From: RussellMc <apptechnz@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
CC: VHome <valmcm@gmail.com>, Kristin <kristin_mcmahon@yahoo.com>, Rod
 Macdonald <essel@slingshot.co.nz>, ApptechNZ <apptechnz@gmail.com>, Iain
	<sir.iain@gmail.com>
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Date: Fri, 22 May 2020 19:58:26 -0700
Subject: Re: [EE] Light bulb moment (literally)
Thread-Topic: [EE] Light bulb moment (literally)
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On Fri, 22 May 2020 at 20:34, David C Brown <dcb.home@gmail.com> wrote:

> I have just realised that the traditional "dynamo" used for bicycle
> lighting (and which is really) a magneto) is, despite being sold as a 6
> volt machine,  a actually a constant current device.
> Since both voltage and frequency - hence reactance - rise
> approximately linearly with speed the current remains constant.
>
> Sort of - and in use, usually.
Voltage is about linear with RPM but reaches Voutdesired at low revs.
Thereafter the output is (usual;;y)* clamped by either a battery and/or a
load - with current rising with revs until the core saturates, setting
Power_max to Vdesired x Isaturation, approximately.
SO, increasing Voutdesired will increase Power_max, with a higher "but in"
speed .

Leading to:

Long ago I had a 90cc Suzuki "Honcho" trail bike. Max speed not much over
50 mph but good enough on a gravel road.
Entered ir in a 24 hour road trial.
Headlight was terrible.
6V maybe 25W tungsten.
No battery - lights loaded alternator "correctly"
I modifed a 12V 50W ish halogen bulb to fit in same socket.
Utterly marvellous.
As long as you didn't miss a gear change :-).

Went trialling.
In the wee small hours we were on an utterly dark  gravel country road
'having fun'. I was following a CB750 Honda.
I missed a gearchange.
Instant very bright and then utter blackness.
I tucked up close to the Honda and followed it for some miles.
Suddenly road looked strangely smooth ahead.
Whoops - ford! (The water sort).
Wheeee. We survived.
I danced with the Honda until we reached some local lighting and stopped
and changed back to the standard bulb. Very sad  ....

I went back some years later to look - the ford had been culverted.

Russell
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