Zener diode, 50 W and stock stator, 130 W(2016)

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If the standard single phase stator produces 130 watts and the standard Lucas Zener diode is only capable of sinking 50 watts, Where are 80 watts going when driving without lights and a fully charged battery?
I couldn't find this particular question answered previously and I apologize if I missed it.

Ed
 
Old Britts say 130W but most other sources say 120W for the RM21 but maximum output would only be achieved around 5,000 RPM.

The ignition system takes about 20-30W.
 
I think the book says that the zener can absorb or turn to heat and have it dissipated the entire output of
the alternator.
 
So Long as the battery is in the system and asuming the ~Zener threshold is around 14.5 volts (maybe more ) then the battery will be absorbing current. If you knew the internal resistance of the battery you could work out roughly how much using good old V=IR. If its fully charged it'll just start making gas.
 
Anybody have any experience with the current "Lucas" branded direct replacement zeners?
tinyurl.com

Triumph BSA Norton Lucas Type 12V Positive Earth Zener Diode | eBay

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tinyurl.com
ebay page is just as for an example and pix.
 
The only time I have experienced a battery being boiled by a Lucas Zener was when the alloy heatsink corroded under the Zener and stopped it from making contact, when the corrosion was removed normal battery charging resumed. So the 50W leakage to heat, or whatever W it is, works with a single phase stator.
 
I was wondering if the new zeners on offer are up to the job.
 
Way over simplified:

Watts = Volts x Amps. (Normally written as P=IE; P=Watts, I=Amps, E=Volts)

The Zener must take everything over it's threshold to ground - not the entire voltage.

To make the math easy, let's say you are producing 15.2 volts DC and the Zener threshold is 14.2. So, you are only dealing with 1 volt to drop. 50 = 1 x Amps. Or Amps = up to 50 at 1 volt.

A 120 watt stator produces around 30 volts AC so 120 = 30 x amps so about 4 amps AC which when converted to DC by the bridge rectifier is about 10 amps at about 15.2 volts DC. Since only 10 amps is available, the 50 amp capacity of the Zener is fine.

This is only meant to give a 50,000 foot view - not an exact explanation - not exact math - and not exactly how a Bridge Rectifier and Zener work.
 
marshg246 said:
The Zener must take everything over it's threshold to ground - not the entire voltage.
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Yes, but the power dissipation is the zener's voltage times whatever average current is flowing through it. So if the zener is shunting 2 amps to ground at 14 volts the diode is dissipating 28 watts. The DO-5 stud case is rated for 50 watts with proper heat sinking.
 
marshg246 said:
A 120 watt stator produces around 30 volts AC so 120 = 30 x amps so about 4 amps AC which when converted to DC by the bridge rectifier is about 10 amps at about 15.2 volts DC. Since only 10 amps is available, the 50 amp capacity of the Zener is fine.
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The Zener is only shunting the power over and above it's rated voltage.

Unregulated, my alternator will produce about 17 volts with a load applied (headlight, taillight, instrument lights, ignition system), and my Zener shunts at 14.5 volts.

So it needs to handle the extra 2.5 volts.

My alternator is optimistically rated at 150 watts - Alton says 90 watts at cruising speed, but I will use the 150 watt rating to be conservative.

At 14.5 volts, this is 10.3 amps.

So the wattage that the Zener needs to dissipate is 2.5 volts at 10.3 amps = 25.8 watts. As stated above, this is a worst-case value, most of the time it will be handling much less power.

This is well within the capability of 50 watt Zener diode.
 
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maylar said:
Yes, but the power dissipation is the zener's voltage times whatever average current is flowing through it. So if the zener is shunting 2 amps to ground at 14 volts the diode is dissipating 28 watts. The DO-5 stud case is rated for 50 watts with proper heat sinking.
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True, but I thought it more confusing to explain it that way. The current it is shunting is the only current flowing through it.
 
There have been reports here that some replacement zeners are too high voltage (like 16 volts). Looking at the wiring diagrams for MKIII and pre-MKIII it looks like the MKIII would want a higher voltage zener to compensate for the voltage drop of the rectifier diodes. So, two 15 volt zeners on a MKIII v.s. a 14 volt zener for older models would be appropriate. Unfortunately, the positive ground polarity is considered "reverse" from a standard diode, and only available by special order and minimum quantity of 25 from electronics suppliers, whereas the negative ground (anode to case) can be had from Mouser, Digikey etc.
 
the diodes currently available from electronics stores are too high 16 volts is the normal operating threshold
a suitable replacement for the original Lucas will have to be a NOS part as no good quality replacement diode has been commercially available for for 20 years
i would use a regulator / rectifier Podtronics or similar
 
@Onder I am surprised to see these being sold as new again.

When Wassell first bought the license to use the Lucas name (2014?) there were a load of zeners that popped up for sale by all the usual suspects.

However, the failure rate was really high, and around a year or so later, Wassell took them out of their catalog.
... your post has got me wondering if they have found a source for something more reliable.
 
Which brings us back to the start of this thread. I hate to drop 40 quid for a sample to test. Yes easier to go for a Podtronics or similar but there was a thread on here a while ago about these units stressing out the stator.
 
Onder said:
Which brings us back to the start of this thread. I hate to drop 40 quid for a sample to test. Yes easier to go for a Podtronics or similar but there was a thread on here a while ago about these units stressing out the stator.
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If you had negative earth, a correct zener from a reputable electronics supplier is only about $10 USD. Positive earth polarity is unobtanium.

The shunt type regulator is the most popular design in the world with a permanent magnet alternator. Though I think there may be better choices than Podtronics.
 
Onder said:
Which brings us back to the start of this thread. I hate to drop 40 quid for a sample to test. Yes easier to go for a Podtronics or similar but there was a thread on here a while ago about these units stressing out the stator.
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Why not put a request in the wanted tread. If you were in the US I would send you a couple of Zeners for the price of postage. Surely there's someone in the UK would do the same.

Although I don't use PODtronics these days, they don't hurt a stator. The better ones, like Tri-Spark's MOSFET compared to the bridge rectifier and Zener are easier on the stator (so what?), provide a little more charging current, run cool, and don't have a parasitic battery drain.
 
Honestly I have had many years of good luck with the Lucas stuff on my bikes. My Norton still has the original zener. I was curious if the clone zeners are any good.
 
I had the stock lucas system on my bike until I installed the Alton E-start. I converted to Podtronics shortly after that, ONLY because of recommendations here that it was a better system. After making the change I discovered that the OEM electrics had produced a bit more charging voltage - .1 to .2V at all RPM - than the Podtronics did. IOW, to me the stock system was better!

However, I didn't convert back simply because that .1V didn't seem worth the trouble. But if the PodT fails, I'll put the EOM Lucas rectifier/Zener(s) back in even though I would have to drill holes in my CNW reset! :(
 
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