Cell phone charger with status indication and automatic output current adjustment. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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Cell phones come with their own chargers. These chargers cannot be called universal. Since there are many varieties of cell phones, the voltage of their batteries is also different.

So a Motorola cell phone cannot be charged using a Samsung or Sony Ericsson cell phone charger, not only because the phones have different external power connectors, but, most importantly, because these phones have different battery voltage ratings.

Most modern cell phones have a built-in "smart" device that automatically stops charging the battery when it reaches full capacity. Therefore, leaving such cell phones on constant charge from the charger is practically safe for the phone itself and its battery. The same applies to the charger included in the 220 V lighting network.

The consumed current (from the 220 V mains) by the charger is very small, and does not exceed 8 ... 10 mA (with a fully charged battery). Outwardly, you can only fix a slight (up to +30°C) heating of the charger case when charging the phone and cooling of this case in the saturated battery mode.

Such a device can be assembled both according to the "classical" scheme, by lowering the mains voltage with a conventional transformer and regulating the reduced voltage, and according to a more modern pulse circuit, by placing a stabilizer and a high-frequency converter in the high-voltage part of the circuit.

The advantage of the "standard" circuit layout is the simplicity of the regulator circuit and the greater safety in setting up the circuit. But there are also disadvantages that are absent in the pulse circuit - a rather large transformer is needed, a strong heating of the control transistor, and the sensitivity of the circuit to fluctuations in the mains voltage.

Switching power supplies operate at a high frequency - tens of kilohertz, so the transformer can be literally "microscopic" (a cube-shaped transformer with a side of 20 mm outputs up to 3 ... 5 W of useful power to the load, i.e. up to 1 A of current; the current in the high-voltage part of the circuit is a transformation ratio of times (30-40) less than the current in the low-voltage part).

Therefore, the heating of the transistor is also much less, especially since it operates in the key mode; well, thanks to PWM (pulse width modulation), the device will be insensitive to fluctuations in the mains voltage within 150 ... 250 V or more. For those who do not have a regular charger (who bought a used cell phone on sale), a home-made charger with status indication and automatic adjustment of the charging current will be useful.

The electrical circuit of this easy-to-repeat and set-up device is shown in fig. 1.7.

Rice. 1.7. Wiring diagram for cell phone charger with status indication and automatic output current adjustment

The diagram shows a "classic" charger for charging nickel-metal hydride (Ni-MH) and lithium (Li-ion) batteries for cell phones with a nominal voltage of 3,6 ... 3,8 V. However, the range of application of this charger can be significantly expand so that it becomes universal and helps to charge cell phones of other companies (with a different nominal battery voltage). To remake the charger (change the value of the output voltage and current), it is enough to change the values ​​​​of only some elements (VD2, R5, R6) in the circuit diagram - this is written a little further.

To understand the nominal battery voltage of your cell phone, it is enough to remove the top cover of the device and examine the record on the battery. As a rule, rechargeable batteries of Nokia, Motorola, Sony Ericsson and some Samsung models have a nominal voltage of 3,6 ... 3,8 V. This is the most popular voltage among modern cell phone models.

The initial charger current is 100 mA. This value is determined by the output voltage of the secondary winding of the transformer T1 and the resistance value of the resistor R2.

Both of these parameters can be adjusted by selecting a different step-down transformer or other limiting resistor.

The alternating voltage of the lighting network 220 V is lowered by the power transformer T1 to 10 V on the secondary winding, then rectified by a diode rectifier (assembled in a bridge circuit) VD1 and smoothed by an oxide capacitor C1.

The rectified voltage through the current-limiting resistor R2 and the current amplifier on transistors VT2, VT3 (included according to the Darlington circuit) is supplied through the X1 connector to the battery and charges it with a minimum current. At the same time, the LED glows. NI indicates the presence of a charging current in the circuit. If this LED is off, then the battery is fully charged, or there is no contact with the load (battery) in the charging circuit.

The glow of the second indicator LED HL2 at the very beginning of the charging process is not noticeable, because the voltage at the output of the charger is not enough to open the transistor switch VT1.

At the same time, the composite transistor VT2, VT3 is in saturation mode and the charging current is present in the circuit (flows through the battery). As soon as the voltage at the battery contacts reaches a value of 3,8 V (which indicates a fully charged battery), the zener diode VD2 opens, the transistor VT1 also opens and the HL2 LED lights up, and the transistors VT2, VT3 close respectively and the charging current in the battery power circuit (X1 ) decreases to almost zero.

Establishment

For a full and effective adjustment of the device, you will need two identical batteries for a cell phone with a nominal voltage of 3,6 ... 3,8 V.

One battery is completely discharged, and the other is fully charged, respectively, with a standard charger that comes with the cell phone.

The adjustment comes down to setting the maximum charging current and voltage at the output of the device, at which the HL2 LED lights up. This maximum current is set empirically as follows.

A deliberately discharged cell phone is connected to the output of the charger (points A and B, connector X1, see Fig. 1.7) through a series-connected DC milliammeter and a current of 2 mA is set by selecting the resistance of resistor R100. For this purpose, it is convenient to use the M260M pointer milliammeter with a total deflection current of 100 mA. However, you can use another similar device, including the pointer ampervoltmeter (tester) Ts20, Ts4237 (and the like), switched on in the current measurement mode at the limit of 150 ... 250 mA. In this regard, it is not advisable to use a digital tester due to the inertia of reading and indicating indications.

After that (having previously disconnected the charger from the AC mains), the emitter of the transistor VT3 is soldered from other elements of the circuit and, instead of a cell phone with a "dead" battery, a cell phone with a normally charged battery is connected to points A and B in the circuit (for this, the batteries are rearranged in one and the same phone).

Now, by selecting the resistance of resistors R5 and R6, they achieve the ignition of the HL2 LED. After that, the emitter of the transistor VT3 is connected to other elements according to the scheme.

About details

Any transformer T1, designed to be powered by a 220 V 50 Hz lighting network with secondary (secondary) windings that produce a voltage of 10 ... 12 V AC, for example, CCI 277-127 / 220-50, TN1-220-50 and similar .

Transistors VT1, VT2 type KT3 15B-KT3 15E, KT3102A-KT3102B, KT503A-KT503V, KT3117A or similar in electrical characteristics. Transistor VT3 - from the KT801, KT815, KT817, KT819 series with any letter index. There is no need to install this transistor on a heat sink.

To points A and B (on the diagram) solder a standard wire from a cell phone charger of the corresponding model so that the terminal connector on the other end of this wire fits the cell phone connector.

All fixed resistors (except R2) type MLT-0,25, MF-25 or similar. R2 - with a dissipation power of 1 W.

Oxide capacitor C1 type K50-24, K50-29 for an operating voltage of at least 25 V or similar. LEDs HL1, HL2 type. AL307BM. Other LEDs can also be used (to indicate the status in different colors), designed for a current of 5-12 mA.

Diode bridge VD1 - any of the series. KTs402, KTs405, KTs407. Zener diode VD2 determines the voltage at which the charging current of the device will decrease to almost zero. This version requires a zener diode with a stabilization (opening) voltage of 4,5 ... 4,8 V.

The zener diode indicated in the diagram can be replaced by the KS447A or made up of two zener diodes for a lower voltage by turning them on in series. In addition, as noted earlier, the threshold for automatically turning off the charging mode of the device can be corrected by changing the resistance of the voltage divider, consisting of resistors R5 and R6.

Clearance at customs

The elements of the device are mounted on a board made of foil fiberglass in a plastic (dielectric) case, in which two holes are drilled for indicator LEDs.

A good option (used by the author) is to design the device board into a case from a used type battery. AZZZ6 (without step-down transformer).

Author: Kashkarov A.P.

See other articles Section Chargers, batteries, galvanic cells.

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