"No resistance" is not obtainable except for a superconductor which does indeed have zero resistance.

You win. Though In context, 0 resistance doesn't exist. No and zero resistance are often confused.
e.g. "No resistance" - unable to measure; "No resistance" - really low or near zero, a wire.
At work, I set up systems that could measure the resistance of a sheet of paper. Ordinary ohmmeters can't.

Let's not get into the discussion of negative resistance regions.

 

To make matters simple, why not just use a relay with a suppression diode built into it. Tnis would be a common Bosch relay commonly used in the industry to suppress voltage spikes when the coil is de-energized. Problem solved.
As for the new coil, is there any portion of it that has an epoxy filler where the diode can be buried? Just a thought.

 

Comments:

Sometimes it's hard to know whether people mean infinite resistance for ZERO. Zero doesn't really exist sometimes. Zero R usually doesn't.
The TVS diode would be infinate, a high value or open in both directions when using resistance. Using the "diode mode" on your meter, it should read "OL" or Overload. In the "diode mode" the meter usually shows a voltage drop in volts, but not always true. I've heard some meters read in mV.



Not between the coil wires, but across or in parallel with the coil.

The voltage across an inductor is given by the equation v= L* di/dt or a change in current induces a voltage multipled by the constant L or inductance. The change in current generates a voltage. So, the current in an inductor can't change instantaneously. This means the voltage has to go up.

TRUE, when the relay opens powering the clutch, the voltage has no where to go. The voltage MAY be large enough to poke though insulation and degrade it. it's not that likely. Formvar https://www.superioressex.com/uploa...merica/Magnet_Wire_-_Winding_Wire/Formvar.pdf (usual magnet wire insulation) won't break down < 9600 Volts. The 12 V automotive wiring MIGHT.

When the relay closes powering the clutch, There is also a change in current in the clutch current which also induces a voltage. In a modern car, the spikes are smaller. In older vehicles like pre 1960's the spikes in a car's electrical system are larger. Most of the come from the alternator. +50 to -200 V are common. http://www.littelfuse.com/~/media/e...n_automotive_environment_application_note.pdf

The activating relay has a coil of it's own. The single diode is usually used there to protect the solid state device driving the relay coil.

I'd need to hunt for variations in older and newer vehicles.

The time of the greatest damage would occur when the clutch is cycling and primarily on activation. That spike may be absorbed by he rest of the vehicle. You want to suppress the spike as close as possible to the source.

This likely is why the choice of a bidirectional TVS was made. Rapid cycling such as when the low pressure switch opens intermittently when the refrigerant charge is low.

Your analysis isn't bad. You need the v(t) = L di/dt; the current in an inductor can't change instantaneously; the notion of breakdown; and the repetiveness of the generating a large voltage on an airconditioning fault (low freon charge).

Furthermore, the modern car is designed with local transient suppression in mind and the alternator being probably the largest contributer is designed with suppression in mind too. AM radio ignition noise is virtually gone with a coil per 2 cylinders because the high voltage wires are really short.



I'll modify your statement to:

bi-directional means voltage suppression in either polarity above and below a threshold voltage (e,g, > 20 V, <-20 V) So, with no measured resistance (i.e. Open) with my meter, in either polar direction on diode I came to the opinion- this circuit is going to short through the diode as soon as the AC relay kicks AND the thresholds above are met.

Since the current in an inductor can't change instantaneously, a voltage must be developed across the coil. The voltage can be developed when the coil is switched on or off. The greatest danger is when the compresser rapidly cycles under a fault condition.



So with no resistance in either polar direction on diode I came to the uneducated opinion- this circuit is going to short through the diode as soon as the AC relay kicks in

Thanks for this detailed response. I did mean zero resistance as in short. I measured 0.7 across coil but that is what I get when I touch my test leads directly together so I treat it like an index error and subtract 0.7 from whatever I read. I did mean parallel and should have said that instead of "between" the coil wires.
Thank you

 

"No resistance" is not obtainable except for a superconductor which does indeed have zero resistance.
The resistance may be some value near zero, indicating a possible short, or the meter indicates an OL or or other indication of a resistance higher than the meter can measure, which is an open.
Please use the terms open or short (or the actual measured resistance) when reporting your resistance measurements.

Understood.
Thanks

 

I've seen the diodes wired into the connectors from the factory. Upper square connector..

https://images.search.yahoo.com/yhs/search?p=chevy+compressor+plug+pictures&fr=yhs-mozilla-004&hspart=mozilla&hsimp=yhs-004&imgurl=http://i.ebayimg.com/images/i/221237827946-0-1/s-l1000.jpg#id=2&iurl=http://i.ebayimg.com/images/i/221237827946-0-1/s-l1000.jpg&action=click

Maybe yours is covered with tape ??

Thanks- I looked there carefully

 

The TVS should effectively measure OPEN in both directions, nothing else connected.

A diode has a forward voltage, a reverse breakdown voltage and a max forward current and a surge forward current.

The forward voltage depends on the material. 0.7V is common for silicon.

Reverse breakdown is "non-destructive" if the current is limited. Zenier diodes control this value precisely. TVS diodes do do, but with high surge capablity.

So a TVS for a 12 V application in SERIES (normal and flipped) with a 12 V thing, the 12 V thing should not turn on.

It's difficult to do diode tests. Diodes used in microwave ovens have about a 9 V forward voltage drop.

Old multimeters used a 1.5V batter and a series of resistors. Newer ones use a constant current source. Newer yet, use a constant current source and measure the voltage.

Resistance measurements sometimes try to keep the maximum applied voltage <0.7 V to AVOID turning on a diode or a nearby silicon transistor.

 

You win. Though In context, 0 resistance doesn't exist. No and zero resistance are often confused.
e.g. "No resistance" - unable to measure; "No resistance" - really low or near zero, a wire.
At work, I set up systems that could measure the resistance of a sheet of paper. Ordinary ohmmeters can't.

Let's not get into the discussion of negative resistance regions.

Not looking for any kind of win. I do understand what you are saying and per your statements and others will say either almost zero or shorted. I will be more than happy to learn your lingo especially considering the help I'm getting but looking to go with the flow by nature anyway.

 

The world is full of falsehoods. We are taught at the level we can understand. Take talking to a kid on an age basis on the concepts of death or sex.

It's difficult to measure what happens with the lack of a TVS,

Just like it was difficult for my boss to even see that a computer system lasted 17 years without a hard drive failure. The system was mostly on 24/7. The only failures were dust and wear. The dust had to be cleaned from the intakes (not a failure) and the floppy drive died (mechanical). There was not a process interruption from the lack of computing except for power failures.

I took an exam in school and I said to teach, "My response is correct". He agreed with his comment : "Your not supposed to know that yet".