Another HT Diode Rectifier Question.

[big_teee]

http://www.valvewizard.co.uk/bridge.html
According to the valve wizard a 10 ohm, or 22 ohm 3 Watt resistor between the rectifier and C1 Cap reservoir, helps on High Freq. Hash noise.
My question is if you blow the HT 1/2 to 1 amp fuse, at 450-490V, will a 3 Watt 10 ohm Resistor still be good or will it be gone up in smoke also.
I keep trying to do the math, and 3 watts seems too small to me.
Please Help me out here?
Thanks,
Terry

[loudthud]

Half an Amp (DC) would be 2.5W in a 10 ohm resistor. But the RMS current between the rectifier and reservoir cap would be higher at half amp DC. You can model the circuit to find the RMS current, or just build and smoke test. IIRC KOC uses a 10W wire wound cement type resistor.

[trobbins]

The other aspect to note is that your query pivots on a 0.5 to 1A fuse blowing.

The 'fault' that causes the fuse to fail could be a current that just sits on or just exceeds the fuse rating, or it could be a 'bolted' short circuit straight after the fuse, or anything in between.

Even for a good short circuit, a fuse may not blow 'instantly' as often the level of overcurrent that is available is not more than a few times the fuse rating (given that the PT has nominally got a HT winding that would be rated to supply only up to about 50% of the fuse rating, and the effective resistance of primary and secondary windings could be quite substantial).

The I2t heating in the fuse is what blows it, and somewhat similarly the internal heating in the resistor is what eventually blows the resistor. And wouldn't you know it, but not all fuses have the same I2t characteristic, and not all 3W resistors have the same surge power rating. Who will win this epic battle to stay alive? That quandary is why as loudthud indicates, some use a resistor wattage rating that has little risk of ending in a question mark.

[exclamationmark]

You can get pulse rated resistors for these types of applications that can eat 100's of watts for a few milliseconds (long enough until the fuse blows) - IIRC most are carbon comp which may arguably defeat the purpose of the noise removing resistor, though you'd have to do the math on that to find if the noise levels are swamped. All resistors have some level of pulse handling capability, but unless their marketed towards automotive or harsh environments they usually don't list the exact characteristics on their datasheet. Alternatively, you could stick a giant ceramic/metal oxide/wirewound resistor in there and just set and forget it.

[big_teee]

So a 5 or 10 watt power resistor and I should be set.
Like you say, at 65 cents, just give it the smoke test.
Thanks,
T

[ReadyTeddy]

A 5 to 10 watt wire-wound resistor will take lots of current for a short time--more than the 1N4007s (or the like) in your power supply, I would think.

[Enzo]

The 1A 1N4007 has a 30A surge rating.

[R.G.]

The Valve Wizard
According to the valve wizard a 10 ohm, or 22 ohm 3 Watt resistor between the rectifier and C1 Cap reservoir, helps on High Freq. Hash noise.

Maybe.

You can also attach high frequency has noise at the source by
(1) changing to fast, soft recovery diodes like FREDs
(2) snubbing the rectifier diodes so their turn-off transients do not cause RF ringing with the inductance/capacitance of the wiring they're connected to
(3) snub the secondaries of the power transformer to prevent the turn-off transients of the diodes from exciting ringing with the secondary leakage inductance

[bob p]

It's interesting that Terry's diagram puts the 100nF bypass cap from B+ to ground. that would help to remove noise from the B+ supply.
The old style noise suppression circuits put a small cap across every diode in the bridge (2).
The current mode for CE certification seems to be putting the small cap directly across the PT secondaries (3) instead of (2).

I'm thinking that (3) isn't really there to help you -- i think it is more designed to protect the primary side (ie: the line and the external world) from noise coming from your amp.

[R.G.]

It's interesting that Terry's diagram puts the 100nF bypass cap from B+ to ground. that would help to remove noise from the B+ supply.
The old style noise suppression circuits put a small cap across every diode in the bridge (2).
The current mode for CE certification seems to be putting the small cap directly across the PT secondaries (3) instead of (2).

I'm thinking that (3) isn't really there to help you -- i think it is more designed to protect the primary side (ie: the line and the external world) from noise coming from your amp.

Yeah. Horses for courses, I guess. It depends on what problem you're trying to solve.

The root issue (I think - ) is the transients caused by the sudden slamming off of standard rectifiers. Standard rectifiers conduct and rectify well, but have a delay in turning off when reverse biased for semiconductor physics reasons, and when they do finally (a few microseconds or so) turn off, the slam off with a bang. This sudden interruption of the current flow in reverse causes the small but always-present inductance of the wiring to the diode to ring with the stray capacitance around it. It's not as good a radio emitter as the old spark-gap transmitters, but it's not quiet either.

There are a few ways of fixing this, or coralling the damage. A ceramic cap across the secondary diverts the energy from the rectifier turn-off into ringing the leakage inductance and self capacitance of the secondary, at a lower frequency than the original squark! from the turn-off. It helps keep the energy in the secondary, and not on the AC power line where it can radiate well with the AC power cords are an antenna. A ceramic cap across the primary helps too, although not as much as the EMI filters used with switching power supplies. And both primary and secondary caps help with preventing RF picked up from the AC power-cord-antennae from getting into the secondary circuits. A cap across the secondary also helps with both killing RF noise from the circuits (if any) and helping prevent RF oscillation caused by high impedance power supply wiring.

Shunting the diodes with caps attacks the rectifier noise issue at the source. Just caps across there reduces the frequency of the ringing from turn off transients. A resistor-cap snubber actually helps eat up the energy; if this is properly tuned, it makes the turn-off almost quiet.

Using FREDs or other fast turn-off rectifiers that also are soft turn-off avoids generating the transient squarks in the first place. They were developed for this purpose in switching power supplies, but they work just as well for it in low frequency supplies, too.

[Leo_Gnardo]

Using FREDs or other fast turn-off rectifiers that also are soft turn-off avoids generating the transient squarks in the first place. They were developed for this purpose in switching power supplies, but they work just as well for it in low frequency supplies, too.

Along those lines try UF4007, almost as dead cheap as 1N4007. Popular amongst the hi fi crowd.

[trobbins]

The source of rectifier noise is in essence the leakage inductance of the PT secondary winding, and that the diode itself causes the current in that winding to try to do a step change in level. Constraining the energy in the secondary winding leakage inductance to a loop directly around that winding is imho the best strategy, rather than focussing on channelling the transient energy out via larger loops that include diode bypasses and large smoothing capacitors and load circuitry etc.

The rate at which the current has to fall to zero depends on the AC mains frequency and the on-voltage of the diode and effective IR change. A valve diode inherently slows the rate down as it takes more time for the diode voltage to drop to zero. Similarly a lower mains frequency will slow the rate of current step. At higher rates of current change from conduction level down to zero current, the reverse recovery current peak in a pn diode gets worse - a well described affect - so the diode can exacerbate the current step by allowing current to flow beyond 0A and go slightly negative. The whole issue is further exacerbated by adding more output filter capacitance, as that increases the peak conduction current in the diode waveform, and shortens the conduction duration - and causes more pain for the mains voltage waveform on the primary side of the PT.

[R.G.]

The source of rectifier noise is in essence the leakage inductance of the PT secondary winding, and that the diode itself causes the current in that winding to try to do a step change in level.

Well, all the stray inductances, but that's a biggie, OK. Being a biggie, it runs the ringing frequency down to quite low in the RF range. The individual wires to/from the diodes and filter caps can do much the same with their own self inductance, at higher frequencies.

Constraining the energy in the secondary winding leakage inductance to a loop directly around that winding is imho the best strategy, rather than focussing on channelling the transient energy out via larger loops that include diode bypasses and large smoothing capacitors and load circuitry etc.

Ideally, instead of constraining the energy anywhere or channelling it anywhere, one would damp the LC circuits so that they didn't ring much. The technique is properly snubbing, which I've heard called "Q-spoiling"; the trick of inserting a resistance to damp just the ring energy to the extent possible.

Just sticking in capacitors anywhere only allows the transient energy to move through different paths as you mention, or lowers the resonant frequency of whatever LC is the predominant ringer. Adding properly designed snubbers actually eats up the transient energy.

Adding properly designed snubbers to the diodes themselves eats up the transient energy in the smallest possible loop, that being right at the diode. It also modifies the diode turnoff a bit by adding in some more external capacitance to give the diode the opportunity to be at a lower current when it does turn off. Standard diodes slam off because they have to sweep out the minority charge carriers from the conducting junction region. When the carriers are gone, conduction stops abruptly. However if external conditions have reduced the reverse current to near zero at the time the carriers finally get swept out, there is little current to slam off and excite a ring. Resonant switching power supplies exploit this kind of action to turn off power switches when their current happens to be at zero from the external resonating parts.

The rate at which the current has to fall to zero depends on the AC mains frequency and the on-voltage of the diode and effective IR change. A valve diode inherently slows the rate down as it takes more time for the diode voltage to drop to zero. Similarly a lower mains frequency will slow the rate of current step. At higher rates of current change from conduction level down to zero current, the reverse recovery current peak in a pn diode gets worse - a well described affect - so the diode can exacerbate the current step by allowing current to flow beyond 0A and go slightly negative.

Substantially everything affects the turn off. Valve diodes don't have minority charge carriers and inherently turn off more quietly.

The longer the diode delays turning off, the higher the reverse current ramps in whatever parasitic inductances are attached to the rectifiers, and the more energy is stored in the external inductances to ring. Simply turning off FAST!! helps because there is less energy in the external inductances to ring, other things being equal.

Line frequency would play a part, but the available change in line frequency is much smaller than the available differences in the turn-off time and characteristic of the diodes. A change from 60 to 50 Hz is only 1/6; it's easy to find diodes 100 or more times faster than standard rectifiers.

The whole issue is further exacerbated by adding more output filter capacitance, as that increases the peak conduction current in the diode waveform, and shortens the conduction duration - and causes more pain for the mains voltage waveform on the primary side of the PT.

One interesting fix I seldom see mentioned is to give the rectifiers a smaller first filter cap to work into, perhaps 10uF, and then a second filter cap separated from the first by a small resistance. This lets the rectifiers work with longer but shorter current pulses into the first filter cap, then the R-C of the second filter cap allows normal filtering for the rest of the circuit. Of course, you pay for this with a higher ripple on the first filter cap and lower DC level out of the second.

But putting in fast, soft recovery diodes fixes a lot of the issues, and is highly recommended. The UF4007 is a great choice, being much cheaper and more available than the IXYS (I think...) FREDS that were first used.

[Steve Conner]

One interesting fix I seldom see mentioned is to give the rectifiers a smaller first filter cap to work into, perhaps 10uF, and then a second filter cap separated from the first by a small resistance. This lets the rectifiers work with longer but shorter current pulses into the first filter cap, then the R-C of the second filter cap allows normal filtering for the rest of the circuit. Of course, you pay for this with a higher ripple on the first filter cap and lower DC level out of the second.

It is mentioned in Bob Cordell's excellent book, "Designing Audio Power Amplifiers". He suggests splitting the filter capacitance in half with a low value wirewound resistor in between. He says that even 0.22 ohms works wonders in a solid-state power amp. Of course the idea originally came from tube amps where we would use two filter caps separated by a choke.

There are two approaches to getting rid of diode reverse recovery transients. You can snub them with a RC snubber: there are several places you can put it, and they all work about the same. I like to put a single snubber, 10 ohms in series with 0.1uF, across the two AC terminals of the bridge. The DC ones are shorted together and connected to RF ground by the filter capacitor.

Or you can just not make the transients in the first place, by selecting ultrafast diodes with soft recovery. These have come down to about the same price as regular diodes, due to the invasion of switchmode power supplies into consumer electronics. The UF4007 is as good a choice as any. There is also a UF5408 for those who fancy a little more current.

Back on the original topic, the idea of protecting a resistor with a fuse makes me uneasy.

[trobbins]

Adding a bypass cap directly across a secondary HT winding will effectively achieve a damped RC snubber, due to the considerable winding resistance that is normally in situ - the damping may not be too bad.

Best to keep the rectifier and bypasses as close to the transformer winding as possible - and if not very close then at least with twisted winding leads as others have often noted here. Adding a fuse in the ac path may make such a loop larger, especially if wiring needs to get to a front or rear panel. If possible, any snubbing across the secondary winding should be placed at the winding, before the fuse and diode circuitry.

The harmonic structure of the current step waveform when the diode is being commutated off by the falling secondary voltage waveform is a fundamental aspect of rectifier noise that is often overlooked when diode noise is discussed. Most conversions about diode noise focus just on the final sub-microsecond of reverse recovery current turn-around and snap off (the highest dI/dt sections), and seem to forget about the major section of commutation with the ramp down in diode forward current from full conduction to zero current.

Adding a series resistance with the winding is different to adding it after the first filter capacitor, for a number of issues. Not being a hi-fier per se, I prefer to put such resistance in series with the secondary when using a ss diode so as to effectively return the diode commutation towards a valve style, and use relatively small main filter capacitance, so pn diode reverse recovery is a much more benign secondary issue.

For sure, if the user wants the least low frequency B+ ripple and puts in a low winding resistance PT, with ss diodes and huge filter capacitance, then a whole sweet of rectifier related noise management measures may be considered by a designer - but sadly some feel that just the use of a SiC device is all that needs to be considered.