I am new to Electronics and have only just starting to getting a grasp of ohms law, I have been given some basic homework to create a basic circuit on a breadboard and to complete reading with my multimeter , It is very basic and in series and only has three resistors - Volt 6v - 4.7k - 5.1k - 10k, . I have finished setting it up i have also started to learn LTspice sim to also plan it out . now my question is ohms law works great but in the real world not everything is accurate , like my power supply reads 6 volts but when i tested it with my multi meter it read 6.50 volts , also resistors don't always add up to what there rated when i test them with my meter as by there colour code , yes we are only taking about small differences but dose this matter? When i measured the current i got 305.6 uA or 00.30mA reading but the numbers would change from a few digits that the closest i could get , the RT adds up to 19.8 ohms, but when i calculate V x I = 20.00 ohms , Is this normal or am i doing something wrong . With the resistors i went with the rated from the colour code and not from the reading that i tested them, i think the 10k read 9.50 ohms the 5.1k read 5.7 k so you get the idea slightly out of what there ratings say .

 

Nothing is ever exact. This applies to test equipment as well as components.
Resistors have an extra colour band that gives their tolerance - how far away from their marked value they are allowed to be.
If you are designing a circuit you will need to design it so it will work even when the values are not exact.
Incidentally, the resistance ranges of multimeters are often not very accurate. Can you check the specifications for your meter?

 

Yes, it is typical. Components have tolerances. Resistors go from ±20% to ±1% and then there are also precision resistors stamped with their exact value and that changes with temperature. Not sure what your power supply is but batteries are never their stated voltage either.

 

we are only taking about small differences but dose this matter?

Sometimes it matters and sometimes it doesn't. For cases where it matters, we use tighter tolerance components.

Regarding your 6V supply measuring 6.5V. That difference is less than 10%. Does 10% make a difference for your instructor?

 

I am new to Electronics and have only just starting to getting a grasp of ohms law, I have been given some basic homework to create a basic circuit on a breadboard and to complete reading with my multimeter , It is very basic and in series and only has three resistors - Volt 6v - 4.7k - 5.1k - 10k, . I have finished setting it up i have also started to learn LTspice sim to also plan it out . now my question is ohms law works great but in the real world not everything is accurate , like my power supply reads 6 volts but when i tested it with my multi meter it read 6.50 volts , also resistors don't always add up to what there rated when i test them with my meter as by there colour code , yes we are only taking about small differences but dose this matter? When i measured the current i got 305.6 uA or 00.30mA reading but the numbers would change from a few digits that the closest i could get , the RT adds up to 19.8 ohms, but when i calculate V x I = 20.00 ohms , Is this normal or am i doing something wrong . With the resistors i went with the rated from the colour code and not from the reading that i tested them, i think the 10k read 9.50 ohms the 5.1k read 5.7 k so you get the idea slightly out of what there ratings say .

Hello,

You could learn a couple things from this experiment.

First, the tolerance of resistors. A 20 Ohm resistor that has plus or minus 1 percent tolerance can be 1 percent off either way, which is 0.2 Ohms. That means the range would be 19.8 to 20.2 Ohms. A lot of times you will find the value to be lower because the temperature range comes into play too and a lot of resistors have a positive temperature coefficient so they value may start out lower so that at elevated temperatures the resistor value stays within range. In the case of the 20 Ohm 1 percent resistor, they want it to stay under 20.2 Ohms even at a higher temperature so they can rate it 1 percent.

The other thing you can get from this is the tolerance of measuring devices. Your power supply meter is probably no match for your multimeter. Power supply meters are often not very good.

What you could do is the following...
You said you have resistors 4.7k, 5.1k, and 10k.
Measure each resistor value with the multimeter and write down the results. You may measure:
4.65k, 5.12k, 9.8k.
Now measure your power supply output, say you measure:
6.51 volts.
Assume those values are more correct, then repeat your calculations using those values only and see if you can get closer to the simulation, using the same values in the simulation.

What you should find is that the results match more closely than before, but it does depend on your meter tolerance too. If your meter is 1 percent on all ranges (probably not though) then you should see results that are roughly within 1 percent or so.
If you dont, then try taking into account the tolerance of the meter on the Ohms range and the voltage range and see if it starts to make sense.

Sometimes you have to use more than one resistor value for each resistor. For the 20 Ohm resistor for example, you could use both 19.8 and 20.2 and see if either come close to the measurement by the meter,
or something in between. Take the tolerance of the meter into account also.
IF the meter is also 1 percent, then you may see roughly 19.6 to 20.4 Ohms. That's because if both the resistor AND the meter are off by a tolerance in the same direction, then the error will be greater than just the resistor tolerance alone.

This exercises are important for working with real components and that means both design and troubleshooting.