How to Make Charging for a Screwdriver

When using a screwdriver, users often encounter damage to the charger (charger). First of all, this is due to the instability of the parameters of the electric network to which the charge device is connected, and secondly, with the failure of the battery. This problem is solved in two ways: by buying a new charger for a screwdriver or by self-repairing it.

Types of Chargers

The popularity of the screwdriver is due to the fact that it simplifies the process of twisting or unscrewing the various fastenersa. Characterized by mobility and small size, it is indispensable in the assembly of furniture structures, dismantling equipment, roofing and other construction works. The instrument owes its mobility to the batteries included in its design.

The advantage of using batteries in the possibility of their repeated use. Batteries, giving up the stored energy to the device, periodically themselves need to be recharged. Chargers are used to restore their capacity.

The screwdriver battery is charged in two ways: with an integrated or external charger. The built-in charger allows you to charge the battery without removing it from the screwdriver. The capacity recovery circuit is located directly with the battery. While remote implies their removal and installation in a separate device for charging. The memory is distinguished by the type of recoverable batteries. Used batteries are:

• Nickel cadmium (NiCd);
• Nickel metal hydride (NiMH);
• Lithium-ion (LiIon).

The final cost of a screwdriver, not least depends on the type of battery used and the capabilities of the charger. The batteries are available in 12 volts, 14.4 volts and 18 volts. In addition, memory devices are divided according to their capabilities and may have:

• Indication;
• Fast charge;
• Different type of protection.

The most used memory devices use a slow charge due to the low current. They do not contain an indication of operation in their design and do not turn off automatically. This is more true for built-in capacity recovery devices. Memories built on pulse circuits provide the possibility of an accelerated charge. They are automatically turned off when the required voltage is reached or in the event of an emergency.

Types of Batteries Used

Nickel-cadmium batteries do not experience problems when charging in accelerated mode. Such batteries have a high load capacity, low price and quietly tolerate work at sub-zero temperatures. The disadvantages include: memory effect, toxicity, high self-discharge rate. Therefore, before charging this type of battery, it must be completely discharged. The battery has a high degree of self-discharge. And quickly discharges, even if it is not used. Currently practically not available due to its toxicity. Of all the types they have the smallest capacity.

Nickel-metal hydride surpass NiCd in all respects. They have less self-discharge, less pronounced memory effect. With the same size, they have a large capacity. They contain no toxic material, cadmium. In the price category, this type occupies a middle position, so it is he who is the most common type of capacitive elements in a screwdriver.

Lithium-ion are characterized by high capacity and low self-discharge value. These batteries do not tolerate overheating and deep discharge. In the first case, they are able to explode, and in the second they will not be able to restore their capacity. They are also capable of operating at low temperatures and have no memory effect. The use of memory with a microcontroller made it possible to protect the battery from overcharging, thereby making this type the most attractive for use. At a price they are more expensive than the first two types.

In addition, the main characteristic of batteries is their capacity. The higher this indicator. The longer the screwdriver works. The unit of capacity is milliamps per hour (mA / h). The design of the battery is to connect the batteries in series and place them in a common housing. For Li-Ion, the voltage on one cell is 3.3 volts, for NiCd and NiMH. 1.2 volts.

The principle of operation of the memory

If the memory device fails, it makes sense to first try to restore it. For repairs, it is desirable to have a diagram of the charge device and a multimeter. The circuitry of many charge devices is based on the HCF4060BE chip. Its switching circuit forms a delay of the charge time interval. It includes a crystal oscillator circuit and a 14-bit binary counter, making it easy to implement a timer.

The principle of operation of the charger circuit is easier to understand on a real example. Here’s what she looks like in the Interskol screwdriver:

This circuit is designed to charge 14.4-volt batteries. It has an LED display showing the network connection, LED2 is lit, and the charging process, LED1 is lit. As a counter, a U1 HCF4060BE chip or its analogs is used: TC4060, CD4060. The rectifier is assembled on power diodes VD1-VD4 type 1N5408. The PNP type Q1 transistor operates in key mode, and the control contacts of the S3-12A relay are connected to its terminals. The key is controlled by controller U1.

When the charger is turned on, the alternating voltage of the network 220 volts through the fuse goes to a step-down transformer, at the output of which its value is 18 volts. Then, passing through the diode bridge, it is rectified and gets on a smoothing capacitor C1 with a capacity of 330 microfarads. The voltage on it is 24 volts. During battery connection, the relay contact group is in the open position. The microcircuit U1 is fed through a Zener diode VD6 with a constant signal equal to 12 volts.

When the “Start” button SK1 is pressed, a stabilized signal through the resistor R6 is supplied to the 16th output of the controller U1. The key Q1 opens and through it, current flows to the relay terminals. The contacts of the S3-12A close and the charging process begins. The VD8 diode, connected in parallel with the transistor, protects it from a power surge caused by the disconnection of the relay.

The used SK1 button works without locking. When it is released, all power is supplied through the chain VD7, VD6 and the limiting resistance R6. And also the power is supplied to the LED1 through the resistor R1. The LED lights up, indicating that the charging process has started. The operating time of the U1 chip is set to one hour of operation, after which the power is removed from the transistor Q1 and, accordingly, from the relay. His contact group is broken and the charge current disappears. Led1 goes out.

This charge device is equipped with an overheat protection circuit. Such protection is implemented using a temperature sensor. Thermocouple SA1. If during the process the temperature reaches more than 45 degrees Celsius, the thermocouple will work, the microcircuit will receive a signal and the charge circuit will break. After the end of the process, the voltage at the battery terminals reaches 16.8 volts.

This charging method is not considered intellectual, The charger cannot determine the condition of the battery.. Because of what, the battery life of a screwdriver will decrease due to the development of a memory effect. That is, the battery capacity decreases every time after a charge.

Homemade devices for charging

Doing it yourself for a 12-volt screwdriver yourself is quite simple, by analogy with the one used in the Interskol charger. To do this, you need to use the ability of the thermal relay to break the contact when a certain temperature is reached.

In the circuit, R1 and VD2 are a charge current passage sensor, R1 is designed to protect the diode VD2. When voltage is applied, transistor VT1 opens, current flows through it, and LED LH1 starts to glow. The voltage drops on the chain R1, D1 and is applied to the battery. The charge current passes through the thermal relay. As soon as the temperature of the battery to which the thermal relay is connected exceeds the permissible value, it trips. The relay contacts switch, and the charge current begins to flow through the resistance R4, the LED LH2 lights up, indicating the end of the charge.

Dual Transistor Circuit

Another simple device can be performed on available items. This circuit operates on two transistors KT829 and KT361.

The value of the charge current is controlled by the KT361 transistor to the collector, which is connected to the LED. This transistor also controls the state of the composite element KT829. As soon as the battery capacity starts to increase, the charge current decreases and the LED accordingly goes off smoothly. Resistance R1 sets the maximum current.

The moment the battery is fully charged is determined by the necessary voltage on it. The required value is set by a 10 kΩ resistor. To check it, you will need to put a voltmeter on the battery connection terminals, without connecting it itself. As source of constant voltage, any rectifier block, designed for a current of at least one ampere, is used.

Using a dedicated chip

Screwdriver manufacturers try to lower for their products, often this is achieved by simplifying the memory scheme. But such actions lead to a quick failure of the battery itself. Using a universal chip designed specifically for the memory of the company MAXIM MAX713, you can achieve good performance of the charge process. Here’s what the charger circuit for an 18-volt screwdriver looks like:

The MAX713 microcircuit allows you to charge nickel-cadmium and nickel-metal hydride batteries in fast charge mode, with a current of up to 4 C. It can track battery parameters and, if necessary, reduce current automatically. At the end of charging, the circuit based on the microcircuit practically does not consume battery power. It may interrupt its work on time or when the temperature sensor is triggered.

Hl1 is for power indication, and HL2 is for fast charge display. Setting up the circuit is as follows. First, the charging current is selected, usually its value is equal to 0.5 C, where C is the battery capacity in ampere hours. The PGM1 pin is connected to the plus of the supply voltage (U). The power of the output transistor is calculated by the formula P = (Uin. Ubat) Izar, where:

• Uin. The highest voltage at the input;
• Ubat. Voltage to the battery;
• Izar. Charging current.

The resistance R1 and R6 is calculated by the formulas: R1 = (Uin-5) / 5, R6 = 0.25 / Izar. The choice of time after which the charging current switches off is determined by connecting the PGM2 and PGM3 contacts to different terminals. So, for 22 minutes PGM2 is left unconnected, and PGM3 is connected to U, for 90 minutes PGM3 is connected to the 16 foot of the REF chip. When you need to increase the charging time to 180 minutes PGM3 short-circuit with 12 legs MAX713. The greatest time 264 minutes is achieved by connecting PGM2 with the second leg, and PGM3 with the 12th leg of the chip.

Charge a screwdriver without a charger

It is not difficult to restore the battery without the help of the charger, but many do not know how. You can charge the screwdriver battery without a charger using any constant voltage power supply. Its value should be equal to or slightly greater than the voltage value of the charged battery. For example, for a 12V battery, you can take a rectifier to charge the car. Using terminal clamps and wires, connect, observing the polarity, to each other for thirty minutes, while controlling the temperature of the battery.

And you can refine and power devices with high voltage, using a simple integral stabilizer. The LM317 chip allows you to control the input signal up to 40 volts. Two stabilizers are needed: one is turned on according to the voltage stabilization circuit, and the second. Current. Such a scheme can also be applied when modifying a memory device that does not have nodes for monitoring the charging process.

The circuit works quite simply. During operation, a voltage drop forms on the resistor R1, it is enough for the LED to light up. As you charge, the current in the circuit drops. After some time, the voltage on the stabilizer will be small and the LED will turn off. Resistor Rx sets the highest current. Its power is selected at least 0.25 watts. When using such a scheme, the battery will not be able to overheat, since the device automatically turns off when the battery is fully charged.

Often you can come across bad advice that you can charge the battery using a diode bridge and a 100 W incandescent lamp. It is absolutely impossible to do this, because there is no galvanic isolation and, in addition to fatal electric shock, there is a high probability of a battery explosion.