Post by thread:[EE] capacitors on a diode bridge

In circuits I sometimes see capacitors over each of the fout diodes in a diode bridge. What is the prurpose and effect of these capacitors? I recently tried to help a client get rid of some interference problem and these capacitors turned out to do a good job. But I would like to know exactly (if possible) why? Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products docent Hogeschool van Utrecht: http://www.voti.nl/hvu

These would be the so called 'snubbing' capacitors, I think. They are supposed to reduce ringing of the LC circuit when the diode switches. (L = transformer winding, C = filter capacitor). I've seen lots of folks arguing about whether they make any difference. I have never found any difference, so I don't use them. Some folks say that an RC network, or just a resistor, works better than a capacitor. Neil On 11/20/05, Wouter van Ooijen <spam_OUTwouterTakeThisOuTvoti.nl> wrote: {Quote hidden}> In circuits I sometimes see capacitors over each of the fout diodes in a > diode bridge. What is the prurpose and effect of these capacitors? I > recently tried to help a client get rid of some interference problem and > these capacitors turned out to do a good job. But I would like to know > exactly (if possible) why? > > Wouter van Ooijen > > -- ------------------------------------------- > Van Ooijen Technische Informatica: http://www.voti.nl > consultancy, development, PICmicro products > docent Hogeschool van Utrecht: http://www.voti.nl/hvu > > > -

On November 20, 2005 12:45 am, Wouter van Ooijen wrote: > In circuits I sometimes see capacitors over each of the fout diodes > in a diode bridge. What is the prurpose and effect of these > capacitors? I recently tried to help a client get rid of some > interference problem and these capacitors turned out to do a good > job. But I would like to know exactly (if possible) why? You didn't mention how-big the caps were, but guessing 100nF, then the "resistance" of the capacitors could be seen as: X = 1 / ( 2 * pi * f * C) X = 1/(2*3.141*50hz*100nF)=31k (sort-off open circuit to AC freq) possible electric motor noise X = 1/(2*3.141*5000hz*100nF)=318ohms (low impedance to hi-freq) possible fluorescent ballasts X = 1/(2*3.141*50000hz*100nF)=32ohms (very low resistance) Electrolytic-type capacitors are fine at low frequencies but have an inductive component at high frequencies, plus being a chemical reaction, don't filter that quickly. Hope that helps.

Jose Da Silva wrote: >> In circuits I sometimes see capacitors over each of the fout diodes >> in a diode bridge. What is the prurpose and effect of these >> capacitors? > > You didn't mention how-big the caps were, but guessing 100nF, then the > "resistance" of the capacitors could be seen as: > > X = 1 / ( 2 * pi * f * C) > possible electric motor noise > X = 1/(2*3.141*5000hz*100nF)=318ohms (low impedance to hi-freq) > > possible fluorescent ballasts > X = 1/(2*3.141*50000hz*100nF)=32ohms (very low resistance) This doesn't explain why they need to be parallel to individual diodes and why there need to be four of them, does it? If you want to filter high frequencies (in or out), a low pass either on the AC supply lines that go to the diode bridge or on the DC supply lines that come out of it would be just as good, I suppose. Gerhard

On Sun, 20 Nov 2005 01:08:01 -0800, you wrote: >These would be the so called 'snubbing' capacitors, I think. They are >supposed to reduce ringing of the LC circuit when the diode switches. >(L = transformer winding, C = filter capacitor). I've seen lots of >folks arguing about whether they make any difference. I have never >found any difference, so I don't use them. Some folks say that an RC >network, or just a resistor, works better than a capacitor. I was recently talking to a guy a an EMC test house. He says that caps across a bridge make a big difference to conducted emissions, and it's the first thing they try when they have a product fail, and it frequently is the only thing they need to do to make it pass.

> I was recently talking to a guy a an EMC test house. He says > that caps across a bridge make a big > difference to conducted emissions, and it's the first thing > they try when they have a product fail, > and it frequently is the only thing they need to do to make it pass. Any hints at the theory behind this, and how do you select the appropriate values (and maybe type) of the capacitors? It was one of the first things I suggested to the customer, but he only placed them after a 4 hour session in which we tried a lot fo other things (with little effect). In this case the problem was not emission from our circuit, but susceptibility to promlems originating from other equipment in the same housing (same mains supply, but different trafo's). >From the fact that they are over each diode and not over the trafo secondary and/or after the bridge I suspect that they might have something to do with the diodes themselves. Maybe prevent nasty effects of high dv/dt spikes? Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products docent Hogeschool van Utrecht: http://www.voti.nl/hvu

At 09:45 AM 11/20/2005 +0100, you wrote: >In circuits I sometimes see capacitors over each of the fout diodes in a >diode bridge. What is the prurpose and effect of these capacitors? I >recently tried to help a client get rid of some interference problem and >these capacitors turned out to do a good job. But I would like to know >exactly (if possible) why? > >Wouter van Ooijen Wouter:- Diodes 101 (or maybe 301). It is due to "snap-off" of the diode reverse-recovery current-- the reverse recovery current can drop abruptly to almost zero as the minority carriers are removed from the junction). The rapid change in current (di/dt) can cause conducted and radiated EMI (and in some cases can cause spikes that exceed the breakdown voltage of the diode due to external inductance). You can also control the problem with soft-recovery diodes. However it's done, it will typically reduce efficiency a bit, all other things being equal. This document has some info re EMI http://www.irf.com/technical-info/whitepaper/murdiodes.pdf Also see page 14 of this document: http://encon.fke.utm.my/courses/see_5433/intro.pdf BTW, there are also diodes *designed* to exhibit this phenomenon called "step recovery diodes" (SRD's). They are used in microwave frequency multipliers, for example. >Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com ->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff

I have no idea about the theory, but here's my experience. I had assembled one of those FM wireless mikes to transmit TV audio, and since this was to be on most of the time, battery operation was not an option. I used a standard bridge for the supply, and the transmitter would transmit a hum. regradless of the size of the filter. Putting those four caps (0.1 uf) across the diodes in the bridge solved the problem. Regards, Anand {Original Message removed}

At 07:22 PM 11/20/2005 +0530, you wrote: >I have no idea about the theory, but here's my experience. I had assembled >one of those FM wireless mikes to transmit TV audio, and since this was to >be on most of the time, battery operation was not an option. I used a >standard bridge for the supply, and the transmitter would transmit a hum. >regradless of the size of the filter. Putting those four caps (0.1 uf) >across the diodes in the bridge solved the problem. > >Regards, >Anand Yes, even at line frequency it can be an issue in low-level circuits. You could also have solved the problem of audible noise by using Schottky diodes for the bridge. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" speffKILLspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com ->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff

Spehro Pefhany 14:26 2005-11-20: >Diodes 101 (or maybe 301). It is due to "snap-off" of the diode reverse-recovery >current-- In other words: the diode continues to conduct during a short period even after the current have reversed. Then, >the reverse recovery current can drop abruptly to almost zero as the >minority carriers are removed from the junction). The rapid change in current >(di/dt) can cause conducted and radiated EMI , and the ringing is pretty HF as it is made up basically only of the capacitance of the diodes and parasitic inductance in the transformer winding (plus leads) >(and in some cases can cause >spikes that exceed the breakdown voltage of the diode due to external >inductance). (But not in a bridge recitfier - unless there is large wiring inductance on the DC side - as they protect each other.) >You can also control the problem with soft-recovery diodes. Or shottkys that shut off very quickly. Shottkys also loose less power so might sometimes be economical choice as you get more electrical power out, and less heat to get rid of. As first step I would guess it is cheaper to use good diodes, than simple diodes + capacitor. For best performance maybe use R in series with the C. If even that does not help also use good diodes as fast soft recovery or schottkys. Alternatively use a RC snubber across rectifier AC side instead of diodes across every diode. /Morgan -- Morgan Olsson, Kivik, Sweden

> This doesn't explain why they need to be parallel to individual diodes and > why there need to be four of them, does it? If you want to filter high > frequencies (in or out), a low pass either on the AC supply lines that go > to the diode bridge or on the DC supply lines that come out of it would be > just as good, I suppose. The diodes produce harmonics when switching hard at low conduction angles. Those capacitors are supposed to filter those harmonics. They also increase the parallel capacitance of the diodes so they are more likely to survive a conducted RFI event's components which are coupled into the secondary. Peter

On November 20, 2005 02:23 am, Gerhard Fiedler wrote: {Quote hidden}> Jose Da Silva wrote: > >> In circuits I sometimes see capacitors over each of the fout > >> diodes in a diode bridge. What is the prurpose and effect of these > >> capacitors? > > > > You didn't mention how-big the caps were, but guessing 100nF, then > > the "resistance" of the capacitors could be seen as: > > > > X = 1 / ( 2 * pi * f * C) > > > > possible electric motor noise > > X = 1/(2*3.141*5000hz*100nF)=318ohms (low impedance to hi-freq) > > > > possible fluorescent ballasts > > X = 1/(2*3.141*50000hz*100nF)=32ohms (very low resistance) > > This doesn't explain why they need to be parallel to individual > diodes and why there need to be four of them, does it? If you want to > filter high frequencies (in or out), a low pass either on the AC > supply lines that go to the diode bridge or on the DC supply lines > that come out of it would be just as good, I suppose. I don't think 4 of them are needed either, just 2 of them should do. You would want to low-pass it to ground (relative to your circuit gnd). Sometimes it just makes sense to see it drawn different versus how it is seen in the circuit So (using ascii graphics) if you look at 2 capacitors grounding noise relative to your circuit/chassis ground: ---AC-hot------+-------|<|- | | --- | --- | | | | | ---AC-neutral------+--|<|-+---+--GND--- | | | | --- | | --- | | | | ----+----------+ | ----- / / /

On Sun, 20 Nov 2005, Wouter van Ooijen wrote: >> I was recently talking to a guy a an EMC test house. He says >> that caps across a bridge make a big >> difference to conducted emissions, and it's the first thing >> they try when they have a product fail, >> and it frequently is the only thing they need to do to make it pass. > > Any hints at the theory behind this, and how do you select the > appropriate values (and maybe type) of the capacitors? Afaik choose C~=50-100 times the capacitance between primary and secondary of the mains transformer (measured with the core grounded if so fitted, else 'in air'). >> From the fact that they are over each diode and not over the trafo > secondary and/or after the bridge I suspect that they might have > something to do with the diodes themselves. Maybe prevent nasty effects > of high dv/dt spikes? That too, when you have a RFI event in the primary it can kill the diodes if the caps are not on. When the diodes switch off hard the transformer secondary still carries current and the impedance it sees is nearly open circuit. The resulting high speed pulse is similar to the one on the drain of a MOSFET being turned off fast and propagates all over the place. It can also mix with other signals (f.ex. radiated and conducted RFI from within the project box which will be mixed with 50Hz ac in the 'modulator' bridge). I never had trouble with this so far but I nearly always put the capacitors in. I usually put in just two caps, from each transformer secondary wire to the circuit gnd. If you have long wires then this may not be enough. If you use real cheap diodes which leak and have large Vf drop, and a small filter cap which increases the conduction angle, then the problem will not happen in the first place ;-) This diode turnoff trick is one of the things that can be modelled nicely with Spice (be sure to model the stray capacitances and inductances). It looks very nice, almost like a spark gap transmitter's output ;-) Peter

On November 20, 2005 11:28 am, Peter wrote: > On Sun, 20 Nov 2005, Spehro Pefhany wrote: > > You could also have solved the problem of audible noise by using > > Schottky diodes for the bridge. > > Shottkys do not have this problem ? I did not know. But who would use > a Shottky in a not-so-low voltage mains rectifier ? It would improve the hum, but I don't think it would cure it. Regular diode drop is about 0.7v while schottky is 0.2v, so the hum would drop to 1/3 it's sound level. If you were to graph a 60hz cycle and look at the crossover points where one diode stops conducting while the next starts conducting, it is not a smooth transition. Yes, 0.2v drop is better than 0.7v drop, but doesn't afford the smooth transition a capacitor provides. Another trick for 60 hz "hum" would be to use a ferrite core choke as suggested by the ARRL manual, but the suggestion was probably made when winding your own choke was cheap and you had a good-sized cabinet to place the ferrite core in... now-a-days, people see a 1cent cap (minimal board space) versus a $1.00 choke (plus some PCB realestate) as the better way to go. I guess the RC resistor solution (vs LC solution) people argue about is a compromise in terms of benefit in board space and money. Cheers!

On 11/20/05, Wouter van Ooijen <.....wouterKILLspam.....voti.nl> wrote: > In circuits I sometimes see capacitors over each of the fout diodes in a > diode bridge. What is the prurpose and effect of these capacitors? There are two explanation and both could fit to your conclusions: 1. the noise from the AC line (which are passing through the transformer, which shows you have used a poor low cost transformer without having a shield connected to ground... or directly mains without LC filters) is killed on the filtering capacitor using the current path created by the capacitors connected in parallel with the bridge diodes 2.the rectifier diodes are poors (they can't switch enough fast the current you're sourcing from the transformer, ie if you're driving motors with PWM technique) and are inducing a noise which can't be killed only with capacitors mounted in parallel with diodes cheers, Vasile

On November 20, 2005 12:17 pm, Enrico Schuerrer wrote: > If there is a bridge rectifier you would probably have 120 Hz, not > 60. Yes, you would have a "pair" of 120hz pulses. :-) You would also have a pair of 120hz for a center-tap 2 rectifier setup. but it might be easier to follow the behaviour of point GND relative to the AC line if you draw a 60hz sinewave versus trying to follow 2 120hz pulses. Most people are probably running 12vac or something, but to exagerate the problem (around zero-crossing), suppose you draw a 3vac sinewave. Then follow the AC sine wave and the DC-out and DC-gnd points relative to the sine wave. When you have more than 1.4v, then you can consider the diodes in proper bias (low-impedance=good-grounding), but then you get to that +1.4->0->-1.4v (and -1.4->0->+1.4v) region where the diodes are in less-"grounded" behaviour of low-impedance going to high impedance to open circuit to high impedance back to low impedance region. If you draw a 12vac sine wave, the region is not so pronounced, but if you draw a 3vac sine wave and trace the signals, you got this 2.8v questionable region if you are using 07.v diodes for a full-wave bridge. It's less pronounced if you use 0.2v schottky diodes because that region is only about 0.8v around the zero-crossing line before the diodes are above the 0.2v conduction range. ...this region would be a point of interest in terms of ac hum, in terms of the microphone example mentioned earlier. > >From my experience the C across the diodes will reduce HF > > interferences - > > and 4 are really necessary, a rule of thumb says between 0.01µF to > 0.1µF will be OK. ...well, if you found 4 necessary and not just 2 enough, then, I guess 4 it is. ...what type of circuit(s) did you fix using 4?

>-----Original Message----- >From: EraseMEpiclist-bouncesspam_OUTTakeThisOuTmit.edu [piclist-bouncesspam_OUTmit.edu] >Sent: 20 November 2005 08:45 >To: 'Microcontroller discussion list - Public.' >Subject: [EE] capacitors on a diode bridge > > >In circuits I sometimes see capacitors over each of the fout >diodes in a diode bridge. What is the prurpose and effect of >these capacitors? I recently tried to help a client get rid of >some interference problem and these capacitors turned out to >do a good job. But I would like to know exactly (if possible) why? > >Wouter van Ooijen One potentialy important function which I've not seen mentioned is that the capacitors provide a stable RF impedance from the point of view of the powered circuit looking back in the mains. The impedance of the diodes obviously varies considerably at a rate of either 50 or 60Hz, and if a circuit conducts any RF back into the mains, this variation can cause problems. A classic case which I've experienced is powering one of the simple FM "bug" transmitters via a "wall wart" style supply. Inevitably the received audio will be drowned in 50/60Hz buzz, that seemingly no amount of capacitive or inductive filtering on the supply lines will eliminate. Bypass the bridge recitifer with small value ceramic caps and the hum all but vanishes. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================