transients in capacitor droppers - Page 1
Forum section: electronics / beginners

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 01, 2019, 07:54:38 pm
content: Is there a simple method to mitigate high frequency transients from conducting large currents?The series current liming capacitor droppers only work when frequency is low, but capacitors are short circuit high frequency. The load on the fullwave rectified side is reactive and can vary impedance rapidly.

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2019, 08:27:19 pm
content: There should be a low value series resistor to limit the turn-on transient, when power is applied at 90º, when the voltage is highest, otherwise the fuse will blow and quite likely the rectifier too.To answer the question: no, a capacitive dropper will always pass higher frequencies, more than lower ones. Swapping the capacitor with an inductor will block high frequency transients, but it will probably work out just as bulky and expensive as a transformer.

author: Gyro
author_rank: Super Contributor
timestamp: September 01, 2019, 08:29:32 pm
content: Yes, a series resistor, and a protective clamp zener across the load. I wouldn't implement a capacitive dropper without them.Many commercial applications use a fusible resistor in that position.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 01, 2019, 08:46:41 pm
content: What frequencies are typical residential transients?Nearly any inductance will attenuate high frequency but that would open a window to a resonant LC band in which the fuse would blow if the LC frequency is a typical mains noise frequency.Maybe residential high frequency transients are all manifest as high voltage transients in which case the MOV should remove them.

author: StillTrying
author_rank: Super Contributor
timestamp: September 01, 2019, 09:15:10 pm
content: We had a similar thread on this trying to protect LEDs from the switch on and and other transients.https://www.eevblog.com/forum/beginners/transformerless-power-supplies-circuit-cheatsheet-and-questions/msg2186906/#msg2186906

author: coppice
author_rank: Super Contributor
timestamp: September 01, 2019, 09:25:42 pm
content: There is a reason most cap drop supplies have a big fat series resistor in them, which normally runs very cool.... it is not at all cool when there are harmonics on the supply rail. Try running many of cap drop based devices from a triac light dimmer, which can produce some really serious harmonic content, and as you twiddle the brightness knob you may be able to get the device to catch fire.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2019, 10:12:18 pm
content: Transient surge is on the order of 0.1 to 10MHz, and EFT and ESD going up into the 100s of MHz. Equally important, surge is low impedance (~2 ohms), while ESD and EFT are modest impedance (~100 ohms).Frequency isn't a terribly useful measure to apply to transient phenomena. You can't get a meaningful impedance for example, if that's where you're going. The rate of change would be more useful, but that also gets complicated by placing a resistor in series -- the current flow is no longer neither just the voltage drop (I = V/R), nor the rate (I = C dV/dt), but somewhere inbetween, depending on how much of each is present. While this situation is still analytically tractable, it's not easily explained, and merely simulating it should be more informative.The equivalent circuit for turn-on during line voltage peak, is approximately the RLC series equivalent of the line inductance (ballpark 100uH), line and capacitor ESR (~low ohms?), added resistor, and the capacitor (and the diodes of course, if you want to model them as such). The resistor should be the dominant part of this (R > sqrt(L/C)), and set to limit peak current to reasonable values (a few amperes?).Incidentally, note that you can combine FWB and zener clamping functions, by building the FWB out of zeners.Tim

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 03, 2019, 09:15:01 pm
content: Quote from: T3sl4co1l on September 01, 2019, 10:12:18 pmAre inrush current limiters ideal to prevent the momentary short circuit conditions a high frequency event can incur?They would reside after the MOV to prevent high voltages from destroying it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 03, 2019, 09:17:57 pm
content: Could, yes. Something like, back-to-back depletion MOS -- it's a rather expensive alternative to a resistor, but in a situation where a resistor isn't suitable, that would be an excellent alternative.Tim

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 03, 2019, 09:27:59 pm
content: Is the current limiter positioned so as it will not incur issues? The fuse and mov seem to prevent it from incurring damage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 03, 2019, 09:44:33 pm
content: Yup, like that.Tim

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 03, 2019, 09:52:16 pm
content: 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.pdfThe logo is almost suggestive of a fire hazard.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2019, 08:35:09 am
content: Bulbs are cheap, if the resistor fuses you buy a new one.Tim

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2019, 10:37:57 am
content: Quote from: LaserTazerPhaser on September 03, 2019, 09:27:59 pmQuote from: T3sl4co1l on September 03, 2019, 09:44:33 pmSurely 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2019, 01:52:29 pm
content: 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

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 04, 2019, 04:59:38 pm
content: Is the zener of any use if the application is for electroplating? Electroplating with this form of supply can be hazardous but its useful since it bares a high DC voltage which readily overcomes electrolytic resistance especially the more series dropper capacitors the smaller the resistance of the fusible resistor.However a TVS zener is needed for loads involving semiconductors after the rectifiers.

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2019, 06:16:13 pm
content: You really needsomethingto kill the supply if one of the series capacitors goes short. The Zener will act as a clamp in these circumstances - either blowing the resistor (hence the fusible, flameproof bit) or, less likely, the fuse. Expect the Zener to go short too.In your circuit , as originally drawn, you show a 470uF (so presumably electrolytic) capacitor on the output of the bridge. Without the Zener to clamp it, the voltage across it would either rise past its rated voltage, or store up enough energy toreallymake a splash.As you say, using this form of non-isolated power supply for Electroplating use is hazardous but you're on your own with that one - at least if you have nobody else to take safety responsibility for.EDIT: This is becoming rather more like Frankenstein's laboratory - I think this is the first time that you have revealed that it's to be used for electroplating.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 04, 2019, 06:17:38 pm
content: For small things such as LED's a resistor is often used to limit the inrush current.With a 10uF capacitor the steady state current is a lot higher hen needed for a LED, and this makes a a simple series resistor less attractive, because of excessive engy loss.You may want to consider to use an inductor to limit inrush and transient currents, and also add a resistor to lower the Q of the LC circuit just enough to limit peaking.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 04, 2019, 06:32:02 pm
content: That 470uF filter capacitor would not be present for electrolytic loads. The filter cap was only for loads which had semis involved in which case a TVS would be implemented along side it, also its voltage rating would be well over the fullwaved peak voltage.The capacitor droppers are 630v and they are self healing. The only things which can cause the series capacitors to short out is over voltage, heat and defects. The zener TVS would likely combust, they are only intended for short duration transients, they aren't made to conduct shorted mains supply.What off the shelf inductor packages & values are ideal? Currently only thru hole torroids appear usable for this. But there are those costly inductors which are made with flat copper wire which can be surface mount, but they have so much ferrite it might saturate and short out for almost any inrush current.

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2019, 06:45:39 pm
content: If the load is a plating bath, you are probably past the point where you care about noise spikes - they will simply be integrated by the plating process. Worst case, the capacitors will self-heal themselvesto death.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2019, 07:12:17 pm
content: I'm rather more concerned now about what kind of plating bath needs so little current that such a circuit would be reasonable.Tim

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 04, 2019, 08:47:06 pm
content: Quote from: Gyro on September 04, 2019, 06:45:39 pmBut for loads involving semis whats the ideal inductor for this?

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2019, 08:59:11 pm
content: No simple answer, it would need to be very lossy to prevent ringing and peaking on different transient edge rates (or made lossy using external components). The advantage of a resistor is that it is more or less broadband.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 04, 2019, 09:32:04 pm
content: Quote from: T3sl4co1l on September 04, 2019, 07:12:17 pmCurrent rises with more capacity between mains and rectifiers.

author: StillTrying
author_rank: Super Contributor
timestamp: September 04, 2019, 10:13:56 pm
content: Quote from: Gyro on September 04, 2019, 06:16:13 pmYes, I hope LaserTazerPhaser realizes thatthe groundson the schem. are always at live mains voltages.It reminds me of the cap dropper battery rejuvenator.https://www.eevblog.com/forum/beginners/please-help-with-switch-selection/msg2490720/#msg2490720