Electronics > Beginners

Ceramic capacitor is of too low voltage?...som,etimes blows up at switch on.

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T3sl4co1l:
Keep in mind that the value is significantly less than rated.  This exaggerates the peak voltage during transient startup or surge conditions.  X7R are bad already, and worse at high voltages.

Note that C0G have more value (at voltage) for the same package volume or cost.

Tim

Doctorandus_P:
My guess is that the inrush current into the capacitor is so high that Ohmic resitance heats a small part of the capacitor too much and it vaporizes some bonding wire or similar.

Did a few experiments a long time ago with a capacitive dropper to light a LED (with anti- parrallel diode) directly from 230Vac. Quite some leds exploded on turn on untill I learned to put in an extra series resistor to limit the inrush current.
A wall power socket can easily deliver several hundreds of Amps without blowing a fuse for a short time such as a switching event.

The LR8 has a current limit between 10mA and 30mA:
http://ww1.microchip.com/downloads/en/DeviceDoc/20005399B.pdf
You can probably use a series resistor between a few hundred Ohm to a few kOhm between 230Vac and your capacitor to limit the inrush current. Make use of this to turn it into a usefull RC filter.

If you have doubts about accidentally wrongly placed caps, then take one of those caps and subject it to a high DC voltage untill it breaks. Compare that voltage to a known good cap.
Beware that you capacitors may explode or release some nasty magic smoke during this experiment, but it is one of the experiments where destroying these components is actually usefull.

chris_leyson:
No room for a film X2 cap ?

Terry01:

--- Quote from: Doctorandus_P on July 05, 2018, 11:04:24 pm ---My guess is that the inrush current into the capacitor is so high that Ohmic resitance heats a small part of the capacitor too much and it vaporizes some bonding wire or similar.

Did a few experiments a long time ago with a capacitive dropper to light a LED (with anti- parrallel diode) directly from 230Vac. Quite some leds exploded on turn on untill I learned to put in an extra series resistor to limit the inrush current.
A wall power socket can easily deliver several hundreds of Amps without blowing a fuse for a short time such as a switching event.

The LR8 has a current limit between 10mA and 30mA:
http://ww1.microchip.com/downloads/en/DeviceDoc/20005399B.pdf
You can probably use a series resistor between a few hundred Ohm to a few kOhm between 230Vac and your capacitor to limit the inrush current. Make use of this to turn it into a usefull RC filter.

If you have doubts about accidentally wrongly placed caps, then take one of those caps and subject it to a high DC voltage untill it breaks. Compare that voltage to a known good cap.
Beware that you capacitors may explode or release some nasty magic smoke during this experiment, but it is one of the experiments where destroying these components is actually usefull.

--- End quote ---

I thought a wall socket was only good for 32 Amps? I don't know for sure myself, i'm still learning. I read it on the forum somewhere.

T3sl4co1l:

--- Quote from: Terry01 on July 05, 2018, 11:35:50 pm ---I thought a wall socket was only good for 32 Amps? I don't know for sure myself, i'm still learning. I read it on the forum somewhere.

--- End quote ---

Some sockets? I guess??

You can easily draw over 2000 amperes from a typical outlet... it's just a question of, for how long.  At that rate, the breaker will trip within a cycle (< 10ms).  Which, mind you, is an electrical eternity -- say a transistor was dropping that voltage (even while drawing far less current -- transistors can't draw fault current, but a fault condition like SMPS shoot-through, or driving a shorted load, will still quickly be fatal), it will die in about 20us.  So the breaker opens after about 500 transistor deaths, by which time the one transistor has thoroughly turned into an expanding plasma ball. :)

Over longer time scales, you want to keep the RMS current below the fuse/breaker rating (whatever that happens to be -- and for UK circuits, I believe that includes a fused plug and cord, versus the US system that assumes any outlet may be asked to deliver full rated current).  RMS is an averaging process, so the peak current can still be much more than rated -- again, it's just a question of, for how long.  :)

In the present case, blowing a ceramic cap will have an equivalent RLC circuit, which will peak at around 1.5A in 3.5us, then 680V at 7us (worst case, switching at the instant of maximum line voltage, and assuming mains inductance ~500uH and capacitance 0.01uF constant).  In practice, the ceramic's capacitance will drop off rapidly as voltage rises, so the peak current will be lower, and the peak voltage will be far higher, probably over 1kV.  At that point, the LR8 is guaranteed to be unhappy; it may well happen that it can bear modest avalanche currents (<1A in this case), but it may also be gradually damaged in the process.  Of course if the capacitor fails internally, you don't get a chance to see if that happened... :)

Tim

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