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| Photodiode Detecting Mains-Powered Filament Light Bulb Output as 100Hz ?! |
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| MikeNye:
Hi everyone, I've been playing around with some photodiode devices (Texas Instruments OPT101) lately for a hobby project that I'm working on, and I've come across a weird phenomenon that had me baffled for a couple of days... Not quite as weird as Dave's weird oscilloscope phenomenon, but still pretty confusing for a beginner. I think I have figured out why this is occurring, but would be keen for someone more experienced to verify my findings... Read on for more info! I have a very simple photodiode circuit (figure 5 from the OPT101 data sheet if anyone is interested), with my oscilloscope measuring the voltage created as a result of light level exposure to the photodiode. When I illuminate the photodiode with a stable light source, such as a battery powered LED, then the voltage generated by the photodiode device is flat DC - this is what I expected. The weirdness occurs when I illuminate the photodiode with a mains-powered halogen desk lamp. Initially, the scope trace showed a sinusoid with a DC offset, which I also expected. The DC offset for the brightness of the halogen lamp, with the sinusoid being caused by the very small changes in brightness (faster than the eye can recognise) as a result of the 50Hz mains alternating current. So, I expected the sinusoid to have a frequency of 50Hz - identical to mains AC frequency. However, when I actually measured the frequency of the sinusoid, it was 100Hz - double mains frequency! Check it out: This had me really confused, because I would've expected the sinusoid to have a frequency of 50Hz, however, upon further consideration, I think the problem can be answered with the following reasoning: Mains AC voltage is at 50 Hz, with a period of 20ms. Therefore, the filament will have current flowing through it in one direction for 10ms, and the other direction for 10ms. Because the filament will glow regardless of the direction of current, we get peak light output twice per AC cycle (once during positive current, once during negative current), giving us the resultant sinusoid with a period of 10ms, or a frequency of 100Hz. I hope my explanation makes sense, and I would be very interested if someone more knowledgeable than I could verify my thought process... Thanks for reading! -Mike |
| TheWelly888:
Have you tried looking at other light sources such as fluorescent lamps? And ordinary light bulbs ( the now banned sort )? Also driving an LED with different waveforms? |
| rossmoffett:
You're correct. |
| DavidDLC:
The mains have a frequency of 50 Hz, for each complete sine wave the light will vary the intensity twice on the positive and the negative part of the wave, that is what you are detecting. |
| kc1980:
Yup, you're right. We've done a few wire arc simulations using 400Hz aircraft power. During the arc event, we would always hear a constant pitch which, after very little thought, we originally believed to be 400Hz. However, when I generated a pure tone via my mp3 player just for kicks, I found that the pitch of the arc was in fact 800Hz. Duh! |
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