6
\$\begingroup\$

I want to do it for a "framed tech" project, it has been lying around in my basement for about 1 year. Now, on the back of the device, there is a sign "do not dissasemble, high voltage". Is it safe to disassemble it after such a long time or could it still have some power on it? Thanks

\$\endgroup\$
2
  • 2
    \$\begingroup\$ I doubt that voltage would be an issue, however there are other dangers - don’t go licking the electronics and watch out for cuts from sharp metal. \$\endgroup\$
    – Kartman
    Feb 20 at 17:29
  • 4
    \$\begingroup\$ Goodness, you must be a lot more sensible than I am: I normally have this thought after I've taken something to bits and am busily probing whatever connection is marked 'danger, high voltage'... \$\endgroup\$
    – 2e0byo
    Feb 21 at 17:20

3 Answers 3

19
\$\begingroup\$

If you don't power it, it is perfectly safe to open (well, I guess you could cut yourself on a sharp edge or choke if you tried to eat small parts, nothing is 100% safe).

If it is somehow powered from the mains a shock is possible. Also, even if it's powered from some safely isolated DC source such as batteries or an AC adapter, don't stare directly into the laser with your remaining eye (from quora.com)

enter image description here

It’s unlikely to cause damage but IR means you don’t have the blink reflex. For what it’s worth the laser is not visible even at close range in one field test I’m aware of, and a subsequent visual field test did not detect any unexpected blind spots in the test subject.

\$\endgroup\$
8
  • 8
    \$\begingroup\$ "Remaining" eye? Are they assuming you already damaged one eye by looking into the laser? \$\endgroup\$ Feb 21 at 8:33
  • 1
    \$\begingroup\$ @pacoverflow.... nah, I think they lost it when playing with a spring \$\endgroup\$ Feb 21 at 10:35
  • 12
    \$\begingroup\$ @pacoverflow, that's the joke. \$\endgroup\$
    – ilkkachu
    Feb 21 at 10:38
  • 1
    \$\begingroup\$ Note that eye damage from lasers is more likely in a dark room vs a bright sunlit room, this is because your eyes let less light pass in a sunlit room \$\endgroup\$
    – Ferrybig
    Feb 21 at 13:24
  • 2
    \$\begingroup\$ @abukaj The spec and the disk parameters allow a range of wavelengths, not just the 780 nm that is often quoted (780 is the most common though). Some early players even used ‘standard’ helium-neon lasers at 632.8 nm, which is very much red and not infrared. I think the spec allows all the way down to 600 nm though. \$\endgroup\$ Feb 21 at 19:55
18
\$\begingroup\$

If it's been unplugged for a year, it's fine. The high voltage warning is only valid when the device is in an energised state. With some exceptions for battery-powered equipment, high voltages don't typically persist in unplugged devices over long periods of time.

One important thing to note is that charge may be retained in capacitors for an hour or so after a mains-powered device is unplugged, and those capacitors may be charged to high voltages. If you're disassembling a device that has been recently powered on, treat it as energised until you have verified that the capacitors have been safely discharged. This doesn't apply to your situation, since you're way past the point where a capacitor could hold its charge, but bear it in mind for future endeavours.

\$\endgroup\$
7
  • 2
    \$\begingroup\$ It's a good habit to get into using your screwdriver (non-insulated) or similar to short any capacitors you're suspicious of to ensure they're discharged. \$\endgroup\$
    – James D
    Feb 21 at 2:01
  • 15
    \$\begingroup\$ Checking with a multimeter first is even better. I can tell you from experience that 200uF at 325V leaves your ears ringing. \$\endgroup\$
    – Polynomial
    Feb 21 at 2:16
  • 5
    \$\begingroup\$ Maybe use a resistor to more or less slowly discharge the capacitor instead of shorting it and risk internal damage by several amps flowing for a very short time. \$\endgroup\$
    – arne
    Feb 21 at 9:23
  • 1
    \$\begingroup\$ Shorting with a resistor is one of those approaches that sort of works, but is really impractical for mains voltages when you only have generic parts available. You need to use a resistor with an appropriate voltage rating, and typical axial carbon resistors may only be rated to 200V. You also need to pick your resistance based on maximum power dissipation through the resistor, using R=V²/P. For a typical 0.5W axial resistor on 325V that's 220k. That resistor would take a whole minute to drop a 200uF cap from 325V to 100V. And you have to hold in in place for all that time, no mistakes. \$\endgroup\$
    – Polynomial
    Feb 21 at 18:14
  • 1
    \$\begingroup\$ If you've got a programmable load with a suitable voltage rating, you can dial in a constant 50mA sink and discharge that same capacitor in 1.5 seconds. There are also cheap dedicated capacitor discharger products that work using constant-current sinking. In both cases you get the convenience of nicely-insulated leads that are far less likely to short to something than an axial resistor held precariously with some pliers. Plus you don't need to break out a calculator. \$\endgroup\$
    – Polynomial
    Feb 21 at 18:22
7
\$\begingroup\$

Sure, go for it. As long as it’s not plugged in you’re not exposed to hazardous voltages. It’s a great learning opportunity.

Before you tear it apart, if you do plug it in with the cover off to see how it works, take note that the AC line voltage is present in the power supply ‘primary’ side, behind a transformer.

While the transformer isolates the electronics from the line, the ‘hot’ area is still there, exposed, which is the reason for the scary ‘hazardous voltage’ warning on the case.

Even when disconnected from power, switching power supplies have primary-side capacitors (big ones - hundreds of uF) that store some charge. Wait for a while for the voltage to bleed off before touching. Check with a voltmeter to be sure.

On the secondary side, the power supply outputs are all low voltage: +5V, +12V, and if present, the VFD (display) cathode bias supply of about -30V. None of these are dangerous.

So if you’re careful and pinky-swear promise to stay away from the power supply ‘hot’ area, you can operate the player with the cover off. For extra safety to avoid shock hazard, make a plastic guard to cover the power supply.

Finally, there is no significant danger from the optical pickup laser. Its focal length is very short (a few mm), it doesn’t emit a coherent beam like a laser pointer. Nevertheless, don’t stare into it (some mechanisms will even have warnings to not do this.)

Curious about the optics? Here’s a tear down of a Blu-ray pickup: http://repairfaq.cis.upenn.edu/Misc/Blu-ray/site1/optics.html

/former DVD player designer

\$\endgroup\$
2
  • \$\begingroup\$ “Wait for a while” — could you be more specific? (That could mean anything from a minute or so up to weeks…) \$\endgroup\$
    – gidds
    Feb 21 at 20:59
  • 1
    \$\begingroup\$ That’s why I added the warning to check with a voltmeter. That said, a bleeder discharge to safe-touch (~60V) from rectified line would be reasonable if it were between 1 and 10 seconds. So waiting say one full minute should be enough. \$\endgroup\$ Feb 21 at 21:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.