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Q & A: What's a 7.2 Volt bulb doing in a 9 Volt flashlight?

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Most recent answer: 05/01/2015
I needed to replace the light bulb in a 6-cell (alkaline D cells) hflashlight. The recommended bulb is a PR-18. I was surprised to find that the bulb's package says that the bulb is for a 6 cell flashlight but it is rated at 7.2 V. I was expecting a 9V rating based on 1.5 volts per alkaline D cell. Why is the bulb not rated for 9V service?
- Bill (age 74)
Schenectady, NY USA

Hi Bill,

The problem is that a D cell battery has an internal resistance of about 0.5 Ohms.   Six of them in series adds up to 3 Ohms worth.   Now the PR-18 is rated at 4 Watts.  That means at 7.2 Volts it draws about 0.56 Amps.  The voltage drop on 3 Ohms is then 0.56*3 = 1.7 Volts.  So  9 - 1.7 gives you 7.3 volts left over.  Seems about right.


(published on 05/01/2015)

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