Stable regulated power supply with overload protection

Many Amateur radio power supply units (PSU) made on chips KREN, CREA, KREN, etc., the Lower limit of adjustment of these chips is 1.2...1.3 V, but sometimes necessary voltage 0,5...1 V. the Author proposes several technical solutions BP on the basis of data chips.

Integrated circuit (IC) CREA (Fig. 1) is an adjustable voltage regulator compensation type in the hull KT-28-2, which allows you to power the device for a current up to 1.5 A in the voltage range of 1.2...37 V. This integral stabilizer has thermostable current protection and output protection against short circuit.

Fig. 1. IC CREA

On the basis of IC CREA you can build a regulated power supply with a circuit (without transformer and diode bridge) is shown in Fig. 2. The rectified input voltage is applied to the diode bridge on the capacitor C1. The transistor VT2 and DA1 chip must be placed on the radiator. The heat sink flange DA1 is electrically connected to pin 2, so if DA1 transistor and VD2 are located on the same heatsink, then they need to be isolated from each other. In the author's version DA1 installed on a separate small radiator, which is not galvanically connected with the heat sink and the transistor VT2.

Fig. 2. Adjustable PSU on the IC CREA

The power dissipated by the chip with heat sink should not exceed 10 watts. Resistors R3 and R5 form a voltage divider, which is the measuring element of the stabilizer, and are selected according to the formula:

Uвых = Uвых.min ( 1 + R3/R5 ).

The capacitor C2 and the resistor R2 is used for the selection of thermostable point VD1) is stable negative voltage of -5 V. In the original voltage is supplied from the diode bridge CCA and regulator 79L05 supplied from a separate winding of the power transformer.

Protection circuit the output circuit of the regulator simply connect in parallel with the resistor R3 electrolytic capacitor of at least 10 μf) and resistor R5 shall be shorted diode KDA. The location details are not critical, but for good temperature stability it is necessary to apply appropriate types of resistors. They should be placed as far as possible from sources of heat. The overall stability of the output voltage is the sum of many factors and usually does not exceed 0.25% after warming up.

After turning on and warming up the device for a minimum output voltage of 0 V is set by the resistor Rдоб. Resistors R2 (Fig.2) and a resistor Rдоб (Fig. 3) must be a multi-turn trimpot from the SP5.

Fig. 3. The switching circuit Rдоб

Opportunities current have chips CREA limited to 1.5 A. currently commercially available chips with the same parameters, but designed for higher load current, for example LM350 - for a current of 3 A, LM338 - for a current of 5 A. the Data on these chips can be found on the National Semiconductor website [1].

Recently appeared in the sale of imported chips from the LOW DROP (SD, DV, LT1083/1084/1085). These chips can operate at low voltage between input and output (up to 1...1.3) and provide at the output a stable voltage in the range of 1.25...30 V At a load current of 7.5/5/3 A, respectively. The nearest on the parameters of the domestic analogue type KREN has a maximum stabilizing current of 7.5 A.

When the maximum output current stabilization mode is guaranteed by the manufacturer when the voltage input / output not less than 1.5 V. the Chip also has a built-in protection against excess current in the load permissible value and a thermal protection against overheating of the body.

These stabilizers provide line regulation of 0.05%/V, the instability of the output voltage when the output current from 10 mA to maximum value of better than 0.1 %/V.

In Fig. 4 shows the schematic of the PSU for home labs, which eliminates the need of transistors VT1 and VT2, shown in Fig.2. Instead circuits DA1 CREA applied chip CREA. This adjustable regulator with low dropout voltage, allowing to obtain in load current of up to 7.5 A.

Fig. 4. Adjustable PSU on the IC CREA

Maximum power dissipation at the output of the stabilizer Rmah can be calculated by the formula:

Rmah = (Uвх - Uвых) IOUT ,
where Uвх - input voltage supplied to the chip DA3, Uвых - output voltage at the load, IOUT is the output current of the chip.

For example, the input voltage supplied to the chip, Uвх=39, the output voltage at the load Uвых=30 V, current at the load IOUT=5 A, then the maximum power dissipated by the chip power at the load is 45 watts.

Electrolytic capacitor C7 is used to reduce the output impedance at high frequencies, and also lowers the voltage level of the noise and improves the ripple smoothing. If this tantalum capacitor, its nominal capacity must be at least 22 µf, if aluminum is not less than 150 UF. If necessary, the capacitance of the capacitor C7 can be increased.

If electrolytic capacitor C7 is located at a distance of 155 mm and is connected to the PSU wire cross section of less than 1 mm, then the on-Board parallel to the capacitor C7, closer to the chip, install additional electrolytic capacitor of at least 10 μf.

The capacitance of the filter capacitor C1 can be determined approximately, at the rate of 2000 μf to 1 A of output current (at a voltage of not less than 50 In). To reduce the temperature drift of the output voltage a resistor R8 must be either a wire or a metal foil with an accuracy not worse than 1 %. The resistor R7 of the same type as the R8. If the Zener CSA is not available, you can use the site shown in Fig.3. Circuit protection solution given in [2], the author was quite happy, as it works flawlessly and tested in practice. You can use any schematics for the protection of PD, such as proposed in [3]. In the author's version when a relay K1 closed contacts K1.1, samarajiva the resistor R7 and the voltage at the output of the PSU is set to 0 V.

The printed circuit Board CB and the arrangement of elements shown in Fig. 5, the appearance BP - Fig. 6. The dimensions of the PCB 112x75 mm Radiator of the selected needle. Chip DA3 isolated from the heatsink gasket and attached to it by means of a steel spring plate, pressing the chip to the heat sink.

Fig. 5. PCB PSU and arrangement of elements

Capacitor C1 type K50-24 is composed of two parallel connected capacitors 4700 mcfh V. it is Possible to apply the imported capacitor type K50-6 capacity 10000 mcfh V. the Capacitor should be placed as close as possible to the PCB and the wires connecting it with the Board, should be as short as possible. Capacitor C7 production Weston capacity 1000 mcfh V. the Capacitor C8 not shown in the diagram, but the holes on the PCB under it. You can use a capacitor with par value of 0.01...0.1 μf, for a voltage not less than 10...15 V.

Fig. 6. Appearance BP

Diodes VD1-VD4 are imported diode micro-RS602 designed for a maximum current of 6 A (Fig.4). In the PSU protection circuit applied the relay RES (passport RS). In the author's version used a resistor R7 type WBS FOR mismatch no more than 5%. Resistor R8 (Fig.4) must have a variance from the specified denomination of less than 1 %.

The power supply settings usually requires and begins to work immediately after Assembly. After warming up the unit resistor R6 (Fig.4) or resistor Rдоп (Fig.3) put 0 In for nominal size R7.

In this design uses a power transformer brand OSM-0,1 ULTRASONIC power 100 watts. Magnetic SL/40-25. The primary winding has 734 loop sew wire 0,6 mm, winding II - 90 turns of wire sew 1.6 mm, winding III - 46 of turns of wire sew 0.4 mm with a branch of the middle.

Diode Assembly RS602 you can replace the diodes with a current capacity of at least 10 And, for example, CDA, b, D or KD A-G (if not to place the diodes separately, will have to redo the PCB). As the transistor VT1 is possible to apply the transistor CTG.

Literature