Q: Why is there a diode in line with a coil of a "vibrating plunger" pump?
A: This question has fooled a number of people. Diodes are also used in this manner on bells being driven from an alternating current (AC) source.
The reason is simple, it is used to "divide the frequency by two". Now for the more technically minded the immediate reaction may well be "oh yea!, pull the other one, how can a simple diode be a frequency divider?". My response; read on and find out. If there are times any explanation seems a little "simple" please understand this paper has been written for everyone, even those who are not too familiar with electro-magnetics.
AC means the electrical current presented swings from a positive to a negative value and back again. A wall plug delivers typically 230volts AC. If viewed using an Oscilloscope (a device used to view waveforms) it would be noticed the waveform of the typical mains supply takes on the shape of a sine wave (as shown in the figure below) that swings about a mean of zero volts. It is not commonly known that the figure of 230volts is the mean voltage and the actual level reached on the peaks is 320volts.
The term 50Hz AC means 50 cycles per second or, simply put, means the alternations take place 50 times per second.
When coil is connected to the supply a magnetic field is set up that resembles the waveform of the AC, first in one direction, building as the voltage increases, then dying off as the voltage reaches zero then reverses direction and builds in the opposite direction as the voltage swings to its complimentary peak. This is shown in the waveform to the right of the above figure indicating the intensity and the N & S showing the magnetic direction.
Now if a magnet was used as the moving part in e.g. a plunger pump then we would have it attracted in one direction as the magnetic field built in one direction and repelled in the opposite direction as the magnetic field reversed. However, subjecting a magnet to this alternating magnetic field has a tendency to "demagnetise" thus weaken it. Soon the magnet would not perform nearly as well as when first implemented.
The next available method is to use a piece of magnetic material, e.g. steel, that simply is pulled into place when a current is present on the coil. This happens regardless of the direction of the magnetic field. This, however, has one disadvantage. If one ignores the direction of the magnetic field (ignore the N & S) this field builds and falls 100 times per second. As the moving part is not sensitive to the direction of the field it would therefor try to move back and forth the same 100 times per second.
If one remembers ones basic rules on kinetic energy it requires 4 times the amount of energy to make something move the same distance back and forth at twice the speed. This means the coil and recoil spring would have to be 4 times as strong (thus using 4 times the amount of current) to move the plunger back and forth in our pump.
In comes the diode! Diodes are used when one desires a current to flow in one direction but not in another. Therefor if we were to use a diode in series with the coil of our pump we would be limiting the building and falling of the magnetic field to only one half of the cycle as seen in the figure below. This limits the amount of times the plunger is pulled towards the coil to 50 times per second and not 100 when no diode is present.
The same principle is used in bells driven by AC. If the bell striker is only pulled towards the gong 50 times per second instead of 100 times the striker will have more time to move to its "resting" position before being pulled once again to strike the gong.
Now, for the more technically minded; There is one other advantage to using this diode throughout e.g. a school building where many bells are used in that the total amount of energy required to ring the bells can be halved as half the bells could be wired to pull the striker on the positive half of the cycle and the rest wired to use the negative half. In a house where two bells are needed the same principle could be used thus saving on transformers as one transformer could power two bells, one on each half of the cycle. Remember to wire the bells for AC operation (the breaking contact is taken out of circuit) else the bells sound intermittent.