In order to increase voltage from 12 to 18 volts using the same power source, you need a step-up converter. The circuit is easy to design, as it employs only one integrated circuit (IC), which has an oscillator and an op amp. The output is controlled using resistors, capacitors and diodes to provide a smooth 18-volt DC. Due to the switching on and off of the IC, a lot of heat is generated, requiring the use of many heat sinks for the different devices within the circuit.
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Things you need
- 330ÂµH inductor
- 3 Capacitors - 56ÂµF, 1 ÂµF, 390uF
- LT1170CT IC
- 5A Schottky diode
- 3 resistors -- 16.8k, 1.24k, 1k
- 3 heat sinks
Attach a heat sink to the IC before laying out anything on the board. It is easier to attach the heat sink while the unit is dismantled to ensure it fits properly.
Attach another heat sink to the Schottky diode.
Lay out the components on the board as shown using the diagram. (See the References section: Meade Portable 12v to 18v Converter.)
Attach a fuse to the input or the output of the inverter. When placed at the input, ensure it has a higher current rating.
Tips and warnings
- To increase the efficiency of the circuit, you might consider adding some extra EMC capacitors at strategic positions as shown in the circuit. (See the illustration of the back side of the board in the References Section: Meade Portable 12v to 18v Converter.)
- To provide protection against reverse voltage protection, an additional Schottky diode in series with the input can be added in case the input polarity is reversed. This diode should likewise have a heat sink attached to it. While this may affect the overall efficiency of the circuit, it will protect the converter from burning out.
- Since the IC cannot protect against short-circuits on the output, there is a chance that excessive current might bypass the IC, pass straight through the inductor and the fuse and to the output, resulting in a total meltdown. This is the case, because most batteries can output huge currents that can result in wire insulation meltdown. Generally, it is recommended to have a fuse rated at 5 amperes when it is placed at the input or a 3-amperes fuse when it is placed at the output of the converter.
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