Designing Embedded Hardware

Switching Big Loads

We’ve already seen how to use an H-bridge chip to switch relatively large voltages (and the corresponding big currents) needed to drive electric motors. In many other cases, you will want to turn large voltages on or off, and in this section I will show you an easy way of doing just that.

The Motorola MC33298 is a chip that is controlled by a microprocessor using SPI and can switch eight power sources on or off. This chip can handle voltages between 5V and 26.5V, with currents as large as 6 Amps. If you need to turn electrical systems on or off, this chip is for you. Its primary use is for industrial and automotive applications, controlling power to subsystems such as heaters, small air-conditioning units, moderate voltage lightbulbs, small pumps, and so on. Obviously, it won’t handle the high AC voltages that come out of your wall socket, so don’t use it for switching power to your home appliances!

The basic schematic for the circuit is shown in Figure 12-35.

Figure 12-35. MC33298 circuit

The MC33298 has two power-supply pins. VDD is a 5V supply and powers the chip’s internal digital logic. It’s decoupled to ground using a 100nF capacitor. V _PWR is the supply voltage for the external subsystems (represented in the figure by each LOAD rectangle) and can range from 5V to 26.5V. There are eight switch outputs, labeled OUT0 through OUT7. When a given switch is activated, ...