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MG TD TF 1500 - Keeping the Clock Ticking

I have a battery master switch which isolates all the electrics including the clock. I know I could run a specific wire from the input side of the switch to the clock but this defeats the reason for the switch. I wonder if any of you electrical gurus out there have ever designed a circuit using a rechargeable battery which would permanently drive the clock and be recharged when the main battery was switched on. I know small, rechargeable batteries are now easily obtainable. Another option may be to simply use a small, normal battery and simply replace it when it runs out.


Jan T
J Targosz

Leave the connection to the clock permanently live and put a suitable fuse in the line?
Ian Bowers

Jan, I can see that being accomplished with only a few parts, e.g., rechargeable 12v battery, diode and resistor. A couple of questions - which side of your battery do you disconnect? How long a period of time are you thinking about? Positive/negative ground? Bud
Bud Krueger

why not just put a battery tender on? What is the purpose of the master disconnect?
L Rutt

Or wire in a 12v rechargeable battery with a 12v charger. The charger will keep the clock battery charged when power is connected. Bulkier but easy.
Geoffrey M Baker

Is your clock original or does it have a transistor modification or mechanism? If modified, great. If original, the carbon brush burns the balance wheel contact pin off over time. If the battery is disconnected and the clock stops it will last many years longer. George
George Butz

Or you could put a PolySwitch pPTC across the master shut off switch that will allow up to about 5 amps of current flow as a keep alive circuit (clock, presets in a modern radio, etc), but it someone tries to start the car, the switch will trip. After a few tries the person trying to start the car will give up and after a minute or so, the switch will reset itself. Rick Astley has information about this component and I have a stock of them that I can provide for people.

Personally, I am not so sure that all these anti-theft items are all that necessary - we have been driving our TD all over the place for the past 41 years, parking it just about everywhere and have never had any theft attempts at all. Besides the TD came with the best anti theft device from the factory - it's called a flyoff hand brake. I have yet to see anyone, not familiar with the hand brake in the T series cars, figure out how to release the hand brake. Cheers - Dave
D.W. DuBois

Hi Bud,

The car is converted to negative earth and the master switch breaks the wire to earth. The clock is original. I want to switch all the electrics off more for safety than security and I suppose it could be off for a week or so. I went into Maplins today and they sell a unit to connect between a solar panel and a battery which breaks the circuit when the battery is charged. It was expensive though at £50 and bulky. As an asside I wonder how many years it will take to recover the £50 when connected to a solar panel.


Jan
J Targosz

As has been said the cheapest solution and probably the most effective is to fit a fused supply bypassing the masterswitch. The fuse needs to be the smallest that will sustain constant operation - I suspect something like 100mA.
Dave H.
Dave Hill

Jan, perhaps there's something I've not thought of, but I think it's not a complicated problem. Your major expense should be the rechargeable battery that could power the clock for a week or two. You already have a battery charger, it's your generator. You'll need a common diode, a 120 ohm resistor, and a fuse holder w/.1A fuse. The diode will isolate the car battery from the rechargeable battery. The 120 ohm resistor will limit any current to no more than about 100mA. The fuse can act as a switch and protect against shorts. Here's a rough sketch of what the schematic would look like. Bud



Bud Krueger

/Quote

Besides the TD came with the best anti theft device from the factory - it's called a flyoff hand brake. I have yet to see anyone, not familiar with the hand brake in the T series cars, figure out how to release the hand brake.

/end quote

And what about starting it? How many people know to pull the knobs for the choke and starter? :-)
Christopher Couper

I found this while designing a system for one of my other cars to protect a small auxiliary battery from discharge while the engine is being cranked and using it to supply power to the ECU:

http://www.caravantechnology.com/%2F12V-Voltage-Sensitive-Relay-P125.aspx
R J Woollaston

LOL, The starting procedure for the T series reminds me of a preflight check list, brakes on, mixture rich, throttles set, prime engines, start. I've thought of this many times. Funny. PJ
Paul S Jennings

This thread was discussed between 02/06/2015 and 03/06/2015

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