What accounts for for these weird scope readings? - Page 1
Forum section: electronics / beginners

author: FenderBender
author_rank: Super Contributor
timestamp: April 13, 2013, 12:40:31 am
content: A while back I just built up a bridge rectifier circuit to play around with. I hooked it up to a transformer but did not attach a load across the output.I remember getting some really strange images on the scope. Lots of weirdly deformed sine waves and flickering and all sorts of garbage.When I hooked up a resistor, it all went away. It almost seemed that the diodes were rapidly switching on and off or something a long those lines. Why does adding a load solve this problem?Thanks.

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2013, 12:44:37 am
content: The AC will happily transfer across the internal capacitance of the diode if there's no load for it to drop into. Also, all diodes will conduct a very small current in the reverse direction, so if there's no forward current to swamp the reverse current, it's going to look a good bit like a resistor instead of a diode.

author: FenderBender
author_rank: Super Contributor
timestamp: April 13, 2013, 12:50:50 am
content: But how is there any current at all if there's an open circuit on the output?

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2013, 12:51:10 am
content: Through the oscilloscope probe.

author: FenderBender
author_rank: Super Contributor
timestamp: April 13, 2013, 12:53:39 am
content: Quote from: c4757p on April 13, 2013, 12:51:10 amObviously. Very obviously.

author: FenderBender
author_rank: Super Contributor
timestamp: April 13, 2013, 12:53:58 am
content: Much thanks.

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2013, 12:56:45 am
content: Also through the aforementioned reverse leakage (depending on the diode type, possibly more than the current through the scope probe).

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2013, 01:09:06 am
content: Quote from: FenderBender on April 13, 2013, 12:40:31 amThere's also the possibility that the trigger was configured wrong. That's often the cause of flickering.

author: FenderBender
author_rank: Super Contributor
timestamp: April 13, 2013, 01:38:50 am
content: Perhaps it was, but when I hooked up a resistor, it displayed fine. I didn't touch the trigger.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: April 13, 2013, 01:40:25 am
content: What you're describing could be signal reflections, with a poor trigger setting so they're not stable on the screen. Was the probe set to 1x or 10x? Ground lead attached or not? Pictures would help, too

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2013, 01:41:17 am
content: If the trigger was just barely in the right position, the presence or absence of the resistor could have caused a small voltage difference and caused the trigger behavior to change.Or the trigger had nothing to do with it.Quote from: Galaxyrise on April 13, 2013, 01:40:25 amSubstantial signal reflections at 60 Hz? I don't think so...

author: FenderBender
author_rank: Super Contributor
timestamp: April 13, 2013, 01:43:34 am
content: Oh boy this was a long time ago. I just thought about it recently. From what I remember, ground lead was(?) attached. Not sure about the probe.c4757p, you may very well be right. ACTUALLY, now that I think of it, I'm not sure if it was flickering at all. I remember taking a picture of it. Let's see if I can dig it up.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: April 13, 2013, 01:44:08 am
content: He's looking at pulse dc, right? The frequency components right at the 0v bounce are going to be a lot higher than 120Hz, aren't they?

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2013, 01:48:46 am
content: They will be a lot higher than 120Hz, but "a lot higher than 120Hz" is still pretty low, especially if the diodes were the typical 1N400x slow rectifiers. 120Hz is barely fast enough to get a solid display on an analog scope.

author: FenderBender
author_rank: Super Contributor
timestamp: April 13, 2013, 01:50:49 am
content: Oh I remember what was bothering me. The scope appeared it was stable. No flickering at all, but the signal it was displaying seemingly had "two functions". I guess you could say that it didn't pass the vertical line test.If I recall correctly, it looked something like this. Sorry for asking a question and I barely remember the situation myself.

author: FenderBender
author_rank: Super Contributor
timestamp: April 13, 2013, 01:51:23 am
content: They were 1N4004s.

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2013, 02:17:32 am
content: That is definitely an odd signal. (Usually having two superimposed like that just means that it's alternating between them.) Honestly, I couldn't tell you with certainty what caused that, and I don't want to speculate and make an ass out of myself.Quote from: FenderBender on April 13, 2013, 01:51:23 amSlow as hell diodes, intentionally, to avoid introducing high frequency switching noise. Not good for much other than mains frequency rectification, but theyarepretty boss at that one purpose.