Electronics > Beginners

transients in capacitor droppers

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T3sl4co1l:
Yup, like that.

Tim

LaserTazerPhaser:
The LED bulbs dont seem to be concerned for over voltage many of them have a simple fusible resistor series between line and constant current led driver. The led driver is typically ~=>500V but voltage transients can exceed that. Howcome they can operate without damage from overvoltage?

Perhaps the CC LED drivers have a builtin TVS to reduce costs, this is one such driver found in a bulb when I dissasembled it.

https://datasheet.lcsc.com/szlcsc/Bright-Power-Semicon-BP5136H_C161793.pdf

The logo is almost suggestive of a fire hazard.

T3sl4co1l:
Bulbs are cheap, if the resistor fuses you buy a new one.

Tim

Gyro:

--- Quote from: LaserTazerPhaser on September 03, 2019, 09:27:59 pm ---Is the current limiter positioned so as it will not incur issues? The fuse and mov seem to prevent it from incurring damage.
https://www.eevblog.com/forum/beginners/transients-in-capacitor-droppers/?action=dlattach;attach=826494;image

--- End quote ---


--- Quote from: T3sl4co1l on September 03, 2019, 09:44:33 pm ---Yup, like that.

Tim

--- End quote ---

Surely an inrush current limiter, aka NTC termistor, isn't going to help with spikes. If it runs cold in normal operation then it will have some useful resistance (assuming it is high enough value in the first place). If it runs warm or gets warm due to a large number of spikes, its resistance will drop, making it less and less effective.

A PTC thermistor would be no good either as it would have a low initial resistance and probably too much thermal inertia to be able to respond to individual damaging spikes. It would also make the fuse ineffective.

A good old fashion resistor is what you want (preferably fusible as I said in my initial reply, certainly flameproof). It is the perfect part for the job.


P.S. Don't forget the Zener after the bridge.

T3sl4co1l:
I was assuming some sort of active current limiter.  Thermistors of either type aren't very useful or applicable at mains voltages (they do however make suitable PTCs in some kind of ceramic, thus in MLCC / chip format; the tempco isn't as sharp as polymer based types).

Tim

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