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Too many components
As a young engineer I was asked by my manager (a mechanical type) why I had to use so many different parts in my designs. Couldn’t I just use resistors and capacitors. The comment was tongue in cheek, of course, but I wonder what he would have said if I had started talking about Varistors. This is a class of device that you don’t generally come across every day. It also might be interesting to talk about how many things we can do if we just use resistors and capacitors. Hmm, I will have to give that some thought.
Voltage Dependent Resistor (VDR)
The most basic use of a VDR, see featured figure, also known as a Metal-Oxide Varistor (MOV), is to protect a circuit from over voltages. The MOV has a very high resistance at low voltages and a lower, and non-linear, resistance at higher voltages. It behaves the same for both directions of current. At low voltages there is a very small leakage current. The curve is essentially flat until the voltage reaches the clamping voltage.
The Varistor got started in 1927 when two physics (L.O. Grondahl and P.H. Geiger [no not that Geiger]) working at the Union Switch and Signal Company in Swissvale Pennsylvania, developed a new type of rectifier based on a cuprous oxide layer on copper. Another type of varistor was developed by R. O. Gridale in the 1930’s based on silicon carbide while working for Bell Laboratories to protect telephone lines from lightning strikes.
So, you mean it’s a TVS diode?
The MOV is not the same as a TVS diode, although they share many similarities. A TVS diode is a diode with a breakdown voltage above which the resistance become very low, i.e. it conducts. The device can absorb more than a kilowatt for a very short period of time. They come in both unidirectional and bi-directional flavors, having one or two diodes. The device employs an effect know as avalanche breakdown (the subject of a future newsletter).
In contrast the MOV is made up of grains of zinc oxide in a ceramic matrix. There are also other metals present in the matrix such as bismuth, cobalt and manganese. These are sandwiched between two opposing electrodes that lead to the outside world.
Where the grains meet you have a diode junction. This configuration is similar to a network of back-to-back diode pairs. Each pair is in parallel with its neighbor. At low voltages there is a small leakage current. At higher voltages the diode junctions break down, not due to the avalanche effect but because of thermionic emission and electron tunneling. This allows a large current to flow, protecting the circuit from the over voltage condition.
Thermionic emission, as the name implies, has to do with electrons, or ions, that are released due to heat. Electrons are normally bound to a material, the atom, by the electrical field attraction between the electron and the nucleus. The amount of energy needed to free an electron is its ionization energy. This is also known as the work function. It is the thermodynamic work, or energy, needed to move an electron from a solid, to a point in the vacuum just outside the solid surface, but not so far away that it can be affected by exterior electrical fields.
An exploit of this phenomena was used in vacuum tubes where a hot cathode was used to produce a source of electrons that were then swept through a vacuum by an electric field. This is also known as the Edison effect. The description here is that of a tube transistor.
Electron tunneling, also know as quantum tunneling, is a quantum mechanical phenomenon where particles will “tunnel’ though a barrier that they cannot normally get over. This is a difficult topic to explain in a short paragraph. Imagine a ball that you are trying to get to go over a hill. If the ball lacks the energy to overcome the effects of gravity it will roll back down the hill. If you try to throw it through the hill it will bounce back at you. If you impart enough energy you can embed it in the hill. In an unlikely scenario, the ball can barrow energy from other balls on the same side of the hill and use it to push through the hill. The ability to do this arises from the wave-particle nature of the electron and brings in such concepts as the Heisenberg uncertainty principle which makes many engineers want to cry, so I will skip that part.
Drawbacks of the MOV
The MOV is designed to conduct significant power for short periods of time, like 8-20 micorseconds. It does not have the ability to operate over extended periods of time. For example, when a neutral line is lost or where power lines are shorted. There have been many examples of MOV’s catching fire. There are techniques to prevent these scenarios (see UL1449). They also do not prevent inrush current surges, over currents, or voltage sags also known as brownouts.
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