Q:

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

- Bill (age 74)

Schenectady, NY USA

A:

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.

LeeH

*(published on 05/01/2015)*