How come the lamp won't light?

[decuser]

OK. I'm hoping this is a good place to ask this, if not and somebody has a better forum to suggest, I'm all ears.

I have gotten hold of an old 200-in-one from Radioshack and I'm putting together a simple circuit:

Positive Terminal 4.5V->Lamp->100 ohm resistor->Negative Terminal

The light won't come on. But, if I connect the lamp to the negative terminal (bypass the 100 ohm resistor), light on! The same circuit with an LED works fine, but not the lamp. What's up?!

 

[Bluejets]

What is the lamp?
Voltage, wattage and what is the battery capacity(type) if you like.

 

[decuser]

The red lamp is 2.5v, 300mA. The battery is selectable at 1.5v, 3v, 4.5v. I tried it at 3 and 4.5v.

 

[decuser]

Your questions and some others on another forum, gave me food for thought and I eventually worked it out:

I figured out what was up with the dissonance between what I thought I was working on and what I was actually working on. Hang on tight... I have a 1981 kit. I was following the instructions in the 1987 kit. The 1987 kit uses an LED in connections 169 and 170, the 1981 kit uses a lamp. The newer instructions show the LED symbol in the schematic and include the 100 ohm resistor - I = E/R, so I = 3v/100 = 30mA - reasonable for an LED if a little on the high side, but not ok for the lamp - I didn't read the schematic before and just blindly followed the wiring instructions, silly me. I was using a nice, clean printed pdf of the new kit cuz it's legible. The 1981 book and pdf are difficult to read for the wiring diagrams, but since I'm doing the schematics, the book is just fine. The 1981 book doesn't include the resistor in the lamp circuit, so that's nifty - live and learn. I'll be sure to pay attention to the components referenced in the schematic when I wire stuff up.

 

[Bluejets]

So the red lamp is an LED or incandescent?

If the latter, 2.5v @ 300ma approx, 8 ohm....8 ohm in series with 100 = 108
4.5v @ 108 ohm approx. 40ma which does not allow your (2.5 * 0.3 = 0.75w) bulb to light.

To calculate 5mm or 3mm common LED current/ resistor values, common red led voltage is approx 1.7v ( not 3v as you quote) which must be subtracted from the supply voltage before working out resistor and therefore led current.