Please explain this resistance reading

[Tha fios agaibh]

Although measuring resistance is useful in determining a short, it can be confusing to a beginner trying to figure out resistances greaterthan a few ohms. When possible, try to measure voltage. Start at the source with your dmm and follow along one rail at a time to see where along the circuit your voltage disappears.

A short circuit can be parallel to the source which often causes a circuit to overload, or in series which may not have an impact on current but make circuit act funny, like a shorted switch or something.

To confuse matters more, what many mistakenly refer to as a "short" is actually an open circuit. Like when an extension cord has a "short"

 

[Martaine2005]

To confuse matters more, what many mistakenly refer to as a "short" is actually an open circuit. Like when an extension cord has a "short

Hehe John, wouldn’t that depend on whether ‘the extension cord’ had something plugged into it?.

Martin

 

[Tha fios agaibh]

Hehe John, wouldn’t that depend on whether ‘the extension chord’ had something plugged into it?.

Martin

Yes, ...and its turned on.

 

[Martaine2005]

Yes, ...and its turned on.

That would have been my nice question! You beat me to it

 

[Martaine2005]

My emoji’s don’t seem to work off my phone.
Glad you saw where I was going John hehe

 

[davenn]

Sorry if I came on as being "nasty"

I am an old timer too. I am a combat Vietnam Veteran, I know what "nasty" is. I think your statement was more judgemental than my statement to Audioguru.

Thanks
Sorry if I offended anyone.

you weren't and you didn't

bluejets must have had a bad day, have deleted his unwarranted outburst

 

[stspringer]

Please, let's not let this thread wander off-topic into back and forth accusations that may or may not have any substantial substance, relevant to measuring short circuits with a digital multimeter.

I think it more important that we try to discuss why anyone would even want to do this. No one I know tries to measure a short-circuit. They may try to identify a short circuit, or remove a short-circuit, or prevent a short-circuit from occurring in the first place, but once identified who cares how much resistance a short-circuit measures? And will measuring resistance help you find a short-circuit? Someone please explain (better than I have been able to do so far) just how that works. I know it works because I have used my dmm to locate short-circuits.

Hello hevans1944,
Thank you for your reply. As a learning experience I wanted to learn "They may try to identify a short circuit"


Thanks

 

[hevans1944]

Hello hevans1944,
Thank you for your reply. As a learning experience I wanted to learn "They may try to identify a short circuit"


Thanks

It is commendable that you are trying to learn the basics of electricity before attempting to build or repair anything. Short circuits are basic to learning the art of electricity. So let's discuss them a little bit more.

I will now provide my definition of a short circuit, which may differ a little from what you find using Google, but it works for me and has worked for more than sixty years: A short circuit is any unintentional low-resistance path that supports the conduction of electricity.

The key words in my definition are unintentional and low-resistance.

So what do you expect to learn by creating deliberate short-circuits? What have you learned so far? Please answer the following two questions, so we can see the extent of your understanding of short-circuits: (1) What is the minimum resistance necessary to constitute a short-circuit? (2) What is the maximum resistance that will constitute a short-circuit?

You seem to be confused, based on earlier posts, about the correct terminology we use here on Electronics Point to describe circuits. Where did you get the idea that LEDs have a positive terminal and a negative terminal?

When used as a source of light radiation, as you did in your breadboard experiment, LEDs do not exhibit polarity. All LEDs have an anode and a cathode. These may be connected to terminals that are positive or negative with respect to another terminal, but the two wires (or leads) connected to an LED do not have a polarity. Neither one can be said to be "positive" or "negative" because they do not source a current between their terminals... unless you try to use the LED as a photo-diode. If you do that, the LED still has an anode and a cathode but now those two have a polarity with respect to each other, a voltage can be developed between the anode and the cathode, and power can be delivered if the LED (now being used as a photo-diode) is illuminated with the right range of wavelengths. However, LEDs do not generally make very good photo-diodes because they are not constructed for that purpose.

Perhaps you were confused by the markings on some rectifier (diode) packages, especially four-terminal bridge rectifiers. These sometimes have two terminals marked ~ to signify where the alternating voltage to be rectified is connected and a + terminal and a - terminal to indicate the polarity of the rectified current, but there is no polarity associated with any of the four diodes that constitute the bridge rectifier.

I hope this clears some things up for you. Please answer my two questions before trying to continue this thread.

 

[stspringer]

It is commendable that you are trying to learn the basics of electricity before attempting to build or repair anything. Short circuits are basic to learning the art of electricity. So let's discuss them a little bit more.

I will now provide my definition of a short circuit, which may differ a little from what you find using Google, but it works for me and has worked for more than sixty years: A short circuit is any unintentional low-resistance path that supports the conduction of electricity.

The key words in my definition are unintentional and low-resistance.

So what do you expect to learn by creating deliberate short-circuits? What have you learned so far? Please answer the following two questions, so we can see the extent of your understanding of short-circuits: (1) What is the minimum resistance necessary to constitute a short-circuit? (2) What is the maximum resistance that will constitute a short-circuit?

You seem to be confused, based on earlier posts, about the correct terminology we use here on Electronics Point to describe circuits. Where did you get the idea that LEDs have a positive terminal and a negative terminal?

When used as a source of light radiation, as you did in your breadboard experiment, LEDs do not exhibit polarity. All LEDs have an anode and a cathode. These may be connected to terminals that are positive or negative with respect to another terminal, but the two wires (or leads) connected to an LED do not have a polarity. Neither one can be said to be "positive" or "negative" because they do not source a current between their terminals... unless you try to use the LED as a photo-diode. If you do that, the LED still has an anode and a cathode but now those two have a polarity with respect to each other, a voltage can be developed between the anode and the cathode, and power can be delivered if the LED (now being used as a photo-diode) is illuminated with the right range of wavelengths. However, LEDs do not generally make very good photo-diodes because they are not constructed for that purpose.

Perhaps you were confused by the markings on some rectifier (diode) packages, especially four-terminal bridge rectifiers. These sometimes have two terminals marked ~ to signify where the alternating voltage to be rectified is connected and a + terminal and a - terminal to indicate the polarity of the rectified current, but there is no polarity associated with any of the four diodes that constitute the bridge rectifier.

I hope this clears some things up for you. Please answer my two questions before trying to continue this thread.

Thank you Sir,


So what do you expect to learn by creating deliberate short-circuits? What have you learned so far? Please answer the following two questions, so we can see the extent of your understanding of short-circuits: (1) What is the minimum resistance necessary to constitute a short-circuit? (2) What is the maximum resistance that will constitute a short-circuit?

Google_1:
A short is a 0 resistance connection between two terminals. This is never physically possible, though, so in reality, a short is any connection that has a terribly low resistance such that there is a higher than normal current.

Google_2:
The voltage drop of any component is directly proportional to the resistance. Now in case of short circuit, the voltage goes to zero. Since Power is always constant, the current rises to highest practically possible value (Theoretically it is infinite) and practically it ranges between hundreds of amperes to thousands of amperes.

Google_3 This explanation is interesting:
In simple terms, voltage is the potential difference or force required to move the charge from one place to another.

So assume that all the charge has moved to the other places since there are no obstacles to stop these charges completely moving, so there are no forces required which is typically the voltage, hence in short circuit this condition has been taken into the account where voltage ceases to exists because of abnormality in the circuit and there are no external forces being blocking these charges flow from other places, hence maximum current has been assumed to take place.

(1) What is the minimum resistance necessary to constitute a short-circuit? - Zero
(2) What is the maximum resistance that will constitute a short-circuit? - Infinity

 

[BobK]

(1) What is the minimum resistance necessary to constitute a short-circuit? - Zero
(2) What is the maximum resistance that will constitute a short-circuit? - Infinity

Huh?

Bob

 

[davenn]

(1) What is the minimum resistance necessary to constitute a short-circuit? - Zero

​

​

Hi @stspringer

Why did you write that ? as that's not really correct when you consider what you posted from google in the #1 quote ( which is correct)

look again as what it said, particularly the bolded part ....

Google_1:
A short is a 0 resistance connection between two terminals. This is never physically possible, though, so in reality, a short is any connection that has a terribly low resistance such that there is a higher than normal current.

(2) What is the maximum resistance that will constitute a short-circuit? - Infinity

this is not correct, not even sure how you came up with that ?


Dave

 

[stspringer]

Huh?

Bob

I guess I am confused,as usual "A short is a 0 resistance connection between two terminals."

So Bob, maybe you could tell me the correct answer? Is it milli-ohm?

 

[stspringer]

Hi @stspringer

Why did you write that ? as that's not really correct when you consider what you posted from google in the #1 quote ( which is correct)

look again as what it said, particularly the bolded part ....





this is not correct, not even sure how you came up with that ?


Dave

So Davenn please tell me the correct answer. Is it milli-ohm?

 

[davenn]

So Davenn please tell me the correct answer. Is it milli-ohm?

so did you reread the bolded part ?

a short is any connection that has a terribly low resistance such that there is a higher than normal current.

it isn't a specific fixed value, as the above quote indicates

what would be deemed a short (in terms of resistance value) depends on the circuit being measured and also depending if the circuit is AC or DC
in most DC circuits, unless there are specific low resistance components present, would be anything less that say ~ 10 Ohms. More commonly less than a couple of Ohms.

In an AC circuit and particularly RF ( radio circuits), DC short circuits are quite happy at RF frequencies.

Take a loop dipole antenna DC zero/near zero Ohms but at RF freq's it's perfectly OK and radiates or receives a signal
This is because with AC circuits, we are now using the term impedance

 

[hevans1944]

The correct answer is there is no minimum resistance that constitutes a short-circuit, nor is there any maximum resistance that will prevent a short-circuit. The resistance of a short-circuit is relative to the circuit in which it occurs. You must always consider this before deciding whether or not a short-circuit exists.

Most of the time we think of low-resistances as being less than an ohm or so. When we do that, and place a one-ohm resistor between the black and white wires of our 120 VAC convenience outlet, then 120 amperes of current will flow (momentarily), a fifteen or twenty ampere circuit breaker or fuse protecting the convenience outlet wiring will open or trip, and just about anybody would say, "Oops! That's a short-circuit." And they would be right. That is a short-circuit.

But what if this "convenience outlet" and its wiring is rated for, say, 200 amperes? Then that 120 A current would cause a power drain of 120 x 120 x 1 = 14,400 watts. Plenty of industrial applications need that much power for heating or operating large motors, so this would not be a short-circuit for a load like that.

There may be practical reasons to use a different circuit to provide 14,400 watts, but the load is certainly not a short-circuit. The load is both intentional and appropriate in value for its purpose, hence it is not "low resistance" in this application.

Going the other way, it is common to use circuits with a very large input impedance or input resistance to measure voltages. Your digital multimeter or dmm has an input impedance of one million ohms, so most people would say there is no way that can constitute a "short-circuit". It's certainly not an infinite input resistance, but it's still pretty large. But what if you are trying to measure the output of a voltage divider across a 100 megohm resistor? Your dmm has just converted that 100 megohm resistor into something a bit less than one megohm by virtue of being connected in parallel with it! The dmm has for all intents and purposes short-circuited the 100 megohm voltage divider resistor! Whatever voltage the dmm measures is not the voltage that is produced when the dmm is removed from the circuit. Your dmm has created an unintentional, low-resistance, short-circuit path across the 100 megohm voltage divider resistor.

If you can wrap your head around the concept that a short-circuit is not an absolute thing that you can just whip out a meter of some sort to measure, and then also use that meter to determine if it exists, you may be able to begin to understand short-circuits. Such meters do not exist. Thoughtful inspection is required to determine the existence of short-circuits.

 

[Bluejets]

Another good example of when an apparent "short " is not as shown on a multimeter is the windings on an smps transformer.
In this instance , because of the frequency, inductive reactance is the current limit.

 

[BobK]

I guess I am confused,as usual "A short is a 0 resistance connection between two terminals."

So Bob, maybe you could tell me the correct answer? Is it milli-ohm?

Look at the second part, the maximum resistance for a short is infinity? I guess I have a short circuit between all of the fingers then, no wonder I am so clumsy at typing.

Bob

 

[Tha fios agaibh]

I suggest getting an assortment of resistors and experimenting with a battery and an incandescent lamp.(Not an led) What resistors alow a current to flow and make the lamp glow?

There is no real answer to the question because it depends on the voltage and acceptable current in the circuit.

For example, If you have a 3 volt circuit and its "shorted" with 3000Ω you get a current of just 1ma, which could be considered insignificant.
Now take 600 volts, and "short" it with 3000 and you get 200ma or nearly 1/4 amp which is significant.

So the resistance that may be considered a "short circuit" when trying to fix automotive wiring is completely different than it will be to the utility guy working on a high voltage pole.

The question that should be asked, is how much current is acceptable.

 

[stspringer]

I suggest getting an assortment of resistors and experimenting with a battery and an incandescent lamp.(Not an led) What resistors alow a current to flow and make the lamp glow?

There is no real answer to the question because it depends on the voltage and acceptable current in the circuit.

For example, If you have a 3 volt circuit and its "shorted" with 3000Ω you get a current of just 1ma, which could be considered insignificant.
Now take 600 volts, and "short" it with 3000 and you get 200ma or nearly 1/4 amp which is significant.

So the resistance that may be considered a "short circuit" when trying to fix automotive wiring is completely different than it will be to the utility guy working on a high voltage pole.

The question that should be asked, is how much current is acceptable.

Good simple explanation thanks

 

[hevans1944]

Another good example of when an apparent "short " is not as shown on a multimeter is the windings on an smps transformer.
In this instance , because of the frequency, inductive reactance is the current limit.

This is so true, and it has misled many a novice down the primrose path to making an assumption that there is something wrong with their switch-mode power supply transformer simply because their multimeter "says" the winding is "shorted."

As you noted before, it is "shorted" for DC, which it is "designed" to be. You do want a low-resistance charging path for the inductance, which is periodically storing energy from the power line and later releasing that energy for the smps output. Whether that energy be for boosting or bucking the DC input voltage, you need a high enough impedance at the switching frequency (the switch part of smps) so as to not actually short-circuit the DC power input from the rectified AC line input! Those seemingly conflicting design decisions require a LOT more understanding of circuit theory than how to find a simple DC short-circuit!

But let's not jump the shark here. This thread is not about reactance, impedance, or even resistance per se. It's about how does a novice determine whether a short-circuit exists, and how this is corrected. Comments and suggestions from those who have been there, done that, maybe (but hopefully not!) suffered arc-flash burns in the process, could perhaps be helpful to @stspringer. I doubt he is learning anything by creating "short circuits" on a breadboard with little bits of wire.

A little side note on circuit board repair: most high-end electronics use multiple layers of circuit conductors on their "printed circuit boards" with tiny little holes (called vias), plated through from conductors on one side to conductors on the other side, using an alloy of copper and tin, to carry signals and current from one layer to the next. It is virtually impossible to repair such boards, so don't even try.