| Electronics > Beginners |
| LED voltage measurement question |
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| hamster_nz:
Hi Lucky-Luka, Electronics works by assumptions and approximations. Things like "oh, this wire just conducts electricity" or "current though a resistor varies linearly with voltage", or "9V batteries put out 9V" and so on. These assumptions don't always hold true, but when people design electronics they work with the simplified models that apply at their level of design, so an auto electrician will use different models than a radar engineer for things like wires. So pretty much everybody will agree that a 1V DC over a 1000 ohm resister does cause 1 milliamp to flow through that resistor (+/- however much that resistor isn't exactly 1000 ohms due to manufacturing errors). However, if that is 1V AC of a WiFi signal rather than DC, then that model will be wrong. So there are "first approximations", more true-to-life "second approximations", "third approximations" and so on, each more true then the previous ones. The first approximations are usually the "ideal" device. For a diode it is "A diode lets current only flow one way". The second approximation for a dioide is "A diode lets current over 0.7V flow one way" The third approximation might be "A diode lets current over 0.7V flow one way, and has a low-valued resistor in series with it" Further approximations might include leakage (where a tiny amount of current can flow the other way), and capacitance (where the diode can store a small amount of charge). Everything you are using in your design - the breadboard, the wires, the switch, the power supply, the LED, even the DVM is like this. Some are more complex than others, but all will behave oddly under unusual operating conditions. Modern DVMs are pretty sensitive devices. They are made to detect really small voltages and really small currents. They are sensitive enough to show things that prove that the assumptions and approximations are just that. This is what you are seeing - it is a phantom reading. I mean it is real, but it is most likely of no actual consequence - like when you just hold the probes up in the air, they will read something but not something that is useful. I really don't know what is going in your case - is the LED acting like a solar cell? is the wall wart (that may be a 'smart' device) constantly probing to see if a load is attached? Is some of the mains frequency leaking through the Wall Wart? Is the breadboard acting like a capacitor, not a bunch of passive wire? Do you have a WiFi router at the edge of your desk that is causing this? Do you have flouro lights? The guess that the power supply has something to do with it is pretty good one, as everything else is pretty simple. So you need a simple power supply that works much like the first approximation for an ideal battery... Batteries are about as simple as they get, they pretty much act as a voltage source with a low-valued resistor in series. So try it with a battery and see if the effect remains. This is the "interesting" part of electronics. Finding weird things, analyzing unexpected problems and learning. However, it is also the frustrating part too! One thing I am pretty sure I know is this: if you put a 10k resistor over the legs of the LED will make the problem go away. It will still light when you press the switch, and it will read (very close to) zero when the switch is open. The 10k resistor will swamp out any of these minuscule effects, and make things look more like the "ideal" LED. Sometimes you will find out really odd things, like I found out can't use my Oscilloscope in the wall socket that was in the same circuit as a baby monitor, because it does something weird and adds quite a bit of RF ripple to all the readings. That took me quite a while to understand that it wasn't my circuit at fault! |
| Wimberleytech:
After further experimentation, I conclude that the issue is simply electrostatics. I can move my hand around the probes and modulate the voltage...kinda like a visual theremin. As suggested, I put a resistor across the LED (a 100k was handy) and the problem went away. |
| ModemHead:
I have one of those cheap 9V adapters, possibly even the same one. The scope shot represents the voltage to earth from the 9V output (either terminal). I measure about 75uA leakage current. Nothing unusual for these types of devices. So if one lead of the LED is connected to the power supply and the other one is left floating, you'll get a little positive voltage from the photo-voltaic effect, but possibly a much larger negative voltage based on how much capacitive coupling there is between the free end and earth. |
| Lucky-Luka:
--- Quote from: hamster_nz on March 30, 2019, 08:08:10 am --- One thing I am pretty sure I know is this: if you put a 10k resistor over the legs of the LED will make the problem go away. It will still light when you press the switch, and it will read (very close to) zero when the switch is open. The 10k resistor will swamp out any of these minuscule effects, and make things look more like the "ideal" LED. --- End quote --- Can you explain me why? thanks Luca |
| Lucky-Luka:
To clarify the situation I have discovered that touching the power module the voltage drops to mV values. If I measure the voltage differential at the power module + - pins I read 0 V. So I have upgraded a little bit my system. The problem is gone: 59mV voltage read, which is ok. Stray voltages seem to be the culprit. Am I right? But what does it mean? Thanks everybody for the contributions: I'm a beginner so everyhing I read here helps me a lot. Luca |
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