Diode at AC compressor

[007]

The device you guys have been discussing is depicted as back to back zener, probably in a common package.
When the machines were upgraded to these it probably corrected more than the wiring issue Niege experienced.
When the current flow stops from the control device the magnetic field collapses across the winding in the clutch causing a back EMF in the order of 200V and several amps.
The problem using a convention diode across the coil to dissipate this current is that the magnetic field doesn't just collapse to zero.
It actually oscillates like AC with a sine wave shaped current many times on the way down to zero.
So a zener being bidirectional would suppress the reverse surge in both directions and have the effect much quicker suppression.
A conventional diode will probably be ok as a replacement but you may well hear a noise in your radio when the clutch disengages, and system will likely have a little more back EMF going back in the wiring.
If it is back to back zener it is testing faulty with your meter as should be high ohms in both directions.

 

[CatKC]

The problem using a convention diode across the coil to dissipate this current is than magnetic field doesn't just collapse to zero.
It actually oscillates like AC with a sine wave shaped current many times on the way down to zero.
So a zener being bidirectional would suppress the reverse surge in both directions and have the effect much quicker suppression.
A conventional diode will probably be ok as a replacement but you may well hear a noise in your radio when the clutch disengages, and system will likely have some back EMF going back in the wiring.

I liked Niges story. . . the last diagram also shows a standard diode.

John C. wrote that the ones he is familiar with has 'three legs'. That is called a 'Triode'. It acts like a 'solid state switch'. When a specific level of voltage is applied to the third leg the "Triode" allows current to pass.

You are correct about the collapse of a magnetic field and the back to back reference, a 'basic' Zener is a 'one way short' but the function is the same. In addition when a magnetic field collapses it can produce 'induced voltage' much higher than the 'applied voltage'. Think of an ignition coil where the spark plug does not 'spark' when voltage is 'applied', but does as soon as the voltage is 'removed'. The ignition 'points' also experience a higher voltage which is 'absorbed' by the condenser/capacitor.
The 'control relay' of a 'magnetic field/coil' probably also has a capacitor but with so many unknown variables (unlike a single wire ignition coil) the voltage spike could exceed the rated voltage of the capacitor and short it out, hence the Zener Diode.

Personally I would not try and replace a Zener with a standard diode because a control relay contact can 'arc' if a 'reverse volt spike' occurs.

Now if you want to talk about the same type device for AC, that would be a MOV (Metal Oxide Varistor) which 'could' be used in a similar circuit but they are not quite as 'sturdy'.

 

[007]

Sorry CatKC, i don't agree with a lot of your references.
The basic zener function has a forward conduction voltage across the junction in one direction and begins to slowly avalanche current as you near the reverse voltage rating in the other direction.
Modern ignition systems work totally different, but referring to the old points system you referenced.
A standard coil would produce around 500V back EMF but this was not wasted across the capacitor.
The 12V primary winding is in series with the secondary high voltage winding and that voltage is added to the HT voltage to help increase spark performance.
The capacitor across the points does help with arcing points but its main purpose is to absorb the oscillations i spoke of to keep the speed of the magnetic collapse as fast as possible.
Most manufactures don't give a crap about arcing relay points or thermostat points wearing out due to arcing.
With the increase of electronics and microprocessors in the system the suppression of arcing has become important, to remove high voltage transients coming back through the DC and interfering with logic cards.
You are correct about the MOV although good at clamping high voltage transients can not deal with the current which will be in the order of several amps and will occur every time the clutch is disengaged.
As long as the wiring was checked to not have the original error in polarity a conventional diode is a way more robust device to replace his zeners.
The OEM would have had to select a zener over 30V reverse in both directions where a diode will be .6V in one direction and maybe 60V in the other direction as the coil collapses.
So the zenner would be roughly same ripple appearing at device i would wager.
OEM prob chose zener as it fixed there wiring issue, so win win.

 

[Birken Vogt]

The device you guys have been discussing is depicted as back to back zener, probably in a common package.
When the machines were upgraded to these it probably corrected more than the wiring issue Niege experienced.
When the current flow stops from the control device the magnetic field collapses across the winding in the clutch causing a back EMF in the order of 200V and several amps.
The problem using a convention diode across the coil to dissipate this current is that the magnetic field doesn't just collapse to zero.
It actually oscillates like AC with a sine wave shaped current many times on the way down to zero.
So a zener being bidirectional would suppress the reverse surge in both directions and have the effect much quicker suppression.
A conventional diode will probably be ok as a replacement but you may well hear a noise in your radio when the clutch disengages, and system will likely have a little more back EMF going back in the wiring.
If it is back to back zener it is testing faulty with your meter as should be high ohms in both directions.

I'm not sure can I agree with this statement.

Lets say the Zener suppressor is set for 40 V.

If the contacts open and the back EMF rises to conduct the suppressor, it will conduct and dissipate energy until the voltage across it drops to where it doesn't conduct any more. Then the field will collapse and may pull the other way but I can't see how the field is going to jump up so high in the opposite direction to make it conduct again, since all that energy was dissipated in the first go-round. It MIGHT come high enough to maybe TOUCH that point but I doubt even that. After that the remaining cycles are not going to come near the Zener point.

I have seen a hundreds or thousands of silicon diodes in this service and I always knew you could use a double Zener but never seen it done until now as a coil clamper.

One other point to make is that the silicon diode does not dissipate much energy because its forward voltage drop is only around 0.5, where the Zener assembly will be dissipating 80 watts whenever it clamps a 2 amp coil for a fraction of a second. Probably not much but it is something.

 

[CatKC]

Sorry CatKC, i don't agree with a lot of your references.
The basic zener function has a forward conduction voltage across the junction in one direction and begins to slowly avalanche current as you near the reverse voltage rating in the other direction.
Modern ignition systems work totally different, but referring to the old points system you referenced.
A standard coil would produce around 500V back EMF but this was not wasted across the capacitor.
The 12V primary winding is in series with the secondary high voltage winding and that voltage is added to the HT voltage to help increase spark performance.
The capacitor across the points does help with arcing points but its main purpose is to absorb the oscillations i spoke of to keep the speed of the magnetic collapse as fast as possible.
Most manufactures don't give a crap about arcing relay points or thermostat points wearing out due to arcing.

I was making a reference to an ignition coil only to make things simpler, not to be technical . . . an analogy.
I am quite aware of the dynamics of such devices . . . back as far as the Selenium Rectifiers used IN TV'S when you knew they went out because they smelled like 'rotten eggs'.
I only designed/worked on solid state devices and I am not posting here to argue about semantics or anything else.

I only post here to try and help the -non technical workers- and help them understand how to repair equipment.
End result is. . . . . the Zener is bad.

 

[CatKC]

Lets say the Zener suppressor is set for 40 V.

If the contacts open and the back EMF rises to conduct the suppressor, it will conduct and dissipate energy until the voltage across it drops to where it doesn't conduct any more. Then the field will collapse and may pull the other way but I can't see how the field is going to jump up so high in the opposite direction to make it conduct again, since all that energy was dissipated in the first go-round. It MIGHT come high enough to maybe TOUCH that point but I doubt even that. After that the remaining cycles are not going to come near the Zener point.
------
One other point to make is that the silicon diode does not dissipate much energy because its forward voltage drop is only around 0.5, where the Zener assembly will be dissipating 80 watts whenever it clamps a 2 amp coil for a fraction of a second. Probably not much but it is something.

A technical response only:
Just my opinion - you are correct.
Once the forward field voltage/current collapses (and with to the -short- the Zener provides) there is little or no reverse voltage/current -in a coil-. . . . UNLESS like in a motor (a reverse generator) there is a -force injected- , like a -rotating magnetic impulse-.
A 'coil' (without external influence) only has so much 'pent up energy', which when 'shorted' is gone. And -anything below the threshold- of the Zener is 'absorbed' by the capacitance of the connected devices.

Your last point is very good (of course the voltage drop would be dependent on the applied volts) as (even though a Zener can be made of silicon, germanium or gallium arsenide) that is -one reason- to use a Zener, it transfers/dissipates a -lot of volts-*amps/watts-.

 

[Vetech63]

Well, I DAM sure didn't expect the thread to go like this! Let me know whos right and Ill go with it.

 

[CatKC]

Well, I DAM sure didn't expect the thread to go like this! Let me know whos right and Ill go with it.

I completely understand that -the -thread- is getting DEEP (digressing) into technical electronics, and there is a lot of technical give and take, right snd wrong comments.

The technical BS is not relevant.
End result, the diode (whatever type) is defective.

 

[Birken Vogt]

I'll bet it is just a straight silicon diode and not the Zener contraption that Cat came up with.

Show us a picture with dimensions and we can give some idea of a replacement.

 

[oceanobob]

I wish to thank all ye who posted the information about this topic ... since there will likely always be coils and on/off control actions with these items.
Understanding the various strategies and theories is very helpful.
~
An image comes to mind of an aerial lift with a literal array of these types of contrivances.

 

[CatKC]

I wish to thank all ye who posted the information about this topic ... since there will likely always be coils and on/off control actions with these items.
Understanding the various strategies and theories is very helpful.
~
An image comes to mind of an aerial lift with a literal array of these types of contrivances.

I am in agreement with you. It interests me in how various devices are used in different applications and the reasons. Sometimes you just scratch your head and say 'why'? Seeing other peoples opinions sometimes helps explain the 'why'.
And maybe this discussion will help prevent someone from paying for unnecessary parts and repairs.

 

[Birken Vogt]

I build my own controls and such, and on any relay or anything with a coil, AC or DC, I will always apply some sort of snubber to protect the control upstream.

 

[crane operator]

You know- I like to think of myself as a fairly sharp guy. I don't drool at the table, can carry on a fairly decent conversation about a wide array of subjects. But I've read this whole thread through, a couple of times, really really slowly.

It's like a different language. I get a few words here and there, and I get that this is being used as a switch (kind of).

But my hat is off to you guys that really, really, understand how this all works, because I'm willing to admit that I don't. I just appreciate the knowledge you guys have. Thanks for the time spent posting, I'm always up for learning something- and actually learning how little I really know, is never a bad thing either.

 

[Birken Vogt]

Flyback! Magnetic lines of force cutting copper windings! Inductance! PN junction! Silicon! Zener! Diode! Triode! Forward/reverse bias! Power dissipation! Avalanche!

I don't know how they work inside either. (Some people know how the electrons go.) I just know how they behave when voltage and/or current are applied to them.

 

[John C.]

The issue is why does an air conditioning system need to be controlled by a micro processor which requires the added cost of the diode and associated wiring. How many Red Dot systems have been installed world wide with simple switches and cable controls for the ducting? The only conclusion to be made about the solid state systems is it means vertical integration of all parts of the machine. The functions are not changed. It is likely detrimental to the reliability. It only adds cost to the purchase, operation and maintenance of the machine.

 

[Birken Vogt]

Air conditioning systems, even with only mechanical systems, still have diodes on compressor clutches (sometimes). It saves the contacts of the switch upstream and adds to reliability. I install them across any coil on systems I build be it a relay or a solenoid coil.

I agree an air conditioning system does not need a microprocessor. It needs an activation switch, a clutch cycling switch, and a blower speed switch.

 

[RZucker]

It's not the same thing (maybe sort of...) I remember the hydraulic steer Wabco scrapers had a pile of capacitors and a resistor to drain off the alternator field when the elevator was switched off. If the resistor went bad (open circuit) they would blow the blocking diode that supplied the initial excitation from the batteries due to the reverse voltage spike.

 

[CatKC]

Air conditioning systems, even with only mechanical systems, still have diodes on compressor clutches (sometimes). It saves the contacts of the switch upstream and adds to reliability. I install them across any coil on systems I build be it a relay or a solenoid coil.

I agree an air conditioning system does not need a microprocessor. It needs an activation switch, a clutch cycling switch, and a blower speed switch.

What you and John C. sy is 100% correct. Unfortunately "electronics" integration and controls have gone to the point of being ridiculous. Some controls of cars and airplanes are 'control/fly by wire' where the motor/control/function (like steering or brakes) is operated by a wire connected to a control computer. While this saves a mechanical connection it adds complexity to the point to where they are no longer 'serviceable' to the average person and many times are not even by a trained person, except by 'replace parts till it works'. Heavy equipment is headed in the same direction.

 

[Swetz]

The pic below is the ac connector from a gm vehicle. The various US (don't know about foreign) manufactures used Zener diodes over the years. I am not sure how they get rid of the magnetic field spike these days on cars.

A real good site to understand how the Zener diode works.
https://www.homemade-circuits.com/zener-diode-circuits-characteristics-calculations/

 

[MrJRC]

Revitalizing an old thread, my 289c a/c system is not getting power to the clutch, found everything good up to the device (17) CONT AC PROTECTION 212-2204. Does anyone know what the function of this device?