[ELECTRON] Power / current in a lamp

[Andrew Back]

On (10:47 13/05/10), Seb Bacon wrote:

> ...would it be correct to say that a 12k resistor "draws" 1mA of
> current at 12V?  Could the inscription on my lamp, "0.3A, 3.5W"
> meaningfully be paralleled with an inscription on a resistor that says
> "1mA, 0.012W"?  What do you call that inscription -- is that also a
> type of rating?  Would it be equivalent to say that a typical 0.25W
> 12k resistor ("5mA, 0.25W") is rated at 55V? (Or something similar)?

If you put a 12k resistor across a 12v supply it would draw 1ma of current.
However, I can't think why you would ever want to do this, apart from to
create a "dummy load" and test a power supply. But, yes, you can just think
of an incandescent light bulb resistor or "resistive load". 

Note that context is important when you see figures stated. A, V and W with
a bulb will be the current it draws at the rated supply voltage, rated
supply voltage and power dissipation. With a resistor V would almost
certainly be max voltage that can be put across it before it breaks down,
and W will be max power it can dissipate before burning out / shortening the
life. And of course you can easily calculate the power rating of resistor
required by knowing it's resistance in ohms and the potential difference
across it. Bearing in mind that resistors are usually in series with other
components which also have a series resistance.
 
> Why do we talk about a lamp in terms of a "rated" voltage, but we talk
> about resistors in terms of a power "rating"?

A lamp's rated voltage is its max recommended working voltage. A resistor's
power rating is the max recommended power dissipation. You select bulbs
according to what voltage supply you have available and required brightness.
You select resistors according to a required resistance and power
dissipation (and in rare cases voltage rating also - e.g. in equipment that
works operates with thousands of volts). 
 
> A 0.25W resistor will blow out somewhere above 0.25W; can I know when
> the lamp we're discussing would blow from the information I've given?
> Is that what "rated" voltage means, somehow?

Yes. Go higher and the current flow increases and thus so does the power
dissipated. As you increase voltage the life of the bulb shortens.
 
> Is the only difference in how we talk about them that the "work" we're
> interested in with a resistor is its resistance at any current up to
> its maximum, and the lamp its power at a given voltage?  Why are we
> presumed to care about the lamp's power (rather than, say, its
> lumens)?

That's it.

I think watts are just handier for lamps. For one, you want to know how much
your electricity bills are going to be or how long your battery will last.
And people tend to know how bright a 40w or 100w etc incandescent lamp is.

Cheers,

Andrew 

-- 
Andrew Back
a at smokebelch.org