Laboratory power supply with integrated protection. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Laboratory power supply with integrated protection. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Every radio amateur involved in the development and repair of radio equipment would like to have a universal power supply in his laboratory, as they say, for all occasions. Such a source must have a widely adjustable output voltage, high current, high voltage stability, low ripple, reliable protection (against overcurrent, overvoltage and overheating), which guarantees the safety of both the powered equipment and the source itself. The power supply should be simple and not contain scarce, expensive and bulky components.

Attempts to find a description of the finished device that meets the above requirements were unsuccessful, so the author had to develop such a unit on his own. What came of it, judge for yourself.

The main attention in the development of the proposed laboratory power supply (PSU) was given to the protection unit. According to the author, to ensure maximum reliability, electronic and electromechanical protection should be used in combination.

In the described power supply unit, protection units for current, against overvoltage at the output, as well as thermal ones are implemented.

To protect radio equipment from electrical damage in a wide range of load current, current protection must be adjustable. During the development process, certain difficulties arose with the implementation of the current sensor. In the classic version, this is a resistor included in the power circuit, the voltage drop across which is monitored by the protection control unit. To implement an adjustable current sensor, a variable resistor of a very high power with a resistance from units to tenths and even hundredths of an ohm would be required. So, for example, with a current sensor resistance of 0,1 Ohm and a current of 15 A, more than 20 W of power is dissipated on it! There is an option with switching resistors, but in this case, the switch must withstand the maximum load current. In addition, the resistance of the switch contacts is unstable and commensurate with the resistance of the switched resistors, therefore, the protection threshold will be unstable, and the switch itself will be very bulky. Of course, you can use a fixed resistor of very low resistance and amplify the voltage drop across it with an adjustable DC amplifier, but in this embodiment, the device will become much more complicated.

The solution appeared after reading the article [1] and is as follows: an additional winding is wound on the body of the reed relay RES-55, which is included in the power supply circuit before the stabilizer. The direction of the current in the main and additional windings of the relay is chosen so that the magnetic fields created by them are summed. Then, by changing the current in the main winding, you can adjust the level of operation of the PSU current protection.

In overvoltage protection nodes at the output, a powerful zener diode or trinistor is usually used, which, at increased voltage, open and close the PSU output. As a result of a sharp increase in current, a fuse installed in the power circuit is activated.

In the proposed unit for protection against overvoltage at the output of the power supply unit, an additional low-power stabilizer is introduced with the same output voltage regulation law as that of the main stabilizer. The output voltage of the additional stabilizer should be slightly higher than the main stabilizer. Both voltages are supplied to the simplest comparison node. Exceeding the voltage at the output of the main stabilizer leads to the operation of the protection.

The thermal protection unit is assembled on thermal switches.

The main technical characteristics of the PSU:

Output voltage regulation intervals, V......1,2...15; 1,2...30
Maximum load current (in the range of 1,2 ... 30 V is guaranteed at a voltage of 15 ... 30 V), A ...... 15
Voltage stabilization factor, not less than ..... 100
Ripple level at an output voltage of 12 V and a load current of 10 A, mV, no more than......30
Protection actuation current regulation interval, А......0,5...15
Exceeding the output voltage at which the protection is triggered, V, not more than ...... 2
Cooling fan activation temperature, °С......50
Temperature of operation of thermal protection, °С......60

The power supply circuit is shown in the figure. From the secondary winding of the network transformer T1, the alternating voltage is supplied to the rectifier bridge VD1. The output voltage intervals are switched by jumper S1: in the left position according to the diagram - 1,5 ... 15 V; in the right - 1,2 ... 30 V. Capacitors C1-C4 reduce multiplicative interference. The rectified voltage, smoothed by capacitors C6-C9, is fed to the inputs of the main and additional stabilizers, which are assembled on DA3 and DA1 microcircuits, connected according to a typical circuit [2]. To increase the output current of the main stabilizer, control transistors VT1-VT4 are used, in the emitter circuits of which current-leveling resistors R9-R12 are installed. Diodes VD2, VD3, VD10 and VD11 are protective. The output voltage of the main and additional stabilizers is regulated by a dual variable resistor R2. Resistor R3 sets the minimum excess voltage of the additional stabilizer over the voltage of the main one, which is necessary for the correct operation of the protection unit.

(click to enlarge)

The voltage at the PSU output is measured with a voltmeter PV1, and the output current is measured with an ammeter RA1.

To increase the stability of the operation, the current protection unit is fed from the DA2 stabilizer. Resistor R4 regulates the current in the main winding 1-2 of the reed relay K1, as a result of which the operating current in the additional winding 3-4 changes. If the output current of the PSU exceeds the set value, relay K1 will work, contacts K1 1 will turn on relay K2 and self-blocking through diode VD8. Relay K2 will work and contacts K2.1 will disconnect the main stabilizer from the rectifier. In this case, the color of the HL1 LED will change from green to red and the sound alarm will turn on (sound emitter HA1 with a built-in generator). The audible alarm can be turned off with the SA3 switch. After eliminating the cause of the current protection operation, the PSU is returned to its original state by pressing the SB1 "Reset" button. Diodes VD7 and VD9 limit the self-induction voltage of the relay windings K1 and K2.

In the node for comparing the voltages of the main and additional stabilizers, a thyristor optocoupler U1 is used. The stabilizer voltages are applied to the emitting diode of the optocoupler, which is closed in the initial state. If the voltage at the output of the main stabilizer increases for any reason, the thyristor of the optocoupler will open, which will trigger the protection, as described above. Diodes VD4-VD6 protect the emitting diode of the optocoupler from overload, and the resistor R8 limits the current.

Thermal protection is provided on thermal switches SF1 and SF2. The SF1 switch is activated if the heat sink temperature has reached 50 °C and turns on the M1 fan motor. If the temperature of the heat sink continues to increase, at 60 °C, the SF2 switch will trip, which will turn on the protection. The fan motor M1 can be forcibly turned on by switch SA2.

The main element that determines the electrical parameters and dimensions of the PSU is the network transformer T1. The author used a ready-made rod transformer with an overall power of approximately 600 W, having a secondary winding with an output voltage of 30 V with an average output. In the PSU, you can use any transformer with the necessary characteristics.

Diode bridge MB351 (VD1) can be replaced by any rectifier of the MB or KVRS series. In extreme cases, the bridge can be assembled from individual diodes that provide the required load current.

The output voltage interval switch S1 is made of three instrument terminals connected by a jumper.

KR142EN22A stabilizers can be replaced by any of this series or imported analogues of the SD1083 DV1083, LT1083, SD1084, DV1084, LT1084 series, and the KR142EN8B stabilizer can be replaced by an imported analogue 7812.

Relay K1 - RES-55B version RS4.569.600-00 (passport RS4.569.626). The relay versions RS4.569.600-05 (passport RS4.569.631), RS4.569.600-01 (passport RS4.569.627) and RS4.569.600-06 (passport RS4.569.632) are also suitable. If the relay does not operate at a voltage of 12 V, the voltage of the DA2 stabilizer must be increased until the relay reliably operates (with a margin of 1,5 ... 2 V) by connecting one or two low-power silicon diodes between output 2 of the microcircuit and the common wire. The output of the relay housing is removed . An additional winding is wound on the relay case with a PETV (PEV) wire. When choosing a wire diameter, one should focus on a current density of 10 A / mm. In the author's version, an additional winding contains 16 turns of wire with a diameter of 1,4 mm. The winding is fixed with a heat shrink tube. The calculated winding resistance is 0,006 Ohm, the voltage drop at a current of 15 A is 0,09 V, the maximum power dissipation is 1,35 W. Relay K2 - automotive 90.3747-01, capable of switching current up to 30 A. Thermal switches SF1 and SF2 - RB5-2 with a response temperature of 60 ° C, previously widely used in EC computers. One switch is adjusted to a response temperature of 50 °C. Thermal switches can be replaced with imported B1009 for the appropriate temperature, but since their contacts are NC, they must be switched on through inverters. The M1 electric motor is a fan used to cool the power supplies of IBM computers.

The ALC331A (HL1) LED can be replaced with an imported two-color one or any two single-color ones (red and green, respectively). Transistors KT818GM (VT1-VT4) are replaceable by powerful p-n-p transistors with a maximum power dissipation of at least 100 W, for example, from the KT825, KT865, KT8102 series.

Resistors R9-R12 - C5-16MV with a power of 2 watts. They can be replaced with home-made ones made of nichrome wire with a diameter of 0,8 ... 1 mm. You can do without these resistors, if you select transistors according to the equality of collector currents at equal base-emitter voltages.

For reliability reasons, wire-wound variable resistors PPZ-45 (R2, R4) and trimmer multi-turn resistors SP5-ZV (R3, R5, R13, R17) were used, but they can be replaced by any. Diodes KD522A (VD3-VD8, VD11) are replaceable by any silicon low-power ones, and diodes KD258A (VD2, VD9, VD10) - by any with a maximum current of at least 1 A.

To measure the voltage and current, M4203 measuring heads with a resistance of 500 Ω and a total deviation current of 1 mA were used. The use of other measuring heads will require recalculation of the resistance of resistors R13, R16, R17.

Capacitors C6-C9 - K50-37, but it is permissible to use any others. It should be remembered that their total capacitance must be at least 2000 microfarads for each ampere of load current, and the rated voltage must exceed the output voltage of the rectifier at the maximum mains supply voltage.

Capacitors C5, C10-C12, C14 - tantalum K52-1, K52-2 and K53-1A. In the case of using aluminum oxide capacitors, their capacitance should be increased several times. The remaining capacitors are any ceramic ones.

Switch SA1 - T2 or another, rated for a current of at least 3 A. Switches SA2, SA3 - MT1, button SB1 - KM-1, but they can be replaced by any others.

Instead of the thyristor optocoupler AOU103A, it is permissible to use any optocoupler from the AOU115 series.

The PSU is assembled in a rectangular metal case measuring 230x120x300 mm. Ventilation holes are drilled in the top, bottom and side panels of the case. The front panel is equipped with measuring devices, output terminals, terminals of the output voltage interval switch, a mains switch, switches for the fan motor and sound alarm, regulators for the output voltage R2 and protection trip current R4, as well as a protection trip alarm LED.

The back panel is made of 3 mm aluminum. On it, through mica gaskets coated on both sides with KPT-8 paste, transistors VT1-VT4, microcircuits DA1-DA3, a rectifier bridge VD1 and thermal switches are fixed. The fan is mounted on the rear panel above the VT1-VT4 transistors on the racks. Ventilation holes are drilled in free places under it. Fuses FU1 and FU2 are also placed on the rear panel.

Mounting of the device is mainly hinged, on terminals and insulating racks. The installation of power circuits is made with a stranded wire with a cross section of 2,5 mm2 of minimum length. Capacitors C6-C9 are screwed to the board made of foil fiberglass, which is attached to the side panel with brackets. A copper wire with a diameter of 1,4 mm is soldered to the printed conductors between the terminals of the capacitors along the entire length. The transformer is fixed on the bottom panel with the help of corners.

Establishing a PSU comes down to adjusting the protection unit and calibrating the ammeter and voltmeter. This will require a 35 V voltmeter, a 20 A ammeter, an auxiliary regulated power supply with a maximum output voltage of 35 V, and variable load resistors (rheostats) with a resistance of 10 and 100 ohms or equivalent load. The protection unit is adjusted in the following sequence.

1. First adjust the overvoltage protection unit.

1.1. The variable resistor slider R4 is set to the maximum resistance position.

1.2 Connect a voltmeter with a positive lead to the output of the stabilizer DA1, and with a negative lead to the output of the stabilizer DA3.

1.3. By changing the output voltage of the PSU within the intervals 1 2 ... 15 and 1,2 ... 30 V, using the resistor R3, they ensure that the measured voltage is always positive, and its value is minimal and not exceeding 1,5 V. If this could not be done, the resistors R2.1 and R2.2 should be swapped or the resistor R2 should be selected with a smaller mismatch.

1.4. Set the PSU output voltage to 30 V

1.5. The output of the resistor R8, right according to the scheme, is disconnected from the PSU output and a voltage (slightly less than 30 V) is applied to it from an auxiliary source.

1.6. By smoothly increasing the voltage of the auxiliary source, the moment of protection operation is fixed by changing the color of the LED glow. The output voltage of the auxiliary source in this case should not exceed 32 V.

1.7. Restore the connection of the resistor R8 with the PSU output.

The serviceability of overvoltage protection can also be checked during operation. The capacitance of the capacitor C12 of the main stabilizer DA3 is greater than the capacitance of the similar capacitor C5 in the additional stabilizer DA1. The increased capacitance helps to reduce the level of ripple at the output of the main stabilizer, but at the same time increases the inertia of adjusting the PSU output voltage. If the slider of the resistor R2 is sharply turned in the direction of decreasing the voltage, then due to the larger capacitance, the output voltage of the PSU will briefly exceed the output voltage of the stabilizer DA1, which will trigger the protection.

2. Then adjust the current protection unit.

2.1. The circuits are broken between resistors R4 and R5, between terminal 4 of the additional winding of relay K1 and contacts K2.1 of relay K2.

2.2. Between terminal 4 of the additional winding of relay K1 and a common wire, a load resistor with a resistance of 10 ohms and an ammeter connected in series are connected.

2.3. By reducing the resistance of the load resistor, measure the protection current, which should be within 16 ... 18 A. This is achieved by changing the number of turns of the additional winding 3-4 of relay K1.

2.4. Restore the connection of resistors R4 and R5. The load resistor with a resistance of 10 ohms is replaced by 100 ohms.

2.5. The slider of the variable resistor R4 is set to the position of minimum resistance, and the trimmer resistor R5 is set to maximum resistance.

2.6. By changing the resistance of the load resistor, the current is set to 0,5 A.

2.7. By moving the engine of the tuning resistor R5, the protection is activated.

2.8. The 100 ohm load resistor is replaced with a 10 ohm. The variable resistor slider R4 is set to the maximum resistance position.

2.9. By changing the resistance of the load resistor, the protection operation current is measured. If its value differs from 15 A, the selection of the resistor R4 will be required.

2.10. By setting several values of the load current, calibrate the scale of the variable resistor R4.

2.11. Disconnect the load resistor and ammeter. Restore the connection between terminal 4 of relay K1 and contacts K2.1.

The ammeter and voltmeter are calibrated according to the generally accepted method. Note that the ammeter scale is non-linear.

In conclusion, it should be noted that almost any PSU can be equipped with such a protection unit or its individual elements.

Literature

Nechaev I. Compact powerful voltage converter. - Radio, 2003, No. 2, p. 29, 30.
Shcherbina A., Blagiy S, Ivanov V. Application of microcircuit stabilizers of series 142, K142 and KR142.- Radio, 1991, No. 3, p. 47-51; No. 5, p. 68-70.

Author: E. Kolomoets, Irkutsk

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