8 June 2017
AAutomotive power sources are indeed quite dirty. There are several sources of electrical noise that can creep in to make running a device on 12V automotive power problematic.
This is particularly true if you care about audio quality, and
it’s a problem that many of us ham radio operators run
into.
Typical sources of noise in a vehicle application (in no
particular order) are as follows. The ignition system is the
first one usually mentioned. High voltage generated by
the ignition coil eventually causes current to flow across
the spark gap; both of which, in turn, cause significant
electromagnetic fields that — because of the shape of
the pulse — are fairly broadband. This noise is sometimes
visible on vehicle 12V systems. The current required
as input to the ignition coil can cause corresponding
fluctuations in the 12V supply.
More modern fuel injection systems used on most
vehicles these days use solenoids to activate the injectors.
Those solenoids operate at much lower voltage than the
ignition system, but require much higher current. The
inductive voltage increase when the current is turned
off also increases the noise. This generates broadband
electromagnetic energy that radiates from the injector
wiring and the injectors themselves. The current required
to fire these injectors can again cause the 12V supply to
fluctuate. I have some personal experience with this. I’ve
tracked this noise down with a Handi-Talkie set to receive
and demodulate AM in the VHF spectrum. Without an
antenna attached forming an attenuator, the receiver helps
locate the source of the noise.
Some of the DC motors in a car can also cause noise
because of the pulsing current required — especially if
they are brush type motors. Examples might be windshield
wiper motors, the electric engine cooling fan, fuel pump,
etc. Some that are only on for a short time but can be quite
noisy might be the pump for windshield wiper fluid and the
motors that raise and lower windows.
A major offender is the alternator. This is particularly
true if the battery is old or its terminals are dirty as this
increases the ESR (equivalent series resistance) of the
battery. The battery acts somewhat like a capacitor to
stabilize the sinusoidal voltage coming from the alternator.
My older car is a big offender in this area — particularly
when a large current is drawn from the battery.
When transmitting at 50W and drawing about 8A,
my two meter FM radio has a quite audible whine on the
transmitted signal. This is despite (or perhaps because)
it is being powered off a double-fused pair of #8 wires
connected directly to the battery. This puts the radio
electrically close to the noise source, though it also
provides a low resistance path to the hopefully stable
battery voltage. I haven’t yet been able to solve this
problem to my satisfaction.
Of course, there are other sources beyond what
I’ve mentioned, including even car door hinges making
intermittent connections with the body (yes, that’s real!).
Fixing this isn’t always easy. It depends on both the nature
of the noise and the requirements of the devices or circuits
that are powered. Complicating things is the low 12V
supply. Small fluctuations end up being a large fraction of
the total voltage. So, what can we do?
If there is some voltage headroom and the current
requirements are not high, regulating down from 12V to
something lower like 10V or even 5V using a pass regulator
is an easy way to clean things up. In that case, a good dose
of capacitive and/or inductive filtering before the regulator
can minimize any bleed-through. If there is no real room to
use a lower voltage, other measures must be employed.
For low to medium current draw, a buck-boost
converter could be used. The LTC3780 is a device that
implements this, giving a fixed output voltage from
a fluctuating input voltage. That device needs a fair
number of outboard components, and since it works on
the principle of switching current into an inductor like a
switching power supply, it may generate its own noise. It
must be carefully filtered. Consult the application notes for
a circuit for that device.
If the current draw is high and intermittent, sometimes
one of those large multiple farad capacitors marketed for
high power car audio can be used. Those capacitors don’t
have super low ESR and may need some help at higher
frequencies with a different type of capacitor, but they
still can supply significant current. Pairing them with one
or two inductors as in the configuration in Figure 3 might
help.
This circuit uses a big 1F capacitor, but smaller may be
okay for lower current applications. There is a secondary
parallel 1 µF non-polar capacitor to
provide some RF stability. The bottom
inductor may not be useful if the
chassis connections are all common
to the car chassis. I’m considering that
for my two meter radio problem. The
inductor will probably want to be a
large value — something on the order
of 100 mH (X = 2p f L 63Ω at
100 Hz). Therefore, a ferrite toroid
core will probably be best. NV
n FIGURE 3. Inductive and capacitive
filter example.
