Hello,

If I measure the resistance of a resistor with my multimeter when power is on or off to the circuit, will I still get a fairly
good estimate of it? Does it matter whether the power is on or off? Or will it give me a incorrect reading?

Thanks

 

It DOES matter. The reading with the power on will depend on how much current is flowing through the resistor. The life of the ohm meter may be very short too!

 

Hello,

If I measure the resistance of a resistor with my multimeter when power is on or off to the circuit, will I still get a fairly
good estimate of it? Does it matter whether the power is on or off? Or will it give me a incorrect reading?

Thanks

You are making two fundamental mistakes.

#1 Don't ever attempt to make a resistance measurement on a unit while its power is on. Your meter might not survive the experience.

#2 If the resistor is still in the circuit, even with the power off, you are not measuring the resistance of just the resistor. You are also measuring the resistance of the rest of the circuit that is connected to the resistor. Furthermore, some circuits can become powered from the battery inside your resistance meter. Whether you get a reliable resistance reading will depend on the rest of the circuit.

To make a proper measurement, remove the resistor from the circuit. At the very least, disconnect one leg of the resistor from the circuit.

 

Hello,

If I measure the resistance of a resistor with my multimeter when power is on or off to the circuit, will I still get a fairly
good estimate of it? Does it matter whether the power is on or off? Or will it give me a incorrect reading?

Thanks

You can expect it to give you an incorrect reading in either case, though the particular incorrect reading will likely be quite different. When the circuit is powered, there is also a good likelihood of damaging your meter.

Imagine you have a 10 kΩ resistor in parallel with a 1 kΩ resistor. If you want to know the value of the 10 kΩ resistor and put your meter leads across the 10 kΩ resistor, do you expect the meter to give you a reading of 10 kΩ? No. You expect it to give you a reading that is somewhere in the 900 Ω range.

If you want to read the value of a resistor with an ohmmeter, then that resistor needs to be the only thing connected to the ohmmeter probes.

 

You can expect it to give you an incorrect reading in either case, though the particular incorrect reading will likely be quite different. When the circuit is powered, there is also a good likelihood of damaging your meter.

Imagine you have a 10 kΩ resistor in parallel with a 1 kΩ resistor. If you want to know the value of the 10 kΩ resistor and put your meter leads across the 10 kΩ resistor, do you expect the meter to give you a reading of 10 kΩ? No. You expect it to give you a reading that is somewhere in the 900 Ω range.

If you want to read the value of a resistor with an ohmmeter, then that resistor needs to be the only thing connected to the ohmmeter probes.

Thanks guys.

I really didn't know that doing this was trouble. I think other hobbyists, particularly non-professionals like myself, would like to know this too.

 

A digital ohmmeter measures resistance by putting a known (constant) current through the resistor, and measuring the resulting voltage to display the resistance using Ohm's law.
Obviously any other current going through the resistance from the the circuit being powered, will affect that reading, making it inaccurate.

 

Thanks guys.

I really didn't know that doing this was trouble. I think other hobbyists, particularly non-professionals like myself, would like to know this too.

Some of us other hobbyists knew that 57 years ago.

 

Some of us other hobbyists knew that 57 years ago.

... and some of us knew even 70 years ago!

 

Thanks guys.

I really didn't know that doing this was trouble. I think other hobbyists, particularly non-professionals like myself, would like to know this too.

Hobbyists in any field of endeavor have similar problems -- how do you learn the ins and outs of that field without any kind of formal training?

How many things do tennis players that do it as a hobby not know that are actually pretty important to playing tennis really well? Lots of things. Same with playing a violin, or cooking, or knitting, or electronics.

So how do you learn these things? Lots of people never do -- and their ability to do well at that hobby is forever limited as a result. Not the end of the world, and if they enjoy what they can do within those limits, they may not even care that those limits exist. Others seek out guidance from people, often other hobbyists with more experience. That's one of the things that forums like this provide. Others seek as least some formal education to further their hobby -- they sign up for a tennis clinic at a club, or a cooking class at the community center, or an electronics class at a community college.

 

It seems that "WB" neglected the benefit of textbooks on the subject. The good news is that even obsolete textbooks still have the right information, although they lack current explanations . Circuit theory texts from 1965 are still OK, but they probably do not consider CMOS devices. But since first you need to understand voltage an current and resistance, that has not changed at all. The math for a multimeter resistance circuit is identical, except no microprocessor in 1965.

 

It seems that "WB" neglected the benefit of textbooks on the subject. The good news is that even obsolete textbooks still have the right information, although they lack current explanations . Circuit theory texts from 1965 are still OK, but they probably do not consider CMOS devices. But since first you need to understand voltage an current and resistance, that has not changed at all. The math for a multimeter resistance circuit is identical, except no microprocessor in 1965.

That's true -- and an oversight on my part. The use of texts and references is such an engrained part of my life that I don't even think of them consciously very often.

 

The problem with that is that today’s audience is more likely to seek answers from a YouTube video or social media on a smart phone. ChatGPT only makes conscious learning even worse.
People expect instant answers instead of not having to expend the effort of thinking through things themselves.

 

(First line erased)

The rest of this comment is deleted to avoid

 

For those unwilling to focus attention long enough to learn, they are simply OUT OF LUCK! They are also courting Natural Selection.
The problems with social media and especially the cartoon channel are primarily that they lack any form of useful quality control. The operating systems can censor words and phrases but it has zero capability to validate truth or accuracy.

 

I really didn't know that doing this was trouble. I think other hobbyists, particularly non-professionals like myself, would like to know this too.

To measure resistance, a DVM applies a known current to the resistor and displays the resistance (voltage drop).

 

An amazing amount of useful information is often found in the instruction sheet or booklet that come with most multi-meters, both digital and analog types.