Page 336 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Designing a decorative serial lamp for a given supply

voltage

Objectives: At the end of this lesson you shall be able to
• calculate the number of bulbs to be connected in series for a given supply voltage.

Serial set design
We have to design a row of 6 or 9 volts lamps. If these lamps are connected directly to the 240V supply, the
lamps will get fused immediately. Therefore the lamps are to be connected in series. The calculation as shown
will be -
1 For 6 volts lamps
240
Total No. of lamps required = = 40 lamps.
6
Taking 5% allowance for fluctuations in the supply voltage

Total No. of lamps = 40 + (5% of 40)
= 40 + 2 = 42 lamps.
2 For 9 volts lamps
240
Total No. of lamps required = = 26.6 or 27 lmps.
6
Taking 5% allowance for fluctuations in the supply voltage
Total No. of lamps = 27 + (5% of 27)

= 27 + 2 = 29 lamps.
The circuit for a series lamp connection of 6V lamp and supply voltage 240V. (Fig 1)

Fig 1

Precautions
• Never connect the low volt lamps directly to the mains.
• Never touch the exposed wires.

In the above case we discussed for 6V and 9V lamps. In the market we get for 6 volts different current ratings viz.
100mA, 150mA, 300mA, 500mA. The shape of the lamp for the above current ratings however remains the same.
For the series lamps to work satisfactorily the current rating of all the lamps should be the same.

We can prepare serial lamps with different voltages but of the same current rating.
Example
You have 25 lamps of 6V, 300mA rating and 20 numbers of 9V,300mA lamps. How will you design a ‘serial lamp’
circuit for 240V supply mains
a using all the available 6V lamps and for the rest of 9V lamps.
b using all the available 9V lamps and for the remaining 6V lamps.

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CITS : Power - Electrician & Wireman - Lesson 54-59