Capacitor for non isolated "capacitor" power supply is X cap required if fused? - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: April 19, 2019, 09:51:48 pm
content: For a circuit like this (assuming UL approval is required), is a X-rated safety cap required if a series fuse is before the circuit?The circuit is completely inaccessible to the user, so isolation is not required.Edit: ——- zener is 12V ——-

author: coppercone2
author_rank: Super Contributor
timestamp: April 20, 2019, 01:58:07 am
content: won't a X cap take surges and stuff better?

author: Gyro
author_rank: Super Contributor
timestamp: April 20, 2019, 09:05:53 am
content: There isn't actually a requirement to use a X cap in a capacitive dropper, in fact some capacitor manufacturers discourage it due to the thinner metallization and progressive capacitance loss (it needs to be rated rather higher than 400V though). You can have UL (flammability) without X rating. The onus is on the manufacturer (you in this case) to ensure that the entire circuit is safe, not just individual components.Let's look at your circuit, as drawn. If they capacitor shorts (even briefly) then full mains current will flow. The 6V2 Zener will take the brunt of it and probably fry first, then the either the bridge or the NTC thermistor will blow (assuming the zener fails short). Clearly protection is needed.You also have the issue of mains spikes (even turn-on spikes at mains peak). The capacitor impedance will pass a nice predictable current on a 50/60Hz sine-wave. When you hit it with a fast edge, it will pass a great deal more current. If there's sufficient energy, it will start taking out parts, and a fuse won't be quick enough. You need a series resistor to limit these spikes. If you put, for example, a 330R resistor in series with the input, this will limit these spikes to less than 1A. With properly rated parts, the circuit can mop those up.Now we come to the issue of the capacitor shorting again. The resistor will try to dissipate all the power between mains voltage and the 6V2 clamp. To overcome this you should make this resistor aflameproof fusibleone. It will go open circuit cleanly and without fuss in these circumstances.Yes, it would be advisable to fit a fused inlet too - the PCB might break or something come loose if dropped for instance, but you wouldn't normally expect it to act fast enough to open with a component fault, the fusible resistor will go first. You should pretty much always use a fuse at a mains input, it's the last line of defence.P.S. Do you really mean an NTC thermistor there?

author: Jester
author_rank: Frequent Contributor
timestamp: April 20, 2019, 12:22:58 pm
content: Gyro,Thanks for your insight.The load current varies from about 1mA to as much as 80mA, the thermistor warms up enough to allow 80mA. The zener is actually 12V. Mains are 120V.

author: Gyro
author_rank: Super Contributor
timestamp: April 20, 2019, 02:25:29 pm
content: You're welcome.Ouch, 80mA is a lot to be pulling on a capacitive dropper though, well over what most applications use it for. ... I think you're looking at around8uF2.5uF for that at 60Hz, if my maths is right, so you'd be looking at a motor-run Polypropylene film rather than an X cap anyway (probably the reason for your question).I'd think twice, you will probably find that a small transformer based or off the shelf SMPS wall wart would be more compact, cheaper and more satisfactory.

author: Jester
author_rank: Frequent Contributor
timestamp: April 20, 2019, 04:12:14 pm
content: I put together a breadboard version yesterday with a 2u2 and an unspecified 100 Ohm NTC and was able to pull 80mA. I actually only need 80mA for 4 s per hour, with nominal current between 1 and 10 mA. I’m very space confined and an external wallwart is not an option. One of the LNK parts might end up being smaller, but then need additional filtering etc.This is turning into one of those piece of cake to hmmm.... this is going to be tight scenarios.I'm going to look at this part as a possible solution:https://www.mouser.com/ProductDetail/Power-Integrations/LNK3202D-TL?qs=%2Fha2pyFadugWa4O%252BeeNnoeocbir1MDK9Jh5bTXhWL2Nct%252B%252BMnPM7DA%3D%3DIf anyone has suggestions for a better approach I'm all ears.