Resistors - Page 1
Forum section: electronics / beginners

author: M0BSW
author_rank: Frequent Contributor
timestamp: April 13, 2013, 08:30:15 am
content: I wanted to add 0.1 ohms to my resistance decade box , so ordered 10 x 0.1 ,1% metal film resisters, they arrived and with all excited I measured them on 3 meters, they actually measure 0.46 ohms, am I been to picky, I ecpected them to read 0.1 0r 0.1.5, where can you buy from that actually sells what they say it is, am I right in thinking that 1% of 0.1 is 0.001.

author: Kremmen
author_rank: Super Contributor
timestamp: April 13, 2013, 08:42:37 am
content: You need something else than a regular multimeter to measure resistances that low. What did you actually use?Also, if the parts came from a reputable manufacturer and vendor, i find it highly unlikely that there could be that great variation between nominal and actual values. Assuming you don't have fake components, that is.

author: M0BSW
author_rank: Frequent Contributor
timestamp: April 13, 2013, 08:46:50 am
content: Quote from: Kremmen on April 13, 2013, 08:42:37 amNo this could be my mistake I used a regular meter,Think I'll take them down to a TV repair friend of mine , who I know has a pure ohm meter, I thought the regular multi would do it, guess I learned somthing today , thanks for replying

author: Strada916
author_rank: Frequent Contributor
timestamp: April 13, 2013, 08:48:40 am
content: did you short the leads out and take the lead resistance into account? as Dave would say "Something young players might get caught out on."

author: kripton2035
author_rank: Super Contributor
timestamp: April 13, 2013, 08:49:48 am
content: I measured my multimeter probes : 0.1 ohm each.so that makes 0.2 ohms more to add to any measureif your multimeter does zero ohm, you can touch the probes, make the zero, and then measure your resistors.you will have a better valuebut yes to measure this low, a 4 wires method is far better .

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 13, 2013, 08:52:13 am
content: perhaps your multimeter leads have 300mohm , that totally possible , please pass around 1A and measure the voltage drop . you should get 101mV to 99mV .I was in need of 10W 0.1ohm resistors , they are fairly costy if 0.1% or better accuracy is neede , all I did was buying 14FT of 11AWG nichrome and I've wound them by myself , they work great and they cost me few dolars to makeNOTE: although you have got 1% 0.1ohm resistors , did you took into play the switch's resistance ? I think you didn'tnote the resistance can be few tens of mili-ohms easilyQuotenot really , have you forgotten the louzy accuracy when measuring lower then 10-20% of the range ? ( Ie not specified accuracy ! )CHeers !

author: M0BSW
author_rank: Frequent Contributor
timestamp: April 13, 2013, 09:03:13 am
content: So this is what I've done meter with leads free measuring .0.32 touched together down to .05, then I rel now it reads zero, now i've put on resistor the reading is 0.09 the meter has a 22000 count , so would the 0.09 be right.Paul

author: Wytnucls
author_rank: Super Contributor
timestamp: April 13, 2013, 09:04:50 am
content: Just do a 4-wire measurement with 2 multimeters and a power source.

author: M0BSW
author_rank: Frequent Contributor
timestamp: April 13, 2013, 09:08:49 am
content: Quote from: Wytnucls on April 13, 2013, 09:04:50 am,I would if I knew how to do thatPaul

author: Unknown
author_rank: Unknown
timestamp: April 13, 2013, 09:28:03 am
content: It uses thesame principlethat expensive bench meters use. You need a constant current source (eg. bench supply) and two DMMs. Set the power supply to deliver a constant current: adjust the current limit to about 1A and the voltage limit to the max value. The exact value of the current does not matter, as long as it's stable. Now connect the 10A current terminal of your first DMM to the positive terminal of your power supply. Set it to measure current. Connect the common terminal of this DMM to one leg of the resistor. Connect the other leg of the resistor to the negative terminal of the power supply. Connect a second DMM (set to measure DCV) across the resistor.You now know both the current through the resistor and the voltage across it, allowing you to derive the resistance using Ohm's law. Keep the dissipation in mind: 1A should be fine for a 1/4 W 0.1 ohm resistor, but not for a 1/4 W 10 ohm resistor.

author: M0BSW
author_rank: Frequent Contributor
timestamp: April 13, 2013, 09:52:22 am
content: Quote from: alm on April 13, 2013, 09:28:03 amThank You now I understand, having a play with it now.Paul

author: M0BSW
author_rank: Frequent Contributor
timestamp: April 13, 2013, 06:55:17 pm
content: All resistors have been checked on a german meterwhich is so accurate,never seen anything like it they are all 0.1, so fitting to decade box, thanks for the replies and help.

author: Rick Law
author_rank: Super Contributor
timestamp: April 17, 2013, 02:07:02 am
content: One thing I failed to put into account at first was the Winter.When I first got my UNI-T 61E, I got a 5-pack of 0.1ohm 1% current sensing resistors. When I first got it, they all measured 0.1 ohms after I delta out the test leads' resistance. A month or so later, I found them all to be a good bit higher (0.12 to 0.17). Even my 61E test leads went from 0.06-0.10 ohm to 1.0 to 1.5 ohms range.After making a post here about why test leads has such high impedance, another EEvBlogger posted that he tested the UNI-T lead to be (around) 0.03ohms each. It was that post that I recall - wait, when I first got the meter, the two leads where in the 0.0x range, not 1.x range.As I was puzzling over it, I feel my dry hands again, and I reflectively reach for the hand lotion on my desk, then it hits me... In the cold dry winter, I use a lot of hand lotion. Particularly when I am using my hands to do things like holding a wrench, stripping a wire, so forth, it get so uncomfortably raw. Every couple of hours, I pasted my hands with hand lotion or Vaselin to stop my skin from cracking. The Vaselin was getting on everything. I soap-cleaned what I could (with alcohol wipe after), and life was better - the leads were back down to 0.08 ohm range when I touch them together. It is a month (or two) later now, my test leads have since added about 0.4ohms worth of hand lotion.I would ask the store clerk if they have low impedance hand lotion, but I think it would probably be more than I want to pay.