From: Andreas Mehlhorn (a.mehlhorn@t-online.de)
Date: Sat Sep 28 2002 - 11:53:15 PDT
Patrick Jankowiak schrieb:
> Low tech solution - wire two 15 ohm 15 watt resistors in series, bottom goes to ground. Top goes to MV powerswitch. Middle goes through a 100 volt 5 amp diode ($1.00) to feed the positive terminal of
> the 12V battery. This is an old fashioned voltage splitter. The two resistors will get hot, but mount them under the dash somewhere. The regulation is not as good as the zener diode solution, but it
> is alot cheaper and more 'period' as far as electronics goes.
>
> note: use fixed width font to see this properly, such as 'courier new'
>
> 24V from switch>__/\/\/\/\/\______/\/\/\/\/\____>ground
> |
> _|_
> _\_/_
> |
> |
> 12v battery
Ok, very low tech. It works but remember, that the current you can get to charge your 12 V Battery
is only around 0,2 A (200 milliamperes). At 12 Volts this is 2,4 Watts. Not very much!
Why this: You have 2 x 15 Ohms on 24 Volts, this makes a current of 0,8 Amperes flow through
both resistors. This means, that the two resistors make 19,8 Watts of heat all the time (0,8 A x 24 V)
The current you may take off at the middle is normally 1/5th - 1/4th of the current going through the
resistors. So you can take 0,2 x 0,8 A = 0,16 A or 0,25 x 0,8 A = 0,2 A.
In reality you can take a little bit more but not much. If you take more, you will have less than 12 V
on the middle connector and this means your 12 V battery won't charge.
Best regards
Andreas
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