>-----Original Message-----
>From: Russell McMahon
>Sent: 06 January 2006 14:22
>To: PIC List
>Subject: [EE] Sealed Lead Acid charger~~
>
>
>The LM340/LM7805 or LM317 with scaling resistors that one would 
>otherwise be tempted to use is cheaper BUT have nasty problems with 
>good accuracy without adjustment . EG the LM317 adjustment current is 
>50 uA typical and 100 uA max so even with a stiffish reference divider 
>the variation in set point voltage at 12v can vary by about half a 
>volt depending on initial adjustment current. This is far too much too 
>ignore for SLA float purposes. Also, back feed down reference chain 
>from battery is far too high too be tolerable so a series output diode 
>is needed, detracting from regulation accuracy. PB137 is rated at 10 
>uA draw AFAIR.

How about MOSFETs on the output of the regulator to disconnect it from the battery when no input voltage is present? Something with a nice low Rds(on) should (in my head at least!) cause minimal regulation impairment compared to a diode.  Or even, *shock horror*, a good old electro-mechanical relay?

How about the old LM723, cheap with built in current limiting, through will need an external pass transistor for the kind of currents you require.

Regards

Mike

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