nuckolls.bob(at)aeroelect
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 Posted: Wed Jul 01, 2026 7:44 am Post subject: Starter spikes redux |
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| Quote: |
cluros(at)gmail.com wrote:
Has anyone recorded an engine startup with oscilloscope
quality voltage data? Do we know what these starts in fact
look like voltage wise? |
Excellent question!
Been there, done that, many times. Spent 45 years
wrangling electrons in aircraft of all sizes from
Rotax ultralights to Hawker 4000 . . . not to
mention studies conducted on instrumented test
benches.
Startup 'spikes' simply do not exist.
| Quote: | Eric wrote: I haven't done so in an airplane, but I did sample an engine start
in my car using a high-sample-rate data acquisition module.
There were no spikes of any kind. Just the voltage dip you would
expect during cranking, then a small rise when the alternator
went to work. The car was a 4-cylinder 1998 Honda Accord. |
Bingo!
The universe of 'common knowledge' in physics
is badly contaminated with erroneous notions
re-enforced by a dearth of data and exacerbated
by random definitions of vocabulary amongst
the debaters.
First a definition:
When qualifying an appliance for suitability
to task in certified aircraft, one is obliged
to conduct tests where the device under
study is subjected to . . .
SPIKES: These excursions from normal bus voltage
are very low energy, short duration events (100
microseconds or so) but they can be 'alarmingly'
large (40 volts in a 14v system). These
events are invariably generated by the collapse
of magnetic fields in inductors. Relay coils,
solenoid valves, generator/alternator fields,
etc. The fact that they are so short an generated
by high resistance devices (low energy) makes
them easy to mitigate with rudimentary techniques
(catch diodes, transorbs, snubbing capacitors, etc.)
One would THINK that the biggest electromagnetic
device on the airplane charged with perhaps
200 amps of supply current would be a disaster
in hiding poised to annihilate ship's
delicate electronics. Doesn't happen.
Motors of any style are NOT significant sources
of spikes. Yes, they have lots of wires wound
on magnetic cores but . . . when the supply
voltage to a motor is interrupted, the motor
is spinning and generating a counter-emf
(voltage) that greatly reduces the differential
voltage across the spreading contact of the
controlling device. By the time the motor's
magnetics have a chance to couple any significant
energy back into the source, the contacts are
sufficiently gapped to prevent propagation
of mischief.
My first job in GA was as a tech writer
at Cessna's single-engine facility in Wichita
circa 1965. Cessna's house brand of
avionics were just beginning to include
those new 'solid state' thingys in the
audio and power supply sections while
the remainder was still vacuum tubes.
There was an 'airplane patch' east of
the plant were delivery ready aircraft
were parked. Sometimes a hundred new
airplanes were tied down out there.
They all had the best lead-acid batteries
of the time (ugh!). To make matters
worse, the state of charge could
be anything. None of these airplanes
had much time on them and they were
short flights.
These latest-and-greatest radios
were often left ON when the airplane
was parked. When a delivery pilot
hopped in to fly it away, it was not
uncommon for a radio to be inoperative
after the engine was started. A breaker
would pop and the radio had to be pulled
for repairs before the airplane could
be delivered.
Aha! Must be 'spikes' from the starter
whacking these delicate chunks of
GERMANIUM buried in the bowels of
the radio.
This was the situation that birthed
the AVIONICS MASTER SWITCH. Sho 'nuf.
If that switch was OFF when the engine
was started, the radio would still be
working when the switch was turned ON.
None of our electrical/avionics design
staff had ANY experience with transistors
yours truly included. In fact, Cessna
hired a Wichita University EE professor
to conduct a several weeks seminar
on 'industrial applications of
transistors'. I got to take the course.
Didn't learn squat about the physics
that would explain the radio failures.
In years since, we came to understand
that the rather delicate, germanium
power transistors of the time . . . while
'rated' for 30v max applied voltage . . .
were NOT being killed by an over voltage
event generated by starters.
It was LOW VOLTAGE condition caused
by cranking the engine with a soggy
battery. Power supply transistors would
come out of saturation and go into
'second breakdown' (melt).
Our lack of understanding generated
(1) a band-aid Master Switch and
(2) launched the myth that starters
were radio killers.
In years since, I've designed dozens
of solid state appliances for aircraft
where NONE had to hide from starters
behind any sort of 'spike wall'.
Batteries are much improved as are
the electronics. We know how to make
transistors sit in the bleachers and
taunt the ship's electro-gremlins.
Significant 'spikes' simply do not
exist in vehicular DC power systems.
Now, surges and over voltage (alternator
runaway) are HIGH energy conditions
also addressed with application of
stone-simple design techniques and
modern components. That's another
topic.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
for the-best-we-know-how-to-do based
on physics and repeatable experiment.
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