7805 for sound card input protection? - Page 1
Forum section: electronics / beginners

author: ledtester
author_rank: Super Contributor
timestamp: November 03, 2022, 02:23:11 pm
content: This video:SoundCard Oscilloscope Probe Adapter ➕ Software Reviewhttps://youtu.be/BopocGfIPFYproposes to use a pair of 7805's as input protection for a sound card oscilloscope probe as shown here:It's such a weird use of a 7805 I'm not sure how to analyze it.Apparently it works as the rest of the video demonstrates but how?

author: radiolistener
author_rank: Super Contributor
timestamp: November 03, 2022, 02:38:31 pm
content: this is a bad solution. Because 7805 add non linear signal distortions.If you want to protect input from overvoltage, you can put protection diodes. One diode has about 0.5 V drop, but depends on the exact diode type. So, if you're needs 1 V cut-off, then just put two diodes in series. When Voltage is going above diode drop, diode will be opened and made short circuit.Also you can use TVS diode for overvoltage protection. If overvoltage can remain for a long duration, you can add fuse which will be burned out when protection diode will be opened.

author: golden_labels
author_rank: Super Contributor
timestamp: November 03, 2022, 03:37:23 pm
content: Someone correct me, if I miss something obvious, but how is 7805 supposed to provide protection on the negative side of the waveform? The D1 seem to be added to cut half of the signal out, but that also blocks positive voltages below 0.7V and introduces distortion from signal present on the other channel. In other words, how would that setup work for anything other than signals sitting entirely in the 850±150mV range?For the same reason, remember that the diodes suggested by radiolistener must be placed both directions. You are dealing with AC signals.

author: magic
author_rank: Super Contributor
timestamp: November 03, 2022, 04:32:53 pm
content: I'm not sure what D1 is for and the outcome depends on whether there exist a ground loop bypassing itLM78 will clamp the signal to one diode drop below its ground pin. Every NPN collector on the schematic below is the N of a diode whose P is ground.The whole idea makes no sense to me. I expect lots of distortion, possible disappearance of weak signals (like tens of mV), poor input impedance (current going to ground rather than to the output).

author: CaptDon
author_rank: Super Contributor
timestamp: November 03, 2022, 08:14:08 pm
content: That has to be about the stupidest way to protect a sound card input using 7805's!!!! There are so many other better solutions!!! And then the guy posts it on youtube as if he just re-invented the wheel. It may work, but it's like designing a suspension system so you can run square wheels.

author: golden_labels
author_rank: Super Contributor
timestamp: November 03, 2022, 09:23:06 pm
content: Quote from: magic on November 03, 2022, 04:32:53 pmGuess what that 2kΩ resistance between ground pins of 7805s is for!The longer I look at that, the more it reminds me of programs written by beginners. It doesn’t take much to learn making computer do what you want it to do. But it takes expertise to make it not do what you don’t want it to do. Someone had good EE intutition addressing particular issues, without noticing they’re digging deeper and deeper into even larger problems.The good thing is that this is a publicly available, open project: it can be studied and criticized by anyone. Now imagine that would be something one buys, hidden in a glued case.

author: magic
author_rank: Super Contributor
timestamp: December 04, 2022, 05:31:09 pm
content: Quote from: golden_labels on November 03, 2022, 09:23:06 pmI honestly have no ideaI found that video opened in a browser tab that I forgot about and I watched it today.The author doesn't explain much about the circuit (maybe it's obvious) so I'm not any wiser.I think it's some original invention which will remain misunderstood by mortals forever.I would try more conventional solutions instead.editI don't know what the 2kΩ is for but I know what it will do.If you apply enough voltage to the input, the quiescent current of the 7805 will flow through that resistor. That's 5mA×2kΩ = 10V. So the output will be regulated to 15V rather than 5V, that's what it will do.I am nevertheless surprised that this gadget is able to pass 250mVpp signals without visible distortion. What sort of path is that current taking through the chip?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 05, 2022, 10:01:48 am
content: Using voltage regulator for protecting a signal input is a rahter poor idea, even if it works somehow. It may as well be a "free energy type" hoax, just as click bait.The more usual, cheaper and safer way is a series resistor and than diodes (possibly 2 in series each to get a little larger useful range) for clamping. So something like 10 K and 2 or4 x 1N4148 for each input.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 05, 2022, 10:39:35 am
content: Radiolistener is right.A TVS is the simplest way to get this function. The 7805 guy or gal (or other) is nuts.Look at 2V line protectors like the PESD2V0Y1BSF.Note that mic "gnd" is not 0V on some sound cards but Vdd/2, commonly 1.65V. That would make the 7805 solution even more useless.

author: magic
author_rank: Super Contributor
timestamp: December 05, 2022, 11:09:05 am
content: Quote from: Kleinstein on December 05, 2022, 10:01:48 amIt absolutely doesn't work. It clamps the input to one diode below ground (maybe the 2kΩ resistor is meant to prevent magic smoke with negative inputs?) and any signal coupling from IN to OUT of a voltage regulator is more or less accidental, inaccurate and poorly controlled. He said he had problems getting it to work with more than 250mVpp and that he "will work on it". I'm surprised he even got that far.I'm sure he genuinely thinks it's a good idea, just like the original inventor he copied it from.Quote from: Terry Bites on December 05, 2022, 10:39:35 amLOL, good point. This is exactly the case with those USB dongles of the sort that this guy usesHe basically shorts out the virtual ground. But the signal passes through to the ADC because it's AC coupled, so it appears to work just fine.

author: amyk
author_rank: Super Contributor
timestamp: December 05, 2022, 02:33:22 pm
content: I can see how someone thought it would work after looking at what the dropout characteristics of a voltage regulator look like, but as others have mentioned, it won't work well and is far more complex than a pair of antiparallel diodes.But it does beg the question, what happens if you use an LDO instead...

author: magic
author_rank: Super Contributor
timestamp: December 05, 2022, 02:46:23 pm
content: Nothing good, of course.Still no worky with negative voltages.Zero guarantee that an LDO keeps being LDO with 0.5V or 1V input voltage.It's typically some PNP/PMOS and it needs base/gate drive to turn on.You may get some wimpy parasitic coupling through some mysterious path, like with that LM7805.

author: EPAIII
author_rank: Super Contributor
timestamp: December 08, 2022, 07:13:49 am
content: Perhaps the magic smoke IS how it works. The 7805s turn into dead shorts.