Why does this resistor act crazy? - Page 1
Forum section: electronics / projects

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 02:49:49 am
content: Hi Everyone,Got a little puzzle going on that is making me crazier than normal.In the attached schematic photo there is a 22k resistor connecting PIN 1 and PIN 2 of IC1 (op amp). Important: the op amp is not in the circuit at this time of measurement.When I took a resistance reading across the 22k resistor it read 74.5k..... hmmmm.... that's not right it should be 22k or less. So I figured it must be open. So it was replaced and remeasured .... still 74.5k!What the heck is going on here?There's NO VOLTAGE present (other than the Ω meter's). Zero volts measured with voltmeter. This doesn't appear to be a complicated circuit here so I'm completely baffled why this reading is so odd.The circuit traces are intact and read zero resistance from each pin to the resistors solder pad.I've never run across this in the 45+ years as an electronic tech.Pin 1 is in the lower right corner.Maybe someone here has an idea.Thanks!Gary

author: amyk
author_rank: Super Contributor
timestamp: September 06, 2025, 08:09:44 am
content: What meter are you using? Have you measured the resistor out of circuit? Does the reading change if you reverse the probes?

author: Sorama
author_rank: Frequent Contributor
timestamp: September 06, 2025, 11:46:48 am
content: Then measure it with one leg unsoldered, as well the resistance between the two pcb pads.Can’t be complicated.

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 12:35:34 pm
content: Thanks for the advice, it's something I already thought of but looking at the circuit it looks really straightforward so I can't understand where the weird is creeping in.

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 12:43:47 pm
content: Quote from: Sorama on September 06, 2025, 11:46:48 amThanks but that is basic electronics 101 .... well beyond that test procedure.

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 12:46:03 pm
content: Quote from: amyk on September 06, 2025, 08:09:44 amI actually used 3 different digital multimeters with different needle probes. The readings don't change if I reverse probes.

author: Sorama
author_rank: Frequent Contributor
timestamp: September 06, 2025, 12:51:12 pm
content: Quote from: Voxman on September 06, 2025, 12:43:47 pmIndeed…and what are the numbers doing so ?

author: wraper
author_rank: Unknown
timestamp: September 06, 2025, 12:53:06 pm
content: Resistor on the photo looks mechanically broken into two large and one small pieces. Other than that, it can measure higher if there is some capacitor or battery applying voltage to it. It will become obvious if you switch multimeter probes to change measurement polarity.

author: tooki
author_rank: Super Contributor
timestamp: September 06, 2025, 01:18:50 pm
content: Quote from: wraper on September 06, 2025, 12:53:06 pmAhem:Quote from: Voxman on September 06, 2025, 12:46:03 pm

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 02:53:11 pm
content: Quote from: wraper on September 06, 2025, 12:53:06 pmOh sorry you missed the info posted before this. That particular resistor is long gone although to reiterate, when I removed it, it read 22k well within spec.

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 02:58:49 pm
content: Well if you read and understood what I described in detail you'd know the resistance was 74.9k across the original resistor and the new one I soldered in. With resistor disconnected both original and new resistor read 22k again. Without any resistor it reads 74.9 again.So with that info, do you have any further information to share or questions I can answer?

author: Terry Bites
author_rank: Super Contributor
timestamp: September 06, 2025, 04:01:38 pm
content: Is the gate voltage high enough to turn off the fet?I imagine you are charging it up with the meter.

author: wraper
author_rank: Unknown
timestamp: September 06, 2025, 04:09:06 pm
content: Quote from: Voxman on September 06, 2025, 02:58:49 pmThen if it still reads the same, then new resistor is either broken too, or it's not 22k but something much higher

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 04:13:28 pm
content: I'm getting ready to delete post as I found out what the problem was. I'm too embarrassed to even describe my error but trust me, I'm a dumbass. I apologize to everyone who contributed to this thread.I going to go stand in a corner now for an hour or two.Gary

author: Ian.M
author_rank: Super Contributor
timestamp: September 06, 2025, 04:19:23 pm
content: Don't stand in the corner, you are not a dunce, and its a waste of time! The penalty you *should* pay for your mistake is to explain it here so others can learn from it . . .

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 06, 2025, 04:20:53 pm
content: I'd say add an oscilloscope to it, and observe what happens when you measure the resistance with the DMM.Do you see any difference when you reverse the probes?Then do the same with an unconnected 22k resistor, and compare those two.Still curious to what it was though...Were you measuring the wrong resistor?

author: gamalot
author_rank: Super Contributor
timestamp: September 06, 2025, 04:40:11 pm
content: Quote from: Ian.M on September 06, 2025, 04:19:23 pmMy guess, not soldering properly or measuring another component.

author: DimitriP
author_rank: Super Contributor
timestamp: September 06, 2025, 04:54:56 pm
content: Quote from: Voxman on September 06, 2025, 04:13:28 pmThere are plenty of people that keep making the same mistakes over and over. At least in this case it appears this is the first time you have meade this mistake.That's something to be proud of, not embarrased!!!Personaly I feel good when I make "new" mistakes" instead of repeating old ones.Embarrasing would be to repat old mistakes

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2025, 05:59:41 pm
content: Quote from: Voxman on September 06, 2025, 02:49:49 amQuote from: Voxman on September 06, 2025, 04:13:28 pmGood luck with thatStalinist revision of history is A Bad Thing, as summed up in the old Soviet joke "Students of Soviet history know how difficult it is to foretell the past".Your civic duty now is to helpotherpeople feel less bad about themselves when they make similar mistakes

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2025, 06:03:40 pm
content: Quote from: DimitriP on September 06, 2025, 04:54:56 pmI taught my daughter precisely that

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 06:47:28 pm
content: I know you're all on the edge of your seats wondering how I managed to completely miss the flashing lights in front of my nose.Apparently one should pay better attention to the Ω meter's range indicator. Big difference between 74K and 74 Ω. I can't fathom how I missed that on 3 separate meters too. Tunnel vision or more like cave vision.Sorry folks!

author: Ian.M
author_rank: Super Contributor
timestamp: September 06, 2025, 07:05:24 pm
content: Yep, that will do it. I had to throw out a formerly favorite pocket DMM last year because the 'k' indicator on the LCD had gone intermittent, and cleaning and reseating the zebra strip hadn't fixed it.If you'd followed Sorama's advice and measured it out of circuit, then across the pads you might have got through yourDoh! moment sooner . . .The low reading is of course from the J113 (T101) FET in parallel. With no gate bias its fully ON, and most JFETs are symmetric enough to be reversible drain-source so its not surprising you got similar readings both ways round.

author: gamalot
author_rank: Super Contributor
timestamp: September 06, 2025, 07:19:13 pm
content: Quote from: Voxman on September 06, 2025, 06:47:28 pmTo be fair, I'll share a silly thing I did myself. Years ago, I bought a used electronic load. When I got it, I noticed the bias settings on the segment LCD were incorrect, making it difficult to read. I opened the case and dug around, finally finding a solution— cut a PCB trace and put a resistor in series.It worked perfectly for the rest of the time, and I was quite pleased with my work. Until one day, I noticed a contrast adjustment knob on the front panel.

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 06, 2025, 07:22:44 pm
content: This is exactly why I manually place the meter in the range of the expected result as I’ve dumbassed myself too many times to count.

author: Voxman
author_rank: Contributor
timestamp: September 06, 2025, 08:23:48 pm
content: Well this never would have happened if I had been using my old trusty BECKMAN 360 meter. It's not auto-ranging so one has to manually change ranges. Pretty sure I would have caught it easily. But no, Murphy had to enter into the game and mess with my mind which is fragile and feeble enough without his help.Plus I must be half blind and can't see out of the other.

author: kite31
author_rank: Regular Contributor
timestamp: September 06, 2025, 10:25:01 pm
content: Quote from: WattsThat on September 06, 2025, 07:22:44 pmA good thought in that one of the meters I use is a new Brymen 785 while the other is a lovely analog Sanwa 460ED from the early 1970s. The latter I have no choice but to set to expected range then consciously read the matching upper or lower scale. Any failure on my part will not be common-source.