[v12-engine] Zener Diodes

An XJ-S owner recently purchased a brand new AB14 ignition amplifier, not
from a Jag source but from Advance or AutoZone or some similar auto parts
chain, and he opened it up and sent me a photo of the innards. Everything
looked essentially similar to the OEM amp with one notable exception: The
zener diode pressed into an opening in the top of the case was missing. In
fact, this being a new casting, there was no hole there – although the boss
was still there, you could see where to drill a hole if you chose to.

What was there instead was a wire that disappeared into a length of heat-
shrink tubing, and what emerged from the other end was an eyelet that was
screwed down to one of the HEI module hold-down screws to ground it.

From the shape of the heat-shrink it was evident that there were two
cylindrical devices hidden therein. The owner reports that, testing with his
VOM, this arrangement appeared electrically identical to the OEM setup.

My presumption is that the single 350V press-fit zener diode has been
replaced with two inline zener diodes, perhaps 175V each or perhaps one
200V and one 150V. I did a brief search for zener diodes online and I
couldn’t find ANY that were press-fit like the original used here. I found stud-
mount zener diodes, which are similar in configuration, but I couldn’t find any
rated higher than 200V.

This brings up several questions for the electrical gurus here: First, is it
kosher to connect zener diodes in series? Do the voltages add? Does the
cumulative current rating become the lowest current rating of the individual
zener diodes in the series?

Second, I had long presumed that the press-fit zener diode was pressed into
the case to serve as a heat sink. Is this not necessary? Is the current rating
of a zener diode independent of its cooling? If someone tries to start their
Jaguar with the HT lead disconnected from the coil, are these little inline
zeners likely to melt down the heat-shrink tubing surrounding them?

Does anyone know what amperage the original zener diode was rated at?
And what amperage rating this application truly calls for? Is the amount of
current drawn by the ignition system during normal operation a good
indication? Or would there be current spikes here that call for higher
amperage ratings?

I didn’t ask this owner to cut open the heat-shrink tubing, although it’d be
easy to do; after checking out the components, a new piece of heat-shrink
could be slid on and shrunk into place. But ideally I’d like to know what
readily-available zener diodes could be used for this job. If we can learn
from whatever this aftermarket manufacturer did, it might provide a scheme
by which we all could rebuild our own AB14 amps. The GM HEI module,
capacitor, resistors and wires are all easy enough to find, the ONLY difficult
item to source has been that pesky zener diode. True enough, it doesn’t
often fail, but anyone looking to renew everything would want to know what
can be used in its place. A pair of inline diodes wrapped in heatshrink would
be easy. The original zener could simply be disconnected and left to plug
the hole in the case, keeping the appearance original.

– Kirbert

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In reply to a message from Kirbert sent Sun 24 Apr 2016:

There is no reason why you can’t connect them in series. Each of
them will have to be good for dissipating heat caused by the full
current passed through them and their individual voltage drop as
they are in series.

The reason components are bolted into a piece of metal will be for
heatsinking purposes and the chunkier the components, the higher
the wattage rating they’ll have when compared to their peers. In
that respect, starting with smaller replacement parts and
insulating them in heatshrink doesn’t sound too clever.

If you can see the original part number, then it should be easy to
just look it up. Failing that, you’ll have to put a scope on it to
see what current is being dumped.

kind regards
Marek–
The original message included these comments:

This brings up several questions for the electrical gurus here: First, is it
kosher to connect zener diodes in series? Do the voltages add? Does the
cumulative current rating become the lowest current rating of the individual
zener diodes in the series?
Second, I had long presumed that the press-fit zener diode was pressed into
the case to serve as a heat sink. Is this not necessary? Is the current rating
Does anyone know what amperage the original zener diode was rated at?


v12 E-type running MS3/3X sequential lpg and petrol
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In reply to a message from MarekH sent Mon 25 Apr 2016:

I don’t know the answers to Kirby’s questions, but I have
looked at the AB14 waveforms with an ignition oscilloscope.
The primary ringing voltage is well below 350V (maybe 100
IIRC) so the Zener doesn’t conduct at all. That’s with the
plugs connected.

I would also add that back in the day, high voltage
transistors were just being introduced and were pricey. The
technology is improved, so I would expect more recent GM HEI
modules to have much higher voltage ratings on their output
transistors as did those available to Lucas. That might
obviate the need for the Zener altogether.–
The original message included these comments:

just look it up. Failing that, you’ll have to put a scope on it to
see what current is being dumped.


Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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I don’t know the answers to Kirby’s questions, but I have
looked at the AB14 waveforms with an ignition oscilloscope.
The primary ringing voltage is well below 350V (maybe 100
IIRC) so the Zener doesn’t conduct at all. That’s with the
plugs connected.

That’s the intention. The problems occur when plugs are disconnected and
the spark energy has noplace to go.

I would also add that back in the day, high voltage
transistors were just being introduced and were pricey. The
technology is improved, so I would expect more recent GM HEI
modules to have much higher voltage ratings on their output
transistors as did those available to Lucas. That might
obviate the need for the Zener altogether.

I just read an article that confirms this. The original HEI modules had
Darlington output transistors rated at 400V, so the output voltage could spike
above that when the plugs were disconnected. The newer units, though,
supposedly have Darlington transistors rated at 600V, and since the system
can only spike to about 450V, the zener is no longer necessary with such
units.

The same article also suggested that, if you can’t find a suitable zener diode,
you can substitute a regular ignition capacitor rated at 0.2 microfarads. This
will clamp the output voltage down to about 300V. The spark performance is
slightly reduced, but not significantly.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 25 Apr 2016 at 9:23, Robert Wilkinson wrote:

Regarding Zener protection …

Many of the modern driver “transistors” (usually IGBT’s) have integrated
protection devices built in. This, in theory, allows doing away with the
need for external protection. The amount of protection available is limited
and rated in Joules (1J = 1 Watt for 1 second). A prudent designer might
still have a back up, although in modern automotive, cost is king and there
is profit in spare parts.

The highest stress on an ignition system is when a spark plug is
disconnected or misfiring. In this case, the energy that has built up in the
coil has to go somewhere and that will either result in an insulation
breakdown, or a high energy kick to be suppressed in the primary driver. (It
has always puzzled me why people group all their HT leads together in order
to tidy up the engine … a recipe for crosstalk … a spark will jump 1mm
per kV in the air. About 1" at ignition voltages)

Running an engine in a misfiring (or plug disconnected) state WILL rapidly
result in component failure.

The actual spikes that are present have a lot to do with the coil(s), wiring
and layout of cables etc.

A diode can be imagined as a one way valve. It passes current in one
direction, but not the other.

A Zener acts like a diode in one direction and a high voltage clamp in the
other. (Using the same analogy, it flows current in one direction and blocks
it in the other until the pressure builds up to a point where it “leaks” or
breaks down.)

Often you can get away with running a circuit without the protection zener.
This is because, under normal circumstances, the “pressure” may never build
up enough to have it break down. But under abnormal conditions it acts as a
safety valve. In doing so, it protects the switching transistor which is
optimised in its normal operating range and not designed to withstand high
“pressure” transients. (high voltage transients)

Zeners can be run in series (or back to back). The power dissipated in each
zener is equal to its clamping voltage x clamping current. Two zeners in
series are both subject to the same current. So a higher voltage part will
dissipate more energy (heat) than the lower voltage part.

There might be a case where a designer would wish to add a series resistor
to the zener, perhaps trying to produce a softer clamping response or trying
to limit the clamping current.

There are two transients at work. The major one is positive and the other is
negative (or an oscillation made up of both).

In the event of a negative transient, the zener will act like a diode and
pass current. However, you might NOT want the zener to do this, or it might
not do it very well. Therefore you might add an specialised diode in series
(or parallel) with the zener (blocking or bypassing forward current) but
leaving the zener in place to clamp the high voltage transients. The
specialised diode is likely faster to act and has lower forward voltage than
the zener, thus being more efficient.

Note that the use of a big zener is a fairly crude solution. For example, a
smaller zener can be connected slightly differently to re-trigger the
switching device (dissipating the fault energy in the transistor switch) and
this is often the approach employed by the built in protection.

Some modern systems will be monitoring the feedback “spike” to detect spark
breakover. From this they can detect spark voltage and duration (=energy)
and are capable of reporting to the ECU a weak or malfunctioning coil.

Rgds
Mark

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In reply to a message from Mark Eaton sent Mon 25 Apr 2016:

Mark,

Thanks for the very nice post! Expanding on one of your
points:

The modern generation of power transistors–IGBTs and
MOSFETS in particular–not only improve on the AB15 but they
have IMHO help make possible the revolution in electric
cars, wind and solar power, etc. that we are now experiencing.–
The original message included these comments:

Regarding Zener protection …


Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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In reply to a message from Mark Eaton sent Mon 25 Apr 2016:

Thank you Mark for your reply to Kirby’s question. It was I
that sent him the picture of a new replacement electronic
ignition amplifier that was missing the diode in the case.
In looking at several different parts suppliers at least
three of them where selling the same part as the part
number is shown on the cover. Lucas AB-14 Electronic
Ignition Amplifier Made in the UK 40839A. Prices varied
greatly. After installing car ran great for less than 30
miles and upon stopping for some time would not start.
Towed home this past Sunday. Do not know cause of this not
start as have not looked into as yet but may this weekend.
A friend that teaches engine maintenance loaned me his
SUNPRO CP9080 Sensor Tester Plus with which the module can
be checked. It can also be used to test the reluctor. The
module inside checks good. Printed on its’ edge is IG-D1906
05010131 England. The module from the original AMP was
replaced in 2009 and it did test as being bad.
Thanks again for sharing your knowledge of electronics.–
The original message included these comments:

Regarding Zener protection …
Many of the modern driver ‘‘transistors’’ (usually IGBT’s) have integrated
protection devices built in. This, in theory, allows doing away with the
need for external protection. The amount of protection available is limited
and rated in Joules (1J = 1 Watt for 1 second). A prudent designer might
still have a back up, although in modern automotive, cost is king and there
is profit in spare parts.
The highest stress on an ignition system is when a spark plug is
disconnected or misfiring. In this case, the energy that has built up in the
coil has to go somewhere and that will either result in an insulation
breakdown, or a high energy kick to be suppressed in the primary driver. (It


1987 XJS Coupe
Weston Lakes/Texas, United States
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Don,

This link :

http://www.worldphaco.net/uploads/CUSTOMISING_A_RELUCTOR_STYLE_DISTRIBUTOR.p
df

has a totally EXCELLENT write up on the HEI unit. The author is the
unfortunate lover of Triumph motor vehicles and has a passion for fixing
their Lucas electronics …

The Zener is a 350V part and the output darlington is a MJ10012 (see circuit
diagram page 11 of the above).

From an electronic design point of view, it frustrates me greatly that there
are only a few $$ of components in these modules and yet prices are very
high.

We are entering into a very interest era where electronics (or lack thereof)
are going to play a very important part in maintaining classic automobiles.
And not just old clunkers. I was looking at a Ferrari F40 the other day and
considering the poor block trying to keep the electronics going. 30 yrs old
next year! $1,000,000 for your supercar and a $1 transistor is going to
render it useless.

If you have time for bedtime reading, the author has uploaded another
excellent write up here :
http://www.worldphaco.net/uploads/CAPACITIVE_DISCHARGE_IGNITION_vs_MAGNETIC_
DISCHARGE_IGNITION…pdf

He concludes that the HEI is still the best ignition solution (doesn’t
appear to have looked at coil on plug).

Regards
Mark

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Lucas AB-14 Electronic
Ignition Amplifier Made in the UK 40839A.

I’ll bet the “A” is important. Like, it implies the use of a newer model HEI
module or something.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 27 Apr 2016 at 16:48, Don Franke wrote:

In reply to a message from Kirbert sent Wed 27 Apr 2016:

What’s the difference, if any, between a Zener and a
clipper diode? The only real familiarity I have with
Zeners is their use as a ‘wastegate’ on British bikes.
Triumph were the first to use alternators (rotating
permanrnt magnet) around 1955 and solved the varying
demand by a big Lucas master switch that matched stator
coils in circuit to the 6V system demands. Two coils were
always in circuit to the silicon plate rectifier to match
ignition and stoplamp load. Another two were connected
when the big switch was set to sidelights and all six
stator coils when headlights were used.

About ten years later in 64 the whole Brit bike industry
switched to Lucas 12V electrics with full time use of all
stator coils. They fitted a 14V stud mount Zener to a heat
sink and permanently wired it to the +ve pole of the -ve
earth battery. The idea was that it didn’t conduct until
the charge went over 14V and then dumped the excess
current as heat.

No switches, no fancy wiring or regulator - just a nice
juicy rectifier and one-wire Zener. You could even
kickstart the coil ignition bikes with no battery, because
they used a capacitor across the battery leads to store
rectified kickstart alternator output, enough to deliver a
spark via the coil(s) when the points opened.

I’ve often thought of using a similar Zener charge control
system on classic Jags, even Dynamo systems but never got
round to researching diode ratings. Is it a feasible
option? I’d imagine a simple one-wire finned alloy gizmo
with one or more Zeners in circuit would be an attractive
replacement for the electro mechanical RB340 or other
styles of voltage regulator?–
66 ‘UberLynx’ D, 70 FHC, 79 S2 XJ12L, 97 XJ6L
Gaithersburg, Maryland, United States
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In reply to a message from PeterCrespin sent Thu 28 Apr 2016:

A clipper circuit prevents a waveform from exceeding a
certain voltage. Diodes, zeners, transistors, and
operational amplifiers can be used. One example is the ‘‘fuzz
tone’’ circuit used by Jimi Hendrix and others. It adds
distortion by clipping the waveform.

In the motorcycle application that you describe, Pete, the
zener clips any voltage that exceeds 14. This application is
called a shunt regulator, because excess current that would
develop more than 14 volts across whatever load resistance
is present is shunted through the diode. That current, I,
results in 14xI watts of power dissipated as heat in the zener.

And therein lies the problem for use in cars IMHO. The
dynamo operates at full output all of the time. If the load
(fog lights, etc) is great enough, all of the watts are
used, with none left for the zener. But with low loads, the
zener would have to dissipate a lot of (wasteful) power. So
the zener regulator has the advantage of simplicity, but is
applicable only if the load doesn’t vary much.–
The original message included these comments:

What’s the difference, if any, between a Zener and a
clipper diode? The only real familiarity I have with
earth battery. The idea was that it didn’t conduct until
current as heat.


Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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I’ve often thought of using a similar Zener charge control
system on classic Jags, even Dynamo systems but never got
round to researching diode ratings. Is it a feasible
option?

On a vehicle with a 75 amp alt, you’d be wasting more than one horsepower
constantly. The impact on fuel economy alone would be unacceptable. Of
course, back in the day you’d be lucky if your alt produced 20 amp, and you
didn’t care a whit about fuel economy.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 28 Apr 2016 at 8:21, PeterCrespin wrote:

Hi Pete,

People forget how much technology has advanced and how little “modern stuff”
Lucas had to play with back then. We’re talking valve radios.

On my old MkII I built something like you describe into the Lucas regulator
box. I think it was the RB340 you describe. Three solenoid/relay type
things.

A simple power schottky diode replaced the cut out relay. (Big 75A stud
mounted thing) That got rid of most of the problems (I recall the cut out
relay sticking)

There was a power transistor that regulated the field coil current based
upon a simple voltage reference. I think that was about it. Seemed to work
fine. I also had the feeling that the dynamo was the happier for it, with
less stress (as was the battery). There was also MUCH less electrical
interference.

This was about 25 yrs ago now. You could make it even smaller and more
efficient with FETs now. I assume someone has these commercially available??
It fitted easily into the existing case and kept the same spade terminal
connections.

Mark-----Original Message-----

I’ve often thought of using a similar Zener charge control
system on classic Jags, even Dynamo systems but never got
round to researching diode ratings. Is it a feasible
option? I’d imagine a simple one-wire finned alloy gizmo
with one or more Zeners in circuit would be an attractive
replacement for the electro mechanical RB340 or other
styles of voltage regulator?

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !

In reply to a message from Kirbert sent Thu 28 Apr 2016:

OK. Glad I asked, but no joy for that idea then. Yes,
semiconductors were in their infancy for sure. The old
rectifiers were a stack of big slices of something (silicon
or germanium or something like that?). Looked like a stack
of 4 mini pancakes on a bolt that formed the earth stud.
They shrank from about 3-4 inches diameter to half that
size and weight but you look at ant BSA/Triumph catalogue photo from about 64 to 71 and you see either a flat finned heat sink
photo from about 64 to 71 and you see either a flat finned
heat sink or a starburst oval one, just under the headlight in
in the breeze.

The alternator stators were resin encapsulated by then and
by the early 80s went to three-phase with triple diode
rectifiers. The whole shebang was very reliable compared to
dynamos, especially once the first Boyer Bransden 'Hall-effect points replacement kits came out.

I was an early adopter but the downside
effect points replacement kits came out.

I was an early adopter but the downside was the capacitor
system didn’t give a steady enough current in the absence
of a battery to power the electronic ignition. Even running
sand bumping very fast would usually get you misfires and
nor much more. There’s nothing quite like the relief of
climbing onboard a rapidly-accelerating bike, with the wind
drying your sweat, when you know that was the very last
push you had the energy for!–
The original message included these comments:

course, back in the day you’d be lucky if your alt produced 20 amp, and you
didn’t care a whit about fuel economy.


66 ‘UberLynx’ D, 70 FHC, 79 S2 XJ12L, 97 XJ6L
Gaithersburg, Maryland, United States
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In reply to a message from Mark Eaton sent Wed 27 Apr 2016:

Neither of the sights would open.–
The original message included these comments:

http://www.worldphaco.net/uploads/CUSTOMISING_A_RELUCTOR_STYLE_DISTRIBUTOR.p
http://www.worldphaco.net/uploads/CAPACITIVE_DISCHARGE_IGNITION_vs_MAGNETIC_
DISCHARGE_IGNITION…pdf


1987 XJS Coupe
Weston Lakes/Texas, United States
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87 xjs 5.3.

Ignition amp modules failing. The ab14 got hot, the engine died and would not restart even when cold, so something has failed. I’ve got two ab14s I carry with me just in case. Both had new gm ac delco d1906 modules. Used old Zener and condensers and didn’t open the white blob cause the tachometer works fine and fuel injection to the ecu works so those resisters must be ok. Right? Also have the single style coil installed a few years ago, but may be an aftermarket so am replacing it with the official Jag single coil and testing resistance before install. I did not test resistance of the first single coil when installed, but should have and will when I remove.

I’ve ordered 2 new ac delco d1906 professional ignition modules and an oem single coil that is in its original jaguar packaging.

Based on the comments in this thread about newer ignition amps with better transistor technology not needing the Zener, I am considering leaving the Zener disconnected this time. Kirbert mentioned that if you have good spark plug wire that are not removed, the Zener is not required. I will probably replace the condensers while the ab14 case is open. I have found many Zener diodes on the market, but they are not the press in type so heat disapation would be lacking, and I don’t know which one would meet the spec anyway.

My understanding of electronics is minimal, so I apologize if my questions are silly.

Question 1: has anyone had success eliminating the zener diode in their ab14? Just disconnect, tape up the wire and leave it pressed in the hole?

Question 2: can I test the Zener with a volt meter without heating it up to 350 degrees?

Question 3: is there a way to test the resistors in the white blob with a volt meter?

Question 4: the new coil should test at .63 ohms resistance or less. Right? If the old coil tests out higher, could the coil be causing the ignition module failures?

Thanks for any insights you may have.

Q2: Most definitely, but you have to apply a variable, current-limited voltage in the range of 350 volts. Power supplies made for experimenting with valve-type electronic circuits are suitable.
Q3: Yes, ohmmeter to each wire terminal and other side to blob input. The Rs are a few Kohms IIRC; their values are in the archives. They are for isolation, so if they read any reasonable value they are probably OK. If you read FI wire to tach wire, you should get them in series.
Q4: Higher R would be unlikely to cause module failure IMO. In fact, it’s an “easier” load. IMHO.

I should add that, IMO, if you test the zener with an ohmmeter and it tests like an ordinary diode–reads one way and infinite R the other, it is almost certainly good. The normal failure mode is a short circuit. It could conceivably fail so that it is open circuit when reversed biased, even above the Zener voltage–but that would be equivalent of disconnecting it without the bother or removing the wire.

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I have what might be a related issue with the ignition amplifier. There seem to be several versions of the Lucas AB14 amplifier - the one I have purchased recently has no zener diode and no resistor block as on the earlier ones which are advertised as being for the same car. Can anybody post a helpful comment

Without an A-B comparison of the two different circuit diagrams, it’s hard to know what the difference in the two AB14 units will be.

The modern equivalent may be using components of much higher voltage specification or it may have a physically much smaller zener hidden in there which is used to sample the voltage and trigger a high voltage transistor to conduct in place of the old style large chunky zener (a so-called “active clamp”).

(One thing that may not be obvious to the layman reading this thread is that when a zener is said to “break down”, this means it conducts in the opposite direction to the arrow when the threshold voltage across it is exceeded - it doesn’t mean it becomes “broken” or “defective”.)

kind regards
Marek