electricity 101

["Ed Prytherch" <dixiedean@domain.elided>]

Brian Vogt is half right in his comments about solenoid currents. He is right
about voltage spikes created by opening the switch in a solenoid circuit.
There is fortunately a simple solution in the form of a clamping diode. The
diode is an electrical check valve. It allows current to flow in one
direction, but blocks it in the other direction. Since the voltage spike is in
the opposite sense to the energizing voltage, a diode will conduct the harmful
electrical energy to ground if it is connected with the cathode to the
solenoid terminal and the anode to ground. Modern cars depend upon clamping
diodes to prevent damage to their electronics by solenoid spikes. If you want
to fit one to an Alfa starter solenoid, choose one with about 200volts/3 amps
rating. The peak current capacity will be about 200 amps. 1N5402 is a suitable
diode. The cost is about 50 cents. Wire it with the circular banded end to the
solenoid and the plain end to ground.

Brian has got it wrong about inrush current. DC solenoids do not have inrush
current. It is a property of inductance to oppose changing currents. The
current to a solenoid, when the switch is closed, rises more slowly than the
current to a purely resistive load. Brian may be getting DC solenoids confused
with AC solenoids. AC solenoids have high inrush current that is caused by
changes in the reluctance of the magnetic circuit as the armature moves. AC
solenoids that stick before they reach the home position have prolonged inrush
current which can cause them to burn out.

The slowly rising charging current of an inductor can be a problem with
ignition circuits that operate at high rpm. It can be a cause of loss of
performance in a high speed 2 stroke engine that fires twice as often as a
four stroke at the same rpm. In my motorcycle racing days (70s) we wired
resistors in series with ignition coils to improve the performance. The trick
was to use a 6 volt coil and a 12 volt battery with a resistor of the same
value as the resistance of the coil. This trick is also used with industrial
dc solenoids to improve initial force. The resistor in this application is
called a forcing resistor.

For more information on solenoid operation, go to
http://www.blpcomp.com/solenoid/pdfs/guide1.pdf


On a different electrical note, www.partsexpress.com have Bosch 30 amp SPDT
relays for $1.99 and no-name "Bosch type" 30A SPDT relays for $0.99. They also
have 5 pin relay sockets with color coded wires for $1.50. They also have a 5
pin socket with a diode wired to the coil for $1.60. I used the Bosch relays
in the light circuits and the electric fan circuit on my GTV.

Ed Prytherch

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