Inductor, Relay and Transformer

We will learn inductor and similar components relay and transformer.

An inductor is a coil of insulated metal (Copper/Aluminium) wire over a spindle where the hollow part of spindle can be air filled (i.e. no material filled inside), it can have filling of iron metal or ferrite etc. This part is called the core of the coil; accordingly they are categorized as air core, iron core or ferrite core coil etc. Mostly these coils are used to block high frequency signals or to pass certain frequency of signal. Property of inductor is called Inductance denoted as 'L' and its unit is “Henry”, mostly we have coils having inductance in range of Nano-henry or Micro-henry. When current is passed through any coil, a magnetic field is created around it, magnetic strength changes if the current inside coil is changed.

 

 

Inductor

Inductance can be calculated as L = N² x µ x A/l, where 

L- in Henry

N- Number of turns in coil

µ = µ₀ x µ_r where µ₀ = 4πx10^-7 is permeability of free space

µ_r- relative permeability of core of coil, click here for more details.

A- Area of circular cross section of coil as seen from side (here Pi x r²) in square meter

l- Length of coil

Symbol

Inductor does not have any polarity, so any terminal can be connected to any point in circuit. They are represented in circuit by a helical symbol denoting its physical construction.

The material filling the hollow portion of coil changes the inductance as we can see in the formula it is directly a multiplying factor. An air core coil will have less inductance but a ferrite core or iron core coil will have more inductance. In simple language the material in core of coil decide how easily magnetic field can be established if a current flow in the coil. This property allows us to create an electromagnet that is a magnetic field created when the current flow in coil and the magnetic field collapses when the current is stopped.

Series and Parallel Connection of Inductor

Series Inductor

Similar to Resistor and opposite to Capacitor series connection, the net inductance increase when two or more inductors are connected in series as it is sum of all inductance in series.

Parallel Circuit

Similarly, in parallel the net inductance is reciprocal sum of each inductance, hence the final inductance is less than the minimum inductance in parallel connection.

Relay

Relay

Using electromagnetic nature of a coil mechanical relay are made which can be driven with small power supply e.g. 3V, 5V,12V etc and switch loads of high to very capacity voltage and current loads, light electric bulb, motor etc. Refer the diagram which shows a relay (Within dotted box) and two lamps connected with it. Relay has a coil around an iron core; there is a flexible iron trip which is attracted towards coil core when current is supplied in relay coil. Till the strip is in normal position as seen in diagram, it is connected to contact marked as “NC” i.e. Normally Closed contact, in this position Lamp-2 is getting supply from cell connected to “COM” i.e. Common point of relay. 

So in this position, Lamp-2 will glow and Lamp-1 will remain off. If we supply current through relay coil, the electromagnet will be build and the

Relay Symbol

Flexible strip will be pulled towards the coil core hence the flexible strip will leave “NC” contact and will stick with “NO” contact supplying current to Lamp-1, hence it will start glowing and Lamp-2 will go off. We can connect larger load to relay based on its “NO” and “NC” capacity. A relay has basically two specifications, one is the driving voltage and current of the coil, which is normally small, and the other is current carrying capacity of “NO/NC” contacts which is normally high to run bigger loads. See the animated relay circuit below for more clarity.

Transformer

Transformer

A transformer is component which can convert an AC voltage into higher or lower AC voltage based on a the number of turns in the input (Primary) coil and number of turns in output (Secondary) coil. If an AC supply is applied to one coil of transformer, it creates a magnetic field in iron core and since the secondary coil is also physically in same magnetic field hence voltage is created in the same. If number of turns is less in primary and more in secondary coil, it will boost the supply voltage and the transformer is called Step-Up transformer. Similarly if number of turns is more in primary coil and less in secondary coil, it will reduce the input voltage and is called a Step-Down transformer. Please note if voltage is boosted, current will be reduced and vice-e-versa, because we know Power = Voltage x Current.

Power supplied at primary side will be transferred to secondary power with minimal losses in form of heat, so Power_Primary = Voltage_PrimaryxCurrent_Primary

Similarly Power_Secondary = Voltage_SecondaryxCurrent_Secondary, in short V_prix I_pri = V_secx I_sec.

So a transformer cannot boost power hence to maintain the input and output power same, if it boosts the voltage the current is reduced and if it reduces the voltage, current is increased. Also as we said earlier that voltage in transformer is dependent on number of turns, hence we can say V_pri/V_sec = N_pri/N_sec. So, a transformer has typically four specifications i.e. primary voltage, secondary voltage and primary current and secondary current. So, when we buy transformer, we need to mention input voltage, output voltage and output current only, input current is already decided based on output current requirement.

Transformer Working

A transformer looks like this where the primary and secondary coils are on same iron core. Here the number of turns in primary coil is more and wire is thin, that means it can take high voltage and low current as wire is thin. At the same time, the secondary coil is having less number of turns but the wire is thicker, means it can supply lower output but higher current as wire is thick. So, you learn one more things here that a thick wire can carry larger current and a thin wire can carry smaller current. We can compare a wire as a pipe and current in the wire as water flowing in the pipe. A pipe with small diameter can carry little water and a larger pipe can carry large amount of water.

Transformer Symbol

Next we learn about AC and DC voltage or current and their behavior. I should have covered this topic earlier, but missed out so will complete it before moving to next component.

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