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First build question - resistor values change after solder
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author First build question - resistor values change after solder
3pand
Dreaded question from a first time builder haha.

So I did a fair amount of post reading and video watching until I felt like I knew what tools and solder to buy and how the basics work, and so I've just started my first build (Befaco VC ADSR)!

Anyway, I've been double checking everything and I've got about 20 resistors on the pcb so far. I noticed, though, that three of the resistors now give different values on the multi meter than they did before I soldered them on.

Before, they were the correct value and now they are quite a bit lower lower.

I'm pretty sure I've ruled out the multi meter being faulty. Also, the new values are consistent - they don't change each time I test the resistor.

I have noticed though, that the multi meter is pretty picky about when it displays the correct value for a resistor. I have to apply decent pressure and not move along the leads, etc.

Anyway, just wondering if maybe I damaged the resistors or didn't solder the joint correctly or something? From everything I've read, it doesn't look like the soldering needs to be redone but the weird readouts are making think something is wrong.

Any help would be greatly appreciated.
authorless
You are measuring across multiple resisters once it is in a circuit. Ohmmeters also sent voltage out and can potentially send voltages where you don't want them, damaging components, be careful with that. The resisters are most likely fine still, they are quite robust.
Koa
Some valuable advice:

Do not worry over every little thing.

It might seem prudent to test every component before and after, to scrutinize and reflow 200x your solder joints, etc..

All that does is slow your progress down.

trust the manufacturer (mouser?) to sort your components right. Troubleshoot only after something comes up.

Solder like you're confident. Put that damn iron to the pad and the component, flow some solder into both, and release. After the solder is liquid, give it a tiny bit more heat (stay for another second or so), but release and then don't worry about it. Keep going. Keep pushing forward!
CONQUER THE WO- errm. ahem.

... where were we? Dead Banana
mskala
When you measure a resistor that is already installed in a circuit, you're really measuring that resistor in parallel with everything else in the circuit. So the reading will often be less than what you would get for just the resistor alone.

However, as long as there's no power source in the circuit other than your ohmmeter, the rest of the circuit can only decrease the apparent resistance. If it was 27k before you installed it, it might read 23k afterwards, but it shouldn't read 100k.

It's pretty unlikely that any modern general-purpose ohmmeter would produce enough voltage and current to damage a circuit. The voltages are no more than a volt or two and the currents are limited to microamps. Consider how much fun the meter's manufacturer would have dealing with the complaints if this were a significant issue...

Another useful tip is to get some clip probes for your multimeter, the kind that look like this:


Those will reduce the awkwardness of needing to press the probe tips into place to get a good connection.
3pand
Thanks all for the info and advice. Makes total sense that they are now part of a circuit. And yeah, those clip probes look great! (Can't believe I'm getting excited about such a thing haha)

Thanks for the wisdom Koa, I think your advice applies to so many things in life. It seems like a lot of times it's experiencing the breaking point or limit of something that gives us the certainty to move freely. Like when you're first learning to drive and it's raining and you drive way too slowly because you don't know how much it takes for the car to lose control.

I will push forward!!
calaveras
that was a goof I made once or twice too.
It was explained to me that when you measure resistors in circuit, it is very rare that you will find one which is isolated from other resistors, and is also the easiest path between probes. More often than not the electrons can find an easier path, so you are measuring the intended path, along with whatever other paths are possible.
fuzzbass
I used to check resistors before, but now I just look at the color code and multiplier before I solder them in. I buy all my resistors from reputable dealers and I have never yet seen one with wrong value indicated. If you force yourself to do this from memory, in a very short time you will have the code memorized.
mskala
fuzzbass wrote:
I used to check resistors before, but now I just look at the color code and multiplier before I solder them in. I buy all my resistors from reputable dealers and I have never yet seen one with wrong value indicated. If you force yourself to do this from memory, in a very short time you will have the code memorized.


I haven't seen a definitely wrong value indicated on a colour-coded fixed resistor, but I've seen:

* "Brown" 1 and "violet" 7 that are indistinguishable
* Colour sequences that could be read in either direction to get different values that are both valid standard values, such as brown-black-black-brown-brown "1k 1%" or brown-brown-black-black-brown "110ohm 1%"
* Resistors with one of the colour bands missing (minor manufacturing defect)
* Vishay multiturn trimmers that were supposed to be 50k, came in a bag from Mouser with other 50k trimmers, and on testing they really were 50k, yet the printed markings said "10k"

So my trust in resistor markings is limited.
StudentsOfTheFuture
fuzzbass wrote:
I used to check resistors before, but now I just look at the color code and multiplier before I solder them in. I buy all my resistors from reputable dealers and I have never yet seen one with wrong value indicated. If you force yourself to do this from memory, in a very short time you will have the code memorized.


Seconding this, and it's very easy to spot the edge cases mskala mentioned once you have a bit of practice. Basically things will just look off (at which point I reach for my meter).

I've built things in groups and I (and others that do this) tend to finish resistors (and so, the build) much quicker, and it makes troubleshooting easier (as does lining up your color codes!)

It's basically a rainbow that expresses digits in scientific notation, if that helps. Its not really a mnemonic device but thats what helped things click for me.

YMMV though - I find color very helpful memory-wise, and use rainbow progressions to organize patch bay cabling, etc.
duff
Koa wrote:

trust the manufacturer (mouser?) to sort your components right. Troubleshoot only after something comes up.


After receiving resistors packaged incorrectly from most of the big sellers and wasting hours troubleshooting I now never trust them until I have tested them. I know a few common values by colour code and have a look up card on my desk but usually just grab my meter and check one per batch on first use.
Addam
duff wrote:

After receiving resistors packaged incorrectly from most of the big sellers and wasting hours troubleshooting I now never trust them until I have tested them. I know a few common values by colour code and have a look up card on my desk but usually just grab my meter and check one per batch on first use.


This is what I do, too, not because I've been burned but due to my own paranoia. I also find it very hard to determine which color is which sometimes (especially on some of those tayda resistors - I think they're so cheap because of all the money they save on paint by making the markings razor thin).

I will agree with not being too afraid of ruining basic components like resistors.. especially when you're first getting used to soldering (it does seem a little wild at first) it's easy to be scared of overheating which can lead to not properly heating the component or the trace and then you end up with a bad solder joint which may not even appear as an issue until you've completed and been using it.
pricklyrobot
If you haven’t stumbled across something comparable already, here are a couple handy resources I keep up on the laptop when I have to sort components:

http://www.hobby-hour.com/electronics/resistorcalculator.php

http://www.muzique.com/schem/caps.htm
unrecordings
duff wrote:
Koa wrote:

trust the manufacturer (mouser?) to sort your components right. Troubleshoot only after something comes up.


After receiving resistors packaged incorrectly from most of the big sellers and wasting hours troubleshooting I now never trust them until I have tested them. I know a few common values by colour code and have a look up card on my desk but usually just grab my meter and check one per batch on first use.

Same here. And because I'm buying resistors on tape, I always always check a few on a meter then write the value every few inches all the way down the tape
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