5 ms·

I did the exact same thing - took apart a camera and noticed the flash capacitor had a shock warning writing right on it, so I thought to myself "I better disch

I did the exact same thing - took apart a camera and noticed the flash capacitor had a shock warning writing right on it, so I thought to myself "I better discharge this before it shocks me." I figured shorting it out using a screwdriver across the terminals would do the trick. A huge flash arc mini-explosion blasted that screwdriver and left me seeing spots for a few hours.

tajen 11y ago
Smaller electricity lesson. We were studying noise on resistor-capacitor circuits at uni (waveform), with an oscilloscope. Teacher says to the most "experimental" student of the class: "You know, even 220V has noise at the top of the curve". The second the teacher turns his back the student put the probes in the power point.
- mschuster91 11y ago
  What happened? Did the scope have over-voltage protection or was it fried?
  - jacquesm 11y ago
    It should have survived, depending on the probe in use and the input circuitry of the scope. But most quality scopes would just display a sine wave after you scaled down the Y axis a bit. Mine certainly would (elderly Tek).
    - mschuster91 11y ago
      Wow. How does that work? I mean, stuff like diodes etc. all have maximum voltage levels, not to mention PCB trace clearance or the rated voltage of coil isolations. And a scope can measure stuff in the mV range, so any op-amps or other electronics should be fried in a second when hit with a 1000-fold increase in voltage.
      - VLM 11y ago
        There's a classic book called "art and science of analog circuit design" and there's a whole chapter on scope inputs. The TLDR is at the input you limit lightning / hand of god level of voltages using the usual RF techniques for lightning protection, so you never have to protect against more than a couple hundred volts, then implement a ridiculously high impedance / voltage attenuator such that it'll arc over before the output exceeds a volt or so, then you limit the output with (optimistically) low capacitance diodes to the power supply. Then a buffer that converts from high impedance in to low output impedance and from there on its pretty boring analog stuff, gain stages etc. Or the TLDR of the TLDR is you have multiple protection stages and (controllably) attenuate the heck out of the input signal. Also you rely on some ohms law tricks... a voltage big enough to damage in series with a ridiculous resistance will be a very low current which any protection diodes can survive. Its a tricky business to work around all the RC time constants that occasionally are not fun at all. Three things to think about. Everyone uses 10x 100x 1000x scope probes, so when you sniff 460V VFD power you're probably using a 100x probe and only measuring 5 volts or so. Also protection circuits tend to protect against the usual 60hz and DC overvoltages... shoving the output of a MRI amplifier at hundreds of MHz will just arc across and blow it to bits. Finally its possible to build RF front ends that sniff RF down to thermal noise limits, and scopes throw all that away to survive hundred volt transients on the input; I assure you ultra low noise microwave preamps will not survive more than a couple volts input, but those are bazzilionths of a picowatt not mV range. Oh and a warning... you pick up a 100x scope probe, your scope handles up to 500 V input (perhaps) so you think you're good to 50000 volts in, but the probe has its own input voltage limit and unless you have bottomless pockets it probably tops out around 2.5 KV, so if you try to measure a 35 KV flyback transformer for an old fashioned CRT using that 2.5 KV probe, kaboom...
  - tajen 11y ago
    The oscilloscope actually survived, as a surprise to me. The power went out at the building, level, classroom and desk level, but no actual damage.

tajen 11y ago

Smaller electricity lesson. We were studying noise on resistor-capacitor circuits at uni (waveform), with an oscilloscope. Teacher says to the most "experimental" student of the class: "You know, even 220V has noise at the top of the curve". The second the teacher turns his back the student put the probes in the power point.

mschuster91 11y ago
What happened? Did the scope have over-voltage protection or was it fried?
- jacquesm 11y ago
  It should have survived, depending on the probe in use and the input circuitry of the scope. But most quality scopes would just display a sine wave after you scaled down the Y axis a bit. Mine certainly would (elderly Tek).
  - mschuster91 11y ago
    Wow. How does that work? I mean, stuff like diodes etc. all have maximum voltage levels, not to mention PCB trace clearance or the rated voltage of coil isolations. And a scope can measure stuff in the mV range, so any op-amps or other electronics should be fried in a second when hit with a 1000-fold increase in voltage.
    - VLM 11y ago
      There's a classic book called "art and science of analog circuit design" and there's a whole chapter on scope inputs. The TLDR is at the input you limit lightning / hand of god level of voltages using the usual RF techniques for lightning protection, so you never have to protect against more than a couple hundred volts, then implement a ridiculously high impedance / voltage attenuator such that it'll arc over before the output exceeds a volt or so, then you limit the output with (optimistically) low capacitance diodes to the power supply. Then a buffer that converts from high impedance in to low output impedance and from there on its pretty boring analog stuff, gain stages etc. Or the TLDR of the TLDR is you have multiple protection stages and (controllably) attenuate the heck out of the input signal. Also you rely on some ohms law tricks... a voltage big enough to damage in series with a ridiculous resistance will be a very low current which any protection diodes can survive. Its a tricky business to work around all the RC time constants that occasionally are not fun at all. Three things to think about. Everyone uses 10x 100x 1000x scope probes, so when you sniff 460V VFD power you're probably using a 100x probe and only measuring 5 volts or so. Also protection circuits tend to protect against the usual 60hz and DC overvoltages... shoving the output of a MRI amplifier at hundreds of MHz will just arc across and blow it to bits. Finally its possible to build RF front ends that sniff RF down to thermal noise limits, and scopes throw all that away to survive hundred volt transients on the input; I assure you ultra low noise microwave preamps will not survive more than a couple volts input, but those are bazzilionths of a picowatt not mV range. Oh and a warning... you pick up a 100x scope probe, your scope handles up to 500 V input (perhaps) so you think you're good to 50000 volts in, but the probe has its own input voltage limit and unless you have bottomless pockets it probably tops out around 2.5 KV, so if you try to measure a 35 KV flyback transformer for an old fashioned CRT using that 2.5 KV probe, kaboom...
- tajen 11y ago
  The oscilloscope actually survived, as a surprise to me. The power went out at the building, level, classroom and desk level, but no actual damage.