General > General Technical Chat
Just Replace the Fuse and You're Good!!
engrguy42:
Wow, these smartphone cameras are amazing...
I zoomed in on a 4k image from my phone, and you can see the apparently intact element running down the middle.
engrguy42:
--- Quote from: Alti on May 21, 2020, 08:26:39 pm ---
--- Quote from: engrguy42 on May 21, 2020, 07:50:32 pm ---So I took it apart, and first thing I tested was the 0.2 A fuse that feeds the measuring circuit. Son of a gun, it was open!!
(..)
Maybe the fuse broke in shipping? Or it was bad from the start?
--- End quote ---
It might be this is a capacitive dropper, has a (1uF?) capacitor in series with resistor, fuse, bridge, zener and then tied to mains voltage. When you plug it near peak of sin(x), the current is only limited by drop resistor and ESR of a capacitor, resistance of fuse and inductance of mains wiring.
Put a jumper over the fuse, take a scope with current probe (small shunt will do), sqrt(2)*110V DC and measure the integral of i2t (square of current * time) on inrush. Fast 0.2A fuses have melting i2t of around 0.02 A2s (Littelfuse 0217.200). If your i2t is over 50% of that value then this is not a coincidence.
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Alti, you sure about the inrush with the capacitive dropper? You're right, the capacitor is 1uF (I show it as 0.1uF), but I did some simulations in LTSpice with that plus the 100ohm resistor in series, and even with the source at peak of the sine wave I just show a 60mA constant peak current, no big inital peak. And the fuse is 200mA.
engrguy42:
BTW, here's the screenshot of my LTSpice simulation of the circuit. Basically 60Hz AC, 170V peak (120RMS), applied to the capacitive dropper via a time controlled switch. The source AC (yellow trace) is shifted 90 degrees to ensure a voltage peak on closing. I even included some source inductance.
As you can see there's a nice clean current with a 60mA peak. Now if you increase the C greatly you'll get some initial offset in current due to the LC stuff going on. But in this case I'm not seeing the big inrush.
Alti:
--- Quote from: engrguy42 on May 22, 2020, 12:20:55 am ---Alti, you sure about the inrush with the capacitive dropper? You're right, the capacitor is 1uF (I show it as 0.1uF), but I did some simulations in LTSpice with that plus the 100ohm resistor in series, and even with the source at peak of the sine wave I just show a 60mA constant peak current, no big inital peak. And the fuse is 200mA.
--- End quote ---
Yes, the discharged capacitor across power line is a dead short at t=0. That is why there has to be inrush current limiting resistor or adequate ESR in a cap. Otherwise inrush overwhelms a fuse. So it should show you 156V/100R=1.56A peak and a bit more if you include 10% mains voltage tolerance.
Now, 1uF cap in series with 100R resistor connected to 110V+10% AC gives i2t of around 4e-4A2s and the fuse withstands 2e-2A2s so with 100R resistor it is 1/50 of a kill. So 200mA is just too big for 100R, lets hope it protects electronics adequately.
engrguy42:
Alti, you're applying a DC to the capacitive dropper. Why? We're talking about AC.
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