Author Topic: Lucas red dot ammeter  (Read 514 times)

Online Rex

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Lucas red dot ammeter
« on: 28.10. 2017 23:06 »
I have what appears to be a Lucas Red Dot ammeter on my 1951 A7, though the red dot is actually a clear dot. It seems rather pointless fitting this type of ammeter to a bike fitted with a mag and without an ignition key, so was it original, or a part fitted on the assembly line as no regular ammeters were available, or just something someone has pressed into service at some point in the bike's history?
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Online Greybeard

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Re: Lucas red dot ammeter
« Reply #1 on: 28.10. 2017 23:33 »
From my quick internet research it would appear that this ammeter incorporates a red charging light similar to older motor cars. The charging on these bikes has nothing to do with the magneto by the way. On old cars charge indicator glows red when current is passing from the battery, through the bulb, into the dynamo to energise the windings and then to earth. When the dynamo starts to pump current back into the battery the voltages either side of the bulb begin to balance so the bulb gets duller until it goes out. I'm not sure if our machines work the same way as we have no ignition switch to stop the dynamo discharging the battery.
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Online Rex

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Re: Lucas red dot ammeter
« Reply #2 on: 28.10. 2017 23:39 »
And that would be the problem GB, ie the light would be on all the time if there was no ignition switch. Square Fours, BSA C11's and various Vincents used these items but I don't see them as being original to a mag-equipped A7 somehow....
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Offline duTch

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Re: Lucas red dot ammeter
« Reply #3 on: 29.10. 2017 01:35 »

 
Quote
... I'm not sure if our machines work the same way as we have no ignition switch to stop the dynamo discharging the battery...

 As far as I can tell,  that's one of the functions of the cutout in a mechanical reg.... but if it doesn't do the ligh alsot, I'm sure a switch could be added ...?. *dunno*


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Online Black Sheep

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Re: Lucas red dot ammeter
« Reply #4 on: 29.10. 2017 06:38 »
I think we can fairly say, it wasn't an original fitment. When I got my one owner unmolested 1951 Star Twin, it just had an ordinary Lucas ammeter. No red dot.
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Online Greybeard

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Re: Lucas red dot ammeter
« Reply #5 on: 29.10. 2017 08:53 »
And that would be the problem GB, ie the light would be on all the time if there was no ignition switch...
I'm sure the dynamo is not permanently powered on our bikes; they must be self energising.
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Online Bsareg

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Re: Lucas red dot ammeter
« Reply #6 on: 29.10. 2017 09:31 »
And that would be the problem GB, ie the light would be on all the time if there was no ignition switch...
I'm sure the dynamo is not permanently powered on our bikes; they must be self energising.
Spot on Greybeard. The regulator is permanently connect but does the switching via the cutout using the residual magnetism in the dynamo. This why after a long period of storage, the dynamo needs re-energysing by flashing the field to the 'live' side of battery
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Online Rex

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Re: Lucas red dot ammeter
« Reply #7 on: 29.10. 2017 10:23 »
And that would be the problem GB, ie the light would be on all the time if there was no ignition switch...
I'm sure the dynamo is not permanently powered on our bikes; they must be self energising.

They are, but the "red dot" lamp also acts as an "ignition on" warning as on older cars, but if the bike is fired by a mag rather than battery coil and points, it's somewhat pointless.
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