Snubber Circuits

Snubber Circuits are circuits which are placed across thyristors to protect it from transient high voltages which may occur during switching off.  When the current flowing through a thyristor is interrupted, the voltage across the thyristor can increase rapidly according to Faraday's second law.  This high voltage can cause damage to the thyristor or to other components in the circuit.

The snubber circuit works by preventing the voltage from rising too high across the component to be protected.  The snubber is normally used in circuits with inductance.

The Basic Snubber Circuit is a capacitor and a resistor connected in series.  The Snubber works on the principle that the current across a capacitor cannot fall to zero instantly.  Hence, a small current keeps flowing through the capacitor.  This current is enough to prevent the voltage from rising to high levels.

The Flyback resistor is also another example of the snubber circuit.

Shielding in Instrumentation Cables

Shielding in Instrumentation Cables is vital in order to prevent stray coupling from other circuits nearby. 

When an instrumentation cables is laid beside a power cable, there are chances of  electromagnetic coupling taking place which can induce voltages in the instrumentation cable.  This can distort the signal in the instrumentation cable.  To prevent this, instrumentation cables are provided with a shield, usually made of aluminium.  

This shield magnetically shields the wires inside it.  This eliminates the chances of stray coupling.  The shielding needs to be grounded at one end.  Grounding the shield at two ends will cause grounding loops which can cause interference.

What is a Backplane ?

A Backplane is a system of parallel electrical conductors in a PCB which are used to connect the connectors.  Backplanes are advantageous over traditional cable-type connections as they are mechanically stronger.  The cards are mounted on backplanes on slots.  This facilitates easy replacement of the cards.  

Backplanes are widely used in computers and in industrial automation such as PLCs, DECS systems, etc.

Backplanes can be classified into Active and Passive Backplanes

Active backplanes are backplanes which have some computing circuitry in built into them.  Passive backplanes are backplanes with no computing circuits.  

A mid-plane is a backplane which has got slots on both sides.  

What is a ribbon cable ?

A ribbon cable is a cable with multiple insulated wires on a single flat plane.  It is called a ribbon cable as it resembles a ribbon.  Ribbon cables are used in computer peripherals such as printers, hard drives.

Ribbon cables are flexible and hence are suited for application such as dot-matrix printers where the printing head has to move back and forth across the page.  Ribbon cables also take up very little space as they are flat and can be accommodated even in cramped locations.  

By convention, one end of the cable is marked red.  This is to be connected to terminal no. 1 in the connector.  One downside of the ribbon cables is that it does not allow flexing in the sideways.  The cable can only bend up or down.

Polarized and Unpolarized Power Sockets

Non-Polarized plug and socket. 

Polarized Power sockets refer to sockets where the phase and neutral connections are fixed.  That is, the plug can be inserted in any direction.  Polarized connections, on the other hand, are designed that the phase and neutral pins can go into specific holes.  Three pin connectors are in general polarized connectors. An exception is the Italian mains connector with three pins which can be connected either way.

Polarized power sockets are used in applications where interchanging the phase and the neutral conductors can result in shocks as in a toaster.  Since the switch is connected to the phase line, send the phase supply through the neutral would mean that the appliance will have power even when it is switched off.

Observe the difference in pin width

A table lamp needs a polarized socket as interchanging the phase and the neutral lines would result in supply reaching the lamp holder even in the power off condition.

What is the difference between a condenser and a capacitor ?

A condenser and a capacitor are essentially the same thing.  They are just two different names.  Capacitors were called condensers earlier.  The term "condenser" is still used in the automotive industry.  A capacitor is a electric component which consists of two conducting plates separated by a dielectric.

Capacitors find wide application in the electric and electronic industry.

What are polarised and non polarised capacitors

Polarized capacitors are capacitors which can be connected only in one polarity.  That is, the positive and negative terminals cannot be changed. Electrolytic capacitors which use capacitors which use rubber, paper or glass are examples of non-polarized capacitors.

Electrolytic capacitors are examples of polarized capacitors.  In these capacitors the polarity is fixed.  The positive and the negative terminals of the capacitor should be connected properly.

Interchanging the positive and negative terminals can cause the capacitor to explode.