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Old 10-08-2009, 16:50   #1
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Zap Stop - Fact or Fiction ?

West Marine is discontinuing the venerable ZapStop alternator diode protector. After talking with some very bright guys in the area of alternators and the mad ramblings of electrons, I am finding that there are some folks who dispute the value of the ZapStop, intimating that the damage to the diodes of an alternator might be heat from overexertion, rather than a possible shutting off of the ignition switch on a diesel while running. Oh my gosh! Is there anyone who can solve this issue?
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Old 10-08-2009, 22:48   #2
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I have had one for years and years. It's basically a Zener diode which under normal circumstances, is invisible to the alternator and the rest of the boat.
If you accidentally turn off the load from the alternator while it's running, it's very possible the output voltage could almost instantly but briefly high enough (about 25 volts as I remember) to short out the alternator diodes.

The zap stop prevents this as it goes into conduction at somewhere around 17 volts.
The transient overvoltage is very brief, so the protection diode doesn't get hot.

Steve B.
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Old 11-08-2009, 05:43   #3
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Quote:
Originally Posted by Roy M View Post
West Marine is discontinuing the venerable ZapStop alternator diode protector. After talking with some very bright guys in the area of alternators and the mad ramblings of electrons, I am finding that there are some folks who dispute the value of the ZapStop, intimating that the damage to the diodes of an alternator might be heat from overexertion, rather than a possible shutting off of the ignition switch on a diesel while running. Oh my gosh! Is there anyone who can solve this issue?
These are two different causes of Alternator failure:

Open circuiting the Alt' output causes a very rapid voltage rise, which will dammage (instantly fry) the diode trio rectifier.
The 'Zap-Stop' (Medium Current Overvoltage Transient Suppressor - Motorola #MR2535L zener diode) conducts this transient voltage to ground, protecting the alternaor.
An alternator field disconnect, on your batteruy selector switch, can also help prevent the most common cause of open circuiting the alternator (when Batt Sw set to 'Off', /w engine running).
Install a Zap-Stop.

The other issue is thermal insult. chronically over-loaded, or under-ventilated Alternator will fail prematurely. Electrical components don’t like heat.
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Old 11-08-2009, 09:22   #4
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Quote:
Originally Posted by Roy M View Post
West Marine is discontinuing the venerable ZapStop alternator diode protector. After talking with some very bright guys in the area of alternators and the mad ramblings of electrons, I am finding that there are some folks who dispute the value of the ZapStop, intimating that the damage to the diodes of an alternator might be heat from overexertion, rather than a possible shutting off of the ignition switch on a diesel while running. Oh my gosh! Is there anyone who can solve this issue?
Killing the excitation current (turning ignition switch to off) won't do anything to the diodes. It's lifting the load that kills them. It's better to have a 1-2-Both switch and an seperate battery disconnect. I have a 25 year old Prestolite on my W58 where the fuel solenoid and the field winding are paralleled to "run' at the key. No problems yet.

Anyone interested in a used W58?
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Old 11-08-2009, 09:53   #5
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I must admit, I've always been somewhat confused on the concept of alternators. So I Googled some sites this morning. They helped enormously to understand the basics. Maybe they should be included in the Sticky (or someone can suggest more appropriate ones? The video is my favorite.


http://auto.howstuffworks.com/alternator1.htm
I'm still confused about the pathology of a dying alternator. What's the difference between shutting off the ignition switch, overheating the internal diodes, and losing the belt?
A little background here: I was taking a customer's boat to the boatyard to have some work done. The voltmeter showed 12.5 volts under full throttle. I assumed the alternator was not putting out enough voltage. The belt tension, at rest, was good. The owner had reported a problem with the ignition switch a month earlier. I assumed it was a diode problem. The alternator shop showed no problems, whatsoever. It turned out to be a glazed belt, that under load, slipped and didn't turn the alternator fast enough to function. The regulator, sensing a problem, excited the alternator, increasing the friction and ramping up the originating problem, eventually creating more heat and deforming the plastic safety shield covering the alternator pulley. Replacement of the belt restored full activity to the alterator, but the question of the possibility of blowing the diodes in the future now became the issue. I have always installed ZapStops to prevent this issue. West Marine and other sources, suddenly stopped stocking these items that I had simply taken for granted. The folks at the alternator shop were surprised, and this lead to a discussion among my peers about how ZapStops (and other similar devices) worked. No one could offer the "killer" explanation, hence this thread.
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Old 11-08-2009, 10:12   #6
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Gord, your explanation is the clearest, so far, and thank you. Here's my dilemma: The alternator is going round and round. The ignition switch provides the voltage to "excite" the windings in the rotor to build the electromagnetic field that swings past the stator windings, breaking the field and creating the electrical impulse that flows to the diodes and on to the batteries (correct?). If the ignition is suddenly shut off, the current to the rotor is also immediatly shut off, at the speed of light. Is the problem that the magnetic field lasts momentarily longer before it collapses? Wouldn't this just mean that the voltage to the battery would just peter out as the current dropped to zero? Where is the "spike", that I read about, originating from? I realize, now, that the fan belt breaking would simply stop the alternator from creating juice, so that's no longer an issue for me. Understanding the dynamic of shutting off the ignition switch, or suffering a massive failure of the internal diodes, is what is confusing me. What is actually happening to the diodes (if anything), and how does the combination of a zener diode and a 20 amp fuse working to protect the internal diodes of the alternator. I deeply appreciate the information coming from this forum. We have so many informed, effective, communicators with tons of real life experience who share their collected wisdom.
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Old 11-08-2009, 10:25   #7
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Electricity is generated by the rate of change of the magnetic field so cutting the field will generate an electric field (voltage) as the B field collapses. However, as much as they try, stators are still reactive (the magnetic equivalent of resistance) so there will be some damping as the field will not instantly go to zero. In any event, the spike (more like a bump) will be small. Having the field collapse faster would be more of a problem, not less.

Lifting the output also creates a spike but with a lot more current (which is what fries the output diodes) a zener on the output will sense an increase in voltage (really "back EMF") and short it to ground.
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Old 11-08-2009, 10:27   #8
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I have a battery isolater, which keeps my three battery banks connected to the alternator through diodes regardless of the battery switch position. If you use a large enough isolater this system is pretty much idiot proof, and eliminates the need for a Zapstop.

I can see how the Zapstop works if you have an internally regulated alternator or an externally regulated alternator which senses voltage at the alternator output. In these cases turning the battery switch off will cause a transient overvoltage which the Zapstop can handle, keeping the alternator output voltage below the point where it will burn out its diodes.

However, if you have your external regulator sensing wire connected to one of the batteries (better for accurate charging and voltage stability), if you turn the battery switch off when the motor is running the regulator is not going to see the higher alternator voltage, and the alternator will probably fry both the Zapstop and its diodes.
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Old 11-08-2009, 10:39   #9
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It's not the shutting off of the ignition switch that does it, it's opening up the load from the alternator that's the diode killer. What happens is a loaded alternator has a certain amount of field current being applied to sustain the alternator output at a specified voltage. The instant the load is removed, (opening the battery switch for example) the voltage at the output of the alternator will go up because of the intensity of the magnetic field which was required for the (now nonexistant) load.
The regulator responds by cutting back on the field current, but the slight delay in the response time of the regulator and the decay time of the magnetic field in the rotor is the cause of the transient overVOLTAGE. We're talking fractions of a second for all this to happen, but it's enough to zap the alternator diodes. The zapstop absorbs this momentary overvoltage by providing a load to the alternator output.

The other thing which can fry alternator diodes is overCURRENT condition which is simply putting too much load on the alternator, causing an overheating condition.

In the commonly wired charging system, there is no problem generated in turning off the ignition switch before stopping the rotation of an alternator. All that happens is power is cut to the regulator which shuts off the field current and the alternator stops charging while the engine continues to run.

Steve B.
30 year retired electronics tech and total DIY'er
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Old 15-06-2010, 20:38   #10
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OK, so I looked up Gord's Motorola diode part number, which seems to be obsolete.

It has a "stand off" voltage (this would be the max it would see in normal service) of 20VDC.
It has a breakdown of 24VDC. That is the maximum voltage that should appear across the terminals.
It seems to be rated at 6Amps, which seems a little meager to me.

The closest replacement I can find at DigiKey is:
Digi-Key Part Number 495-3413-6-ND Price Break Unit Price Extended Price 10.150000.15100.118001.181000.073507.3510000.05880 58.80 Manufacturer Part NumberB72500D200A60V7 DescriptionCERADIODE 22VDC 30A 0603

Stand off of 22VDC
Breakdown of 25VDC
Rating of 30Amps, I think.

So, does this do the trick? Or is the stand off too high?

It seems like the next size down is 16VDC stand off, which seems too close to normal battery voltage for comfort.

BTW I googled Zap Stop and didn't find anything for sale. Except from Xantrex, who wanted $50. Seems a little steep for me.
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Old 15-06-2010, 22:59   #11
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OK, so I looked up Gord's Motorola diode part number, which seems to be obsolete.

It has a "stand off" voltage (this would be the max it would see in normal service) of 20VDC.
It has a breakdown of 24VDC. That is the maximum voltage that should appear across the terminals.
It seems to be rated at 6Amps, which seems a little meager to me.

The closest replacement I can find at DigiKey is:
Digi-Key Part Number 495-3413-6-ND Price Break Unit Price Extended Price 10.150000.15100.118001.181000.073507.3510000.05880 58.80 Manufacturer Part NumberB72500D200A60V7 DescriptionCERADIODE 22VDC 30A 0603

Stand off of 22VDC
Breakdown of 25VDC
Rating of 30Amps, I think.

So, does this do the trick? Or is the stand off too high?

It seems like the next size down is 16VDC stand off, which seems too close to normal battery voltage for comfort.

BTW I googled Zap Stop and didn't find anything for sale. Except from Xantrex,
who wanted $50. Seems a little steep for me.
The part you researched would probably work fine. As far as the current rating of the original zap stop, that's the continuous rating. Surge rating for solid state diodes is considerably higher, usually on the order of x 10 or more. Surge in this instance is a fraction of a second, so there's no time for any significant heat buildup.
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Old 16-06-2010, 05:48   #12
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Thanks Steve.
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Old 16-06-2010, 10:27   #13
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These devices are still available at Balmar; p/n TSP-12

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Old 16-06-2010, 15:46   #14
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Well that's better, $22.
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Old 16-06-2010, 16:29   #15
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Deep fried alternators

The simplest solution is to install make before break battery switches to eliminate the chance of inadvertently switching under load.
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