WE’VE MENTIONED, THROUGHOUT the previous sections, the bad stuff that too much ripple can do to your dc system. Here’s a quick summary:
- High ripple currents can cause unnecessary battery heating, possibly shortening life.
High ripple voltages can degrade the performance of equipment connected to the dc bus.
If the battery is removed (or becomes defective), ripple voltage could be high enough to permanently damage connected equipment.
You’ve probably seen that we talk about both ripple voltage and ripple current. They’re two sides of the same issue. In the next section, on filters, we’ll describe the charger circuit components that we use to reduce both the ripple current and the ripple voltage that the charger delivers to the load and/or battery.
Before we go on, though, we should review the role the battery plays in reducing ripple voltage. Note that we’re now looking at ripple voltage, not ripple current. The charger is basically a current source, and that includes the component of ripple current in the dc output. Thus, the charger delivers some amount of ripple current to the connected load. The resulting ripple voltage that you can measure on the output terminals depends on the nature of the load.
If the load is purely resistive (there aren’t many, but incandescent lighting comes to mind), then the ripple voltage is directly proportional to the ripple current, obeying Ohm’s law [VRIPPLE = IRIPPLE × RLOAD]. We’ve seen that in an unfiltered rectifier, ripple current can be pretty high, leading to high ripple voltage for a resistive load.
Most dc loads, though, have some inductance or capacitance. The granddaddy of capacitive loads is a storage battery; a healthy battery has lots and lots of effective capacitance. As you’ll see below, that capacitance acts to reduce ripple voltage by 90% or more when compared to a purely resistive load.