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Power supply with battery backup for DMM - del20010This is a simple charger circuit which will work for a light load like a DMM, the Battery can be a sealed maintenance free battery of 9V-2AH or better. The circuit will work best if the Unit is powered on many times daily on regular use, else battery will drain down. Voltage Regulators LM7812 and LM317 The 555 Astable is used to generate a AC signal from which a negative voltage is generated, A 79L05 which is a low power TO92 equivalent of 7905 a negative -5 volts regulator is used as -5 volts load is less. A TO220 7805 is used for the +5V supply.  To view circuit below, Click the Link of PNG or PDF and view the Circuit, PNG can be Scrolled with Mouse |
| PNG-Schematic
PDF-Circuit DSN-Source |
| Power supply with battery backup for DMM - del20010 |
Many dual supplies are derived from one DC Source. A SMPS solution is the best. A Series Regulator is simple to troubleshoot in comparison to Switching Types, there is no EMI-RFI too. +5 to -5 using a 555 Astable Multivibrator This simple circuit is ok only for Low Current gadgets, Whereas SMPS is green and efficient. In SMPS a greater care for Product Safety is required. In a Linear Supply with Step-Down Mains-Frequency Transformer. The Transformer is the only place, where you look into safety the most. In SMPS it is the PCB, the feedback components and also The High-Frequency (200 kHz) Mains Ferrite Transformer. |