Capacitor calc for powering 110Vac, 60Hz device from 230Vac, 50Hz - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 17, 2018, 03:09:06 pm
content: I have a PIR detector and which rated at 110Vac, 60Hz input. The power supply in my part of the world is 230Vac, 50Hz.The device has a capacitor transformerless powersupply, with value of 564K (0.56uF).The impedance of this at 60Hz turns out to be around 4.7Kohm.1. If I want to power this unit on 230V, 50Hz would it be as simple as choosing a capactor with double this impedance that is 9.4Kohm, Which turns out to be around 0.33uF.2. The existing cap seems to be red exoxy coated pillow shaped, seems to be a polyester cap. Would I be better off using a X2 cap with 250V rating?TIA

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2018, 04:41:35 pm
content: The instant the device is powered, the uncharged series capacitor might be exposed to the highest instantaneous voltage of the sinewave line voltage and this determines the surge current the capacitor and the diodes used to rectify are exposed to. Depending on the ratings and the value of the series resistor to the capacitor, this series resistor value may need to doubled as well, or you may see smoke one time(maybe not even the first time) this thing is turned on. It all about the winning the line voltage lottery on turn on, where all the winners are all getting zero.X2/250V would be preferred to win the lottery too. Pillow-shaped caps are quite fashionable, but I prefer square ones 'cause they don't move around so much, even though its harder to find them, well, red. Just dull yellow or green or gray, rarely pink, seems to be the popular choices.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 17, 2018, 05:02:25 pm
content: BigClive has posted numerous videos on the subject of capacitive dropping from mains power.Your calculations may be correct(?)It would be nice if there were some way to test/verify it without putting the gadget at risk.But the major take-away I learned from watching BigClive is to use properly rated (voltage) capacitors.At least 400V minimum for your mains voltage (240V is RMS, not peak).I would use a special Class-X capacitor because it is designed for direct mains connection.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 17, 2018, 06:00:57 pm
content: Quote from: Paul Price on May 17, 2018, 04:41:35 pmCurrently there is a 56ohm, 1W resistor in series. I intended to use a 100ohm, 1W over there. Its difficult for me to see the part numbers since the parts are mounted on the board and some of the numbers are occluded.I have with me a 1uF, X2 box capacitor that has 250V/275V rating on it. Is this an rms rating?For 1uF the impedance at 50Hz is around 3K which is lower than that of the original 0.56uF which was 4.7K. I intended to use 0.33uF but I have heard that the current sourcing of the power supply depends on the capacitance value. So now I'm confused should I go in for 0.33uF which seems to be logically correct with around 9K impedance at 50Hz or go for a higher value capacitor like 0.47uF or 1uF.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2018, 06:07:48 pm
content: Stick with the .33 uf, you got it right, yes it is RMS, and the 56-ohm resistor already in circuit is already a decent value.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 18, 2018, 08:42:19 am
content: Quote from: Richard Crowley on May 17, 2018, 05:02:25 pmWhan you say say 400V minimum? That would be a peak rating right? the X2 capacitors that I have are rating for 250V / 275V.

author: glarsson
author_rank: Frequent Contributor
timestamp: May 18, 2018, 11:15:04 am
content: Normally capacitors are specified to handle a certain number of volts DC, but X and Y capacitors are commonly (always?) specified with volts AC. An X capacitor specified for 250V AC should by ok for 230V mains.