Multimeters and Resistors

[paulca]

As my eyes are deteriorating rapidly in the last 10 years, due mostly to very heavy VDU usage I find it impossible to read the small blue resistors, completely impossible, even with a magnifier inspection lamp. 

The 5% brown carbon resistors (like from bitsbox) are much easier to read.  Even then sometimes they are tricky.

So here is what I did.

I bought a large compartment box and a pack of known resistors.  I placed them in decades eg:

10, 100, 1k, 10k, 100k, 1M
22, 220, 2k2, 22k, 220k, 2M
47, 470, 4k7, 47k, 470k, 4M

Most are still on tape strips, but used ones lie loose in the trays under the tape strips.

These get me by.  Should I ever decide to get the other decades I'll just start another box.

When I take a resistor out, I give it a quick glance that it looks "probably right", if I have any doubt I stick it in the breadboard and test it with the meter.

When I'm tearing down a breadboard I put them all back where they belong. 

Of course mistakes happen, I drop resistors into random slots and so forth.

What I am planning to get round to is a resistor jig for the meter.  Just a couple of old breadboard rails or similar that I can croc clip onto and press resistors against to check them quickly before use or putting back in the compartment box.

[Vtile]

You can also make something like this. It is made from solid-core mains copper wire and quick glued to some fiber board. The original idea is variation from IIRC Conrad Hoffman's "Minimetrology lab" articles, Conrad is frequently hanging on the Metrology section here at eevblog.

[kalel]

You can also make something like this. It is made from solid-core mains copper wire and quick glued to some fiber board. The original idea is variation from IIRC Conrad Hoffman's "Minimetrology lab" articles, Conrad is frequently hanging on the Metrology section here at eevblog.

That looks interesting, where do the measurement/connection to instrument leads go? Unless I got the purpose wrong.

[Vtile]

You can also make something like this. It is made from solid-core mains copper wire and quick glued to some fiber board. The original idea is variation from IIRC Conrad Hoffman's "Minimetrology lab" articles, Conrad is frequently hanging on the Metrology section here at eevblog.

That looks interesting, where do the measurement/connection to instrument leads go? Unless I got the purpose wrong.

Like Paulcas plan, I have used just typical croc-clips to these jig arms.

[6PTsocket]

I could read a resistor colour code from 4 feet away on a carbon film resistor ... but when they introduced the metal film with the blue body, I'm challenged to read some values with my head magnifier at times!!

I'd really like to knee-cap the bozos that came up with that.

Not to worry. Leaded resistors are becoming obsolete and everything is going SMD. They all have the value printed on them. The last digit is the number of zeros and R is used as a decimal point. The down side is you need a microscope to see the #!$%= thing and a whole new set of tools to replace them.

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[6PTsocket]

I think you better get the meter squared away first.On a manal ranging meter, the lowest range that will display the value will give the greatest accuracy. You don't have to do any calculating. Just read the meter. Depending on the range the range the meter might express 1.2k as 1200 ohms. 500,000 ohms is .5 megohms. It is no different than going from mm. to cm. to meters. Work with resistors long enough and the color code will become second nature. Just memorize the colors There are only 12, including gold and silver.

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[Jwillis]

A really bright cool white lamp and a pair of 4X reading glasses works for me .A loupe for those smd.s and 1/8W axials.But still measure with an DMM or milliohm meter.One day I'll invest in a Digital microscope.

[6PTsocket]

A really bright cool white lamp and a pair of 4X reading glasses works for me .A loupe for those smd.s and 1/8W axials.But still measure with an DMM or milliohm meter.One day I'll invest in a Digital microscope.

Reading them is only half the battle. After that you have to replace those little #%=× SMD's. They have special desoldering solder, heat guns, tweezers, flux, vacuum pick ups; ARGHHH!!!! I guess I am going to have to bite the bullet and develop some new skills. With leaded components you could fudge it if the replacement was not the exact same physical size but SMD resistors come in a while bunch of physical sizes.

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[JS]

A really bright cool white lamp and a pair of 4X reading glasses works for me .A loupe for those smd.s and 1/8W axials.But still measure with an DMM or milliohm meter.One day I'll invest in a Digital microscope.

Reading them is only half the battle. After that you have to replace those little #%=× SMD's. They have special desoldering solder, heat guns, tweezers, flux, vacuum pick ups; ARGHHH!!!! I guess I am going to have to bite the bullet and develop some new skills. With leaded components you could fudge it if the replacement was not the exact same physical size but SMD resistors come in a while bunch of physical sizes.

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Desoldering TH parts makes pretty likely to break traces, specially in single sided PCBs where they usually grow. SMD parts also can be exchanged between a few sizes, some pretty tight PCB might not allow it but in many they will fit one size up and down. One problem could be power dissipation for parts with tight ratings, where you can't use smaller ones.

JS

[ArthurDent]

One cheap item that could be a big help to you, that could verify what you think your multimeter is telling you, is a component tester. There are a lot of them on Ebay, generally for between $15-$25, and they work well. I have lots of test equipment but for a quick check on a questionable capacitor or a resistor where all the colors are almost indistinguishable, it comes in very handy. The one thing to be very careful of is to make sure any capacitor you are testing is totally discharged or it can zap the tester. Other than that, the tester is almost foolproof and has a very good display with all the information you'll need on any component.

[tooki]

I thought the resistance of my body was too high to have the parallel effect (just like how air doesn't count). But, now that i think about it, it does make sense.  The resistance is still present across the same circuits, but if i can let even the slightest amount of current from that 9v through me, it'll take some of the resistance away.

The resistance of your skin varies wildly based on thickness, where on your body, how dry or moist the skin is, how sweaty it is (sweat = salt = ions = lower resistance), how oily it is, etc.

I still do wonder what the threshold for current flow is. In a pure math world, everything's constantly conducting in the slightest amount (too small to measure), but in the real world that's not the case (at least by our understanding). There's a noticeable threshold for when an arch happens and when it does not. Presumably, this should be the case for all materials. So at what current value (according to ohm's law) does current actually flow?

It’s the voltage that pushes current flow, and it’s voltage that causes dielectric breakdown and allows conduction to begin across an insulator. That’s why high voltage can jump across an air gap, ionize the air, and produce a conductive path.

I’m no physics whiz, but basically you can consider things to be either conductors (any resistance lower than infinity) or insulators (infinite resistance). For most low-voltage purposes (and certainly the level you’re at), a sort of ballpark is that any resistance up to about 10Mohm is a conductor, and anything much above you can effectively treat as an insulator. Look at the ranges of values that normal resistors come in: from about 1/2 ohm to about 100Mohm: that gives you a decent idea of what is used in most circuits, and even the extents of that are fairly rare. Any resistance below that, we can usually treat as “zero resistance”, and anything above as a de-facto insulator.

Of course, there are specialties where this does not apply (like insulation testing for safety in mains-powered devices, or anything high voltage), but you’re not ready to be doing those things quite yet anyway.



As for color bands: I can’t emphasize enough how important the light source is. You need lots of light, and it needs to be good quality, as others have said. A little halogen desk lamp can be a lifesaver in this regard.