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Fuses

I read every post the search turned up. My question is which lead from the dynamo should get a fuse?
I got started on this because when working on my horn I noticed if the points are closed when 6v is applied things can get hot fast. Thankfully I was using a very thin wire when applying voltage. If for some reason the points close and do not open the wiring will burn up I guess along with the horn. So I installed a fuse. Also adding fuses to the battery main power feed and the lights.
That leaves the dynamo. I has two wires. Which one?
Thanks

email (option): cbhaws@verizon.net

Re: Fuses

You may be over thinking this.

One fuse on the positive battery terminal is normal.

Rob

email (option): robmiller11(a)yahoo.co.uk

Re: Fuses

Your BSA had no fuses at all when made. I have a number of old bikes that also had no fuses when made and still don't. If you feel the need to have one, the positive battery terminal as Rob stated is where to put it. The reason the horn gets hot is the points are supposed to vibrate. If they stay closed the winding will eventually burn up.

My BSA M21 is now going on 70 years without a fuse and has never needed one. I understand the reason to have one but most bikes did not have them until the 1960's without any problems. The dynamo does not make enough current to hurt anything. The LUCAS regulator already has over current protection to save the dynamo.

Should the battery get shorted to ground from wiring problems a fuse can be a goo idea. But really how often does that happen?

Re: Fuses

I have modernized the wiring and electrics a touch.

While most end connectors are original, I've soldered them onto modern wire which is thus wrapped in cable housing to clean up the bike.
I've also velcro fastener'ed it to the various points on the frame so if I need to get at it, it is easy enough.
Battery is a modern 6V YUSA battery, with an inline 15amp fuse on the positive terminal I have also installed a spade connector between the bike mains and the batter in case the battery goes all Sparky McBurny and I can disconnect it very quickly.
Lighting is LED entirely. I also use modern sealed switches on the brakes to ensure water ingress isn't an issue.

I know it isn't entirely period original, but that isn't what I was looking for. I want a safe, reliable bike that isn't going to flake out on me during a ride.

Brad

email (option): You can look me up in past posts :)

Re: Fuses

Thanks for the input. I have fused the positive on the battery and the horn so far.
My worry is if the battery fuse fails where does the power generated by the dynamo go. Some short caused the battery fuse to fail so now the dynamo Is pumping amps to the save shorted wire.
I may not understand how the dynamo works if there is a shorted wire. I have seen a few pictures of burnt down M20’s floating around the internet.
Thanks.

email (option): cbhaws@verizon.net

Re: Fuses

Hello Charlie.
There is no need to put a safety fuse on the dynamo output. There is no build up of dynamo 'power' if the battery fuse blows. A dynamo produces current by moving a wire through a magnetic field. The wire from the dynamo that feeds your electrics via the regulator is in effect an extension of this moving wire. The current consists of a flow of moving electrons. For the electrons to flow the wire has to make a circuit, else they have nowhere to go. This circuit will be through the battery, lights, horn or whatever other load. If your battery fuse blows (and your lights are off), then there is no circuit, the electrons cannot flow, therefore there is no current, and as power is current multiplied by volts, no power either. The dynamo stops generating. The real situation is a little more complicated due to the action of the regulator but nonetheless the dynamo doesn't need a battery to absorb 'spare' power.
I put an in-line fuse in the battery supply wire, but really to act as an isolator. I remove it when the bike is not in use to keep the battery's energy in the battery, eg in the event of somebody fiddling with the light switch.
Incidentally, one of the reasons I stick with electro-mechanical regulators is that they give an output without a battery. If your battery goes west, your dynamo/regulator will keep your lights on if the engine is running. I've read that electronic regulators won't do this, they need a voltage from the battery to function, so a dead battery means dead lights. Happy to stand corrected if this is wrong.

email (option): John.childs1@btopenworld.com

Re: Fuses

John
I've just fixed (I hope) an issue that drained & seems to've destroyed my battery. It doesn't seem to be taking a charge but with all the other circuitry in OK condition the lights work with the engine running. I'm using a DVR2 regulator.
And I'll vote for installing a single fuse on the positive side. I've also got a male-female connection near the battery so I can disconnect things easily.

Re: Fuses

Thanks Gentlemen,
I have a fuse on the battery and the horn.
I love this little old bike. It starts with ease, runs around town and trips over to the beach (only a mile). I does not handle like my Ninja or is comfortable as my Harley Fatboy, but it does get more looks.
The bike will get used for WWII re-enacting when this Covid thing is over. For being 81 years old, through a war and who knows how many owners it is a survivor.
Thanks again.

email (option): cbhaws@verizon.net

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