Question. Wheatstone bridge for a bulb.

117

01:42:24

This is the Wheatstone bridge.
it allows you to measure the resistance of the light bulb with a comparison method.
by varying Rc it goes up to the value of the bulb resistance. to understand when you get the result you must observe the amperometer. when the voltage at its terminals is zero, ie when no current flows means that we are at equilibrium:
Ra * Ia = x * Ix; Rb = Rc * Ia * Ix.
calculations show that
Rbulb * Rc = Ra / Rb

749nA are acceptable and approximated to 0A. due to the lack of resolution of the change in resistance you can not arrive at 0A.

Rbulb=(3,34 +- 0,01) kohm

confirmation of a result you can look in
P = I ^ 2 * Rbulb
the bulb is 30mW
from the circuit you can see that it is crossed by 1.5mA
with the inverse formula 
Rbulb = P / I ^ 2 = 13,33 kohm

so I wonder: where is the mistake? it is a bug?
what is certain is that the Wheatstone bridge can not go wrong.
Bye.

published 9 years ago

BillyT

9 years ago

Your maths are impeccable, but you have over looked one thing. The 30 milli Watt rating is at 10V. At 10V the lamp will draw 3 mA. R=E/I =3,333 Ohms. The investigation did however find that EC's modeling of a light bulb is not really correct thought, a bulb is a PTC device and as such the 5V current should have been higher than 1.5mA.

hurz

9 years ago

Yes right Billy, Rbulb = P / (2 * I)^2.

zaccoman2

9 years ago

Thank you!:)

thebugger

9 years ago

Keep in mind that in reality this method won't get you very accurate results. The bulb resistance varies widely with the applied voltage. When idle, the resistance will be much lower than at full power. Sometimes it'll vary by a very large degree. This is called a Positive Thermal Coefficient (PTC) as billy already mentioned.

zorgrian

9 years ago

correction! The bulb's resistance varies with the amount of current that passes through it and NOT the applied voltage

thebugger

9 years ago

I=U/R if you know what I mean

hurz

8 years ago

7.49mW at 5V bulbs are were available? http://everycircuit.com/circuit/4595048133165056

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