DC UPS
Or how to keep your networking gear going 24/7

Typical networking devices use "Black Block" type wall adapters to drop the main voltage to 9V-18V or higher. A wall transformer presents a large inductive load for inverters and is not recommended for use with computer UPS systems. The off-line UPS typically comprises a battery charging circuit, fast switch over in the event of utility failure, and an inverter (12V DC to 110/220AC) inverting DC. The inverter can easily be regulated to the desired DC voltage, reducing the need for an inverter and hence overall system cost. The DC UPS presented here forms the focus of a relatively simple real world use case. The challenges:

• Designing a power supply with 4 DC outputs.
• Setting the battery charge current and limiting the charge voltage for continuous "float" operation.
• Simulating the battery
• Finding a suitable model for the LM317 regulator

DC UPS circuit (download: A/D 2000 | A/D v4)

The circuit was drawn following a search for a suitable LM317 model. A number of compromises were made with regards to simulating the transformer and battery. The toridal transformer is modeled using coils. A large value capacitor has been used to represent the battery.

Brief circuit description:
Main power, 3 variable voltage regulators, charger circuit and battery.
Top Left - transformer and rectifier output. This is smoothed by capacitors C1 and C8. The timed switch XX2 is used to represent a power failure! Panel voltage and current meters are shown in the circuit for reference and are actually present in the real UPS. Diode D7 stops reverse voltages coming back from the regulator which can be seen bottom left. The LM317 is configured to provide an charge current of less than 1A. The voltage present in capacitor C10 can be changed to represent a low battery voltage less than 11V or charged battery greater than 13V.
On the right hand section of the circuit the LM317's are configured to provide 9V, 9V and 12V.

Theory of operation:
The main DC line around +24v is feed to the 3 regulators on the right these provide the power for a 3COM Office Connect Dual Speed Switch 8, LinkSys Cable/DSL Router and DSL Modem. The charger circuit continuously float charges a 7AH Gel battery. Diode D5 prevents the MAIN supply line directly charging the battery. Diode D6 prevents the battery attempting to self charge itself via the charger regulator. In the event of a power failure current is drawn via D5 to the main regulators and the equipment connected continues uninterrupted.