EVERYONE Needs a Multi-Meter

18 Steps

Step 10: Cords and other things

I have added alligator clip attachments to the probe ends on my meter. They are a very handy accessory I got at Radio Shack. They are like an extra hand.

This toaster works well, and the meter shows a resistance reading due to the heating element inside the toaster. This is normal and good. This test could indicate there is a break in the circuit within the toaster. Further testing would be necessary to determine which part of the circuit is defective. Then a decision would need to be made as to whether the appliance can be repaired and whether it is worth the effort.

Whether it is a toaster, or a phone charger, or a power cord for a computer; cords often fray internally within an inch or two of the plug. This is very understandable. That is the area of the cord that is frequently flexed back and forth. If something, like a phone charger, works some of the time, but not at other times; attach the meter with a resistance scale setting. Gently flex the cord back and forth near the plug end. Make fairly sharp bends. Watch the meter reading to see if it fluctuates between a normal reading and no connection. If it does, ask someone with a soldering iron to cut the cord back and reattach the plug end for you.

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ProCactus says: May 19, 2011. 1:01 PM
Dont repair the cord. completely replace it.. Though you must really love your toaster to do this.

Looking at the back/Bottom of the toaster. Im going to use OHMS Law to tell you what the reading on the meter should be close too.

My toaster is 220-240Volt 700Watt

remember R(OHMS) = V (Volts) / I (Amps) .... .... And P(Watt) = V * I .... .... And I = V / R
(Dont be put off by whats in the brackets or the capital letters), Have a go at it.

..so Whats my current flowing though the toaster ?
Is... .. I = P / V
IE. .. . 3.18Amps = 700W / 220 Volts
...............................................................................................................
... so the toaster uses 3.2Amps at 220Volts
OHMS law tells me that R =V / I
. . . . . . . . . 220 / 3.18 = 69.18Ohms

= = = = 69.18 Ohms at 220Volts

That is what ( I just farted ) the resistance should read on a typical 2 slice toaster at 220Volts

A toaster at 700W at 110V will be
.. .. 700 / 110 = 6.36Amps
.. .. . 110 / 6.36 = 17.3 OHMS
= = = = 17.3Ohms at 110Volts

Its not hard, Use Ohms law to check before or after. The result will be a little different but will be in the range. And besides if it is actually broken it will read ZERO ohms.

-Cactus
sschoemann in reply to ProCactusJul 25, 2011. 10:13 PM
due to an error by the system I couldnt reply to your subsequent post. You seem to got what I was getting at. While voltage and current are irrelevant to resistance, resistance is 100% relative to current, (but not voltage, the other constant in the equation). While voltage can be changed easier than resistance, these two are the constants that determine current. And at the same time voltage and resistance have no direct relevance to each other. They simply "inter act" to determine current. Current on it's own can not change without a change in voltage or resistance, hence V and R are directly relevant to the current, and theoretically voltage and resistance are fixed without external actions changing them (in the real world we know that battery supplies weaken, and resistances are affected by temperatures but that is beyond the scope of the discussion at this particular point).

A correction however on the statement you made "And besides if it is actually broken it will read ZERO ohms".... while its a common error due to no reading on the meter, it isnt zero ohms, it's infinite ohms or an open circuit. I'd say 99% followed what you meant and didnt even catch that. I didnt until just now, passing over it, knowing what you meant... It's like proof reading your own work, it just doesnt help since you know what you mean. But keep up the good work!
Phil B (author) in reply to sschoemannJul 26, 2011. 4:36 AM
I appreciate the knowledge and experience all of you have, but be aware you have gone way beyond the original intent of this Instructable, which was to avoid highly intimidating theoretical discussions of such things so that a very non-technical person could gain the confidence safely and profitably to use a multi-meter for the performance of a few simple tests around the home and automobile in order to save much trouble and money. My aim was to remove intimidation, not add to it.
ProCactus in reply to sschoemannJul 26, 2011. 12:04 AM

I think i was drunk when i wrote that.
sschoemann in reply to ProCactusJul 22, 2011. 12:20 AM
To simplify, Voltage and Current are irrelevant in reference to resistance. The resistive load is always a constant regardless of current or voltage. However between the three values, you only need two of them to calculate the third. The power (wattage) can also be calculated if any two of the values are known, since you will be able to calculate the third value if it happens to be needed. as power is simply the voltage multiplied by the current flow. (Didnt mean to step on your explanation ProC, we just tend to forget that some people dont understand how simple Ohms law really is, and those of us who use it every day thake the power relationship for granted)
ProCactus in reply to sschoemannJul 22, 2011. 10:23 AM
Urm, Im not quite sure what your saying there.
But its all good.

Resistance is directly relevant to Current and Voltage.

R = V / I

One person already commented 'skrubol' about the Resistant changes as metals rises in temperature, is true.

But im not sure how to measure the impedance of a wire glowing red hot . So im not sure how much of a difference it makes.

Maybe heat a wire with a lighter and measure its Resistance??

digbysellers in reply to ProCactusMay 29, 2011. 7:23 AM
=) "I just farted" - Awesome!
skrubol in reply to ProCactusMay 21, 2011. 8:24 PM
Heating elements may not read exactly what you expect. Resistance changes with temperature (with metals it goes up as temperature goes up, some things are the opposite.) Incandescent light bulbs for instance read much lower resistance through a meter than using ohms law would make you think.
If it's reading zero ohms it will be tripping the breaker. If the conductor is broken it will read infinite ohms. Most meters represent this as 0L or sometimes as a 1 or 0 with no extra digits. Basically it will look the same as when the leads aren't connected to anything.
sschoemann in reply to skrubolJul 2, 2011. 11:10 PM
Incandescent bulbs, as a general rule (Most but not all) are not purely resistive, the majority of their impedance is actually due to inductive reactance. If you look closely most have coiled elements, this supplies a sufficently reactive load until the element heats up enough for the pure resistive element of the load to increase due to the heat... A typical ohm meter wont read this.
Phil B (author) in reply to skrubolMay 21, 2011. 8:44 PM
I made the same mistake regarding zero ohms when I meant an open circuit and infinite ohms inside the body of the Instructable, but also caught it about the 2nd day it was posted and made the correction.
NaturalCrafter in reply to ProCactusMay 20, 2011. 8:05 PM
I am really enjoying this as I have not done much with the knowledge since I took the class. Love the Instructables and the comments.
Phil B (author) in reply to NaturalCrafterMay 20, 2011. 9:34 PM
There are quite a number of Instructables dealing with electricity and electronics. Many are related to LED projects. If you are interested, here are some I have done: repairing an electronic fly swatter, building a timing light for an automobile engine, using diodes to drop a voltage for keeping a battery charged, automotive 6 or 12 volt battery charger, using a multi-meter to test a transistor, making a GPS power adapter work again, how 3-way and 4-way switches work, and charging CD player batteries without removing them from the player,  There are also many Instructables on DC power supplies, solar chargers, and other chargers for low voltage devices. I hope you enjoy some of these.
NaturalCrafter in reply to Phil BMay 24, 2011. 12:28 PM
Thank you! I am also sending a few links to my sons face book page so he and his friends can take a look
Phil B (author) in reply to NaturalCrafterMay 24, 2011. 2:39 PM
At last I have mastered the art of shameless self-promotion cleverly disguised as being helpful! ;-) I forgot to mention a number of Instructables by various folks on high voltage projects, like Tesla coils, and various kinds of audio projects, like small audio amplifiers and small speakers. Another favorite topic area is making arc welders by ganging two transformers from old microwave ovens together. And, there are the numerous Arduino (a personal digital processor) projects.

Several times I thought I wanted to learn electronics theory really, really well. Yet, I found my needs were always merely for some DC power adapter between 3 and 12 volts. As long as I knew how to do those projects, I was pretty well set.