Comment by Neywiny
1 day ago
Good to give mains respect. It can deliver kilowatts of power without breaking a sweat, and breakers are slow enough you could see multiple joules of energy into your device. It's an expert magic smoke emancipator.
Disagree on the surprise that the setup worked, though. Mains is only regulated to a few % in frequency from what I've read. But you can see 0.05 Hz deviations (or 1%aka 1000 ppm). Even a junky crystal at ~100ppm is an order of magnitude better. A 10 ppm oscillator isn't hard to find, so the computer is likely somewhere in the middle. The math all checks out.
> Mains is only regulated to a few % in frequency from what I've read.
You've read wrong. While it's a different network, there were articles talking about how if the Texas grid stayed under 59.4Hz for a few minutes longer, some generators would have started cutting out to prevent damage, and the whole thing might have collapsed. So that's a 1% deviation being defcon 1.
And I found a page saying this about the European grid: "The allowed mains frequency range in normal operation is thus obtained at 49.8 Hz to 50.2 Hz." "short term deviations until 800 mHz are allowed (49.200 Hz to 50.800 Hz)."
> But you can see 0.05 Hz deviations (or 1%aka 1000 ppm).
That's 0.1%
You're right. Within a few 0.1%. shouldn't do mental math late at night. That said, my PPM math was correct, so I'm sticking by my point, which was based on PPMs.
> you could see multiple joules of energy into your device
Is that supposed to be a lot? Your phone receives multiple joules every second when charging, even with a slow charger.
One watt is one joule per second. Most things designed to consume power, especially related to wall devices and consumer electronics you'd charge, are capable of ingesting multiple watts. Most things that aren't, aren't. The difference that I'm not sure I'm seeing from the other comments is that a phone takes that energy and stores it as a chemical reaction, with some losses as heat. For everything else, it's all heat. Also, it's uncontrolled. The phone charger circuit can be upwards of 95% efficient, so very little power is getting turned into heat. The ADC input to your scope, on the other hand, would turn 100% of that into heat, which is why it'd blow up.
Practical example is the 50 ohm term. Most scopes I've seen max that at 5 Vrms. P = V^2/R, so 0.5W being dissipated. Now assume you hooked your scope to mains and accidentally turned on 50 ohm term. A low mains voltage is 100Vrms. That's 200W. 400x the maximum. Could a device take 200W? Sure. Could that device? No
It's a question of energy density. Multiple joules into your big phone battery is nothing, multiple joules into a small SMD component means it evaporates immediately in a bright flash!
Yeah, and batteries are deeply weird in that they're a component you put power into and they mostly don't get warm.
"slow" is still measured in milliseconds. That said, the energy is probably in the low hundred Joules. which is a lot when theres no time to dissipate it.
> and breakers are slow enough you could see multiple joules of energy into your device.
The magnetic part of a miniature circuit breaker will trip in nanoseconds with enough fault current or over voltage, but the thermal elements can take longer to trip for a lower amount of fault current or voltage. Instantaneous trip ratings are generally max out at 16.67ms to clear the fault in one cycle.
Large frame circuit breakers have protection relays that detect fault current and over voltage and trip the breaker.
Breaker trip curves for Cutler Hammer BR breakers: https://www.eaton.com/content/dam/eaton/products/low-voltage...
I am not sure if you're agreeing with me or not. Assuming you are, thank you. Nanoseconds could save you but milliseconds will likely not. It takes very little to explode a chip that isn't fortified and designed for robustness.