Why won't the lamp come on?

[decuser]

After seeing an EEVblog video about Radioshack's 200-in-1 https://www.youtube.com/watch?v=r1gTu5r8mkQ, I went and bought one on ebay. I may be fifty-some years older than Sagan, but I don't know quiet as much as the kid, so I figured I might learn something.

Anyhow, after a bit of unpacking, I tried my hand at a project, #24 Light Telegraph, as a way to test the kit out. It worked fine for the 6 LEDs, but when I tried it with the lamp, no go. I simplified the circuit to:

Positive terminal 5V -> Lamp -> 100 ohm resistor -> Negative terminal

... nothing, but the LED's would light up.

Then I bypassed the resistor and the lamp lights up nice and bright. I thought I understood the resistance, voltage, current stuff and that the resistor, being tiny wouldn't get in the way of the lamp, but it appears that I was wrong. Thoughts on what's up?

Thanks,

Will

[TimFox]

The 100 ohm resistor is needed for the LED, since one must limit the current from a 5 V switched source through a device whose voltage is relatively insensitive to the current.
Incandescent bulbs are resistors whose resistance rises quickly as they heat up.
The maximum current you can get from 5 V and 100 ohms is 50 mA (reasonable for a LED set):  what bulb are you using?

[decuser]

It says 2.5V and the booklet shows it's 300mA.

[Analog Kid]

There's your problem:

You've got a voltage divider with the 100Ω resistor and the lamp. Both have 2.5 volts across them (5 volts total from the supply).

So how much current can flow through the resistor? Ohm's Law tells us that

I = E/R

meaning that current (in amps) equals voltage (E) divided by resistance.

2.5 / 100 = 0.025 = 25 mA. But the bulb "wants" 300 mA. That's why it won't light with the resistor; the resistor is limiting the current. Just what we want it to do with a LED, but in this case we're "starving" the lamp.

So what size resistor would you use to light the bulb from a 5V supply?

In that case, we know two things:
o The current we need (300 mA)
o The voltage across the resistor (2.5V)

So we use a different form of Ohm's Law, since we need to calculate R:

R = E/I

So R = 2.5 / .3 = 8.3Ω. That's not a standard value, but 8.2Ω is.

[decuser]

OK. Y'alls questions and comments helped tremendously. I get it now why it wasn't working.

Also, 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.