This circuit was originally designed to power a motorcycle intercom
from the vehicle supply system. This type of intercom, which is used for
communication between driver and passenger, generally requires quite a
bit of power. In order to improve intelligibility there is often
elaborate filtering and a compander is sometimes used as well. The
disadvantage is that a battery doesn’t last very long. You could use
rechargeable batteries, of course, but that is often rather laborious.
It seems much more obvious to use the motorcycle power supply instead.
9-V Battery Replacement Circuit Diagram
A
9-V converter for such an application has to meet a few special
requirements. For one, it has to prevent interference from, for example,
the ignition system reaching the attached circuit. It is also
preferable that the entire circuit fits in the 9-V battery compartment.
This circuit meets these requirements quite successfully and the design
has nonetheless remained fairly simple. In the schematic we can
recognise a filter, followed by a voltage regulator and a voltage
indicator. D1, which protects the circuit against reverse polarity, is
followed by an LC and an RC filter (C3/L1/L2/C1/R1/C2). This filter
excludes various disturbances from the motorcycle power system.
Moreover, the design with the 78L08 and D3 ensures that the voltage
regulator is operating in the linear region. The nominal sys-tem voltage
of 14 V can some-times sag to about 12 V when heavy loads such as the
lights are switched on.
Although the circuit is obviously
suitable for all kinds of applications, we would like to mention that it
has been extensively tested on a Yamaha TRX850. These tests show that
the converter functions very well and that the interference suppression
is excellent.