MRE 12 Volt UPS Circuit
The project was to build a UPS relay switchover for a 12V router from junk box parts. I took a 120VAC relay and wired my 12VDC Power Supply and 12V battery through the relay in a simple fashion using a capacitor to maintain voltage to the router while the points were in between contacts and wa-la, it didn't work. It turns out these little switching power supply warts have a delay of about 1 second when AC power is applied before they start outputting DC power. When the relay switched back from the battery, the PS was not "on" yet and the router rebooted...
The first solution was to ditch the 12V switching PS for an old regulated linear supply that was "instant on" and sure enough, this worked, but I really wanted to use the original PS that came with the device. I dug up a relay with a 12VDC coil and wired the coil directly to my power supply thinking that it could not pull in the relay until it was producing output so all would be well when the switchover occurred. It didn't work. The problem with this design is the filter capacitors in the DC supply, the router, and my points capacitor itself, maintained DC voltage on the circuit long after the AC power had failed. The relay did not drop out until the voltage was around 5V ... and the router rebooted.
It became obvious that if I was going to operate with a 12VDC relay a circuit to monitor the 12V power supply voltage was needed. I liked using the low voltage relay since the whole device could be contained in a small box with no need for a connection to the AC power line. After some research and experimentation I settled on using a voltage comparator chip and decided that instead of having the relay energized during normal AC power operation I would have it energize only in backup power mode. The circuit shown in the schematic diagram on this page is my current design.
The LM393 was selected because I had one on an old circuit board. It turns out that it works nicely in this application. R1 can be adjusted for a hair trigger but it is suggested that the power supply voltage be allowed to drop a few tenths of a volt before switching the relay. Setting it too close can cause unwanted switching and even oscillation. Most digital devices, like my router, operate internally with a 5.1 or 3.3V regulator and can tolerate a substantial supply voltage drop without problems. The circuit above is working very reliably so far and the added LEDs indicate the mode in which the UPS is operating at a glance. Resistor R3 was chosen to saturate the transistor with a 75ma coil load. If you use a different relay or don't have the exact resistor some recalculation may be in order but in most cases a 1500 to 2200 ohm resistor should work. Any suitable PNP switching transistor such as a 2n2907 can be used in the circuit. Capacitor C1 maintains output voltage while the points are in between contacts. For most small loads, like a router, 500 to 1000uf should suffice. Zener diode ZD1 was selected to provide a reference voltage of 5.1V for three reasons. (1.) They are plentiful and cheap. (2.) The voltage is near the mid-point of the 12V supply. (3.) The voltage is below the pull-in voltage of the relay which prevents the relay from falsely triggering in the case of low battery voltage. That said, any 4 to 6 volt 0.5W Zener would work for this circuit. Adjust R1 accordingly if you use a different Zener.
I hope you find this information useful. As usual I end by noting the fact that I am not an electrical engineer, just a hack tinkerer ham radio guy so take that into account when considering this design. If you damage your equipment or burn down the block trying to use something you find on this site, don't come crying to me. Heck, how do you think my junk box gets supplied? Enjoy!
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THE ABOVE CIRCUIT AND TECHNIQUE FOR IMPLEMENTING A 12VDC UPS SWITCH SHOULD BE DONE AT YOUR OWN RISK. THE AUTHOR, THIS WEBSITE, NOR THE WEB HOST ASSUMES ANY RESPONSIBILITY FOR THE SAFETY OR ACCURACY OF THIS INFORMATION. THERE IS NO WARRANTY EITHER EXPRESSED OR IMPLIED THAT THIS METHOD AND PROCEDURE IS SAFE AND NO LIABILITY WILL BE BORNE BY ANY PARTY INVOLVED IN PREPARING OR PUBLISHING THIS DOCUMENT. 12VDC SWITCH FAILURE MAY RESULT IN DAMAGE TO THE EQUIPMENT ON WHICH IT IS IMPLEMENTED AND OR PERSONAL INJURY MAY RESULT. THIS CIRCUIT AND SYSTEM SHOULD BE CONSIDERED EXPERIMENTAL AND IS OFFERED HERE FOR INFORMATIONAL PURPOSES ONLY.
USE AT YOUR OWN RISK!