Just when I thought I was getting it,

Thread Starter


Joined Sep 29, 2017
I tried another exercise with those pesky zeners. The exercise had some things that give me grief, So i thought it would be a good test.
Here is the link to the exercise and the schematic.
I wired it up as shown using exactly the specified components other than the lamp and it's socket.
I expected to get a light then I was gonna put in the metering points. Alas no light. I checked lots of times and was unable to discover any error.
So. I substituted 16v instead of 9v and it lit up though the resistor did not last long. No surprise there.
So, could the makeup of the lamp be causing my grief ? In that it is quite different than what was specified.


Joined Apr 5, 2008

What lamp are you using?
The given page states this:
One lamp rated for approximately 25 mA at 6 volts.



Joined Nov 30, 2010
Isn't it 150 mW
Ah yes. I ran that twice on my calculator and still read it wrong.:oops:
Probably shouldn't try to do math right after a nap.:D
But the premise is still good. I would have to scratch through a drawer of tiny, bayonet base light bulbs to find one which uses so little power as to be useful in this experiment...and I still might not find one.;)


Joined Mar 31, 2012
Since it appears you have some meters, I would recommend measuring a few points on the operating characterstic of that light bulb.

Measure it's cold resistance. You will probably find that it is pretty low (compared to what its wattage and voltage ratings would indicate). This is because the filament in an incandescent bulb operate at very high temperatures (the have to be literally white hot in order to give off white light, after all) and the resistance goes up significantly as you go from room temperature to operating temperature.

Now apply some low voltages, say a 1.5 V battery, to the bulb and measure of the voltage and current. So the same for 3 V, 4.5 V and on up to the rated voltage of the bulb (and perhaps a bit above, but not too much unless you have additional nominally identical bulbs to use).

Now go ahead and put your original circuit together and make the measurements indicated. Are they consistent with the numbers you measured for the bulb?