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This circuit was developed to power an AVR microcontroller from a 12 V lead-acid battery. The regulator itself draws only 14 µA. Of course, there are dedicated ICs, for example from Linear Technology or Maxim, which can be used, but these can be very hard to get hold of and are frequently only available in SMD packages these days. These difficulties are simply and quickly avoided using this discrete circuit. Circuit diagram : The series regulator component is the widely-available type BS170 FET. When power is applied it is driven on via R1. When the output voltage reaches 5.1 V, T2 starts to conduct and limits any further rise in the output voltage by pulling down the voltage on the gate of T1. The output voltage can be calculated as follows: UOUT = (ULED + UBE) × (R4 + R2) / R4 where we can set ULED at 1.6 V and UBE at 0.5 V. The temperature coefficients of ULED and UBE can also be incorporated into the formula. The circuit is so simple that of course someone has thought of it before...

Protect your expensive batteries from discharge damage with this mini-sized electronic cutout switch. It uses virtually no power and can be built to suit a wide range of battery voltages. Main Features Disconnects load at preset battery voltage Automatically reconnects load when battery recharged Ultra-low power consumption (<20ma) Miniature size 10A maximum rating Suitable for use with 4.8-12.5V batteries Transient voltage protection (optional) Suitable for use in... Cars, boats & caravans Security systems Emergency lighting Small solar installations Camera battery packs Many other low-power applications Picture of the project: Back in May 2002, we (Silicon Chip) presented the "Battery Guardian", a project designed specifically for protecting 12V car batteries from over-discharge. This unit has proven to be very popular and is still available from kit suppliers. This new design does not supersede the Battery Guardian – at least not when it comes to 12V car batteries....