A sodium vapor lamp is a low pressure and cold cathode lamp i.e its electrode is not electrically heated by a filament. A sodium vapor discharge lamp consists of a U-shaped tube enclosed in a double-walled vacuum flask, to keep the temperature of the tube within the working region. The inner U-tube consists of two oxide-coated electrodes, which are sealed with the ends. These electrodes are connected to a pin type base construction of a sodium vapor lamp is shown in Fig.



This sodium vapor lamp is low luminosity lamp so that the length of the lamp should be more. In order to get the desired length, it is made in the form of a U-shaped tube. This long U-tube consists of a small amount of neon gas and metallic sodium. At the time of start, the neon gas vaporizes and develops sufficient heat to vaporize metallic sodium in the U-shaped tube.

Neon Lamp
This is a cold cathode lamp, in which no filament is used to heat the electrode for starting. The neon lamp consists of two electrodes placed at the two ends of a long discharge tube.



The discharge tube is filled with neon gas. A low voltage of 150 V on DC or 110 V on AC is impressed across the two electrodes; the discharge takes place through the neon gas that emits light or electromagnetic radiation reddish in color. The sizes of electrodes used are equal for both AC and DC supplies. On DC, neon glow appear nearer to the negative electrode; therefore, the negative electrode is made larger in size. Neon lamp electric circuit consists of a transformer with high leakage reactance in order to stabilize the arc. The capacitor is used to improve the power factor. Neon lamp efficiency is approximately 15-40 lumens/W. The power consumption of the neon lamp is 5 W

If the helium gas is used instead of neon, pinkish white light is obtained. These lamps are used as night lamps and as indicator lamps and used for the determination of the polarity of DC mains and for advertising purpose.


The tap changer is placed on high voltage side because:

1)      The HV winding generally wound over LV winding hence it is easier to access the HV winding turns instead of LV winding.

2)      Because of high voltage the current through the HV winding is less compared to LV windings, hence there is less “wear” on the tap changer contacts. Due this low current, in on load tap changer the change over spark will be less.


Tap changer Primary side:
In this type the tap changer circuit is placed in primary side or supply side. As we know;

    Turns ratio        = secondary winding turns (Ns)/ primary winding turns (Np).

           Secondary voltage = (supply voltage or primary voltage) / Turns ratio.

By the above formulas it is stated that if the primary turns decreases the turn’s ratio increases hence then secondary voltage decreases. Opposite for the reverse case i.e. primary turns increase leads to turns ratio decrease which increases the secondary voltage.

Figure shows the tap changer on primary winding with tap changing interval of 2.5 % per tap. With this we can understand three conditions:

Transformer Tap changer on Primary Side
Transformer Tap changer on Primary Side

1)      In normal operation the tap changer will be at 0% position to provide required designed secondary voltage.

2)   If the supply voltage increases or load current decreases there will be an increase in supply voltage which is not desirable. At this case the tap position in the primary winding will rise towards positive direction i.e. +2.5%, and hence decreases the Np. This will increases the turns ratio (Ns/Np) further decreases the secondary voltage.

Consider the load voltage decreased then the tap changer shift towards negative side to increase the primary turns and hence decreases the turn’s ratio. The secondary voltage will increase to compensate the change.

Tap changer Secondary side:

In this the tap changer is placed in secondary side of the transformer. This type of taping is used in step-up transformer where low voltage winding is in primary side and high voltage winding is in secondary side. Figure shows the tap changer circuit on secondary side with tap interval of 2.5 %. In some distribution transformers the tap changer resolution can be up to 1% for fine adjustments.

Transformer Tap changer on secondary Side
Transformer Tap changer on secondary Side

In this the case is reverse compared to primary tap changer. To increase the secondary voltage the tap changer will move towards positive direction and it moves in negative direction to decrease the secondary voltage.

Resistor values can be known by the color band.
0 black
1 brown
2 red
3 orange
4 yellow
5 green
6 blue 
7 violet
8 grey
9 white
First color = digit
Second color=digit
Third color= power of ten
Fourth color =tolerance
Now in the given question 50 × 100  ± 2%
5 = Green
0 = Black
100 = Black
± 2% = Red


https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg RRB JE, SSC AE/JE UPSSSC JE, SSC JE, CIVIL ENGINEERING MCQs, ELECTICAL ENGINEERING MCQs, preavious year quesion papers, dmrc, lmrc, drdo,rrb ntpc, ntpc, pgcil, dsssb, states board, GATE IES EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA