New Type BTH Magneto

roy the mechanic

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Having had a problem on starting-up my new toy, a phone call to the current guy at bth, said this is a very robust unit, you cannot hurt it by spinning the motor on the rollers without the h-t leads being connected. Is, Was he correct? I'll report back after the next attempt to "make noise"
 

nkt267

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I may be way off with this but reading the posts so far I personally would have no worrys about earthing the ht lead directly to earth.
The power from either coil ignition or magneto goes to earth via the spark plug,jumping the plug gap to earth makes a spark but the power still goes to earth.
So if the power goes to earth, in my view, it makes no difference whether the plug gap is closed or the plug is connected to the ht lead and earthed away from any ignitable fumes or the ht lead connected to earth..
Electricity is basically lazy and wants to find the easisest way to complete a circuit ..Apoligies in advance if I am wrong,I'm sure i will soon be put right..John
 

roy the mechanic

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earthing out disconnected h-t leads is certainly the safe way to go, I have blown a couple of car ignitions by not doing this. But the bth guy was adamant-you cannot hurt this unit in this way. As he makes them I figure he knows. (at least I hope he knows!)
 

Magnetoman

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So if the power goes to earth, in my view, it makes no difference whether the plug gap is closed or the plug is connected to the ht lead and earthed away from any ignitable fumes or the ht lead connected to earth..
Actually, there is a difference, although any ignition system will be designed to easily survive the difference. Whether the plug gap is closed and it is earthed, or the HT lead is connected directly to earth, it is the same. However, this is different than if the HT lead is connected to a functioning plug that is earthed.

In the former case the HT lead is shorted directly to earth so the voltage inside the coil never rises above ~0 Volts. In the latter case it rises to ~1 kV (for a gap of 0.018" at room pressure) before a spark occurs and it pulls the voltage inside the coil back down to ~0 Volts.

Again, any functional ignition system will easily survive this difference, but there is a difference. Also, I wrote ~0 Volts because current is flowing so there is a voltage drop across the coil, but it is quite small compared to the 20 kV overvoltage situation we're worried about if a HT lead is left dangling in air.
 

roy the mechanic

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Hi, Magnetoman, the modern bth is reckoned to produce 60kv at the plug. In an open circuit situation it can survive without any damage. That has got to be one tough system!
 

Magnetoman

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Hi, Magnetoman, the modern bth is reckoned to produce 60kv at the plug. In an open circuit situation it can survive without any damage.
That actually is pretty remarkable. The highest voltage oscilloscope probe I have goes to "only" 50 kV (and, even then, only when filled with a few precious ounces from of my supply of banned Freon 114) so I wouldn't even be able to directly measure its pulse shape when operated open circuit. The only way their HT coils could survive this is if they are wound with fairly thick insulation between the individual coils. If anyone has one that has failed please send it to me so I can cut it in half to examine the construction. Seriously.
 

Robert Watson

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I think you may find that in the true sense of the word this is not a magneto but a system that has some kind of internal generator that powers an electronic spark system and if I recall it fires a lost spark so if one plug is disconnected it will still fire the lost spark to the other plug. Coincidentally, I did exactly that today on a new BT-H mag and with no ill effect!! It has external coils.
 

Albervin

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That actually is pretty remarkable. The highest voltage oscilloscope probe I have goes to "only" 50 kV (and, even then, only when filled with a few precious ounces from of my supply of banned Freon 114) so I wouldn't even be able to directly measure its pulse shape when operated open circuit. The only way their HT coils could survive this is if they are wound with fairly thick insulation between the individual coils. If anyone has one that has failed please send it to me so I can cut it in half to examine the construction. Seriously.
I had a coil fail and it went straight into a bin, propelled by a force that was neither measured nor controlled:) Fortunately that man Greg Brillus was there and he promptly went home and brought back a new coil. This was on the first day of a rally in Queensland so he really made my day and week!
 
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