The simple laboratory power supply
In the offered laboratory power supply two-level transformation of the straightened voltage is applied: ШИМ transformation to an intermediate voltage and the subsequent linear stabilization. The basic characteristics of the device the following: limits of regulation of a target voltage - from 1,3 up to 30 V, factor of instability on a voltage - 0,07 %/V, instability on a current of loading of 0,1 %, the maximal entrance voltage - 27 V, efficiency of transformation at the maximal current of loading - not less than 70 %. The opportunity of change of a threshold of restriction of a current up to 1,2 is stipulated And, there is a protection against short circuit with light indication. The source differs small dimensions, the minimal thermal losses (at a current of loading up to 0,3 A heat-conducting paths are not required).
The block diagram of the device is shown on fig. 1. Entrance voltage Uвх is transformed by ШИМ-CONVERTER DA1 in intermediate Uпр which, in turn, is entrance for analog stabilizer DA2. The feedback through differential amplifier DA3 supports necessary for DA2 a power failure (for LM317 - 2,5 ВV due to what thermal losses on DA2 are minimal.
Fig. 1. The block diagram of the power supply
The basic circuit of the power supply is represented on fig. 2. The straightened voltage from an output of bridge VD1 smooths out condenser S1 and moves on input ШИМ of the converter collected on elements DA1, VT2, VD2, L1. The circuit of inclusion DA1 - typical lowering . Application of microcircuit КР1156ЕУ5 has reduced number of passive elements to a minimum, but has imposed restriction on the maximal entrance voltage which in such inclusion should not exceed 40 V. SHIM with the help of memory throttle L1 and diode VD2 forms intermediate voltage Uпр on condenser S4.
Fig. 2. The basic circuit of the power supply
On stabilizer DA2 the linear regulator of a voltage) is collected. Adjust its variable resistor R12. Diodes VD3 and VD4 protect a microcircuit from return currents and negative voltage and are entered according to recommendations on its application .
Microcircuit DA3 and resistors R7-R10 form the differential amplifier which is watching a power failure on stabilizer DA2. The factor of amplification DA3 is chosen equal 1,5, that allows to support the established value in all an interval of voltage and currents, including at short circuit of an output. Tuning resistor R2 adjust a power failure at adjustment.
On elements VT1, HL1, R1 the signalling device of a short-circuited condition of an output is executed. In a normal mode transistor VT1 is open, and the power failure on it does not exceed several tenth shares of volt. At a voltage reduction on an output of a source up to 0,7 In and less transistor VT1 is closed also light-emitting diode HL1 starts to shine. About the included condition of the power supply light-emitting diode HL2 signals.
The role of resistor R5 is rather interesting. At a voltage on it more than 120 mV (the average value determined by practical consideration) comes into effect the internal terminator of width of pulses of microcircuit DA1, transforming it in a source of a current. It is possible to take advantage of this property КР1156ЕУ5 for restriction of the maximal current of loading. So, for example, at the resistance of this resistor equal of 0,1 Ohm, the source is capable to give out in loading a current up to 1,2 And, and at R5 = 1 Ohm - only up to 120 мА. Having established the resistor resistance 0,5 Ohm and having limited thus a current of loading value 240 мА, it is possible to refuse a heat-conducting path for microcircuit DA2 and from external current key ШИМ of the converter (having excluded transistor VT2, resistor R3 and having connected a conclusion 2 DA1 to a point of connection of throttle L1 and diode VD2). In this case dimensions of a product will be not much more more match a box.
As key VT2 it is possible to apply any transistor with static factor of transfer of a current of base more than 30 and an allowable current of a collector not less than 3 A. The author used КТ805АМ. At it quite good frequency properties, therefore are small losses at switching. Very well "conducts" itself on this place field transistor IRF3205 - the heat-conducting path is not necessary for it at a current up to 1 And.
Inductance of throttle L1 can be anyone from 40 up to 600 мкГн, the unique requirement - it should be designed for a current not less than 1,5 A. Resistors - МЛТ, С1-4 with a permissible deviation of resistance from face value ±10 %, tuning resistor R2 - multitur-naround wire СП5-2ВБ or similar, variable R12 - any type resistance 4,7...6,8 whom. Condensers S1 and С4 - оксидные К50-35 in capacity 220...470 мкФ with rated voltage 63 In, the others - ceramic (КД2, К10-7, К10-17ит. item).
Adjustment of the power supply is reduced to installation by tuning resistor R2 of a pressure 2,5 In between conclusions 2 and 3 DA2 (at 50-percent loading).
The author: С. МУРАЛЕВ, Dimitrovgrad Ulyanovsk обл