Please explain this resistance reading

[hevans1944]

The answer to Question (1) is incorrect. Explanations follow.
The answer to Question (2) is incorrect. Explanations follow.

(1) Except for the special case of superconductors, which actually do exhibit zero resistance, it is impossible to create a short-circuit with zero resistance because all wires (electrical conductors) that are not superconductors have a finite resistance that is greater than zero ohms. Therefore, the minimum resistance necessary to constitute a short-circuit must also be some value greater than zero ohms.

(2) There is no such thing as infinity. Infinity is not a number; it is just a very large quantity that can be approached but never reached: a convenient fiction, useful to mathematicians and confusing and misinterpreted by everyone else. However, your answer is closer to the truth than your answer to the first question. Very large resistances, megohms and larger, can and do constitute a short-circuit, depending on the circuit under consideration. In fact, there is no maximum resistance that cannot constitute a short-circuit. It all depends on the circuit under consideration.

From these two explanations I hope you can see that short-circuits can have any resistance, ranging from almost zero ohms to greater resistances than the largest resistance you can measure with your digital multimeter.

Remember my definition: short-circuits must be unintentional and low-resistance. Unintentional is not always obvious, although with careful circuit design it should be! Low-resistance depends on, and is relative to, the particular circuit under consideration.

 

[hevans1944]

CORRECTION TO POST #28, REGARDING POLARITY

I don't know what I was thinking when I wrote:

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?

I can see now where you got the idea. It is all over the Internet!. And it is correct. I am the one who was confused. Please accept my sincere apology.

Somehow I must have been thinking that a component had to be a source of electricity to have positive and/or negative terminals. However, this is NOT common usage. LEDs do have an anode and a cathode, and the anode is often referred to as the positive terminal while the cathode is often referred to as the negative terminal of the LED. The same terminology applies to every other diode: anode is positive, cathode is negative. This is the voltage polarity that must be applied to make the diode conduct in the forward direction.

A zener (or avalanche conduction) diode follows the same rules, but in normal usage these diodes are always reverse biased (anode negative and cathode positive) and require a current-limiting resistance when the reverse-bias voltage exceeds the zener voltage or the avalanche voltage. Zener and avalanche diodes were sometimes used as voltage regulators, because the voltage drop across the diode in the reverse conduction direction is relatively constant with variations in the supply voltage. However, better voltage references have been invented that are more voltage-stable and less electrically noisy, so you don't often see zener diodes and avalanche diodes used as voltage references in modern power supply designs.

One thing that confounded me early on in my study of electronics was the output polarity of a diode rectifier, typically a half-wave rectifier consisting of just one diode whose anode is connected to one end of a transformer secondary winding. The other end of the secondary winding is connected to a load resistor, and the other end of this load resistor is connected to the cathode of the diode. Simple series circuit. Output voltage is taken across the load resistor, and the output has the diode cathode connection to the load resistor positive with respect to the other end of the load resistor that is connected to one end of the transformer secondary. Huh? How is this possible if the cathode is the "negative" terminal of the diode. Answer is left as an exercise for the student.



Other two-terminal components with polarity exist. Most electrolytic capacitors have polarity and this polarity must be strictly observed when wiring the electrolytic capacitor into a circuit. DC motors have polarity and this polarity determines the direction of motor shaft rotation when power is applied to the motor. Photo-diodes and photo-voltaic power cells have polarity. Batteries of course have polarity. Silicon controlled rectifiers (SCRs), although three-terminal devices, have polarity. I am almost assured there are other examples, but I am somewhat surprised no one here has pointed out my error yet.

 

[(*steve*)]

I prefer to think of a short as an inappropriate (usually additional) load placed on some part of a circuit resulting in some undesired change in behaviour.

I'm pretty sure this wording encompasses all of the descriptions given above.

This description covers the two necessary conditions:

1. The fault is caused by an increase in current or signal through some path.
2. The change causes incorrect operation.

In a very simple case of a battery powering a bulb, a resistor placed across the bulb represents an additional load on the battery (because it needs to supply current to the bulb in addition to that now flowing through the resistor).

Is this a short? Well that depends.

• If the bulb still lights at full brightness, and the additional load is small enough that the battery life is largely unchanged, then this isn't a short.
  
• If the bulb still lights to full brightness, but the battery is consumed faster, then the additional load could be termed a short.
• If the bulb no longer lights to full brightness, regardless of any affect on battery life, the additional load could be termed a short.
• If the bulb no longer lights at all, then it's a short.

The same method could also apply to an amplifier. A short might reduce the maximum volume, reduce the volume to zero, or just make the amplifier run hotter.

In real life, a short is rarely caused by the addition of a component in a circuit (unless we include people connecting up things badly), but some failure that changes the characteristics of an existing component -- including parasitic components.

An example of this might be a failure of a capacitor that becomes a very low value resistor. I recently repaired a couple of pieces of equipment with this fault. The additional load imposed by this caused the power supply voltage to drop, the voltage regulator to be damaged and in one case, another resistor to burn out.

Many semiconductors (transistors, diodes, MOSFETs, etc) will fail as a very low resistance. This failure may be caused by an overload (which may have been caused by a short), and will almost always result in another short. Problems like this can cascade through a circuit. This is why repairing the obvious fault may not be a complete fix.

Another interesting case is what I have called "parasitic components". You know that insulation on a wire stops current flowing through it. However if it gets very hot it can carbonize and become more conductive. This can happen to a PCB when a component gets very hot. The resulting unintended (lower) resistance can change the operation of the circuit.

The is also a possibility that something increasing in resistance can cause a short. A resistor may be placed in a circuit to keep something turned off until some condition is reached. If this resistor increases in resistance, the component it is supposed to keep turned off may turn on. The effect of this uncontrolled switching on can be termed a short. In this case, whilst the failure did not directly cause a lower resistance, the effect was to cause a lowered resistance elsewhere.

So, a short can have a range of causes and effects, and one short may be very different from another.

You also may come across the term "dead short". It is more frequently (and informally) used to refer to a very low resistance short, often with catastrophic impact.

edited for phone autocorrect corrections.

 

[stspringer]

I prefer to think of a short as an inappropriate (usually additional) load placed on some part of a circuit resulting in some undesired change in behaviour.

I'm pretty sure this wording encompasses all of the descriptions given above.

This description covers the two necessary conditions:

1. The fault is caused by an increase in current or signal through some path.
2. The change causes incorrect operation.

In a very simple case of a battery powering a bulb, a resistor places across the bulb represents an additional load on the battery (because it needs to supply current to the bulb in addition to that now flowing through the resistor).

Is this a short? Well that depends.

• If the bulb still lights at full brightness, and the additional load is small enough that the battery life is largely unchanged, then this isn't a short.
  
• If the bulb still lights to fill brightness, but the battery is consumed faster, then the additional load could be termed a short.
• If the bulb no longer lights to fill brightness, regardless of any affect on battery life, the additional load could be termed a short.
• If the bulb no longer lights at all, then it's a short.

The same method could also apply to an amplifier. A short might reduce the maximum volume, reduce the volume to zero, or just make the amplifier run hotter.

In real life, a short is rarely caused by the addition of a component in a circuit (unless we include people connecting up things badly), but some failure that changes the characteristics of an existing component -- including parasitic components.

An example of this might be a failure of a capacitor that becomes a very low value resistor. I recently repaired a couple of pieces of equipment with this fault. The additional load imposed by this caused the power supply voltage to drop, the voltage regulator to be damaged and in one case, another resistor to burn out.

Many semiconductors (transistors, diodes, MOSFETs, etc) will fail as a very low resistance. This failure may be caused by an overload (which may have been caused by a short), and will almost always result in another short. Problems like this can cascade through a circuit. This is why repairing the obvious fault may not be a complete fix.

Another interesting case is what I have called "parasitic components". You know that insulation on a wire stops current flowing through it. However if it gets very hot it can carbonize and become more conductive. This can happen to a PCB when a component gets very hot. The resulting unintended (lower) resistance can change the operation of the circuit.

The is also a possibility that something increasing in resistance can cause a short. A resistor may be placed in a circuit to keep something turned off until some condition is reached. If this resistor increases in resistance, the component it is supposed to keep turned off may turn on. The effect of this uncontrolled switching on can be termed a short. In this case, whilst the failure did not directly cause a lower resistance, the effect was to cause a lowered resistance elsewhere.

So, a short can have a range of causes and effects, and one short may be very different from another.

You also may come across the term "dead short". It is more frequently (and informally) used to refer to a very low resistance short, often with catastrophic impact.

Thank you
Very good explanation to wrap my head around.

 

[stspringer]

CORRECTION TO POST #28, REGARDING POLARITY

I don't know what I was thinking when I wrote:


I can see now where you got the idea. It is all over the Internet!. And it is correct. I am the one who was confused. Please accept my sincere apology.

Somehow I must have been thinking that a component had to be a source of electricity to have positive and/or negative terminals. However, this is NOT common usage. LEDs do have an anode and a cathode, and the anode is often referred to as the positive terminal while the cathode is often referred to as the negative terminal of the LED. The same terminology applies to every other diode: anode is positive, cathode is negative. This is the voltage polarity that must be applied to make the diode conduct in the forward direction.

A zener (or avalanche conduction) diode follows the same rules, but in normal usage these diodes are always reverse biased (anode negative and cathode positive) and require a current-limiting resistance when the reverse-bias voltage exceeds the zener voltage or the avalanche voltage. Zener and avalanche diodes were sometimes used as voltage regulators, because the voltage drop across the diode in the reverse conduction direction is relatively constant with variations in the supply voltage. However, better voltage references have been invented that are more voltage-stable and less electrically noisy, so you don't often see zener diodes and avalanche diodes used as voltage references in modern power supply designs.

One thing that confounded me early on in my study of electronics was the output polarity of a diode rectifier, typically a half-wave rectifier consisting of just one diode whose anode is connected to one end of a transformer secondary winding. The other end of the secondary winding is connected to a load resistor, and the other end of this load resistor is connected to the cathode of the diode. Simple series circuit. Output voltage is taken across the load resistor, and the output has the diode cathode connection to the load resistor positive with respect to the other end of the load resistor that is connected to one end of the transformer secondary. Huh? How is this possible if the cathode is the "negative" terminal of the diode. Answer is left as an exercise for the student.

View attachment 45595

Other two-terminal components with polarity exist. Most electrolytic capacitors have polarity and this polarity must be strictly observed when wiring the electrolytic capacitor into a circuit. DC motors have polarity and this polarity determines the direction of motor shaft rotation when power is applied to the motor. Photo-diodes and photo-voltaic power cells have polarity. Batteries of course have polarity. Silicon controlled rectifiers (SCRs), although three-terminal devices, have polarity. I am almost assured there are other examples, but I am somewhat surprised no one here has pointed out my error yet.

Yes, I was confused on the led part

 

[hevans1944]

The following is a true story from my past. While attending grade school, around the time that the Soviet Union launched their Sputnik low-earth-orbiting satellite, the first of its kind in the world, one portion of our school week was devoted to Science. IIRC, that particular week we were studying electricity and how it could be used to make a magnet you could "turn on" and "turn off" just by supplying or interrupting current to a coil of wire wrapped around a piece of soft iron. Up until then, our class only had experience with permanent magnets, such as bar magnets and horseshoe-shaped magnets. Teacher decided we should make our own electromagnets as a homework assignment, and bring them to class for a "show and tell" demonstration.

Now most of the students in this semi-rural Tennessee middle school were farmer's boys and girls. About as close as they every got to electricity was jump-starting Daddy's tractor. However nails were abundant on farms, as was soft-drawn copper bailing wire. The only difficult thing to come by was a No. 6 size zinc-carbon dry cell with screw-terminal binding posts. These were very expensive, considering how little the children earned selling "butter an' aigs". Still, not to appear like they couldn't afford to do this homework project, most of the students dutifully purchased a No. 6 dry-cell and made their "electromagnet" by scramble-winding copper wire around a nail... with their father's permission of course.

On the day of "show and tell" one of the children complained that his "magnet" wouldn't "pick up" anything... not even a small paper clip. And the wire and dry-cell battery became very hot when he tried. Closer examination revealed the cause. The copper bailing wire, while a good conductor of electricity, was not insulated. Instead of having a few hundred turns of insulated wire wrapped around the nail, the student had wrapped a few hundred turns of bare copper wire around the nail, effectively shorting out the battery! Either he wasn't paying attention when teacher explained why insulated wire was necessary, or teacher somehow forgot to mention it. It was a sad day for the child, who had invested what seemed to him to be a lot of time and money into this homework project. I don't remember whether he was made the butt of cruel jokes that day by his fellow classmates, but that was the norm in this school. Everyone else had built electromagnets that actually worked, some stronger, some weaker, than others. This student was the only one in class whose homework assignment was a total fail.