DC Uninterruped Power Supply

By Mike Rooke


(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.

The Role of B2.Spice A/D

Lets review the challenges

Designing a power supply with 4 DC outputs:
Many reference designs can be found on the Internet. However, estimating total current draw or available voltage when configuring the regulators can be done very quickly using the operating point simulation in B2 Spice. Take an existing design drop it into B2 Spice and customize to your needs!

Battery charge current limiting for "float" operation:
12V gel batteries are typically charged at 13.8V. Aim for 14.0V with no load allowing for the diode voltage drop 13.5 - 13.8V. In this simulation no provision is made to cut the battery power it it falls below a minimum level. R4 & R13 provide crude current limiting in the event the battery is deep discharged. See page 3 for more details.

Simulating the battery:
A large value capacitor has been used. I was not able to find a gel/lead acid battery model at the time of simulation.

Finding a suitable model for the LM317 regulator:
A model can be found in this paper at : LM317 Paper (pdf)

The graph below shows the initial 100ms after power has been turned on. Note that the transformer "thump" due to the inrush of current is present (max 6.362A). The main DC supply ripple is easily obtained by min and max operations. Here it is below 1.3V or +-0.64V.

DC UPS: How B2.Spice Was Used

Trivial examples: 3 front panel LED's were needed to monitor and confirm each of the regulated outputs were active. B2 Spice was used to calculate the current limiting resistor. The components were quickly dropped and wired. A parameter sweep on the current limiting resistor can be easily performed. The correct value can be obtained by checking the manufacturer's recommendation against the graph, at the intersection the resistance will be shown. Alternatively use the operating point mode and modify the resistor value directly. Hit simulate and you will obtain the results in the schematic.

LED circuit (Download circuit: A/D 2000 | A/D v4)

Setting the charge voltage:
Many charging circuits exist for gel batteries. The one presented here is no different. The LM317 voltage output is controlled by the R3 resistor in the diagram below. To obtain the correct value, the datasheet can be consulted and buttons pushed, or a parameter sweep can be performed. The transfer graph shows the point where the output voltage is 13.660V and resistance is just over 2.6K. In the real UPS a variable resistor has been used and the output adjusted with a fully charged battery and monitored until it reached 13.65V.

LM317 circuit (Download circuit: A/D 2000 | A/D v4)

But why a DC UPS?
The whole UPS under load draws 1.2Amps. A 7AH gel battery is sufficient to keep all the author's networking gear up and running for over 5 hours. The main computer UPS has long shutdown before this one runs out of juice, time to break out the laptop!


The ability to really see exactly how a circuit performs before spending money on components and construction is quite an educational experience. The author has come from primarily a digital background with basic analog/passive component knowledge. B2 Spice enabled the simulation of a design concept and helped plan the next component purchases as the project rolled along.

A further improvement to the design would be to replace the LM317's with LM338 devices. This would enable higher loads and also facilitate faster charging. A relay in parallel with the battery diode D5 configured to switch when the main supply failed would remove the diode voltage loss. For situations of long power outage, a protection circuit would be needed to switch off the UPS. This would prevent a highly discharged battery and subsequent damage. Consult the battery manufacture's data sheet to obtain the minimum voltage recommendations.