Quite often there is a need to provide backup power to your device; this article discusses 4 ways to provide this.
Simplest
The easiest way to switch to backup power is 2 diodes
Only one of the diodes will be open, from the power source whose voltage is greater. The advantages of the scheme are simplicity and low cost. The disadvantages of the circuit are obvious: the dependence of the load voltage on the current, the type of diode (Schottky or regular), and temperature. The voltage will always be lower than that of the source by the amount of voltage drop across the diode.
A little more complicated
This circuit is a little more complicated, it works as follows: when the VCC voltage is present, and it is greater than the voltage of the backup source (in this case it is the BT2 battery), then the mosfet is closed, because the voltage at the Gate is higher than at the Source. , the voltage pass to the load and Source is ensured by the opened diode D3. When VCC disappears, the voltage at the Gate will disappear along with it, but the diode inside the mosfet will open, providing voltage at the Source, and since there is now voltage at the Source, but not at the Gate, the transistor will open completely, ensuring switching of the battery without loss of voltage. This method is excellent for switching power for the GSM module, we select the external voltage 4.5V, then 4.2-4.3V will come to the module through diode D3 and the voltage from the battery will flow without loss.
Expensive but no loss
Without voltage loss, you can switch sources using special microcircuits, in particular LTC4412 download datasheet However, this microcircuit can be scarce and expensive.
Optimal lossless
Well, we’ve come to the optimal method, without any losses. First, let's look at the block diagram of the LTC4412
It is immediately clear that there is nothing complicated in it, so why not repeat it on discrete elements? The PowerSorceSelector block is a matrix of two diodes that provides power to the rest of the circuit, A1 is a comparator, AnalogController is not clear what, but we can assume that it does not do anything particularly important; later it will become clear why.
Let's try to depict this.
DA3 is a comparator. It compares the voltages at two sources. Powered by diode D4 or D5. When the voltage at VCC is greater than the battery, the output of the comparator goes high, this closes VT2, and opens VT3 because it is connected to the output through the inverter. Thus, VCC passes to the load without loss. In the case when VCC is less than the battery, the low level at the output of the comparator will close VT3 and open VT2.
I must say a few words about the choice of parts. DA3, DD1 must have a consumption that is acceptable in a given system; the choice is very wide, from a few milliamps to hundreds of nanoamps (for example, MCP6541UT-E/OT and 74LVC1G02). Diodes are necessarily Schottky, if the drop on the diode is higher than the opening threshold of the transistor (and for the IRLML6402TR it can be -0.4V), then it will not be able to close completely.
This device is needed to automatically switch the load between the main (priority) and backup power lines. If it’s really on the fingers, then so that if there is a power outage in a country house, you can start the generator and continue surfing the Internet until the power failure is eliminated :-)
Result: you can take it
Below the cut are several photos with a total volume of approximately 4-5 megabytes
I wanted to write another fascinating review about the power bank, or about the B6 charger, or about the piston from Xiaomi, but that’s in the future. For now, read a boring review about a thing with a motor - Carlson's automatic switch
Although this is a necessary thing, it is quite rarely used in everyday life, so I will try to adhere to the basic principle: “brevity - p. T."
So.
In case of a power outage, the country house has a gasoline generator. There is a special switch in the panel that connects the house either to electricity from the poles or to electricity from the generator. Previously, I used a reversing switch from the well-known company ABB, but this thing is difficult to switch and quite expensive (currently a switch for 63A current costs $100-120). 
A little less than a year ago, 2/3 of the house burned down, so now they have rebuilt it, and again we have to solve the issue of switching between the main line and the generator. I decided to save money and buy a similar device from the Chinese for pennies.
upd. Let me explain separately - the house burned down due to a burnt-out pipe in the bathhouse, and not because of the electrics. And the words about kopecks should be interpreted in such a way that $120 for a simple manual switch, which does not even have a shabby resistor, is, in my opinion, overkill. Its price is $15 on market day, which is what it was calculated on.
In the process of searching, I came across an automatic switch for $16.56, and after a week of deliberation, it was decided to try playing Chinese roulette, since there were no reviews for this thing or any other information, and I didn’t really look for it
There was no free shipping, the cheapest paid shipping was through SPSR, costing $17.25. The Toad, of course, protested, but the total $33.81 is still less than the $100 looming on the horizon, so he showed willpower and clicked the “Buy from this seller” button.
As a result, a week of waiting before sending, then another 12 days of traveling around the continent, and in the end the treasured box lies in a parcel locker 3 minutes from home. For those who haven’t encountered it yet, a parcel locker (post office) is a thing made up of many cells for quickly receiving mail. You enter the secret code from SMS into the terminal, a cell with your product opens, you take it and rejoice 
Packed as standard: a little bubble wrap, a cardboard box, inside a plastic bag the switch itself 
The fact that they charge money for delivery does not mean that it will be treated with any more care. Apparently mine was dropped. The metal, about 1 mm thick, was bent, at least they didn’t drop it with the plastic down, it would probably have cracked 
The first thing that catches your eye is its fairly large weight and dimensions. Height 12 cm, width 15 cm, depth 13 cm. Weight about one and a half kilograms. Thus, in width this module will take up as much as 7.5 DIN space. However, this is not much more than ABB
The kit includes two bolts for fastening the handle and two stickers for it. For what? Apparently, it’s not a pity. Plus the standard “beggar” about “five-star positie feedback” (in the end I gave it 4 stars, but more on that below) 
The second is a body with large gaps. The plastic itself is ordinary, does not smell, and is quite thick. But it is attached to the metal base with weak clips, which is why the gaps between the lid and the base reach 1 mm. 
All you have to do is squeeze the two halves together and the gaps disappear 
Bottom view 
Well, okay, gaps are not a big deal. In the end, it will stand in the shield, where there is especially no need to look. Let's take off our “clothes” in anticipation of technoporn 
Well, not porn, of course, but techno. The simplest device is two packets, two limit switches and a reversible motor. A controversial design, in my opinion, but remembering $16.5, I understand that this is nitpicking 
There are 4 LEDs on the front side of the device. The LED lights up if there is voltage on the corresponding contacts nearby. This way you can monitor the status of the main and backup lines. These LEDs can also be duplicated on the front side of the shield by connecting indicator lamps to the green connector 
Everything looks quite neat, the inner perfectionist is not particularly indignant 
But the soldering was a little disappointing. Uncle Liao minorly sabotaged his monotonous job of soldering LEDs 
One LED is poorly soldered, the solder has fallen off, the leg is dangling, but it’s not very visible in the photo 
The second LED is more sad: they forgot to bite off its leads and hung the noble snot with solder 
Okay, a soldering iron and wire cutters are at hand, it can be removed in half a minute. We inspect the rest of the electronics, there seem to be no problems 
Can be connected to electricity.
The switch works. Moreover, it is quite fast (switching time is around 1 second) and quiet, I expected more noise from such a motor. If you switch the red switch to manual switching mode, the handle turns quite hard, but easier than in the ABB switch mentioned above
To summarize: it’s a completely normal device, I think there will be no problems with it in operation, there’s nothing to break. I recommend, but with a visual inspection before commissioning. On the eve of 11.11 sold raised the price by one and a half bucks, but participates in the promotion with a price of $16.50 :-)
upd. In the comments they recommend replacing Chinese machines with Russian analogues in order to eliminate a potentially weak link, who knows how they are built inside (I will then change and add a disassembled machine to the review)
I forgot to mention one nuance. It's obvious, but still. This thing only switches lines, but does not turn on the generator. Therefore, when the power goes out, all you have to do is go and start the generator (it starts with a button). As soon as there is voltage on the backup line, the automatic transfer switch will transfer the house to power from the generator. If there is power at this time, the switch will switch the house back to it, the generator will need to go and turn off
Automation of this part, although possible, is not relevant, since power outages are mainly associated only with accidents in the city network, which are quite rare and can be eliminated quite quickly
Thank you for your attention. Ask questions, criticize, praise. First post after all
Upd.
Posted a video, thanks madeweb!
Extraneous sounds can be heard in the video - the wife is doing needlework, sewing on a machine. Don't be scared :-)
Upd2.
Although there are circuit breakers here (D63A), the steering knuckle will not allow them to operate in case of current overload. When you turn on the machine, the fist supports it from below, and since the motor itself is very tight, no clicking will occur, the machine will remain in the on state. Keep this in mind.
Upd3.
I was corrected that the circuit breaker should work even when the lever is clamped, so if the circuit breaker is not crooked, the addition of upd2 does not matter
d) Launch Controls. Motor generator installation controls typically provide automatic starting with a main power failure sensor as part of the switching device. In some cases, manual or remote controls are used for facilities and equipment with reduced critical requirements. After starting the engine-generator, the speed and power are automatically regulated by the engine and the electrical load is connected by the switching device. The engine-generator should operate automatically without adjustment or the need to monitor it. Switching to the main power source and stopping the engine can be done automatically or by remote control.
e) Fuel supply. Typically, liquid fuel for backup power is stored in tanks close to the engine-generator location. The capacity of the fuel tanks must correspond to the maximum operating time expected for the engine-generator. Some authorities require food to be provided for a minimum period of 72 hours. Other authorities provide a shorter period of time, but the period of time should generally be at least twice the maximum duration of expected conditions that may require the use of backup power. Fuel tanks and connections must meet all safety requirements and must provide easy access for refueling. These fuel tanks must also have provisions for testing for fuel contamination, especially with regard to the accumulation of water in the tank.
2.3.3 Switching power supply
2.3.3.1 To switch power from the main to the backup source, an appropriate switching device is required. For manual start and control mode, this may correspond to a simple switch or relay that disconnects the load from one power source and connects it to another power source. Automatic switching requires additional controls. As a rule, they are combined into one control unit or panel. Such a unit must sense the failure of the main power supply, begin starting the prime mover of the standby generator installation, determining that the generator voltage and frequency are appropriately stabilized, and connect the load to the generator. This unit can also disconnect non-essential loads and equipment that should not receive power from the backup source and transfer those loads to the primary source once power is restored. Switches or relays for disconnecting and connecting loads must be able to control the rated load of the generator. The operation of these switches or relays is the same for both 2 minute or 15 second and
Regulatory documentation database: www.complexdoc.ru
1 second switching period, although faster relays may be required for shortest switching times. For a 2-minute transfer period, power failure sensors can introduce a delay of several seconds in determining whether the primary power supply has failed or is just jittering, and in determining whether the backup power supply has stabilized. For a 15 second power transfer period, the sensors should respond in less than 3 seconds, as the motors' fast start mode allows 10 seconds to start and stabilize. A transfer time of 1 second or less is too short to start the motor, but the load can be switched from one power source to another running source within this limited period; however, the power failure detection sensor must respond within several periods of AC current.
2.3.4 Uninterruptible Power Supply (UPS) Systems
2.3.4.1 An uninterrupted power supply is required for electronic or other equipment that performs critical functions and requires a constant, uninterrupted power supply to function properly.
2.3.4.2 UPS equipment. An uninterruptible power supply system consists of one or more UPS modules, a charged battery, and the accessories needed to provide reliable, high-quality power. The UPS system isolates the load from the primary and backup sources and, in the event of a power interruption, provides regulated power to the critical load for a specified period of time. (Typically, the battery has the capacity to operate at full load for 15 minutes.) (See Figure 2-2).
a) UPS module. The UPS module is part of the static power conversion of the UPS system and consists of a rectifier, converter and associated controls along with synchronization, protection and auxiliary devices. UPS modules can be designed to operate either separately or in parallel.
b) Reservation. For most operations, a non-redundant UPS system is acceptable. However, if the cost is worth it, a redundant UPS system configuration can be used to protect against module failure or very frequent main power failures (see Figure 2-3).
c) UPS batteries. The battery shall be a heavy-duty industrial unit, lead-cadmium type, having an ampere-hour capacity sufficient to supply the direct current to the converter required by the specifications.
Regulatory documentation database: www.complexdoc.ru
manufacturer to install the UPS system. As a rule, the battery installation is equipped with two-tier racks; however, where space is limited, three-tier shelving may be required.
d) Remote alarm. UPS equipment must be equipped with a remote alarm console installed in the work area served by the UPS unit or in another occupied area, such as a security area. Since UPS equipment rooms are typically unmanned, additional remote alarm devices should be provided to monitor the environmental controls and fire alarm systems of the UPS module and battery rooms.
e) Requirements for premises for placement of UPS equipment and batteries. UPS modules and associated battery installation must be located in separate rooms. The design must be of a permanent type. The wall separating the UPS module room from the battery room must be fire-resistant (withstand fire for one hour). Where feasible, space should be provided in the UPS module and battery rooms to accommodate future installation of additional UPS equipment.
f) Managing external conditions. Both the UPS module room and the battery room must be equipped with an environmental control system to maintain the prescribed room conditions. Each environmental control system must consist of a primary system with the possibility of using a backup system. If the primary environmental control system fails, there shall be an automatic switchover to the backup system and an audible alarm shall be issued to indicate the need for maintenance.
2.3.5 Special backup power devices
2.3.5.1 Other backup power devices that may be used for special facilities include battery backup systems with or without DC-AC converters; solar or wind generators with battery systems and with or without DC/AC converters; independent power generation devices, such as thermoelectric, nuclear or chemical fuel cells; and generators with inertial flywheel. The manufacturer must provide information explaining the operation and nature of the facilities for the use of these devices.
A power failure not only creates inconvenience, but can lead to significant property damage and a threat to human safety. Uninterruptible power supply is provided by two sources of electricity, one of which is usually the mains, and the other is a battery, diesel generator and others.
Reserve connection panel with two independent inputs
Uninterruptible power supply can be created by supplying power from two sources at once. The method has the following disadvantages:
- higher short-circuit current;
- increased electricity losses;
- complication of the protection system.
Automatic transfer of reserve (ATS) allows you to quickly restore the supply of electricity by turning on a switching device that separates the power lines. The actual response time is tens of seconds, but can reach 0.3 seconds. In this case, it is necessary to take into account the power of the additional power source so that it can cope with connecting the consumer system. If this cannot be achieved, the protection circuit is organized in such a way that only the most important loads are connected.
The photo above shows an ATS shield with two independent inputs.
Types and requirements for automatic transfer switches
There are 2 types of ATS switch:
- one-way - one of the power lines is working, and the other is backup;
- two-way - any input can be working or backup.
The ATS is required to have high performance and mandatory switching on, regardless of the reasons for which the voltage has disappeared.
Automatic switching on of the reserve occurs based on a signal from a sensor, for example, a minimum voltage relay. The power supply at the inputs and phase rotation are controlled.
The following requirements apply to the AVR:
- There is no short circuit in the controlled area.
- The ATS is used to connect a reserve whenever the voltage at the input to the consumer disappears. An exception is a short circuit, in which the ATS is blocked.
- One-time operation. The switch cannot be turned on more than once until the short circuit is eliminated.
- Possibility of adjusting the voltage threshold to reduce the impact of voltage drops when starting load motors.
- The switch will only operate if voltage is present in the backup section.
If the listed conditions are met, the ATS logical system sends a command to turn off the input switch and turn on the sectional one. In this case, their simultaneous activation is electrically blocked. Some ATS models are also equipped with a mechanical lock.
ATS operation with a generator
Electricity supply companies divide consumers into three categories based on the degree of reliability of electricity supply. Private houses and apartments belong to the third – lowest category. In apartments, uninterruptible power supplies using batteries are usually used.
For a private home, a gasoline or diesel generator can also be a backup power source. If previously they were put into operation manually, now automatic start-up is possible. It all depends on what price to pay for it.
For automatic backup, it is preferable to use a microprocessor-controlled device. Easy programmable relay controllers are widely used in everyday life and production. The relay input receives signals from voltage sensors. When the power is turned off, the controller starts the generator engine. After reaching the nominal parameters, which takes a certain time, the ATS circuit switches the load to backup power. In this case, there are temporary connection delays. For domestic needs they are acceptable, but for powerful and critical loads the task becomes more complex.
The figure shows an uninterruptible power supply diagram using an additional diesel generator.
Connection diagram of a backup diesel generator to the load
The network and generator are connected to the ATS input, and the output is connected to the load. The main power source is usually the mains. When the network voltage is turned off, the generator starts, after which the ATS connects the load to it. As soon as the power grid is restored, the power switches to the previous mode, and the generator turns off after a specified time. The figure below shows the electrical circuit of an uninterruptible power supply.
Performing ATS on contactors
The circuit is used for a single-phase network of a private house or a small industrial building.
ATS diagram on one contactor for a single-phase network
To put the circuit into operation, automata SF1 and SF2 are turned on. Power is supplied to the contactor KM1 - the switch of the main and backup input. When it is triggered, contact KM1.1 connects the circuit of the main power source, and the backup circuit is opened by contact KM1.2.
The two-pole switch QF1 is turned on, the contacts of which close the circuit of the main power source.
In the event of an emergency, when the main input is de-energized, the contactor KM1 is turned off and the main network is disconnected and the reserve is connected to the normally closed contact KM1.2. When the power to the main input is restored, the loads are switched to it again using a contactor.
If you need to manually connect the reserve, just turn off the SF1 circuit breaker.
The power of the backup source must be taken into account. Typically, it powers the most necessary loads, such as lighting and heating.
Switching the phase and neutral (contacts KM1.1 and KM 1.2 in the figure below) simultaneously makes it possible to completely eliminate the idle input from operation and use an autonomous reserve.
ATS circuit on one contactor with phase and zero disconnection
The switching on of the ATS is carried out as in the previous circuit, only the KM1 switch breaks or connects phase and zero. The circuit is most common for connecting an autonomous voltage source, for example, an uninterruptible power supply or a diesel generator. Here, the connection of loads through two-pole circuit breakers QF2, QF3, QF4 is shown in detail, and the PE grounding wire, which is not connected to the power supply of the loads, is also shown. It connects to the housings of electrical appliances and performs the function of protection against electric shock.
The figure shows a typical connection diagram for the AVR-3/3 module for three-phase power and reserve circuits.
Typical connection diagram for the AVR-3/3 module
The phases on the module are marked L1, L2, L3, neutral – N. The switching contacts of the built-in relays are connected to terminals 11, 12, 14. The device is controlled by a microprocessor that controls the voltage along two three-phase lines.
Video about entering a reserve
You can learn how to assemble an ATS unit for a generator from this video.
Interruptions in the supply of electricity can cause various negative phenomena among consumers. The ATS device allows you to maintain the functionality of objects for which a constant supply of supply voltage is essential.
It could only work when the voltage of the main source disappeared; it could not protect the load from a decrease or increase in voltage. These shortcomings have been corrected in the new version of the device, namely:
- The device will not switch the load to the backup power source even if the main source voltage is low.
The device is not capable of operating at a voltage of less than 6 volts.
The device will not protect the load when the voltage increases above the permissible value.
The new version of the device has significantly improved characteristics.
Capable of operating with main source input voltage from 6 to 15 V.
Load protection from undervoltage or overvoltage. Two comparators are used to control the voltage of the main source. When the main voltage source is turned off, the operation of the device is similar to its previous version.
The current consumed by the load is limited only by the maximum current that the contacts of the electromagnetic relay used can withstand.
The device is powered by a 12 V backup power supply and consumes a current of about 100 mA. If the voltage of the main source is less than 12 volts, you need to use a stabilizer and connect it to the gap shown in the diagram, and also set the protection thresholds using construction resistors.
Device operation
The main source voltage is supplied to resistors R6 and R12 from which the voltage is supplied to the inputs of the comparators, where it is compared with the voltage coming from the stabilizer VR1. A separate stabilizer VR1 is used so that when the voltage of the backup power supply changes, the protection thresholds do not change. I will briefly describe what these trimming resistors are intended for. Resistor R12 is responsible for triggering the protection when the voltage drops below the minimum threshold that is set by this resistor. In my case, this threshold is 10.5 volts, and in order to set it, with an input voltage of 10.5 volts, using this resistor, set the voltage at pin 7 of the comparator to 1.3 V, which is lower than the operating threshold of the comparator, since the voltage at the 6th leg of the microcircuit is 1.65 volts, protection will work immediately. Resistor R6 is responsible for tripping the protection in the event of a critical increase in the voltage of the main source. In my case, the maximum voltage is set at 13 volts. At this voltage, resistor R6 must be set to 4 volts on the 5th leg of the microcircuit, which will trigger the protection and switch the load to the backup source. Thanks to these resistors, the protection is triggered when the voltage drops to 10.5 volts or increases to 13.
The most interesting part of the circuit is the assembly assembled on the DD1 and DD2 microcircuits. It is actually a protection circuit. The two inputs of this node are connected to comparators, but in order for a logical level of 1 to appear at pin 8 of the DD1 microcircuit and the protection to operate, certain conditions must be created. This node is also interesting because a logical one at output 8 of DD1.1 will appear if there are identical logical states at the inputs, either two 0s or two 1s. If there is a 1 at one input and a 0 at the other, the protection will not work.
The protection circuit works as follows. With a normal input voltage of the main source, only the DA1.2 comparator works, since the voltage is above the minimum shutdown threshold and therefore the open output transistor of the DA1.2 comparator closes pins 4 and 5 of the DD2.4 element to ground, which is similar to the logical 0 state, and at the inputs 1 and 2 elements of DD2.3 have a voltage of about 4.5 - 5 volts, which is similar to the state of logical 1, since the voltage does not reach 13 volts and the comparator DA1.1 does not work. Under this condition, the protection will not work. When the voltage of the main source increases to 13 volts, the comparator DA1.1 starts working, the output transistor opens and, by shorting inputs 1 and 2 of DD2.3 to ground, it forcibly creates a logical level of 0, thereby forcing a logical level of 0 to appear at both inputs and the protection is triggered. If the voltage drops below the minimum threshold, then the voltage supplied to the 7th leg of the comparator drops to a level below 1.65 volts, the output transistor will close and stop connecting inputs 4 and 5 of the DD2.4 element to ground, which will lead to voltage setting at inputs 4 and 5 4.5 - 5 volts (level 1). Since DA1.1 no longer works and DA1.2 has stopped, a condition is created under which a logical one level will appear at both inputs of the protection unit and it will work. The operation of the node is shown in more detail in the table. The table shows the logical states at all pins of the microcircuits.
Table of logical states of node elements.
Setting up the device
A correctly assembled device requires minimal adjustment, namely setting protection thresholds. To do this, instead of the main voltage source, you need to connect an regulated power supply to the device and use trimming resistors to set the protection thresholds.
Appearance of the device
Location of parts on the device board.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| DD1, DD2 | Logic IC | K155LA3 | 2 | To notepad | ||
| DA1 | Comparator | LM339-N | 1 | To notepad | ||
| VR1, VR2 | Linear regulator | LM7805 | 2 | To notepad | ||
| VT1 | Bipolar transistor | KT819A | 1 | To notepad | ||
| Rel 1 | Relay | RTE24012 | 1 | To notepad | ||
| R1 | Resistor | 3.3 kOhm | 1 | To notepad | ||
| R2, R3 | Resistor | 1 kOhm | 2 |
