How to Test Resistors?

Hello

I am assuming you want to verify the ohmic value (resistance) of a two-terminal device designed to reduce the flow of current or, stated another way, designed to reduce the voltage available at a given point.

You need a working knowledge of Ohms Law and a means to measure current and voltage. You also need a source of current.

Apply a known (measured) voltage across the resistor and measure the current flowing in the circuit. Use Ohms Law, R=E/I, to calculate R. This is what an ohmmeter does only it reads out directly so that you don't have to calculate anything. Inexpensive multimeters are readily available.

If you are testing for other parameters such as power dissipation or reactance, this discussion would need to go much further.

Lonnie

Ishurtle's given as good an explanation as anyone can give.

In my experience, I've rarely had to test resistors except to verify their value. I happen to be color blind and reading color codes was pure hell for me back in school. Give me color codes on paper and I can get 100% in my exams. Give me a physical resistor and tell me to read it...well, one of my teachers suggested I change my course to accounting (no thanks. I already know accounting...ah-one, ah-two, ah-three...).

On the job, finding a busted resistor was easy. They were usually burned to a crisp. On some occasions, the legs were all that was left plus a hole on the PCB.

Once, I had a problem with a tape deck and there was no burned resistor anywhere. Measuring voltages around the circuit, I came across one resistor that seemed to drop an large amount of voltage. It looked normal. I touched it lightly with the probe and it broke in half.

Oh well, just change it right? Big problem. Since it was broken, I couldn't read its resistance with my multimeter. I'd have to read the color code but I was afraid that I might make a mistake. So, I swallowed my pride and asked one of my mates to read it for me. I received a lot of ribbing but I fixed it so it wasn't so bad.

Mr vulcan has given a good explanation ,but during measurement of the resistance ,if u r using multimeter make sure that no other devices are connected across the resistor terminals,else the resistance value measured will be net resistance if the combination.

make sure that no other devices are connected across the resistor terminals

Yes, of course, but it's good that you've pointed that out. 'Shoulda see that one. Thanks.

I do lift off one leg...usually. Depending on the circuit, I sometimes measure directly without taking one leg off (the resistor's, that is). If I wanted the value then, yes, I do lift off one leg.

Vulcan

I too have encountered the "disintegrating resistor" and the "burned beyond recognition" resistor and sometimes, if you don't have documentation, its a real challenge to figure out an appropriate replacement value.

Some other "toughies" are the carbon comp resistor that has been hot enough to change value drastically but looks fine or the one that has darkened and made the colors difficult to identify.

And then there is the metal film resistor that has arced and carbon tracked between two adjacent spirals of metalic film--usually on the underside next to the PCB where it is difficult to see.

Or the resistor that may or may not be OK but has been warm enough to create a cold solder joint or sometimes a flux incursion in the joint, changing the resistance the circuit actually sees. You can sure spend hours finding some of these.

I don't know of any real general technique to figure it out other than a common sense understanding of the circuit parameters and some years of experience with the resistance values in similar designs. I guess a person could undertake to reverse engineer the circuit but that is time consuming and probably over my head.

Most of my years have been with relatively high powered stuff that makes smoke and soot when it fails. Those are lots easier to find. This surface mount stuff is so tiny and operates with such low power that I am somewhat at a loss sometimes. And when you do find the fault you are unable to repair it without $1500 dollars worth of work station and magnification and lighting and jigs to hold things.

I'm looking at one of those situations right now. A simple chip replacement but I know better than to attempt it with my field-grade tools and hand-held techniques. Oh well...I'm far from the root of this thread so I will get out of here.

L.