How to protect an analog ammeter with a diode - Page 1
Forum section: electronics / projects

author: radioholic
author_rank: Newbie
timestamp: July 22, 2022, 11:37:06 pm
content: See attached article "How to protect an analog ammeter with a diode"

author: Cerebus
author_rank: Super Contributor
timestamp: July 23, 2022, 02:35:16 am
content: Hmm. The OP is probably perfectly innocent, but ask yourself "A one-time only poster is inviting me to download something in a format that is know to have had malware related exploits. Should I download it?".From a long time member with a history of posts and taking part in discussions - sure. From someone who has popped up from nowhere and theirvery first everpost from an account registered "Today at 00:23:43" expects you to download something - I'd give it a miss or at least run it through an online tool that scans pdfs for malware first before risking downloading it.I note that there were already 8 downloads before I posted this.

author: pcprogrammer
author_rank: Super Contributor
timestamp: July 23, 2022, 08:24:31 am
content: And none of the down loaders have reported back, so probably busy restoring their machines from a malware attack

author: radioholic
author_rank: Newbie
timestamp: July 23, 2022, 09:27:01 am
content: It is totally innocent, I promise! It has now been downloaded 18 times , so I can only hope that at least one of them will reply confirming that it is not malware.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: July 23, 2022, 09:41:51 am
content: here is my take on this.. one picture in a safe format

author: mikerj
author_rank: Super Contributor
timestamp: July 23, 2022, 09:44:15 am
content: Virustotal gives it all greens so I'd be reasonably confident it's fine.

author: Ian.M
author_rank: Super Contributor
timestamp: July 23, 2022, 10:21:06 am
content: I looked at it on my Kindle Keyboard, on the basis that its PDF viewer wouldn't have any exploits in common with Chromium/MS Edge or Acrobat Reader, and it appears to be what it purports to be. An *OLD* version of Acrobat Reader with Javascript disabled, didn't warn that it contained any javascript, (which it normally does if you attempt to open an interactive PDF) and I exported the following text from it:Quoteso its better than 99.9% certain its legit, but *DUMB* to post here in that format!@radioholic,Yes, at least some of us have seen the article. The absence of comment on the contents indicates that it is not notable but probably not grossly incorrect.If you have a question:http://www.catb.org/~esr/faqs/smart-questions.htmlIf not, why did you dump a PDF on us with no explanation? Attachments are meant to supplement forum posts, not replace them!

author: Halcyon
author_rank: Unknown
timestamp: July 23, 2022, 12:49:03 pm
content: It is but a mere PDF file -- it's safe. But OP is being watched for future potential spamming.

author: Cerebus
author_rank: Super Contributor
timestamp: July 23, 2022, 01:47:55 pm
content: Quote from: Halcyon on July 23, 2022, 12:49:03 pmThat's rather clumsy English - "Tis but a mere PDF file, so it's safe." makes it sound like pdfs are always innocuous, they are not. The list of exploitable CVEs for pdf files is longer than both my arms. I'm sureyouknow better, as you've many times mentioned your security qualifications, but that phraseology could lull people into a false sense of security.The OP, and others, might like to take on board that any topic started which requires you to download and open something to find out what it's got to say is unlikely to gain much traction. The meat of a posting ought to be in the post, not in an attachment, if you want people to actually read it. At the time of writing, this topic has had 321 views, only 30 downloads, which pretty much makes the case for what I'm saying.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 23, 2022, 03:46:05 pm
content: A lot of paranoia out there. We all were a one time posters to begin with.Still, a pair of back to back diodes across the shunt and a series fuse and or PTC will do.

author: Gyro
author_rank: Super Contributor
timestamp: July 23, 2022, 04:35:03 pm
content: Quote from: Cerebus on July 23, 2022, 01:47:55 pmYou really use an exploitable reader to read PDFs that you've downloaded from the web (in general)? Just use something like Sumatra.I do agree that there should have been significantly more meat in the post though.

author: floobydust
author_rank: Super Contributor
timestamp: July 23, 2022, 05:01:13 pm
content: OP's thread title is incorrect - you need two diodes to protect a panel meter. So I wouldn't take the .pdf to mean anything.

author: Cerebus
author_rank: Super Contributor
timestamp: July 23, 2022, 05:46:01 pm
content: Quote from: Gyro on July 23, 2022, 04:35:03 pmNo, I don't use a pdf viewer withknownexploits, but what of the unknown? Have you not heard of the phrase "zero-day exploit"? Sumatra has had its share of exploitable CVEs in the past and at some point they were allunknownvulnerabilities waiting to be exploited.Sadly one needs to exercise caution online nowadays, and while a pdf attached to a post is not of itself particularly worrying, one from a new poster, with a new account, that requires you to download the pdf to understand the post is enough to raise an eyebrow.

author: radioholic
author_rank: Newbie
timestamp: July 24, 2022, 12:42:57 am
content: Actually, a diode simply shunted across the meter will NOT do in all cases, and this is not widely understood. This is the point of my article. Some meters have a voltage drop at full scale as low as 50mV.Most diodes (silicon) have a Vf of about 0.7V, so the meter will experience a 14-fold current overload by the time the diode is fully conducting. While some meters will tolerate this degree of overload, others will not.There needs to be a series resistor as well, and the article explains how to calculate its correct value.

author: ledtester
author_rank: Super Contributor
timestamp: July 24, 2022, 01:11:30 am
content: Quote from: oz2cpu on July 23, 2022, 09:41:51 amHere's the picture from the PDF:

author: Halcyon
author_rank: Unknown
timestamp: July 24, 2022, 07:10:20 am
content: Quote from: Cerebus on July 23, 2022, 01:47:55 pmHow does that apply to this attachment in particular? As with any file you may choose to download, you should have measures in place to protect yourself from malicious files and not just take the word of someone on a forum that it's safe.

author: Ian.M
author_rank: Super Contributor
timestamp: July 24, 2022, 08:00:11 am
content: There's different grades of protection that are appropriate. Running a locked-down, sandboxed PDF viewer with Javascript disabled is sufficient for PDFs that are probably legit, from an author you've had a dialog with or a reputable parts manufacturer. as the highest risk is the author created them on a system infected with PDF aware malware. However if there's any risk the PDF contains a deliberately crafted zero-day attack, then initial inspection on an isolated system with an uncommon architecture (to stymie code injection by buffer overrun attacks), that you can nuke from orbit if it doesn't contain legit content is only prudent. Whether you trust a cross-platform VM running under emulation for that, or want real hardware, physically isolated depends on your level of (justifiable) paranoia! My Kindle was an acceptable risk - I keep a hot spare ready to go, and it isn't tot much work to restore it from backups then resync current content.As this was a newbie's first post, uncommonly lacking in inline content, apart from a teaser title, it could reasonably have been a targeted exploit, either from one of Dave's few 'black hat' disgruntled ex-members, or from someone spearphishing for one of the regulars here who customarily help newbies. Therefore I'm not surprised it set all Cerebus' guard dog whiskers twitching.

author: Cerebus
author_rank: Super Contributor
timestamp: July 24, 2022, 05:56:44 pm
content: Quote from: Halcyon on July 24, 2022, 07:10:20 amIt doesn't apply to this particular attachment, it applies to your ambiguous statement where "mere pdf, so it's safe" could be taken to mean two things - that pdfs are merely harmless in general, or that this particular pdf is harmless and merely contains what it should. The fact that I have to repeat for you that I'm talking about phraseology is ample proof, if needed, that what one means, and what meaning people will take from what one has written are not always the same.

author: Halcyon
author_rank: Unknown
timestamp: July 25, 2022, 01:36:33 am
content: Quote from: Cerebus on July 24, 2022, 05:56:44 pmYou left out a key word there which changes the meaning behind the phrase (seemingly, to suit your argument), but allow me to clarify: I was referencing the particular PDF in the original post, which fits in with context of this thread. I did not digress and make the claim about "all PDFs in existence". I was not referring to an unrelated PDF in another thread, or a PDF sitting on my neighbours flash drive.

author: Cerebus
author_rank: Super Contributor
timestamp: July 25, 2022, 12:48:28 pm
content: Quote from: Halcyon on July 25, 2022, 01:36:33 amI've said that what you wrote was clumsily written and open to an ambiguous interpretation. I did not ascribe a meaning opposite to your intent, only that itcouldbe interpreted in a way that you did not intend. I wasn't gainsaying you, merely clarifying. Why can't you simply take on board that how you wrote was open to misinterpretation. There is no "argument", I feel no need to change things to "suit my argument" and indeed haven't done so, I'm merely pointing out thathowyou said something could be misinterpreted. You seem to be looking for an argument about "Who's right" when there isn't one.Either you accept that the wording you originally used is open to misinterpretation or you don't. There's no more to it than that, and creating a storm in a teacup by arguing the toss with me about it is the most pointless thing that I've seen today, so far.

author: Halcyon
author_rank: Unknown
timestamp: July 26, 2022, 10:09:26 am
content: Quote from: Cerebus on July 25, 2022, 12:48:28 pmI have modified my original post, just so there are no unintended interpretations. I hope this clears it up for you.Also, just because you misinterpret something, doesn't make it "clumsily written". Although, I acknowledge, I left out an apostrophe. I apologise for my poor grammar on that occasion.

author: Cerebus
author_rank: Super Contributor
timestamp: July 26, 2022, 02:17:22 pm
content: Quote from: Halcyon on July 26, 2022, 10:09:26 amI didn't misinterpret it, I pointed out that it wasopento misinterpretation, not the same thing. As far as myopinionthat it was clumsily written goes, it's just that - an opinion, but one backed up by having been a magazine section editor in a past life. I think I can reasonably claim that experience adds a little weight to my opinions about writing in such a way that the average reader will get one's point clearly and unambiguously.