assume is the current limit resistor for the LED?

Why are you using pins 0 and 1?
They are reserved for program loading.

Thanks.. Tom...

Grumpy_Mike:
That is correct you will not see any voltage drop from one resistor.

You need two resistors to drop a voltage.
Voltage divider - Wikipedia

Eh?? What are you on about. Of course you will see a voltage drop across one resistor as long as there is current flowing. How do you design a simple LED circuit? With a voltage divider or just one resistor in series?

I think you need to read up on Ohms Law as well

Sorry for not being descriptive enough. Thanks to Grumpy_Mike explanation, I ruled out the problem I thought I found.

I took a closer look at my board and sensors and fond a bad connection between my photo interrupter and my board. Now everything is working well.

Let me explain the project. I’m building an arcade spinner with my pro micro acting as a USB mouse.

MarkDerbyshire:
Eh?? What are you on about. Of course you will see a voltage drop across one resistor as long as there is current flowing. How do you design a simple LED circuit? With a voltage divider or just one resistor in series?

I think you need to read up on Ohms Law as well

Mark after 50 years in electronics I think I know ohms law by now.
I will tell you what I am on about.

The OP was measuring the output voltage from a single resistor connected to a high voltage. The only current flowing would be the current caused by the input impedance of the meter, that is, to use a technical term, the current flowing would be SOD ALL. Therefore he was not seeing a voltage drop. Now if you had been a tad smarter you would have spotted that.

The trick to supplying good help, especially to a beginner is to put it in terms that they can understand without turning back somersaults giving unnecessary detail. I notice you, nor I, went into the fact that as current flows through a resistor it heats it up and causes a resistance change which in turn changes the current. I assume you would have gone into that if you had thought about it.

No, the OP made a simple and understandable mistake. Their is no shame in that, he didn't know so he asked. It is you Mark that leaped to a conclusion without looking. Look before you leap.