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Voltage regulators

The weight, for a given energy level, is much less for Li-Ion batteries compared to lead-acid and NiCad batteries, but the output voltage of Li-Ion batteries varies widely, and drops significantly, with usage from a full charge. Lead acid batteries exhibit this nonconstancy of output voltage to some degree (compared to NiCads which are much more self-regulating), but Li-Ion batteries are far worse in this regard, and therefore, almost certainly, need a voltage regulator.

Another reason that a voltage regulator is needed is that various components of WearComp require different voltages. Typically the computational apparatus requires 5 volts while the analog video circuits, and the RF components require 12 volts. It is desirable that a single battery power the entire rig.

With the exception of WearComp0-3, all current versions of WearComp use 12 volt batteriesgif. The original reason for this voltage selection arose from the automotive battery voltage standard, so that WearComp could be operated from an automobile cigarette lighter or accessory outlet fitted with a long cord, either for testing, or for additional runtime when the batteries were low. Furthermore, because much of the peripheral radio equipment operated at 12 volts, this voltage was convenient.

Accordingly, a single ``12 volt'' battery is used to power most of the apparatus, together with a voltage regulator to bring the 12 volts down to 5 for powering the computational portion of the apparatus.

A linear voltage regulator is undesirable, due to the dissipation of excess heat, since much more efficient switching regulators are available.

Regulators may be compared by:

  1. linear versus integrated switching regulators (ISRs);
  2. isolated versus 3 terminal
The most efficient are the non-isolated stepdown ISRs.

Furthermore, the voltage variation of Li-Ion batteries is typically excessive for certain components, which require exactly 12 volts, so it is often desirable to have separate switching regulators, one to provide 5 volts, and another to provide 12 volts. I generally use a so-called ``step down'' regulator to provide 5 volts for the computational apparatus, and a 12v to 12v regulator to take in the varying battery voltage and provide a fixed 12v output for other devices (video, radio, etc.). Furthermore, it is often desirable to use separate regulators for individual components, so that they don't affect each other. (e.g, I often use more than one 12 volt to 12 volt regulator, so that, for example, when the radio transmitter keys up to transmit a packet of data, it doesn't affect other 12 volt components).


next up previous
Next: Specific details about how Up: How to build a Previous: Building the bridge

Steve Mann
Wed Dec 24 05:46:06 EST 1997