Bridge Rectifier caps

[dumpsterdiver]

As I "mess around" with mostly consumer electronics I have often seen small capacitors across the diodes in bridge rectifiers. What are these caps for?

 

[gophert]

As I "mess around" with mostly consumer electronics I have often seen small capacitors across the diodes in bridge rectifiers. What are these caps for?

They clean up any transients in the AC line.

Sometimes there is a disk that looks like a capacitor but it is actually a transient voltage suppressor. Kind of an instantaneous fuse for over-voltage transients.

In either case, they are to clean up the incoming AC. Usually between mains feed-in and transformer primary. I haven't seen them between the transformer secondary and bridge.

Note: if they are placed after the bridge, then they are called filter capacitors to convert the freshly rectified AC camel humps to a flatter DC.

Does that answer your question or is there another configuration that you are trying to describe? If so, a schematic would help a lot.

 

[audioguru]

Capacitors across rectifier diodes have nothing to do with transients on the AC line. The rectifier diodes switch between conducting and not conducting very fast which produces "buzz" on the DC power supply voltage and the buzz radiates to nearby wiring and radio inputs. The capacitors slow the rectifier switching speed and prevent the buzz. I saw the capacitors on European audio products 45 years ago.

 

[spec]

Hi dumpster,

As ag says, the capacitors across the rectifier diodes suppress the commutation spikes that are generated when the diodes suddenly turn on and pass a huge current to charge the reservoir capacitor near the peak of the input sine wave. These suppression capacitors are especially important for sensitive equipment, like audio preamps where commutation spikes can infiltrate the the whole circuit. Because Schottky diodes are very fast, they particularly need commutation capacitors. Commutation capacitors are not always placed directly across the diodes and can be placed across the secondary of the input transformer and across the reservoir capacitors.

As gophert says, you will see all sorts of suppressors filters, inductors, and capacitors sprinkled around circuits these days to stop unwanted signals getting into and out of the equipment to comply with EMC standards. Commutation capacitors can also help suppress interference coming in from the mains line.

All equipment now has to comply with EMC standards. I have been involved with EMC certification on a few equipments, and you often find it necessary to place suppression components in the strangest places in order to pass the tests. In one case, a ferrite suppressor in the mains lead actually made things worse, rather than improving the situation as would be normal.

 

[MikeMl]

A different reason for putting capacitors across power rectifiers:

Here is the power supply schematic of my HF RF amp. Note the capacitors across the HV rectifiers. They are there to equalize the PIV on the non-conducting half of the rectifiers that might occur if a line transient happens. Without them, the rectifier with the lowest junction capacitance in the string could have its PIV exceeded by the transient...

 

[spec]

A different reason for putting capacitors across power rectifiers:

Here is the power supply schematic of my HF RF amp. Note the capacitors across the HV rectifiers. They are there to equalize the PIV on the non-conducting half of the rectifiers that might occur if a line transient happens. Without them, the rectifier with the lowest junction capacitance in the string could have its PIV exceeded by the transient...

Interesting, but that is a specialist application.

 

[spec]

As I "mess around" with mostly consumer electronics I have often seen small capacitors across the diodes in bridge rectifiers. What are these caps for?

dumpsterdiver you would be skipdiver in the UK. I am under treatment at the moment and must not go any closer then 100 yards to a skip (dumpster).

 

[Nikolai Petrenko]

Hmm, seem by to make clean DC out instead of clean transients.

 

[spec]

Hmm, seem by to make clean DC out instead of clean transients.

Hi Nikolai,

Your meaning is not clear. Can you explain what you mean?

spec

 

[Nikolai Petrenko]

Hi Nikolai,

Your meaning is not clear. Can you explain what you mean?

spec

seem be, not seem by as I posted. I mean across capacitor make clean DC out.

 

[spec]

Placing high frequency low ESR capacitors- ceramic for non audio paths and pollycarb for audio paths- makes the source impedance of a large electrolytic capacitor appear to be lower at high frequencies. This has two major benefits:
(1) Presents the audio amp with a lower impedance on the supply rails. Supply rail modulation is one of the biggest sources of distortion, after xover distortion, in class AB amplifiers. This is partly why a common upgrade on some commercial audio power amplifiers of a better auxiliary PSU brings such huge improvements in performance. On the Mission Cyrus range of amplifiers the addition of an external PSX unit transformed an already good amp.

I did an experiment once of placing large LiIon batteries across the supply rails with the bridge rectifier disconnected. This improved the sound noticeably (LiIon batteries have a very low source impedance and the existing capacitors also helped, especially at high frequencies).

I had plans at one time to build an amp where batteries were part of the PSU, but for some reason never got around to it. I did design a headphone amplifier powered by large LiIon batteries but that project is still on the back burner, like many projects.

(2) For the same reason as above, HF capacitors sprinkled around a physical circuit can also suppress interference both in and out of a unit.

An important point to note is that it is practically impossible to make a low distortion, high EMC equipment without a good physical layout, especially earth reference points. By the way, there is no such thing as a resistor, capacitor, inductor or conductor in the real word. Also, the nice clean schematic that you work to is an illusion. That is why sims can be an illusion too if not deployed wisely.

You can see evidence of this by looking at the waveforms inside a classic Difference/VAS/VtoI audio amp reproducing a sine wave. Often you will not see a sine wave but a saw tooth like waveform as the colossal amount of overall negative feedback wrestles with the parasitics etc to make the output voltage sine wave simply a bigger version of the input sine wave. On audio amps which heavily use local feedback, as was the fashion at one time, you will see sine waves though.

And don't even bother poking around the output stage as the drivers and output transistors switch on and off around the center of the input sine wave. By the way I am talking in hifi terms here where very small distortion artifacts can totally wreck the sound of an audio amp.

 

[dumpsterdiver]

dumpsterdiver you would be skipdiver in the UK. I am under treatment at the moment and must not go any closer then 100 yards to a skip (dumpster).

Restraining orders from trash bins??? Sounds like they want you to "skip" on bye!

 

[dumpsterdiver]

They clean up any transients in the AC line.

Sometimes there is a disk that looks like a capacitor but it is actually a transient voltage suppressor. Kind of an instantaneous fuse for over-voltage transients.

In either case, they are to clean up the incoming AC. Usually between mains feed-in and transformer primary. I haven't seen them between the transformer secondary and bridge.

Note: if they are placed after the bridge, then they are called filter capacitors to convert the freshly rectified AC camel humps to a flatter DC.

Does that answer your question or is there another configuration that you are trying to describe? If so, a schematic would help a lot.

I was referring to the small value caps that are in paralell with each of the 4 diodes in a rectifier bridge. I have also often seen the caps and MOVs you mentioned along with some inductors between feed-in and transformer primary.

 

[dumpsterdiver]

Capacitors across rectifier diodes have nothing to do with transients on the AC line. The rectifier diodes switch between conducting and not conducting very fast which produces "buzz" on the DC power supply voltage and the buzz radiates to nearby wiring and radio inputs. The capacitors slow the rectifier switching speed and prevent the buzz. I saw the capacitors on European audio products 45 years ago.

I have an egregious example of what you are talking about. I found a Sanyo LCD analog TV in the trash with no power supply (a brick like a laptop uses). Research showed it needed 12V at an amp or two. Used the 12v output of a PC power supply and it worked great. Ordered a replacement with the proper connector from eBay and it works but it radiates loads of interference. It was a cheapo switching type so there are probably no caps on the diodes or other suppression devices.

 

[spec]

I have an egregious example of what you are talking about. I found a Sanyo LCD analog TV in the trash with no power supply (a brick like a laptop uses). Research showed it needed 12V at an amp or two. Used the 12v output of a PC power supply and it worked great. Ordered a replacement with the proper connector from eBay and it works but it radiates loads of interference. It was a cheapo switching type so there are probably no caps on the diodes or other suppression devices.

dumpster,

You can't necessarily say that the Ebay PSU is deficient, it may just not be comparable with the TV. Of course it may be deficient. (does this make sens?)

 

[dumpsterdiver]

dumpster,

You can't necessarily say that the Ebay PSU is deficient, it may just not be comparable with the TV. Of course it may be deficient. (does this make sens?)

I did check out the voltage and current ratings of the TV and PSU before I bought the PSU. It has a unique ( to me at least) 4 pin connector so I looked for units specifically made for that line of Sanyo LCD Tvs.
When working on things fairly late at night I would listen to AM talk radio. The computer supply caused no interference even close to the receiver. The eBay PSU caused interference from across the room 15 feet away. It runs the TV with no problems.

 

[spec]

I did check out the voltage and current ratings of the TV and PSU before I bought the PSU. It has a unique ( to me at least) 4 pin connector so I looked for units specifically made for that line of Sanyo LCD Tvs.
When working on things fairly late at night I would listen to AM talk radio. The computer supply caused no interference even close to the receiver. The eBay PSU caused interference from across the room 15 feet away. It runs the TV with no problems.

Oh I see- that sounds pretty conclusive- the Ebay PSU was chucking out heaps of hash

 

[gophert]

I did check out the voltage and current ratings of the TV and PSU before I bought the PSU. It has a unique ( to me at least) 4 pin connector so I looked for units specifically made for that line of Sanyo LCD Tvs.
When working on things fairly late at night I would listen to AM talk radio. The computer supply caused no interference even close to the receiver. The eBay PSU caused interference from across the room 15 feet away. It runs the TV with no problems.

The core of the biggest toroid inductor is made of iron powder (bonded by epoxy) or ferrite powder that is sintered together. The permeability of the core is key to getting enough current out of the supply. Any oxidation of the iron greatly decreases the permeability and, therefore, decreases the current where the inductor is magnetically saturated. The strong interference tells me that your power supply cannot generate the current it was designed for because of a bad inductor. Bad because of a cracked or corroded core or bad because the enamel coated wire has shorted within the winding of the inductor. The switching mechanism seems to be working but the inductor is just not giving everything she used to have.

 

[dumpsterdiver]

The unit is sealed up with a glued plastic case, so it's not easy to get inside. It seems to run the TV just fine and doesn't get hot -it just radiates hash. I have the PSU and a cable box on a power strip and plug/unplug the PSU when needed since the hash would go beyond the apartment walls. When I plug in there is a spark/pop which indicates a surge to charge filter caps.

 

[spec]

The unit is sealed up with a glued plastic case, so it's not easy to get inside. It seems to run the TV just fine and doesn't get hot -it just radiates hash. I have the PSU and a cable box on a power strip and plug/unplug the PSU when needed since the hash would go beyond the apartment walls. When I plug in there is a spark/pop which indicates a surge to charge filter caps.

dumpster,

most laptop PSUs have a spark/pop when you plug them in. Have you tried to suppress the errant PSU by placing it in a conductive container foe example and building in a few strategically placed filters. The other approach, of course, in to use a linear PSU. I think I have seen some pretty cheap kits on Ebay.