Power supply current protection device. Encyclopedia of radio electronics and electrical engineering

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The current protection unit described in this article is designed for a power source, the description of which can be found in [1], which works in conjunction with an output voltage and load current meter [2]. The node differs from other similar devices in that, in addition to performing protection functions, it allows you to set and control the response threshold by the load current meter of the power supply without loading it.

In most current protection devices, the threshold is changed by a variable resistor with a graduated scale or a switch with a set of resistors. In the first case, it is difficult to set the required threshold exactly; in the second case, the number of its possible values ​​is limited by the number of switch positions. In addition, its contacts must withstand the maximum load current, and such switches are quite expensive.

The protective device presented in this article allows you to set the protection threshold in the entire range of operation of the load current meter with the accuracy provided by this meter without any calibration and selection of resistors.

The protective device operates in two modes - limiting the load current and turning off the output voltage when the threshold is exceeded (trigger mode). Its scheme is shown in Fig. 1. It is built on the op amp DA1, included in the scheme of a non-inverting amplifier.

Rice. 1. Protective device diagram (click to enlarge)

An exemplary voltage is supplied to the inverting input of the op-amp from the resistive divider R4-R6. The voltage from the amplifier output of the current measurement unit [2] was used as the input signal of the protection device. While there is no load, at the output of this amplifier, and therefore at the non-inverting input of the op-amp DA1, the voltage is zero. Since the voltage at its inverting input is above zero, the voltage at the output of this op-amp is below zero, the transistor VT1 is closed, and the HL1 LED is off.

With the advent of the load current, the voltage at the non-inverting input of the op-amp increases. As soon as it exceeds the exemplary one, the voltage at the output of the op-amp will become above zero and open the transistor VT1. The latter, opening, shunts the output of the parallel voltage regulator DA1 (Fig. 5 in [2]). The output voltage of the power supply, and with it the load current, decreases until the voltage at the non-inverting input of the op-amp DA1 is equal to the exemplary one. The load current will be limited to a steady state. LED HL1 signals the transition to the current limiting mode.

To switch to trigger mode, you need to close the contacts of the pushbutton switch SB2. In this case, when the load current exceeds the set value, the transistor VT2 will open and a voltage of - 1 V ​​will be supplied to the inverting input of the op-amp DA8. A voltage of about +6 V will be set at the output of the op-amp, the transistor VT1 will fully open, the output voltage of the source will become close to zero. The LED in this mode signals the protection operation. To return the source to the operating mode, it is enough to switch the protection to the current limiting mode for a short time. With the values ​​​​of resistors R4-R6 indicated in the diagram, the threshold for its operation can be adjusted from 20 mA to 2 A. To change this interval, the mentioned resistors are selected.

The R11C7 circuit serves to prevent self-excitation of the op-amp. Although it is most likely not possible to completely eliminate it, the R11C7 circuit significantly reduces the amplitude of the high-frequency alternating voltage at the output of the op-amp. So that the generation does not affect the operation of the remaining nodes, the signal from the output of the op-amp is applied to the base of the transistor VT1 through the filter R2C1. Resistor R1 in the emitter circuit VT1 creates a local negative current feedback.

Shunting the collector-emitter section of transistor VT1 (Fig. 5 in [1]) with a 4,7 μF capacitor for a voltage of 63 V will also help eliminate self-excitation. The absence of acoustic source noise indirectly indicates that there is no self-excitation. And self-excitation is accompanied by characteristic sounds, well perceived by ear. In any case, you should check the range of output voltage ripples in the current limiting mode with an oscilloscope and, by selecting corrective circuits, minimize it. It may be necessary to stabilize the supply voltage of the op-amp.

It should be noted that the use of the R11C7 circuit and the resistor R1 is not always required. In one of the copies of the protection device, they did not have to be installed at all, although the amplitude of ripples with a frequency of more than 200 kHz at the output of the op-amp DA1 reached 100 mV. The criterion is the amplitude of pulsations at the output of the source. If, during its operation in the current limiting mode, it does not exceed 10 ... 15 mV, the operation of the protection node can be considered satisfactory, since such a mode is considered emergency in most cases.

The R11C7 circuit and the R1 resistor can also be omitted if the source is not expected to operate in the current limiting mode, but only the trigger mode is required. In this case, the collector of the transistor VT2 should be connected directly to pin 2 of DA1, and the switch SB2 should be replaced with a switch by turning it into a break in the wire connecting resistor R9 to pin 3 of DA1 according to the diagram shown in Fig. 2. With trigger protection disabled, source [1] output current will be limited to about 2,5A.

Rice. 2. The connection diagram of the resistor R9 with pin 3 DA1

Since at a load current equal to the threshold, the voltages at the inputs of the op-amp are equal, in order to determine the threshold for the protection operation, it is enough to measure the voltage on the engine of the variable resistor R5 relative to the negative load wire. To do this, in the meter [2] you should break the circuit between the output of the op-amp DA1 and the resistor R10 and bring the wires to the contacts of the switch SB1. The protection current can be measured in any operating mode.

The protection device is powered by a voltage converter built into the meter [2]. Its power is sufficient for this. Of course, the best option is to use additional secondary windings of the power transformer with appropriate rectifiers and stabilizers instead of the converter.

The power supply, built from the nodes described in [1] and [2], with the proposed protection device is not without drawbacks. Firstly, when it is connected to the network, a voltage pulse appears at the output, the amplitude of which does not exceed the set output voltage. This is a consequence of the power supply of the protection unit from the voltage converter. It starts later than the power supply, so transients in the protection node occur with a delay. At the moment the converter is started, a voltage of +1 V briefly appears at the output of the op-amp DA6 and the transistor VT1 opens, which causes the appearance of a pulse.

Another disadvantage is due to the same reason as the first one, but it manifests itself when the trigger protection mode is on. When power is applied, a voltage pulse appears, the amplitude of which does not exceed the set output voltage, and then the source turns off. If the protection node and the meter are powered from additional windings of the mains transformer, these effects are less pronounced.

To eliminate the influence of these shortcomings, you can simply not turn on the trigger mode and not connect the load until the output voltage of the unit is established. But to completely get rid of them will help the circuit, the diagram of which is shown in Fig. 3. At the moment the unit is connected to the network, the capacitor C9 is discharged, a negative voltage is supplied to the non-inverting input of the op-amp DA1 through the diode VD1, so a pulse does not appear at its output. As the capacitor charges, the voltage across it gradually increases. When it becomes greater than at the input of the op-amp, the diode VD1 will be closed, and the capacitor C9 through the resistor R12 will be charged to the total voltage at the outputs of the converter (16 V) and will no longer affect the further operation of the device. Diode VD2 serves to accelerate the discharge of capacitor C9 when the power is turned off. The time constant of the C9R12 circuit should be chosen to be the minimum at which the trigger protection does not work at the moment the source is connected to the network.

Rice. 3. Circuit diagram

The printed circuit board for the protection node was not developed. When equipping the power supply [1] with this node, instead of the variable resistor R11' (Fig. 3 in [1]), set a constant value of 3,6 kOhm, and exclude the resistor R11''.

The protection unit uses MLT resistors and imported oxide capacitors. Variable resistor - SP3-40. KT3102E transistors can be replaced with SS9014, and instead of the KR140UD708 op-amp, use imported analogues or other domestic op-amps, for example, KR1408UD1A. An op amp with a low slew rate should be preferred.

Literature

1. Gerasimov E. Laboratory power supply from a dot matrix printer power supply. - Radio, 2016, No. 7, p. 24-26.
2. Gerasimov E. Voltage and current meter. - Radio, 2016, No. 5 p. 29-31.

Author: E. Gerasimov

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