Do resistor values change?

[aro]

I have a question that probably is too simple for even a beginner question, but I would really appreciate some help...

I wanted to replace some resistors on a PCB. One of them is a 100k resistor, to be replaced with a 220k. I measured the 220k before starting everything and the measurement showed indeed 220k. After I put it on the board I measured it again...and it wasn't 220k anymore... It was something like 77k

Is this normal? Can the PCB change the value of the resistor based on...something...?

The funny thing, for me at least, is that another resistor I replaced maintained its value after putting it somewhere else on the board.

Could anyone please shed some light on this? Thanks

 

[Mikebits]

I have a question that probably is too simple for even a beginner question, but I would really appreciate some help...

I wanted to replace some resistors on a PCB. One of them is a 100k resistor, to be replaced with a 220k. I measured the 220k before starting everything and the measurement showed indeed 220k. After I put it on the board I measured it again...and it wasn't 220k anymore... It was something like 77k

Is this normal? Can the PCB change the value of the resistor based on...something...?

The funny thing, for me at least, is that another resistor I replaced maintained its value after putting it somewhere else on the board.

Could anyone please shed some light on this? Thanks

Components in parallel. Two 100K resistors in parallel will be 50K, so you have to consider what parallel circuits.

Series and Parallel Circuits

 

[3v0]

Good question.

The resistor itself has not changed. When you measured it prior to mounting you were measuring the resistor itself. Now that it is soldered into the circuit the resistance you measure is not just that of the resistor you installed, but the resistance of the circuit between the two point which are the resistor leads.

You will want to look up parallel resistance. Why, because you put the resistor in parallel with the resistance that was already there.

Here is a that can help you but you should learn how to do it with math.

HTH

3v0

 

[aro]

Thanks guys for the quick response. (At least I know now that I'm not just seeing things...)

I'll try to make sense out of the schematics, but it must be an issue of serial vs. parallel. There's a lot to learn, but it's also a lot of fun.

 

[Leftyretro]

Thanks guys for the quick response. (At least I know now that I'm not just seeing things...)

I'll try to make sense out of the schematics, but it must be an issue of serial vs. parallel. There's a lot to learn, but it's also a lot of fun.

Measuring resistances of devices while still installed in an existing circuit is a more advance technique requiring a through understanding of electronic components and circuits they are being used in.

People first learning electronics should unsolder or otherwise remove at least one lead of a resistor from the circuit board before taking a resistance measurement. The same applies to using an ohm meter to test diodes and transistors, it's tricky to test these devices in circuit without a certain level of knowledge, experience and documentation.

Lefty

 

[3v0]

The OP was trying to figure out why the resistance changed after he installed it.

 

[audioguru]

Maybe a very cheap soldering iron or a fire was used for soldering that got much too hot.
Then the resistance of the resistor was changed by it being cooked to death.

Of course other parts in the circuit are in parallel with the resistor.

 

[Dean Huster]

However, resistors themselves, out of circuit, can change value, especially if they're of carbon composition construction. The Tektronix 2213 and 2215 used several 510K, 1/2-watt carbon comp resistors in their high voltage circuitry. A symptom was the focus drifting off to the point that it couldn't be adjusted with the front panel focus controls. The problem was that many of these resistors had changed value from 510K to 600K, 800K, evern into the mid-megohm range!

Dean

 

[3v0]

By my way of thinking the only real measure of a government is the quality of life enjoyed by the people who live under it. Everything else is a detail.

I admit that quality of life if a rather wide brush, but it is the correct brush.

3v0