Using an optocoupler in the feedback circuit of a voltage stabilizer or charger. Scheme, description

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Using an optocoupler in the feedback circuit of a voltage stabilizer or charger. Encyclopedia of radio electronics and electrical engineering

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

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A simple, low-cost circuit that simultaneously functions as a stabilizer and charger for low-capacity batteries can be assembled without the use of complex voltage sensors. In this circuit, the diode (emitter) of the optocoupler, included in a simple feedback circuit, perceives changes in the output voltage. The circuit generates a stabilized output voltage of 12,7 V at a current of 50 mA and can be used to charge batteries while maintaining the current and voltage limits, which are quite easy to change.

The optocoupler is the optimal device in terms of its application as a voltage sensor. The diode perceives the output voltage without loading the circuit and without violating the normal operating mode, and the voltage across it does not change and has a relatively small value for any changes in the charging or load currents.

As shown in the diagram, the diode bridge and capacitor C1 rectify and filter the AC input voltage. Let's assume that the circuit works as a charger.

(click to enlarge)

When the battery is not fully charged, the voltage on it is below 12,7 V (Vz + Vd). This voltage is set by selecting an appropriate silicon zener diode, which is in series with the optocoupler diode. In this case, the 1N2270 series transistor turns on and passes current to the battery. The 1A current is limited mainly by a 220 ohm resistor.

When the battery voltage exceeds (Vz+Vd), the zener diode turns on and current Iz flows through the optocoupler diode, turning on the phototransistor and turning off the series transistor Q. In the absence of a battery, when the circuit is in regulator mode, the current enters the load at a voltage of 12,7 B. In this case, of course, the output current depends mainly on the load resistance.

The ripple voltage is 25 mV in stabilization mode and 1 mV in charging mode. The circuit provides stabilization of 30 mV / V with voltage changes and 8 mV / mA with load changes in the range from 5 to 30 mA. Both parameters can be improved by replacing transistor Q with a compound transistor.

The output voltage and current can be set by appropriate selection of resistors R1 and R2 and a zener diode. In addition, the resistances of the resistors can be determined if the battery capacity (C) in milliamp-hours and the input voltage (on capacitor C1) are given.

It was experimentally found that for the best operation of the circuit, the current Ia should be equal to 0,25 C. This ratio is typical for batteries that consume a relatively large charging current. Assuming the input voltage V_in much greater than the base-emitter voltage drop of transistor Q, we get

R2>V_in/0,25C-R1/h_fe,

where h_fe is the DC gain of transistor Q. To find the minimum value of R2, assume that Io is known, therefore,

R2<(V_in-Vo/Io.

This equation is valid provided that Io>Iz.

The value of R1 depends on the minimum value of Io corresponding to the off state of the transistor Q. It can be shown that

R1>(V_in-Vo)/0,02C.

This equation assumes that the minimum value of Io is about 0,02C - a ratio determined experimentally and not theoretically.

Author: LA Cherkason; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Chargers, batteries, galvanic cells.

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A few years ago, it was suggested that it is time for humanity to think about an imminent shortage, but not about the oil and gas one, which we are usually afraid of, but about the shortage of an alkali metal - lithium. In our life there are more and more electronic devices and all kinds of gadgets. And all of them, from a mobile phone to an electric car, use electrical energy stored in batteries. Most of these are lithium-ion batteries. Today it is the most common type of rechargeable batteries. And although we are unlikely to see wars over lithium deposits in the near future, its cost may increase. And this means that it is time to think about cheaper batteries that would use other cells. Developers are betting on the closest relative of lithium in the periodic system - sodium, as a much more common and inexpensive metal.

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