Zener diode

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JerryT
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Zener diode

Post by JerryT »

DSC01295.jpg
I need some help on this please; electrics are a mystery to me.

I took the B50 out a few days ago with no problems but yesterday when I tried to start it there was no sign of life.
I firstly checked the battery which seems to be fine and a trace back led me to find that the main fuse had blown and the fuse holder showing signs of melting. When I fitted a new fuse it blew immediately.
I found that when I disconnected the cable to the zener diode? rectifier? the electrics - lights etc worked normally.
You can see from the photo that my electrical system is non standard and is partly situated behind the side panel. The zener diode is housed on an aluminium plate.
Does this indicate a failed zener diode? and is there any reason for the unit to fail? and lastly is a 35amp fuse the correct rating?
TIA
Jerry
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Re: Zener diode

Post by minetymenace »

I disconnected the cable to the zener diode? rectifier?
Which?

You can pull off the wire to the Zener quite easily, if you do this and the fuse still blows it is not the zener, ergo it is the bridge rectifier (they cost about £2.50 so no big deal).
There is no evidence to support the notion that life is serious.
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JerryT
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Re: Zener diode

Post by JerryT »

Thanks for the reply Minety.
When I pull the connector off the zener diode the fuse DOES NOT blow.

Jerry
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Re: Zener diode

Post by JerryT »

Assuming that it is the ZD at fault the next question is why did it fail? Do they do this from time to time or is this indicative of another fault in the electrical system?
If it's a common fault then it would make sense to order additional ones as spares.
Jerry
Jeff K
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Re: Zener diode

Post by Jeff K »

The square unit in the photos is a Full Wave Bridge rectifier. It has 4 wires hooked to it with push on connectors. The round bolt like unit with a single push on wire next to it on the right in the photo is the Zener. The electricity that is coming from your alternator is AC - Alternating Current. That has 2 wires coming from the magneto and they plug unto opposite corners of the rectifier unit. The Full Wave Bridge rectifier then converts this to DC - Direct Current to charge your battery and to power your lights, ignition etc. The Full wave Bridge rectifier has 4 diodes in it, think of them like four separate one way valves. AC flows back and forth, DC only flows one way. The diode/one way valve makes the AC, back and forth only go one way, DC.
Draw a square and then tip it over so it is standing on one of it's corners. Draw a dot on each corner. Now from the left corner draw a arrow on the upper side pointing to the top corner, on the bottom side draw a arrow point back to the left corner. On the right do the same, top side points to top, bottom side points back to right corner. Now think of water moving back and forth and hitting the one way valves. Top valve allows it to move forward only and it goes to the top. And the bottom only allows water moving back to pass to the bottom corner.
You now have AC on the sides and DC on the top (+) and (-) on the bottom.
The english system is brilliant in it's simple design. Magneto makes the AC, The Rectifier turns AC into DC. Now you have the AC to DC, but to stop from making too much voltage and burning parts up you need a regulator. that is the Zener Diode. Think of it a a pop off, pressure relief valve. It holds until the voltage gets to high , say 13.9v and then it pops open and bleeds off some of the excess pressure(voltage ) to the ground.

Now back to your problem. You are blowing a fuse, that indicates a big short to ground. Some where you have a wire touching the frame or a electrical part that has gone bad. The bike came with a Black selenium rectifier that had many fins on it that converted the AC to DC. Yours has been replaced by a common store bought square rectifier. I used to buy them at Radio Shack. Your bike came with Positive (+) ground stock. You have a different system now. Does the positive wire or the negative wire go to ground. Sometimes people when they update the system from positive to negative ground. Hook the battery up backwards from the the way the system is set up will cause your problem , a short to ground.

You are on the path to finding your problem, simple way is to unhook items one at a time and to then replace the fuse. Hook the wire back up, replace fuse until you find where it no longer blows the fuse, that is your problem. Since you seem to think it is the Zener, then you will need to replace that and see what happens. A bad Zener can cause a direct to ground short, so can hooking the battery up backwards.

Does it blow the fuse before you turn the key on? Great then the short is before the ignition switch? Simple, Magneto to rectifier, to zener, to battery and their wires are all you have to check.
Jeff
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Re: Zener diode

Post by JerryT »

Thanks for the technical information Jeff. Armed with this I am much better informed to be able to describe the problem.

Yes, I believe I have a positive earth system.

The supply from the battery goes directly - in parallel - the both the ignition switch and the zener diode.
When I disconnected the supply to the ignition switch only, the fuse still blew.
When I reconnected the supply to the ignition switch but disconnected the supply to the zener diode the fuse did NOT blow and the lights, horn etc work as normal.
Jerry
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Re: Zener diode

Post by Jeff K »

Jerry
It sounds like it is in the "charging" side. Always think of electricity as being water flowing in a pipe. So with the key off, you are still blowing the fuse? The Battery (Stores Pressure) the Bridge Rectifier ( turns the back and forth flow of AC to flowing one way DC) and the Zener ( regulates the pressure) are all hooked up before the key switch. So they are always "on".
The wire with the fuse comes right from the battery and goes to the Negative terminal on the Rectifier. There are two wires on this terminal and the second wire goes right to the Zener and then from here to the key switch. Now on some bikes there also a large blue Capacitor next. I do not see that in your photo, so we will rule this out. Next in line is your key switch. So if you are blowing fuse with the key off. Then the problem is before that the key switch. So it is either a wire shorting to the frame or a bad component. You only have the Zener and the Rectifier left. And if you remove the one wire that is on the Zener and the fuse no longer blows. then you have found your problem :thumb .
The battery just stores electricity(pressure). So for the Alternator to push more electricity into the battery it has to use more pressure then what is in the battery. So if a battery has 12.25 volts in it and your alternator is only pushing 11 volts it will not charge. That is why zener (regulator) is set to push ~14.5 volts before it "pops" off and vents the extra pressure. You need more then what the battery has in it to be able to push more into it. Like blowing up a balloon. So when the battery is full and you do not need any more to run the bikes, lights etc. The Zener vents the excess voltage (pressure) to the frame. The best way to check this is with a VOM meter, the cheap digital ones work fine. Set the VOM to the 20v scale DC. Hook the red lead to the Frame or good ground and the Black lead to the wire going to the fuse. Start bike and slowly rev it up. You will see the voltage climb and then level off when the zener starts to "vent" the excess. This is around 14.5, but can be a bit higher. A dead battery can pull the voltage down because it is dumping so much into the battery to get it back up. Like blowing up a huge balloon. lots of air going in before it starts to inflate and give you a little resistance.
Do not run the bike without a good Zener hooked up, that would allow the voltage to climb to over 20v and start to burn out things like the lights, etc.
I think that you need a new Zener. Here in the USA , I have been using a nice unit called Podtronics. Simple to use and it replaces both the Rectifier and the Zener..
http://www.podtronics.net
Jeff
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Re: Zener diode

Post by JerryT »

Thanks Jeff,
Your way of describing the workings of the charging system has finally given me a reasonable understanding of how it works and what it does.
A trusted source has also recommended the podtronics system.
I would prefer reliability to authenticity so I guess that this is the way to go.
Thanks again for the assistance.
Jerry
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Re: Zener diode

Post by JerryT »

Can anyone recommend a supplier for the podtronics unit? ( I did see the list of suppliers/dealers on your attachment Jeff)
Is there a specific model that I need for my B50?
Jerry
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Re: Zener diode

Post by skippy »

There must be hundreds of Zenor diodes around. I have 2 in my small collection of parts.
Doug
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Re: Zener diode

Post by Jeff K »

Jerry
Doug has a good point. Maybe some one on here who lives by you has a extra one. One nut and one wire and you have it replaced.
Jeff
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Re: Zener diode

Post by BasilF »

Voltage regulators are available from Draganfly, Burton bike bits and several eBay vendors. If your alternator has two wires then you want the single phase, 120 watts model although the slightly more expensive 200watt one won't do any harm.
I've got a 40 year old zener diode in my junk box which checks out as a diode but I can't guarantee it is fully working. I'll post it too you if you want.
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Re: Zener diode

Post by 2manyrides »

Isn’t a failed zenor diode a direct short?
Doesn’t a. zenor diode dissipate excess electricity to heat?
And that aluminum plate is not enough of a heat sink...
Zenor diodes can last a long time. I think this may be the reason for the failure over time. I used to run one of my bsa’s with the zenor on an aluminum plate. One day after stopping during a long ride, it just plain would not start back up.
I had to call in for a ride home for me, the dog and the bike.
I spent hours working on my bike in a parking lot. Even the police dropped by to ask me what I was doing. Embarrassing! I was on my way to my mountain retreat for an extended weekend on my ss converted to dirt road rider.
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74 TR5MX bought in 77-new, I'm the original owner

Which one to ride today?
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Re: Zener diode

Post by beat »

" Pacivico " plastic Tank ?

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JerryT
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Re: Zener diode

Post by JerryT »

Yes please Basil - you know my address.

2manyrides. Thanks for your comments. My ZD is fixed to an aluminium plate behind the side panel and certainly out of the airflow! My experience was similar to yours in that the bike failed to start the day after only a shortish journey.

I have fitted a std OIF electrical box to my machine although it is only for show. I could easily attach the ZD to the electrical box as per the original spec and extend the cable to suit. Would I then need an earthing cable from the box to the frame or do I rely on the bolt fixings?

Back in 1977 I was returning home from holiday on my B40, a journey of about 150 miles. The bike spluttered to a halt a few miles from home and I discovered that the ZD and heatsink were hanging loose after the bracket broke off. I can't remember how I resolved the problem but it wasn't a roadside fix.

Finaly - Is the Podtronics unit a more reliable alternative to system that I have?

Thanks for all the responses so far.
Jerry
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Re: Zener diode

Post by skippy »

Heat dissipation is very important shifting the Zenor away from the rectifier will help a lot.
I really don't know if either is anymore reliable, but I noticed a higher voltage from the podtronics which is not necessarily good for small AGM alarm batteries. 13.8v zenor to 14.5v podtronics
Doug
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Re: Zener diode

Post by 2manyrides »

JerryT wrote: Sat Apr 25, 2020 9:04 pm Yes please Basil - you know my address.
I have fitted a std OIF electrical box to my machine although it is only for show. I could easily attach the ZD to the electrical box as per the original spec and extend the cable to suit. Would I then need an earthing cable from the box to the frame or do I rely on the bolt fixings?

heatsink were hanging loose after the bracket broke off.

Finally - Is the Podtronics unit a more reliable alternative to system that I have?

Thanks for all the responses so far.
Jerry
Ground your box.
"heatsink were hanging loose", That will do it.
I have only ever run the stock components, but I have found them reliable for me.
72 B50SS bought in 74, I'm the second owner
74 TR5MX bought in 77-new, I'm the original owner

Which one to ride today?
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Re: Zener diode

Post by Jeff K »

OIF electrical box is rubber mounted, Run a good ground wire.
Jeff
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Re: Zener diode

Post by minetymenace »

Jef K wrote:The bike came with a Black selenium rectifier
No, definitely not. By 1972 selenium was passé. From the factory they were silicon diodes on square fins as heat sinks, the diodes looking like ticks or lumps on the heatsink. Selenium was a coating applied to the surface of the plates. Selenium was gone by the mid 60's or earlier.

Jerry T your Zener is passed it
There is no evidence to support the notion that life is serious.
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