Low cost dual UPS, for home ADSL routers
and low powered embedded applications

by sv3ora

Recently I wanted to build a small battery backup for my ADSL router, so that when a power failure happened, it wouldn't be disconnected from the internet. This would, among other things, keep my dynamic IP address unchanged for much more time. Not only that, but I also wanted a low powered server I had, to be kept alive on power failures and not to be abnormally shut down, preventing it's OS from crashing. I did not want to spend a fortune on a UPS and since both the router and the server were powered by a 12v PSU, I thought a simpler way to battery backup them. Here is the schematic I ended up with:














The two PSUs, are common wall-types (regulated or unregulated) transformers, for the current that your loads require. They can be switched mode types or not. Obviously switched types will have less weight, but non-switched types will usually provide much less EM noise. Switched types will also provide higher currents whereas non-switched types can reach a maximum of 1A or so and less if they are regulated. However, from my experience, I have seen many switched mode power supplies fail during the years, whereas none of my non-switched PSUs has ever failed. If reliability is important to you, then you might consider using non-switched types, if they are rated for the current of your loads. In my case, reliability was more important, as well as EM noise that had to be kept at low levels in my RF lab, so I have chosen non-switched types. There is no reason why two smaller PSUs must be used instead of a bigger one, other than that the size and especially the height of the transformer is kept smaller by using two smaller PSUs instead of a big one. Also, wall-type PSUs (mine were pulled out of old router equipment), are not rated for currents greater than 1A, but I needed more current, so a second PSU hat to be used. However if you use just one, there is no need to repeat the diode circuit, so you save two diodes. Saying so, the circuit can be extended to as many PSUs as you like. Just make sure your battery can handle the total current drawn by all your devices connected to it.

Diodes D, are 5A Schottky types, for low voltage dropout. The output voltages of the UPS, will be slightly less than 12v, if the input voltage to the diodes is 12v, because of the voltage drop of the diodes. However most routers and embedded systems are quite tolerant in such voltage variations and there should not be a problem.

The volt meter, is a cheap analogue panel-mount volt meter. A digital one is not required, since we only need to roughly estimate the battery life, so as to see if it needs to be replaced or not.

SW1, is a momentary (push-to-make) switch. When closed, the battery voltage is instantly monitored. This operation is very useful, because it allows you to quickly monitor the battery voltage at any time, without disrupting the circuit. It is important to note, that when the circuit operates from the wall PSUs, the unloaded battery voltage is monitored by closing SW1. Likewise, when the circuit operates from the batteries, the loaded battery voltage is monitored by closing SW1. To avoid downtimes, it is a good practice to periodically press this switch to monitor the battery unloaded and loaded voltage, when the mains power is connected and when not.

The battery, is a bank of eight AA or AAA (depending on required current) alkaline batteries, connected in series, for an output voltage of approximately 12v. Rechargeable batteries can be used, but these have to be a low self-discharge types only, so as to keep their charge for a long time, without the need to be recharged periodically. Usually, such batteries have a note "ready to use" on their package, when bought new. SLA batteries, are absolutely no good for this circuit, due to their high self-discharge. Because of low cost, the circuit intentionally provides no way of charging the batteries. If rechargeable batteries are used, ten of them must be connected in series instead of eight, so as to achieve an output voltage of 12v. Note, that the use of rechargeable batteries increases the overall cost of the UPS. This is also because, if using rechargeable batteries, in order to avoid the risk of downtime, you would need a second bank (another 10) of batteries, so that the discharged ones can be replaced instantly with new charged ones. If removing the rechargeable batteries from the UPS and wait for hours to charge them, you risk down dime if a power failure happens at this time interval. For all these reasons, in my version, I used AA alkaline batteries, since the UPS was to be used anyway for periodic short-time power failures, that didn't discharge the batteries very fast.

The enclosure for the circuit is better to be made out of plastic, like any mains equipment, although the wall-type PSUs have their own plastic enclosures.