This is the circuit that I am building ( again ). I am doing these over and over until I can predict and measure them correctly.
The lamp that I used was this.

5v, .06 amps, incandescent. Everything works fine and I get these numbers measured at the lamp.
Voltage drop = 5.25
Current = 61ma
So, using ohms law, 61 = 5.25 / x
Which in my calculator is 86ohms.
My problem is that when I measure the resistance of the bulb I get 12.5 ohms.

Why ? Or am i expecting things to be too exact.

Thanks,
George

 

Hello,

When you are measuring the resistance of a cold bulb, you will find a much lower value.
The resistance will rise when the current rises.

Read this page for more info:
https://www.quora.com/Why-is-the-resistance-of-a-lamp-bigger-when-it-is-lit

Bertus

 

bertus has the major effect. Measurement to one ohm accurately is not necessarily difficult, but it's an effect. The test leads may have a resistance of close to an ohm.

 

Yeah, incandescent bulbs do that. You do need to factor in the cold current to make sure it doesn't exceed the components in your circuit

 

The resistance is generally around a factor of 10-13x higher at operating temperature then when cold.

 

This is the circuit that I am building ( again ). I am doing these over and over until I can predict and measure them correctly.View attachment 140608
The lamp that I used was this.
View attachment 140609
5v, .06 amps, incandescent. Everything works fine and I get these numbers measured at the lamp.
Voltage drop = 5.25
Current = 61ma
So, using ohms law, 61 = 5.25 / x
Which in my calculator is 86ohms.
My problem is that when I measure the resistance of the bulb I get 12.5 ohms.

Why ? Or am i expecting things to be too exact.

Thanks,
George

I think this was pointed out in your other thread -- the resistance of an incandescent bulb filament is a STRONG function of temperature -- at full operating temperature (i.e., literally white hot) they are typically 13 to 14 times the resistance at room temperature.

 

You have to be careful measuring the resistance of lamps at room temperature because the current you pass into the lamp might actually heat it. Keeping the voltage low is therefore important: the best way to measure it is to pass a small current and measure the voltage across it.
You can estimate the temperature of a lamp from its resistance, using tungsten resistance data. Many lamps are rated at 2700K and these show a 13:1 typical change while torch bulbs may run at only 2200K and show a 10:1 change.