Voltage converter 2: Although a 9.5-24 V direct voltage can be brought down to 5 V easily by a standard regulator, 1 the converter described here has the advantage that, since it is a switch-mode type, it hardly dissipates any heat. Its maximum, steady output current at 5 V is 250 mA, although it can cope with peaks of up to 750 mA.
The converter is based on a Motorola ‘Type MC34161 circuit, to which a power stage: T1 has been added. Inductor LI, C5 and D1 remove any ripple from the output. The internal comparator at pin 2 of IC1 is connected to the output of the converter via potential divider R4-R5 to monitor the output voltage. The second comparator (pin 3) is used in the oscillator circuit and connected to pin 6 direct and to pin 5 via R3-C3. When the supply is switched on, the output of the converter, and thus the voltage at pin 2, is 0; the oscillator operates normally. Transistor T1 charges C5 via L1. When T1 is Off, L1 ensures a supply of energy to C5 via D1. As soon as the terminal voltage of C5 has risen to a sufficiently high level, the internal comparator at pin 2 changes state. The oscillator is then switched off via pin 6 so that T1 is also off. After C5 has been discharged to an extent that its terminal voltage drops below the preset level, the oscillator is re-enabled, and C5 is charged again via T1.
The values of L1 and C5 determine the switching frequency; with values as shown, an input voltage of 12 V and a load. a current of 250 mA, the frequency is 18 kHz. With .larger inductances and 4f input voltages, the frequency drops.
It is essential that all earth connections are taken to the negative to minal of C5 as shown the diagram.
The inductor is a standard triac choke, to which a number of turns have to be added. If the inductance of the choke is L pH and its number of turns is n, the number of turns, n’ required for the present inductor is given by n’ = n √ (470/L).
