What exactly is dangerous? (beginner's question)

Discussion in 'General Electronics Chat' started by sharevari, Aug 18, 2011.

  1. sharevari

    Thread Starter New Member

    Aug 18, 2011
    So I'm just starting out with electronics and am quite keen to start fiddling around with old electronic gadgets I have lying around.

    But before I get started, I'd like to get an understanding of whether it could ever be dangerous to open something up and poke around, with the power unplugged.

    I have read that this could be dangerous with CRT monitors but are there other pieces of equipment that I shouldn't play around with if I don't know what I'm doing?

    For example, I have a broken DAB alarm clock radio, which on the back has a big warning sign saying "Warning, risk of electric shock, do not open. High voltage inside" etc. But next to the power inlet it also says "DC 6V". How could this possibly be dangerous? Or is the warning blurb just the manufacturer being over-cautious wanting to scare people off just in case someone sued them?

  2. #12


    Nov 30, 2010
    More like warning people off so they won't repair it.

    anyway...capacitors. A CRT is a big capacitor. Capacitors hold voltage for a long time if they aren't wired to a discharge resistor. They can bite pretty bad. Worse, the capacitor sparks you and you jerk your arm into something hard or sharp or hot (like a soldering iron).

    The good part about capacitors is that they have voltage labels most of the time. They will tell you if they can carry hurtful voltage. You can make an alligator lead with a 1000 ohm resistor in the middle and use that to discharge capacitors so they can't bite. Leave the rubber cap on the alligators!

    General rule? Never get into anything as high as 50 volts and you'll probably survive.
  3. sharevari

    Thread Starter New Member

    Aug 18, 2011
    Cool, thanks for the quick reply.

    So capacitors are the ones to look out for in unpowered equipment.

    Understood, but all these voltage guidelines only apply for powered circuits right? Are there dangers, other than caps, with poking about in unplugged mains-voltage equipment?
  4. #12


    Nov 30, 2010
    Dead critters. I just hate when a mouse doesn't read and obey the warning label!
  5. debjit625

    Well-Known Member

    Apr 17, 2010
    It will not always depend on voltage like a 1000 volt will not kill you but a 200 can, it all depends on the power i.e.. with voltage how much current is their, its a long topic and I will not go further because slowly you will come to know everything.

    Yes, but they are not always dangerous. Like when we open a TV set we short the power capacitor of the power supply section it stores a huge power that can give you a shock but their are 100s of capacitors in a TV set we don't have to take care for all of them because they store a very small amount of charge, yes excluding CRT, it's very dangerous but CRT is not that much exposed unless you do something on it.

    Both could be dangerous, like an unplugged UPS could convert the 12VDC to 120VAC/220VAC.

    What you need is to know about the basic, so here you go it will solve all your questions and help you in your journey for electronic. Here is the free e-book, above this webpage you will find all the links for all the volumes.
    VOL 1 DC
    VOL 2 AC

    Good Luck
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
    Way back when auto companies toyed with 42v electrical systems, I believe it was the SAE that decided any voltage over 42 could be dangerous. So #12's 50v number is good.

    A simple rule to follow when poking around circuits with potentially high voltage is keep one hand in your pocket.

    FWIW, that 42v scheme included combining the alternator and starter into one unit built as part of the flywheel/flex plate. They deemed it so reliable it would rarely if ever require maintenance.
  7. MrChips


    Oct 2, 2009
    Live ones too. We had a mouse that laid a nest on top of the co-processor of an IBM PC. When I opened it up to inspect after the grad student reported the crashed system, we found that the mouse had unwrapped toilet paper from a roll sitting on the lab bench and had built the nest on the warmest component. Under the top layer of tissue were three squirming newborns.

    And this was before the mouse was even invented for the PC!
    This PC begat its own mouse. I have the pics around somewhere.

    And the PC stunk with pee!
  8. Wendy


    Mar 24, 2008
    CRTs have high voltages on their caps because the circuitry and CRTs requires it. Something like a clock/radio generally doesn't have high voltages other that the wires going to the transformer. Their designs generally create a safe low DC voltage for the circuitry, which doesn't require it since they don't have a CRT.
  9. Adjuster

    Late Member

    Dec 26, 2010
    CRTs also present a danger because they are basically very large vacuum tubes. The large glass vacuum chamber represents a fair amount of stored energy, and can "implode" if broken. The shards of sharp broken glass start by falling inward, but some end up flying on outwards, when they can cause serious injury. These things need to be treated with respect.
  10. sharevari

    Thread Starter New Member

    Aug 18, 2011
    Thanks all. The radio has now been dismantled and useful components salvaged without any danger to life and limb.

    One curious thing. I found some white goo sprinkled over one of the circuit boards, kinda foamy like the insides of these things:


    I wonder where that came from...
  11. #12


    Nov 30, 2010
    Might be the guts of a bad capacitor. A photo might help.
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Manufacturers use lots of types of goo to secure components and wires against movement. It might be proper goo, not "bad goo" from an exploded cap.
  13. sharevari

    Thread Starter New Member

    Aug 18, 2011
    Heh, there were two types of distinct goo in this thing. There was lots of proper hardened goo used to hold things in place which was obviously glue of some sort, but this white stuff had not hardened and was just randomly scattered about the place, as if something had exploded.

    Sorry, no pic available. It's not really important either, I was just curious.
  14. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Microwave ovens pose a hi V threat with power off in the aproximate 1μF 2000 V AC cap. Most have multi meg discharge resistors- some do not.
  15. Johnny Nelson

    New Member

    Aug 19, 2011
    I guess the manufacturer puts the label on mostly to warn people so they avoid getting sued if somebody hurts themselves.
    The only thing I can think of which could be a hazard are high-voltage capacitors and then is there are any components containing dangerous chemicals.
  16. Gizzy

    New Member

    Sep 5, 2011