How afraid are u to turn it BACK ON - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 19, 2018, 02:59:47 am
content: I'm plenty new enough to be worried about trying AC powered stuff again after repairs. I have a lightblub current limiter setup. Soon I'll be getting a big variac transformer.In the meantime I'm going to use a wooden blast shield when I fire up a computer PSU I got back together

author: waytec
author_rank: Newbie
timestamp: April 19, 2018, 03:12:44 am
content: HiTo be honest my success rate is only about 25% in repairing SMPS....I give up after a couple of a attempts and throw it away....and just buy another.....After a bad experience a number of years ago ... I still use a small pillow to cover the unit when powering up etc:....Wayne

author: byoungblood
author_rank: Contributor
timestamp: April 19, 2018, 03:26:46 am
content: I usually do a quick continuity test to ground and to adjacent pads on anything I’ve been soldering on a PCB, just for my own sanity before I power something up, regardless if powered off of AC mains, DC supply or even battery powered. Seems to at least lessen the chance of letting the smoke out of something or causing some kind of unintended operation after powering it back up.

author: drussell
author_rank: Super Contributor
timestamp: April 19, 2018, 03:34:38 am
content: Quote from: lordvader88 on April 19, 2018, 02:59:47 amLOL... Yeah, it can be a tad nerve racking when you fire something up that you're not 100% sure what is going to happen when you do... On the other hand, it is more fun that way!Take a moment to review any potential hazards, double check your work.... then engage safety squints, step back and carefully, gingerly apply thepow(erif you didn't just blow yourself up)edit:(or off to theE.R.if you did?)

author: Distelzombie
author_rank: Frequent Contributor
timestamp: April 19, 2018, 03:35:42 am
content: Quote from: byoungblood on April 19, 2018, 03:26:46 amDid you ever find something?

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 19, 2018, 03:59:41 am
content: In my case I've washed the PCB pretty good. In the sunlight tomorrow I'm going to go over it with the 30x loupe to make "sure" there's no bits of leftover solder rolling around

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2018, 05:21:05 am
content: I don't worry about it at all in most cases, I often use a series bulb for initial testing, at this point I've fixed enough stuff that I have a pretty good success rate.

author: metrologist
author_rank: Super Contributor
timestamp: April 19, 2018, 05:21:23 am
content: watch more electroboom

author: Rascal
author_rank: Contributor
timestamp: April 19, 2018, 09:23:39 am
content: I remember my dad using a Cossor 1035 oscilloscope when I was kid. I came across this very unit when I was clearing out some of his stuff and I thought I might see if it fires up (not literally). This oscilloscope is probably circa 1950's and was probably last used in the mid 1970's.I wound up the variac (slowly) and surprisingly enough the beam eventually appeared. It even managed to display a reasonable looking sine wave when I ran a signal into it. Not bad for something that is nearly 70 years old and hasn't seen the light of day for over 40 yearsPaul

author: byoungblood
author_rank: Contributor
timestamp: April 19, 2018, 01:12:34 pm
content: Quote from: Distelzombie on April 19, 2018, 03:35:42 amA couple of times. Usually a small solder ball that didn't get cleaned off the board or when the PCB traces are pretty beat up to begin with. I'm not inspecting the entire board, just around what I was working on.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2018, 01:22:59 pm
content: If I'm afraid to turn something on, it's because I've failed to address the risk factors.There is no perfectly safe thing. But one needn't be a damned fool about it. One can only be as safe as is reasonable.Mind, not even "as safe as possible", because quite safe operations may bepossible, but they will be extraordinarily prohibitive, in time, money and flexibility, to implement.OSHA for example is pragmatic when it comes to workplace safety. They do not prohibit people from, say, reaching inside live breaker boxes with their hands. Not unconditionally so. But you'd better have a damned good reason to do it in the first place -- having no better way to address the problem with tools (e.g., not enough dexterity with a probe-on-a-stick), and taking every other precaution you can (thick rubber gloves TESTED RECENTLY are a must!).While testing a little electronic circuit is hardly a life-or-death situation, the exact same principles can be applied on any scale, for any cost (be it a human life, a multimillion dollar facility, or a ten dollar charger that you don't want to have to get off your ass and buy a new one of just yet).So, identify those risks.For equipment repair, what are you afraid of? Capacitors and transistors exploding? Resistors catching fire? Holes being burned in boards? Burning out fuse after fuse, not learning any new information in the process?Identify possible causes. Create hypothesesAND THEN TEST THEM!Example: a power supply is turning on and off, strobing, even when lightly loaded. This is most commonly caused by an intended mechanism: the controller is self-powered, and when it isn't receiving power, it times out, shuts down for a while, then turns on again and so on. You might test if this has failed, by tacking pigtails onto the circuit, and powering it with an isolated supply. If it's still pulsing, you've at least ruled one thing out, but if not, you now have much more information available to test: is it switching? Is it generating output? Is it generating auxiliary power after all? And from here you can narrow down the possibilities much more rapidly.Say you get a piece of equipment and it's got a blown fuse. Well, it's a good bet that something blew that fuse. So, don't just go replacing the fuse, then plugging it back in to test. That's a low-information result: a likely waste of fuses. Follow the AC power into the circuit, and check for shorted diodes, capacitors, transistors and so on. Only if you have not identified a likely problem, go ahead and plug it in. You've at least then gathered a few bits of data, rather than ~1/2 a bit. If it fails again, you know it's probably not the things you just checked, so get more creative, check more unlikely targets. Instead of ohming it out for shorts or opens, use diode test and see if the semiconductors look reasonable. Apply a stimulus or external power and see what happens. There are many circuits you can test in blocks, this way -- something akin to attaching life support machines to a patient undergoing major surgery.Even at the instant of turn-on, you can minimize risk. Use a current-limited supply; increase the supply voltage gradually; supply aux power to circuits that can't operate so low; etc. If the circuit has large capacitors in it, consider disconnecting them (so that, if one device fails shorted, it doesn't dump the entire charge through the circuit, taking everything else down with it!), or isolating them with current-limiting resistors.Some of this does require extra attention (who wants to pull out capacitors or cut traces?), or extra equipment. I personally have created several current-limiting devices for use on my bench -- I have a 24V battery circuit available here, and they are an excellent way to deal with the hundreds of amperes short-circuit capability!This was rather lengthy for a beginner's thread, but I hope that at least bits and pieces of it shall prove useful to those wondering.Cheers,Tim

author: Jwillis
author_rank: Super Contributor
timestamp: April 19, 2018, 08:51:37 pm
content: I made a mains remote switch for turning on questionable circuits.Just a box with a standard wall socket and a light switch.

author: TheDane
author_rank: Regular Contributor
timestamp: April 21, 2018, 09:40:55 am
content: A well proportioned fuse on the input (fast blow is best, but it will blow if caps are big enough and the inrush current is too big for it to handle - in this case use a slow blow fuse)Another good thing is a polycarbonate blast shield -https://www.google.com/search?q=polycarbonate+blast+shield&tbm=isch&tbo=u&source=univ- it allows you to safely see what blows/burns, whilst reaching for your emergency power-off switch solution.It also shields you and others from accidentially touching live stuff - if used correctlySo, when powered from a current limited source - no worries at all on my part.(Ear plugs can also be a good thing - if the power is large enough, and the devices goes pop a lot)