7
The very best source of power in a typical vehicle is a secondary battery. Note that I didn’t write this as
“the engine start battery”. Your engine start battery will work too if it’s 12V, or you could tap into an
unused branch circuit at the fuse block. Least desirable as a power source is the vehicle’s accessory
electrical system.
A secondary battery that’s well‐isolated from the vehicle’s engine‐start system will work best for the
KX3. There are a number of isolators made for 12VDC systems, and these are used to connect the
secondary battery to the vehicle’s DC system only for charging, as needed. Some isolators also supply
transient suppression and filtering, and over‐current protection. In general, this will eliminate or
suppress conducted emissions from the primary accessory power system. See
http://www.powerstream.com/battery‐isolator‐solid‐state.htm
for an example of an isolator.
In practice, not everyone can fit in a second storage battery to use for the KX3. Small cars in particular
have little space for extras like this. You might be able to fit a small battery under a seat or in the trunk,
but take care to protect its terminals from being shorted, and it should be securely mounted. Most of us
would probably choose to use the vehicle’s engine start battery as the primary KX3 power source. This
approach is generally the best compromise for power quality, cost, and convenience. The KX3 power
distribution and conversion example in this Guide uses the engine start battery as the source.
The KX3 was designed to operate from clean DC power – batteries – and is not immune to automotive
DC power supply impurities (I call this “gudge”). If any DC line noise is present at audio frequencies, it
will be readily audible in the KX3’s speaker and PHONES output. (The microphone amplifier circuits are
not as susceptible to line noise.) One way to mitigate a DC source with audio frequency noise is to use
the KX3’s RX SHFT function to move amplification away from the audio frequencies, up to 8 KHz. This
works amazingly well. But it’s even better if you remove the DC line noise entirely, or lessen it
substantially.
Power distribution to the KX3 and the KXPA100 may be routed via an automotive relay with instrument
panel control via a switch. Shown in the wiring diagram, Figure 3, this separate switching allows the
operator to avoid damaging engine start transients at the KX3. The switch and relay can be purchased
from your local auto parts dealer. See Figure 5 for a photo of the interior penal switch in a Honda Civic.
Figure 5 shows the relay enclosure, mounted in the Civic’s trunk.
Caution: DO NOT use panel switches like the one shown to directly switch loads such as the KX3 or
KXPA100. Switches of this type are designed for the amount of current a relay coil needs to energize,
0.5A or less. Let the relay do the heavy lifting; it only costs a couple of dollars.
Figure 3, below, shows a wiring diagram for a KX3 and a KXPA100 installation – both powered from the
vehicle’s engine start battery. This design uses a relay to isolate the KX3 / KXPA100 branch circuit.
Figure 4 is a schematic diagram of the relay and power interconnect circuits. The diode is used to
protect the contacts on the panel switch shown in Figure 4 from back EMF when the relay coil is de‐
energized.
