I'm playing around with a very simple setup:
- 9V battery
- light diode
- voltage divider (R1 – three 1 kOhm resistors, R2 – one 1 kOhm resistor), which should give me roughly 3V for the diode
The first time I assembled it, everything was fine, but after a while diode got much dimmer and then stopped emitting light. The battery is ok, as is diode, the problem is in one of the resistors, R2. With power plugged in it shows crazy 10-12 Mega Ohm. When I disconnect the battery it returns to its normal resistance at around 970 Ohm. No wonder diode isn't working given that resistance.
Why is the resistance so high?
I had another, more complex setup and there was the same problem, I decided to assemble something simpler, but there is this problem with very high resistance again.
Best Answer
The problem is that you are trying to measure the resistance while the circuit is energized.
First, you have to keep in mind that there is no such thing as measuring resistance directly. The multimeter applies a small current to the resistor being measured and works out the resistance by measuring the voltage drop.
By measuring an energized circuit, you are not only measuring the wrong voltage drop, but also have the risk of damaging the measuring device.
A similar question has been asked before.