How to repair a crazy chinese resistor? - Page 1
Forum section: electronics / repair

author: Beamin
author_rank: Super Contributor
timestamp: October 17, 2017, 07:22:32 am
content: I bought one of those 30$ oscilloscope kits you have to put together. It comes with a bunch of 1/8 watt Chinesium blue through hole resistors. One of them I can not figure out the value (color bands are too small on blue back ground there is a red band on the end and a darker band). First off this resistor doesn't instantly give you its reading like most resistors. I think this is supposed to be a 2M or 1M resistor. When you measure it the value usually starts high but then drops: Some time it starts at 2 then goes to ~1.8 but moves around. Then I goes down to 0.8 then up to 1.2. I tested the leads and they are 0 ohms. I tried stretching out the resistor or squishing the leads together but that doesn't change its value. Why would it do this? I tried another resistor and the meter works fine. Not a great meter; it was 12$ on ebay but it has always worked. Why would it do this? This is the strangest reading I have ever got off a part. The heat from my hand shouldn't effect such a large value.I not going to put this in the project but I really want to know how it could red so strange.Tried another meter the same. If you squeeze the leads really hard the value stabilizes a bit but still is reading around 1.2 +/- 300k which there is not 1.2 in the kit only a 2M and a 800k. When you test the leads by them selves it reads zero quickly.

author: helius
author_rank: Super Contributor
timestamp: October 17, 2017, 07:26:27 am
content: Send him or her to a re-education camp?Seriously, high-value resistors can have significant inductance, which is likely the effect you're seeing. A red band is a 2 so it is a 2M.

author: mc172
author_rank: Frequent Contributor
timestamp: October 17, 2017, 08:32:10 am
content: The reading is varying because you are putting yourself in parallel with the resistor, to a varying degree depending on how hard you squeeze the leads and how acute the angle of your tongue is relative to your lips.

author: wraper
author_rank: Unknown
timestamp: October 17, 2017, 08:39:35 am
content: Quote from: mc172 on October 17, 2017, 08:32:10 amThis

author: kalel
author_rank: Frequent Contributor
timestamp: October 17, 2017, 09:16:04 am
content: I find it easier to measure resistance in one of those little multi testers.I'm sure they are not as accurate as a good meter, but at least there are no test probes used, and once you "close" the socket the resistor leads won't move around, your fingers won't affect the readings, etc.This seems a bit like an advertisement, it's not - I have one of these, and just sharing my experience.

author: wraper
author_rank: Unknown
timestamp: October 17, 2017, 09:23:52 am
content: Quote from: helius on October 17, 2017, 07:26:27 amNot that high, and not that more than resistors in general. Wirewound resistors have a lot more of inductance. Also multimeter does not care about this as uses DC voltage for measurement. mc172 said exactly right, you should touch no more than one multimeter lead when measuring anything above 10k and expect remotely accurate reading.

author: Beamin
author_rank: Super Contributor
timestamp: October 17, 2017, 07:00:05 pm
content: I suspected that I was a part of the circuit but wasn't sure since the meter uses such a low voltage. I don't often use M resistors so that explains why I have never seen this before. The Wikipedia page on resistors doesn't tell you this stuff.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 17, 2017, 07:36:44 pm
content: They do not need to tell this because it has nothing to do with the resistor. It is a user mistake. If values get higher the more difficult the measurement becomes. Swet, dirt, fingerprints can cause leakage current bypassing the resistor and degrade the value of the resistor. And there comes a stage where blinking your eyes will cause a deviation on the meter.

author: helius
author_rank: Super Contributor
timestamp: October 17, 2017, 08:08:36 pm
content: I assumed you were using using alligator clips, as that's what I resort to for measuring loose axial resistors. The resistance of your body is around 200Kohm and will affect the readings of any resistor of that value or higher.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 17, 2017, 08:42:18 pm
content: Just place the resistor on a flat, insulated surface and press the leads down with your probes.

author: Beamin
author_rank: Super Contributor
timestamp: October 18, 2017, 09:13:42 pm
content: Quote from: Cyberdragon on October 17, 2017, 08:42:18 pmThat's what I did and it was spot on and it worked in the project. Sadly I set the board down on a bit of stray lead clippings shorting the bottom of the board out and its now useless. I have an extra screen and I ordered another one with the "0.99 protection plan" to see if they will fix the broken one. 0.99 worth of insurance? I have an idea how you make money off that; starting with not calling it an insurance product but rather a "protection plan contract".