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Oscilloscope oddities
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GadgetBoy:
So, I have this neat DSO138 that I got online, and I've been planning to do some practical demonstrations on AC rectification for my YouTube channel, but I'm running into something a bit weird. So, I have a frequency generator producing a nice 100Hz 8Vpp sine wave (first picture). Now I hooked up the generator to a 1n4007 diode with a 100ohm load resistor, and I don't get a nice half-wave rectified waveform (instead I get picture #2). It's been quite a while since I played with an oscilloscope, so I imagine I'm doing something wrong - can anyone point me in the right direction for establishing a good half wave rectified waveform (and subsequently full wave rectified waveform)?

Thanks!

Gadgetboy

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Mr. Scram:
It looks a bit like it's hitting the breakdown voltage on both sides and conducting again. Are you sure your diode is good? Note that though the DSO138 is a neat little thing, it's limitations can sometimes cause effects to show that aren't really there. It can be difficult to distinguish what's what if you don't have a "real" DSO at hand. Aliasing would be one example where you can get into trouble with the DSO138.
Ian.M:
Try a 1N4148 + a 10K load as I suspect you are overloading the signal generator.
GadgetBoy:

--- Quote from: Mr. Scram on August 13, 2018, 06:33:39 am ---It looks a bit like it's hitting the breakdown voltage on both sides and conducting again. Are you sure your diode is good? Note that though the DSO138 is a neat little thing, it's limitations can sometimes cause effects to show that aren't really there. It can be difficult to distinguish what's what if you don't have a "real" DSO at hand. Aliasing would be one example where you can get into trouble with the DSO138.

--- End quote ---
Didn't think to test the diode, but it's fine. Tried a different diode just in case - with similar results. Tried a 1k resistor, and the waveform is nearly complete with just a little nipped off of the bottom curves. First picture is waveform across the 1k. Second is waveform across the diode.

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JS:
The load might be too much for your signal gen.

You have 4V peak, if your sig gen is 50Ω it should output 2.6V peak into 100Ω, take the 2V- from the rectifier you should be seeing at least 0.6V peaks. Also, the generator needs to source a few mA.

Try taking the load out, or use a much bigger one if the signal isn't that clear. 1k to 10k should do.

Also, with a low voltage AC you won't get a very nice rectified signal, something more like what you are seeing, increasing source voltage, decreasing the load, using lower voltage diodes (Ge or schokty) would make things better.

JS

PS: too many fast replies, just to add, if you can't generate higher voltage you could use a mains AC transformer and take the output from there, sine could be a little distorted as those tend to saturate at the peaks but the picture might end better than with your config.
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