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Re: Lighting questions
> Hella suggests relays with resistors and diodes on the control circuit
> to avoid voltage spikes up to 500 volts produced by the relay closing.
> I noticed the resistor and diode are on the control side. So now my
> relays will blow my brittle headlight switch, I can't win! ;)
If that's happening, what about all the other relays in the car? That
"spike" must be very short lived, generated by the relays coil. Not
The problem is not the relay/coil itself, that's "trivial" (typically;
"sensitive" electronics in the relay may well need protection from the
relay itself). The "big surge" comes from the sudden delta load --
when your alternator is busy cranking out the amps, and you suddenly
yank off a load, all that energy the alternator is pumping into the
circuit(s) does not instantaneously adjust to a lower power/load, it
goes "oof!" first...
Consider the following Intel tech paper . . .
THE AUTOMOTIVE ENVIRONMENT
The automobile presents an extremely hostile environment for electonic
systems. There are several parts to it:
1. Temperature extremes from -40C to +125C (under the hood) or +85C
(in the passenger compartment)
2. Electromagnetic pulses from the ignition system
3. Supply line transients that will knock your socks off
One needs to take a long, careful look at the temperature extremes.
The allowable storage temperature range for most Intel MOS chips is
-65C to +150C, although some chips have a maximum storage termperature
rating of +125C. In operation (or "under bias," as the data sheets say)
the allowable termperature range depends on the product grade, as
Grade Ambient Temperature
Commercial 0 70
Industrial -40 +85
Automotive -40 +110
Military -55 +125
The different product grades are actually the same chip, but tested
according to different standards. Thus, a given commercial-grade chip
might actually pass military temperature requirements, but not have
been tested for it. (Of course, there are other differences in grading
requirements having to do with packaging, burn-in, traceability, etc.)
In any case, it's apparent that commercial-grade chips can't be used
safely in automotive applications, not even in the passenger compart-
ment, and automotive or military chips are required in under-the-hood
Ignition noise, CB radios, and that sort of thing are probably the least
of your worries. In a poorly designed system, or in one that has not
been adequately tested for the automotive environment, this type of EMI
might cause a few software upsets, but not destroy chips.
The major problem, and the one that seems to come as the biggest sur-
prise to most people, is the line transients. Regrettably, the 12V bat-
tery is not actually the source of power when the car is running. The
charging system is, and it's not very clean. the only time the battery
is the real source of power is when the car is first being started,
and in that condition the battery terminals may be delivering about
5V or 6V. As follows is a brief description of the major idiosyncracies
of the "12V" automotive power line.
o An abrupt reduction in the alternator load causes a positive vol-
tage transient called "load dump." In a load dump transient the
line voltage rises to 20V or 30V in a few microseconds, then decays
exponentially with a time constant of about 100 microseconds. Much
higher peak voltages and longer decay times have also been reported.
The worst case load dump is caused by disconnecting a low battery
from the alternator circuit while the alternator is running. Nor-
mally, this would happen intermittently when the battery terminal
connections are defective.
o When the ignition is turned off, as the field excitation decays,
the line voltage can go to between -40V and -100V for 100 micro-
seconds or more.
o Miscellaneous solenoid switching transients can drive the line to
+ or -200V to 400V for several microseconds.
o Mutual coupling between unshielded wires in long harnesses can in-
duce 100V and 200V transients in unprotected circuits.
What all this adds up to is that people in the business of building
systems for automotive applications need a comprehensive testing pro-
gram. An SAE guideline which describes the automotive environment is
available to designers: SAE J1211, "Recommended Environmental Prac-
tices for Electronic Equipment Design," 1980 SAE Handbook, Part 1, pp
22.90 - 22.96.
Some suggestions for protecting circuitry are: A transient suppressor
is placed in front of the regulator chip to protect it. Since the rise
times in these transients are not like those in ESD pulses, lead induc-
tance is less critical and conventional devices can be used. The regu-
lator itself is pretty much of a necessity, since a load dump transient
is simply not going to be removed by any conventional LC or RC filter.
Special I/O interfacing is also required, because of the need for high
tolerance to voltage transients, input noise, input/output isolation,
etc. In addition, switches that are being monitored or driven by these
buffers are usually referenced to chassis ground instead of signal
ground, and in a car there can be many volts difference between the two.
[Several "figures" and graphs are of ASCIIty not shown here; there is
one very striking "Figure 25. Transient Created by De-energizing an
Air Conditioning Clutch Solenoid" ('scope photo) with an incredible
-400V microsecond-sized transient shown. Ye-Ouch!]