question about component ratings

[marchache]

Another newbie question from yours truly. The clouds are starting to clear so hang in there with me.

Bought some bulbs at the local Radio Shack, they have a rating of 7.5V, 220ma.

So does this mean that they are 1.65W bulbs ? I'm assuming that the maximum voltage that should be applied is 7.5V correct ? Or can I apply 9V and the current will drop to 183ma ?

I'm confused.

Next question, a switch has a rating of 1.5A at 250VAC. Eh ? Can I do more current at a lower voltage ? What about DC current, does that make a difference ?

Thanks. Trying to avoid a shocking experience.
Marc

 

[k7elp60]

You are correct about the wattage at 7.5 volts. If you increase the voltage to 9V the current will also increase, and the life of the bulb will be shorter. I have found that if you reduce the applied voltage to incandescent bulbs by 10 to 15% there life is greatly extended.

Switch rating can be real confusing. Generally the DC current rating of the switch contacts is lower than the AC ratings. I have not noticed a
rule of thumb, and sometimes the ratings are for a resistive load or inductive load. Sometimes the voltage affects the contact rating also.

 

[gerty]

I think you are confusing the amp ratings between the switch and the light..The light is a load, consuming 220 ma, the switch is a 'conductor' conducting 1.5 a.. The switch doesn't consume any power and lowering the voltage won't have an effect on it..

 

[marchache]

You are correct about the wattage at 7.5 volts. If you increase the voltage to 9V the current will also increase, and the life of the bulb will be shorter

Of course. I was good in math too :? (just keep repeating v=ir, v=ir, v=ir)

Thanks for the bit about switches. Don't plan on going anywhere near 1A so I should be good (switch will actually be used in the above).

Marc

 

[Dean Huster]

Switches

Switches have a lower DC voltage rating because when arcing starts in DC you can't stop it other than by separating the conductors. In an AC circuit, the current drops to zero twice each cycle, so the power is automatically cut off, stopping the arc, hence the higher AC voltage rating for a given switch.

An incandescent lamp will draw less current with a lower voltage, but will also put out less light and the color will shift toward the red end of the spectrum. An incandescent lamp operated 5% above its rated voltage will have its life expectancy cut in half. One that's operated 5% below its rated voltage will have its life expectancy increased at least four-fold. Overvoltage is a killer for incandescent lamps.

Dean