Voltage

Thread Starter

Lightfire

Joined Oct 5, 2010
690
Hello,

I have read somewhere that it is OK to put three 1.5 V batteries in a bulb rated 3.5 volt because if you put the bulb onto the batteries, the batteries' volt will fall to 3.5. Is that true? How it comes?
 

CDRIVE

Joined Jul 1, 2008
2,219
Hello,

I have read somewhere that it is OK to put three 1.5 V batteries in a bulb rated 3.5 volt because if you put the bulb onto the batteries, the batteries' volt will fall to 3.5. Is that true? How it comes?
That is a ridiculous statement (by whoever wrote it) because it makes no reference to what this bulb draws. Incandescent bulbs are rated in Volts and Watts which will give you 'I'. Your V drop will be Dependant on this Vs the capacity of the batteries.
 

Thread Starter

Lightfire

Joined Oct 5, 2010
690
Voltage rating

This is the supply voltage required for normal brightness. If a slightly higher voltage is used the lamp will be brighter but its lifetime will be shorter. With a lower supply voltage the lamp will be dimmer and its lifetime will be longer. The light from dim lamps has a yellow-orange colour. Torch lamps pass a relatively large current and this significantly reduces the output voltage of the battery. Some voltage is used up inside the battery driving the large current through the small resistance of the battery itself (its 'internal resistance'). As a result the correct voltage rating for a torch lamp is lower than the normal voltage of the battery which lights it!
For example: a lamp rated 3.5V 0.3A is correct for a 4.5V battery (three 1.5V cells) because when the lamp is connected the voltage across the battery falls to about 3.5V.
http://www.kpsec.freeuk.com/components/lamp.htm
 

Bernard

Joined Aug 7, 2008
5,784
There are many LED flash lights using from one to 9 bright white LEDs in parallel running on 3 ea AAA, AA, or N . Some of mine are quite old and never lost a LED.
 

CDRIVE

Joined Jul 1, 2008
2,219
Post #3 would seem to imply he's talking about incandescent lamps, not LEDs. This statement, extracted from the same post implies a specific voltage drop but ignores the large difference in mAH between battery families. The mAH-AH rating of a battery is the governing factor here.
Rich (BB code):
For example: a lamp rated 3.5V 0.3A is correct for a 4.5V battery (three 1.5V cells) because when the lamp is connected the voltage across the battery falls to about 3.5V.
 

Adjuster

Joined Dec 26, 2010
2,148
This is indeed standard practice (but soon may not be, as filament lamps grow obsolete). The nominal voltage of the filament lamp will be lower than that of the battery intended to power it. This leads to lamp ratings such as 1.2V, 2.4V, 3.5V, and 4.5V to about 5.5V, the latter range reflecting the fact that some portable lights used very much higher capacity "6V" batteries.

This is not very precise, and clearly it will not apply unless there is some rough relationship between the current taken and the battery capacity. A considerable variety of lamp voltages and currents are (or were) available.
http://www.lampco.co.uk/results.asp?page=2&section=Lamps&watt=0-2W&type=Torch&volt=0-15V


Typically a torch (flash-light) using small cells might have a lamp using 300mA or less. A bigger torch or portable lantern might use more current, or else use a rather higher voltage rated bulb relative to the nominal battery voltage.

The system would originally have been worked out for zinc-carbon batteries which had a pretty rapid decline in voltage with use. Modern alkaline types hold their voltage better, so the optimum lamp voltage would be higher. In practice, it may or may not be worthwhile to get exactly the ideal voltage, given that it is a compromise between brightness and lamp life.

Of course, these days it might be better to forget about filament lamps and use LEDs instead!
 
Top