Joined: Aug 2017 Posts: 741 Threads: 8
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Location: N W Kent
10-12-2021, 05:41 PM
(This post was last modified: 10-12-2021, 05:50 PM by Stuart Giles.
Edit Reason: missing word
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![[Image: main-qimg-996c9c548f1ea62775a19fbccdb14d4f-lq]](https://qph.fs.quoracdn.net/main-qimg-996c9c548f1ea62775a19fbccdb14d4f-lq)
IIRC, the commercially available diodes to replace the original cutouts are Schotky (sp) clamped ones which have a lower voltage drop than a normal power diode. The above symbol borrowed from the web is for a Schotky diode. The anode is the positive side on any diode for conventional current flow. Easier to remember that that "you can always pour positive down the funnel" might help you to connect it up the right way round -so, positive side to the dynamo on a negative earth car.
Joined: Aug 2017 Posts: 612 Threads: 19
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Location: Hampshire UK
Hi
I believe the "Smart Diode" in question is based on the Texas Instruments LM74611 Smart Bypass Diode device, which claims only 26 mV drop at 8 Amps which is much less than even a Schottky diode. This would give a power loss of only about 0.2 Watts, which is probably small enough that a heatsink isn't required. They were originally designed for use in solar panels.
Datasheet here if you need (lots) more detail: https://www.ti.com/lit/ds/symlink/sm74611.pdf
Joined: Aug 2017 Posts: 425 Threads: 30
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Location: Wellington, NZ
Hi John, yes that was my guess as to what they actually were too. I posted the same data sheet when these were first mentioned last year. The link I posted then is now broken but the failure page is amusing if you get the joke!
Here is a link to where to get them in NZ: https://nz.element14.com/texas-instrumen...dp/3008488
This is what I posted at the time:
Quote:It's not the same as the normal diodes, it's probably one of these. Designed for use in solar power applications: https://www.ti.com/lit/ds/symlink/sm7461...e.com%252F
Clever little chip really. Quite cheap, in NZ the chip is only $6.40 (about 3 quid). If anyone has one of the boards from the Magneto guys they can check the number.
Simon
Joined: May 2018 Posts: 2,954 Threads: 558
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Location: Peak District, Derbyshire
Car type: 1929 Chummy, 1930 Chummy, 1930 Ulster Replica, 1934 Ruby
Solved. It wasn't the cut-out, the dynamo, the battery, a wiring fault, a fuse, or a gremlin - it was the dashboard switch. Fiddling with the lever (it's a Ruby type) found a sweet spot where everything came back to life.
Joined: May 2018 Posts: 2,109 Threads: 110
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Location: Llandrindod Wells
Car type: 29 Special, 30 RK, 28 C Cab
(13-12-2021, 11:47 AM)Tony Griffiths Wrote: Solved. It wasn't the cut-out, the dynamo, the battery, a wiring fault, a fuse, or a gremlin - it was the dashboard switch. Fiddling with the lever (it's a Ruby type) found a sweet spot where everything came back to life.
Well done Tony
Cheers
Howard
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Location: Auckland, NZ
Eons ago the ign sw in my RP became erratic. The car was in everyday use so I was reluctant to dismantle.(And if i had known how it would not increase enthusiasm). Turning the key part on cured it, but the key was not then secure. On the way to work one morning it fell out and thru the handbrake hole. A long search failed to find it. The keys then had a reg no tag attached. It had been handed in at a local shop and a week later i got it back. Some time later the crank broke. The key was in the glovebox and I had to frantically search for it while the motor idled like a gold stamper battery.
Joined: Oct 2017 Posts: 1,505 Threads: 54
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Before you rush to dismantle the switch I suggest that you refer to the instructions on the Cornwall Austin Seven club website Tony and then only dismantle it while sat in the bath with the plug in and no water.
Joined: Aug 2017 Posts: 2,400 Threads: 33
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Location: Deepest Frogland 30960
Car type: 1933 RP Standard Saloon
It's easier, if the ignition switch is dicky, to simply by-pass it and use an on/off switch hidden elsewhere. I would advise against even attempting to take the unit apart, unless you like a challenge. Bits WILL go everywhere, and getting them back togetjher is no easy task.
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(13-12-2021, 09:14 PM)Dave Mann Wrote: Before you rush to dismantle the switch I suggest that you refer to the instructions on the Cornwall Austin Seven club website Tony and then only dismantle it while sat in the bath with the plug in and no water.
doesn't it get a bit chilly? Or are you still clothed?
Joined: May 2018 Posts: 2,954 Threads: 558
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Location: Peak District, Derbyshire
Car type: 1929 Chummy, 1930 Chummy, 1930 Ulster Replica, 1934 Ruby
13-12-2021, 11:38 PM
(This post was last modified: 13-12-2021, 11:40 PM by Tony Griffiths.)
It appears not to be the ignition on/off switch, but the switch that selects low/high charge and side and headlamps. Prising the handle upwards appears to make a contact and the ammeter then shows a good charge. The switch lacks a positive action and is "slack" in the lights-on sector.; in other words, it's worn out. I have what seems to be a good spare, with a decent click action, and will fit that - though it's a really fiddly job in a Ruby!
(13-12-2021, 09:14 PM)Dave Mann Wrote: Before you rush to dismantle the switch I suggest that you refer to the instructions on the Cornwall Austin Seven club website Tony and then only dismantle it while sat in the bath with the plug in and no water.
http://www.austin7.org/Technical%20Artic...witch%201/ - Thanks Dave. I have what seems to be a sound replacement and, when that's fitted, I'll think out trying to fix the old one. Perhaps.
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