The essence of the article is the use of an old computer UPS (uninterruptible power supply) Back-UPS APC 300 (capable of starting without being connected to the mains) as the basis of a home solar power plant.

The role of the UPS is to protect computer equipment from short-term power outages of 220 V. Backup operation is carried out thanks to the battery located inside the UPS. There are different types of uninterruptible power supplies with different operating concepts (for some, the inverter works all the time, for others only when necessary, and others), but it is this one that usually powers the PC directly from the network and switches it to the inverter, and the battery is activated only in case of a power failure. Of course, this will not allow the computer to work for a long time, but at least it can save work and safely shut down the computer.

In this uninterruptible power supply, it is enough to turn on the operating mode with the button without connecting to the network, so that 220 volts is created at the output. In a situation where it is connected to the network and the mains voltage disappears, it works fine, but in order to be able to turn on without being connected to the network at all, you just need to short two contact pads on the board. Other models have a separate “cold start” button, or “cold” start is supported, for example, by holding the main button longer.

Which solar installation needs this UPS? For a 25 W panel, in order to be able to supply current consumers up to 100 W or more. Initially, it is planned to power a laptop and other similar devices from it.

The APC 300 Back-UPS only has a main switch with a standby LED on the front panel. At the back, you can switch between 50 and 60 Hz, as well as between voltages (4 different options) and enable / disable the buzzer.

Only three out of four sockets survive a power loss. One only provides surge protection and has a printer symbol next to it. The battery compartment opens from the bottom. After removing the housing cover, you can see the electronics:

Immediately striking is the huge inverter transformer used to convert 12V to 220V. It may surprise someone why there is no switching power supply? The thing is simple, this power supply gives 50 Hz output, so the most convenient design was to achieve it by manipulating direct current at this frequency through a conventional transformer. This is a very primitive construction, which, by the way, does not give a sinusoid, but a rectangle, but more on that later.

A little lower is hidden the second transformer – the one that charges the batteries.

UPS Circuitry

The full scheme may seem rather complicated, only fragments are given here.

Here we have 4 MOSFET transistors with N channel, 50 V, 0.028 ohm rdsOn, they work in pairs. The gates of Q5 and Q4 are driven together (although each has a separate gate resistor). All transistor sources are grounded, and drains are connected in pairs. This is called push-pull topology.

The middle tap (labeled CT, Center Tap) is in turn connected to 12V from the battery, and the transistors short it to ground (alternating with a certain dead time, sometimes when both are off). Also here you can see anti-interference filters and a relay that switches between mains power and inverter.

The label on the NO relay means “Normally Open” (open when the relay coil is de-energized), and on the NC relay it means “Normally Closed” (vice versa). The rest of the UPS is quite complex. The circuit is implemented without a microcontroller.

We have KA339 (comparator), HEF4066BP (analog switch), CD4001 (NOR CMOS gate), MM74C14N (Schmitt Hex trigger), HEF4011BP (NAND gate) and CS3524AN (PWM controller) among others.

Potentiometers accessible only after removing the housing allow, among other things, to fine-tune the voltage and frequency of operation, which can also be checked on the diagram.

In the case of this power supply, “cold start” can only be enabled by modifying it on the board. Probably, one of its versions had a button for this, and the cheapest version did not have it installed at all. The jumper must be placed on the board as per the diagram.

These types of UPS are not designed for continuous operation. In a typical configuration, the battery usually lasts a few minutes to shut down the computer safely. For this reason, the electronics inside have small heatsinks that are unable to efficiently dissipate large amounts of heat.

Device testing

Before you use your old UPS as a solar power inverter, you should think carefully about overheating it. It is best to test this in practice. For this, several hours of tests were carried out with various loads.

1. Lamp test, 25 W: heating was not noticeable after a long time.
2. The laptop in high performance mode, with the battery charging, consumes about 35 watts, with peaks up to 50 watts: after an hour of operation, the transformer and transistors were the warmest, but they did not exceed 30 ° C.

In practice, the device was used in a summer cottage. There is already a simple solar installation, including:

• solar panel 18V 25W
• 30 A W88-C Solar Charge Controller
• two batteries from the UPS 12 V, 7.2 Ah
• and now this uninterruptible.

The test included the power of an Acer laptop. Basically, full batteries (constantly recharged from the panel) allowed using a laptop for less than 3 hours. The UPS was only slightly warm. Then I managed to recharge the laptop after a break, as the day was sunny and the system worked all the time. In general, thanks to the UPS and its own built-in battery, the laptop worked on the street for more than 5 hours.

Next, it is worth showing what waveform is at the output of this UPS. It does not generate a full sine wave, or even an approximate one, it would be too expensive, in any case, for the loads for which it is designed it would not matter. The output is a dead time rectangle that only pretends to be a sine wave:

This means that it will not be possible to connect to it, for example, an inductive or capacitive load, but in most cases this is not a problem. Under a load of more than 200 watts, the voltage drops slightly.

In general, such equipment is really worthy of respect. After all, he has a separate transformer for charging the battery and all the necessary electronics. And you can also build a wind farm and also charge batteries and get 220 V from such converted UPSs.

Thus, an old computer UPS, although not designed for continuous operation, can still be used as a 12 – 220 V inverter in a low-power solar installation. Commissioning the APC 300 without a mains connection was very simple, and the power supply itself has been working regularly this summer, especially since sunny days are frequent. In addition to the laptop, we load it with chargers for power tools, which also turned out to be very useful in the country.