Measuring Resistors...Just Checking

[NFX]

Can resistors be measured in a circuit or will they give different values than what they are?

[DIY Bass]

Depends on what it's connected to. In other words you can't assume it can be measured in circuit unless you know the circuit pretty well.

[Rob Strand]

If you measure in both directions and it matches what you expect then there's a good chance it's correct.  Sometimes you have to wait for the value to stabilize.

[antonis]

Maybe Rob confused you more, NFX  but he refers to relatively complicated circuits with also diodes & capacitors in some kind of "contact" with resistor under measurement..

Both DIY Bass & Rob well said stand only for not powered circuits, of course.. 

[Rob Strand]

Maybe Rob confused you more, NFX  but he refers to relatively complicated circuits with also diodes & capacitors in some kind of "contact" with resistor under measurement..

Hmmm, yeah maybe.   (To make things worse some circuits you *know* the measurement will stuff-up in one direction and not the other so you can only trust (or maybe trust) one of the measurements.)

The general problem with anything is, the bigger the shortcut you make the more you have to know to justify you can get away with it.

[NFX]

 thought so, sometimes you get a different value than the banding colours and sometimes they measure right.

[PRR]

> Can resistors be measured in a circuit

There's usually other stuff in "a circuit".

Here is a very simple case.


Not only does the 100r resistor measure 66.6r, the 200r resistor also measures 66.6r. Oh no! BOTH are bad!!

But when removed, both read 100 or 200.

That particular combination is not common; we would slap one 68r instead of a 100r and a 200r. But there are other circuits....

As said, if you know the circuit well, you can work it out. Here is an advanced freshman problem:

http://www.electronics-tutorials.ws/resistor/res_5.html

[thermionix]

Answer:  A low number of ohms. 

[Rob Strand]

Here is an advanced freshman problem:

I remember my high school teacher putting one of those up and I called out as soon as he finished writing.
He looked at me like, yeah right, like I was joking, then he realized it was correct.  Everyone was freaked out - BTW I wasn't exactly a studious person in my earlier school days.  The weird thing is I can do that one by sight as well.

[antonis]

Here is an advanced freshman problem:

Next one will be the above with some voltage sources among resistors..
(or current sources to also honor H. F. Mayer & E. L. Norton..)