Electronic – LED and lamp in series – why does the bulb not light


My 6 year old son has just started experimenting with a Snap Circuits style kit and already we have a very basic question.

If we arrange an LED and lamp in parallel powered by batteries then both LED and lamp light up brightly.

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However, if we arrange the LED and lamp in series then only the LED comes on.

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Obviously current is passing through the lamp (if I unscrew the bulb the LED goes off).

So why does the bulb not light?

I'm a bit of a cheapskate so rather than buy proper Snap Circuits I bought a similar generic set from China on eBay (See: Electronic Blocks Kit W-58)

(Apologies if this is too basic for this forum but I haven't yet found the answer via Google)

Best Answer

For the parallel connected LED and lamp, each has the entire battery voltage across.

When series connected, the voltage across each must sum to the battery voltage.

Without any more information than is given, the most likely answer is that the voltage across the lamp, which must equal the battery voltage minus the voltage across the LED, is insufficient to produce visible light.

While typing this answer, I see that you've added some pictures. It appears that the total battery voltage is about 3V. Given that many LEDs have a forward voltage in excess of 2V, this leaves less than 1V across the bulb.

Do you have a voltmeter with your kit? If so, measure the voltage across the lamp for the series connection.

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