Scope Traces of Timer Contacts
The following traces were made with
a Pico 4 Channel PC Oscilloscope connected to a timer being turned by a dc
motor with variable speed control from 1 to 2000 rpm.
-The timer runs at half engine speed.
-The timer is loaded with both filaments of a
Stop/Tail light Bulb #1157 running on a separate 12 volt
battery. The current is about 2.5 amps
when the timer is stopped. At speed, the draw is about 1.6 amps to
simulate the load of a Model T Coil.
A setup for looking at the timer
contacts with a PC Oscilloscope:
From the left is a PC with 4 channel pc scope (blue), ammeter, lamps, timer test
fixture, timer cases and motor speed control.
Here’s the motor, timer mount and
speed controller…….
Here’s the ammeter in series with the battery lead supplying about 1.6 amps of
current at 12 volts to the lamps.
Each lamp has its own timer contact,
and at low speed you can see them ‘fire’ 1,2,4,3
A standard
This design gives good performance
balanced with optimum lifetime of the timer.
The timer running at 500 rpm….one
contact only was logged…..
At 900 rpm the contacts begins to
show a bit of bounce (discontinuity at the leading edge)……
At 1200 rpm (2400 crankshaft rpm)
there is severe contact bounce which will result in misfire…….
In the past, for high
speed operation, such as the
Although it worked well, the
stronger spring caused accelerated wear of the rotary contact, and the contacts
had to be changed frequently.
A new timer configuration promises
to reduce the ‘contact bounce’ which can cause
misfiring and prolong the life of the contacts.
Results at 1200 rpm (2400 engine
rpm).
Note that all 4 contacts are now monitored and
show on the chart.
And at 1500rpm (3000 engine rpm)
Note the blue channel is just starting to show
contact bounce.
Also note the dwell difference between contacts due to equipment problems.