0.1R 1% smt resistors tolerance issue - Page 1
Forum section: electronics / projects

author: stock86c
author_rank: Newbie
timestamp: March 19, 2015, 01:44:32 am
content: Hi all, recently bought a reel of smt 0.1R 1% resistors,and measured a few just to check. To my surprise(maybe I am a noob),some measured like 0.18R, which I think is a tolerance issue. In my knowledge, let's say a 100r resistor with 1% tolerance should vary between 99 -101. So why are these 0.1R res varying that much? Is it normal or did i buy a faulty reel/bad manufacturer etc?

author: Rick Law
author_rank: Super Contributor
timestamp: March 19, 2015, 01:49:04 am
content: Quote from: stock86c on March 19, 2015, 01:44:32 amDid you take into account the resistance for your DMM probe? 0.08 ohm is around what my pair of probes combined to.Down at that low, even twisting the probe's jack may give you some variations. I can turn my jack in the banana socket like I turn a radio dial and get a delta > 0.1 ohm.Try this, touch the ends of the two DMM probes, see what it reads. Twist it around, lessen/increase the pressure of the contacts between the two probes, see what the variations are. Touch one end of the SMD with both probes, see what it reads. (Both at the same end may not be zero, depending on quality of contact between your probe and the SMD). Twist the probe cable, see what the variations are... In short, get familiar with how even the smallest of things could get in the way when measuring something this low in resistance.Rick

author: wraper
author_rank: Unknown
timestamp: March 19, 2015, 01:58:33 am
content: You need 4 wire connection to measure such low resistance reliably. 2 wire connection is no good for this, because both lead resistance and contact resistance. You can compensate lead resistance but as your meter seem to have only 2 digits after point at ohms range it cannot measure such resistance with any precision at all as it's resolution is 10% of the resistance itself.

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 19, 2015, 02:17:59 am
content: What power rating are the resistors ? A wheatstone would be an interesting test, but at that low ohms value, the connectors themselves would have to be very good quality.

author: stock86c
author_rank: Newbie
timestamp: March 19, 2015, 02:47:39 am
content: Quote from: wraper on March 19, 2015, 01:58:33 amQuote from: Rick Law on March 19, 2015, 01:49:04 amThanks guys, I am a noob. -_-"

author: electr_peter
author_rank: Unknown
timestamp: March 19, 2015, 08:15:43 am
content: Very simple method to measure low resistance - connect 1.5V battery in series with a resistor in series with multimeter(1) in current measurement mode. Turn on the circuit and with another multimeter(2) measure voltage drop across resistor. Multimeter 1 gives current in A or mA, multimeter 2 gives voltage drop in V.Then actual resistance is calculated with Ohms law: Resistance = Voltage/Current (R = V/I)

author: paulie
author_rank: Frequent Contributor
timestamp: March 19, 2015, 08:52:21 am
content: Or with one meter and say 1k or 100ohm resistor in series with a known supply just measure voltage. I got almost exactly the same results with a recent lot of .1 ohm but immediately recognized the probe issue.

author: Howardlong
author_rank: Super Contributor
timestamp: March 19, 2015, 08:57:34 am
content: If you have a lab PSU with an accurate constant current setting, set it to 1A, and measure the voltage drop, but make sure you probe the resistor terminals directly themselves and not anywhere else with your DMM!!!That should measure 100mV.Don't be tempted to put much more through the resistor, depending on its power rating.Remember P = VI = I^2 * R, so that's 100mW if you put in 1A. You don't specify the resistor size or power rating, but if it's 0805 or larger 100mW should be fine.To logistically do this test, you'll almost certainly need to solder one to a bit of board to apply the current and probe the part.