The charger has protection against short circuits in the load. The device is a simple current source, it additionally includes a reference voltage indicator on the LED and an automatic current shutdown circuit at the end of charging, which is made on a zener diode VD1, a voltage comparator on the op-amp and a switch on transistor VT1.
Schematic electrical diagram.
The level of charging current is set by resistor R7 according to the formula, which you can see in the original article in the picture (click to enlarge).
Operating principle of the charger
The voltage at the non-inverting input of the microcircuit is greater than the voltage at the inverting input. The output voltage of the operational amplifier is close to the supply voltage, transistor VT1 is open and a current of about 10 mA flows through the LED. As the battery charges, the voltage across it increases, which means the voltage at the inverting input also increases. As soon as it exceeds the voltage at the non-inverting input, the comparator will switch to another state, all transistors will close, the LED will go out and the battery will stop charging. The maximum voltage at which battery charging stops is set by resistor R2. To avoid unstable operation of the comparator in the dead zone, you can install a resistor, shown in the dashed line, with a resistance of 100 kOhm.
Most radio amateurs use digital multimeters that are powered by rechargeable batteries or Krona batteries.
At the same time, taking into account the law of meanness, they are always discharged at the most inopportune moment, when the performance of the entire project depends on the accuracy of measurements.
After visiting the store, I decided for myself that using a Krona battery is more economical than constantly buying and keeping a battery in stock. But this is only if the battery is used correctly.
Therefore, a simple charger was required. It can be purchased in many stores. BUT! Like many of you, I am not looking for easy ways. And it’s much more interesting and useful to come up with a scheme, assemble it, and set it up for high-quality work.
This is the charger I got.
This device allows you to charge Krona type batteries – 2 pcs. separate channels with optimal charging current (1/10 of the capacity) and has LED indication.
The indication consists of two LEDs. The 1st indicates that the battery is more than 50% discharged. 2nd – indicates that the battery is charged and can be removed from the device.
In addition, charging a discharged battery occurs in two stages: constant current charging and constant voltage charging.
Let's analyze the operation of the circuit. The circuit is powered by a constant (rectified) voltage from 12 to 30 V. But an increased supply voltage will cause a higher voltage difference across the LM317, which will lead to its heating and the need to install a heatsink. Therefore, I recommend powering the circuit with 12-15 V.
Turning on the LM317 in voltage stabilization mode allows you to obtain a constant (unchangeable) voltage at the output of the microcircuit when the supply voltage changes.
After LM317, a current stabilizer is made using two transistors. When we connect the terminals to a discharged battery, the voltage drop across the 27 ohm resistor significantly exceeds the opening threshold of the second transistor, which leads to the LED turning on and the first transistor partially closing and, thereby, limiting the charge current.
During the charging process of the battery, the voltage drop across the 27 ohm resistor at a certain moment closes the second transistor, which leads to an almost complete opening of the first transistor, which means that almost all of the input voltage goes to the emitter of the transistor, that is, to the output.
This ensures a safe charging current for the battery Krona.
The operational amplifier OP (LM358) acts as a comparator that monitors the voltage at the battery terminals and compares it with the installed variable resistor. As soon as the voltage exceeds the set value, the second LED will light up, indicating that the battery is charged.
We begin the setup by setting the output voltage. To do this, connect a voltmeter to the output terminals (without load) and use a trimmer resistor (in the LM317 stabilizer circuit) to set the voltage to 9.1-9.2V.
Next, to configure the operation of the LED, signaling the end of charging, we connect a voltmeter to the output terminals and connect the Krona battery. As soon as the voltage reaches 9V, rotating the trimming resistor (in the LM358 circuit) turns on the LED. This operation requires quite a lot of patience and precision, so I recommend using multi-turn resistors.
After adjustment, these resistors are covered with varnish or wax to eliminate the possibility of disrupting the previously made adjustment.
The board layout is made taking into account the available parts.
Many people use standard 9V batteries (Krona) to set up or test many of their electronics projects. Of course, 9 volts are not always used - sometimes you need 5, 3 or even less, but it is either impossible or there is no desire to make up from lower-voltage batteries - after all, it’s easier to poke the crown and see how it will work there. And the excess voltage will simply sag due to the weakness of this galvanic element. But it’s better to do it correctly once - and then you won’t be afraid that something will go wrong on the diagram. Next, we suggest assembling miniature battery attachments - power supply boards. They provide the required reduced voltages and have a convenient form factor for use with a 9V battery.
On the printed circuit board there is a microcircuit - a regulator with wiring components on one side, and contacts for a 9 V battery on the other. In short, the idea is that the power supply will become part of the battery itself!
Several options for stabilizer circuits
This option uses a specialized buck converter:
The second version uses a buck/boost converter:
And this is a prototype that uses a cheap linear regulator LM317:
Printed circuit boards are etched, drilled (the radio components themselves are planar) and after desoldering, the board clings to the crown, providing the required voltage at the output.
In this article I will tell you how you can assemble a very cheap power bank from all sorts of rubbish that you should definitely have on hand. More precisely, something similar to a power bank, but it will perform its main function quite well. This portable charger for a smartphone will operate on a battery or Krona battery, 9 or 12 volts.
What is needed for assembly
- Ballast housing for fluorescent lamps
- Crown terminal
- Battery or battery Krona
- USB socket
- Toggle switch or button
- Voltage stabilizer 7805
- Two 100n capacitors
Preparing the case for the power bank
The ballast housing for fluorescent lamps is very convenient for such homemade products, at least it’s just right for my power bank. The crown and USB socket fit into it very well. By the way, you can take any socket; I had a broken car USB charger lying around and I took it from there. The socket is triple and installed on the board, the board itself is exactly the right size for my case, you can see this in the photo below. You can also take a single socket, make a hole for it in the case and fix it with hot glue!
So, there were a lot of unnecessary parts in my case, and using a hacksaw and a blade, I successfully got rid of them. Namely, I cut off one of the fastenings of the case with the base, and on the other side I removed only the fastening.
Next, we make a small hole in the case to output wires to the USB connector and on the other side another hole for the toggle switch. Well, or in any other place, it all depends on the size of the button used, I had a large one, so it fit right there.)
The body preparation is complete, now let's move on to assembly!
Power bank assembly
First we need to assemble a voltage stabilizer, the heart of our bank! To assemble the stabilizer I used the following diagram:
The circuit is very simple, I think it will be easy to assemble. Any 7805 can be used as a stabilizer; I took the KIA7805. We also need two 100n capacitors, but that’s basically it. We solder the circuit by surface mounting, immediately solder two thin insulated wires to the output and a terminal for the crown to the input. Please note that the terminal must be soldered into the gap on the toggle switch so that our power bank can be turned on and off!
We place the assembled circuit in the case. I glued the stabilizer with hot glue near a small hole in order to bring the wires to the USB socket.
After installing the toggle switch, I realized that the crown would not fit into one of the compartments, where it fit very well, and I had to cut off the partition.
Next, we pass the wires through a small hole and solder them to the USB socket. Be careful not to reverse the polarity!
We glue the socket itself to the end of the case using hot glue.
Close the back cover and that’s it, our bank is ready!))
It turned out very compact and neat! Believe it or not, with the help of one new Krona battery, I was able to charge a completely dead smartphone battery and recharge it a couple of times from half charge.
Of course, this power bank is not comparable to the factory one, but still, it works and charges! The case could easily fit two crowns, and if you take a smaller toggle switch, then all three! What I mean is that the capacity of such a power bank can be increased by connecting two or three crowns in parallel. The only disadvantage of this homemade product is the lack of a charging socket for the device itself. This problem can be solved by using Krona batteries; they can be removed and charged separately, and the batteries do not have to be changed every time.
Well, in general, as it is, I present my homemade product to the court, please do not judge harshly!)) This power bank can be thrown into the car and charge your phone when it dies at the most inopportune moment or when the lights are turned off.
Thank you for your attention!))
Among the many schemes for assembling chargers for Krona batteries, I found one that was relatively simple and affordable. By the way, the 9-volt battery, known in Russia and the CIS countries as “Krona,” has a 6F22 standard.
The battery consists of 7 4A nickel-metal hydride batteries connected in series. The recommended charging current is no more than 20-30 mA.
The charger is manufactured by redesigning a Chinese-made mobile phone charger.
There are 2 types of inexpensive chargers originating from China. They are pulsed, and both are based on self-oscillator circuits capable of delivering 5 V output.
The first type is the most common. It does not have control of the output voltage, but by selecting a zener diode, which is located in such circuits in the input circuit near the 1N4148 diode, you can obtain the desired voltage. Usually there are two types - 4.7 and 5.1 V.
To charge the Krona you need a voltage of about 10-11 V. This can be achieved by replacing the zener diode with one that has the appropriate voltage. It is also recommended to change the capacitor, which is located at the charging output. As a rule, it is 10 V. You need to install a 16-25 V capacitor with a capacity of 47-220 μF.
The second type of such circuits has control of the output voltage, implemented by installing an optocoupler and a zener diode.
Take a look at the principle of redesigning the second circuit.
It is necessary to remove all components located after the transformer, and leave only the unit that controls the output voltage. This unit consists of an optocoupler, a pair of resistors and a zener diode.
It is necessary to replace the diode rectifier, since manufacturers claim a charging current of 500 mA, and the maximum diode current is no more than 200 mA, although the peak current is about 450 mA. It's dangerous! In general, you need to install the FR107 diode. Thus, charging will produce the required voltage.
The next thing to do is to assemble a current stabilization unit, using the LM317 microcircuit as a basis. In general, you can get by with one quenching resistor instead of assembling a stabilization unit.
But in this example, preference is given to reliable stabilization, because the Krona battery is not the cheapest.
Resistor R1 affects the stabilization current. The calculation program can be downloaded in the Attached files at the end of the article.
The operating principle of this circuit is as follows:
When the Krona is connected, the LED lights up.
A voltage drop is created across resistor R2. Gradually, the current in the circuit decreases, and the voltage that allows the LED to light suddenly becomes insufficient. It simply goes out.
This occurs at the end of the charging process, when the battery voltage becomes equal to the charger voltage. The charging process stops and the current drops to almost zero.
The LM317 chip does not need to be installed on a radiator, unlike , because the charge current is very tiny.
All that remains is to attach the battery connector to the case, which can be made from a non-working battery.
If you use a DC-DC converter, you will get a charger for the Krona via a USB port. like this.
Attached files: .
Soldering the plug to the shielded audio cable Universal protection for batteries
