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Old 6th February 2016, 07:29 PM   #1
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Default Rectifier Diode Snubber;

I soldered small film caps between each bridge rectifier legs and before plugging in to full mains I tried with DBT. Well, bulb stays bright and each transformer secondary is 7V AC instead of 18V AC. Is it because of the bypass caps between rectifier legs? I checked my work, there is no short between legs. If I remove these small bypass caps DBT stays dim..

Did I done something wrong?

I followed this schematic;

Click the image to open in full size.
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Old 6th February 2016, 07:44 PM   #2
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by starbender View Post
I soldered small film caps between each bridge rectifier legs and before plugging in to full mains I tried with DBT. Well, bulb stays bright and each transformer secondary is 7V AC instead of 18V AC. Is it because of the bypass caps between rectifier legs? I checked my work, there is no short between legs. If I remove these small bypass caps DBT stays dim..

Did I done something wrong?

I followed this schematic;

Click the image to open in full size.
I see no valid reason for the behavior you report; it has to be bad or wrong components, unsuspected short or something of the kind.
Anyway, the preferred implementation of what you want to do is with small ceramic disc caps (1 to 10nF, 500V), preferably soldered directly on each diode for good VHF suppression.
Film caps are not as effective
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Old 6th February 2016, 08:04 PM   #3
Mooly is offline Mooly  United Kingdom
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Is the circuit just as shown or have you still got it coupled to and feeding whatever it supplies ?

Edit... sorry, hadn't noticed the bit "If I remove these small bypass caps DBT stays dim.."

Ignore
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Old 6th February 2016, 08:13 PM   #4
AndrewT is offline AndrewT  Scotland
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Those capacitors are not snubbers.

All they do is add capacitor and diode capacitance togther, to end up lowering the resonant frequency. Often resulting in worse ringing.

You need a resistor to damp the ringing. It's the resistor that acts as the snubber.
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Old 6th February 2016, 08:23 PM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by AndrewT View Post
Those capacitors are not snubbers.

All they do is add capacitor and diode capacitance togther, to end up lowering the resonant frequency. Often resulting in worse ringing.

You need a resistor to damp the ringing. It's the resistor that acts as the snubber.
You have been brainwashed by the "snubbering brigade": try to feed an ordinary AM/FM radio with the the "crude" capacitor version and then the nice, cleanly snubbered one (as seen on an oscilloscope screen).
You will understand why asian makers of cheap sets got it right as early as the sixties.
Note that you can do both if you find it comforting, but only one is actually necessary
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Old 6th February 2016, 08:51 PM   #6
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I'm sorry but I forgot to mention something.. And I think this is the root of the symptoms that I have.

This is F5.. with two secondary,two bridge rectifier and fully biased (1.3A per channel) amp. And I soldered caps to bridge as it is and tested with dbt with 25W bulb. And it stayed bright. And I think this is because of amp was fully biased already.. I removed the caps from the bridge, while I'm in there I zeroed offset and bias to readjust. Tried with dbt again and it goes dim all the way... with zero bias...
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Old 7th February 2016, 07:44 AM   #7
Mooly is offline Mooly  United Kingdom
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Thanks for explaining what had happened. So all is good
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Old 8th February 2016, 02:54 PM   #8
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Quote:
Originally Posted by Elvee View Post
try to feed an ordinary AM/FM radio with the the "crude" capacitor version and then the nice, cleanly snubbered one (as seen on an oscilloscope screen).
I recommend a multi-band shortwave receiver instead. The ringing can occur at whatever frequency the L's and C's dictate; there is no a priori reason to be confident that this ringing must fall within the AM band or the FM band.

Changing the value of L and/or C will change the frequency of ringing, which can move it into the AM band or the FM band, leading to the incorrect conclusion that the modification made the situation worse. It is also possible that changing L and/or C changes the frequency of ringing, moving it out of the AM band or the FM band, leading to the incorrect conclusion that the modification made the situation better.

In my own experiments, I've seen transformer leakage inductance L as low as 10uH and as high as 100mH, a range of ten thousand to one. I've also seen secondary resonant capacitance C (= trafo Cmutual + Crectifiers) without any "protection" capacitors added, as low as 60pF and as high as 10nF, a range of 160 to one. So the secondary's natural frequency of oscillation omega_naught, varies by more than a hundred to one; sometimes it falls in the AM or FM band, other times it does not. The claim that an AM/FM receiver is absolutely the best analysis and measurement tool for every transformer+rectifier combination, is not supported by experimental data.
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Old 8th February 2016, 06:07 PM   #9
Elvee is offline Elvee  Belgium
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Rectifier diodes typically cause three distinct types of disturbances.

First type is caused by the diode switching exciting the resonance of the transformer's leakage inductance with various capacitances, explicit or implicit.
It is the most visually obvious with an oscilloscope, and it is also relatively easy to cure with an appropriate damping network.

Second type is caused by the snap-off effect of the diode, generating a spray of HF and VHF energy at each current zero crossing. Because the HF level is small, and the recurrence is low (compared to the frequencies of the energy), it is completely invisible on an oscilloscope and even on a traditional, swept spectrum-analyzer. However, it causes a characteristic 100Hz (or 120Hz) buzz in receivers. A damping network for type 1 will dissipate some of the RF energy, but since the damping network has a series resistance, it will leave a significant residue.
In addition, the damping network is normally used once, across the transformer, but each diode (and its associated wiring) acts as a RF generator.

Third type is even more evasive: it is caused by the rectifier diodes behaving as PIN diodes. They change the grounding configuration of the antenna 50 or 100 times per second, and they modulate the apparent level of the incoming RF signal at the same rate, also causing a buzz that typically varies with the placement of cables, hand effects, etc.
There again, a damping network intended for type 1 will mitigate the effects, because there will be some equalization in the impedance variations, but some switching effects will remain.

The cure for type 2 and 3 is more or less the same: a small bypass capacitor directly across each diode. This capacitor acts as a short at RF. For type 2, it confines the RF disturbances to a small space. In principle, a capacitor cannot dissipate anything, and it can only reflect incident energy, but in practice with the values and components used, the confined energy is quickly dissipated. Matters can be improved by adding a small dissipating ferrite bead to the diode: it increases the impedance of the generator, and provides an explicit dissipation means.
For type 3, the capacitor acts by shorting completely all modulation effects.

Typically, the capacitor value is 4.7 or 10nF, but the range found goes from ~1 to 100nF.

See below an example of a typical commercial product.

These capacitors are the only mandatory ones. A "proper" damping network is rarely found on commercial products, because it is generally unnecessary on otherwise properly designed equipment.
Things can be different for amateur gear, for which there is also the "before/after" effect: when the cure is so visually effective, it can only be "a good thing".
It can do no harm anyway, and will always improve matters, but by order of importance the capacitor on each diode (very close) comes first, followed by the ferrite bead, and finally for perfectionists, the optimal damping network.
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Old 8th February 2016, 06:54 PM   #10
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So, there's a type of diode-generated RF noise that's too small for oscilloscopes too see, even with AC coupled (high pass filtered) input and scope trace sync'd/triggered to zero crossings? Not only is this noise too small to see on a scope, it's too small to see on a spectrum analyzer? However it's not too small for a high gain radio receiver to detect it?

How well does a typical audio power amplifier (gain = 21X) detect this RF noise? Phono preamplifier (gain = 2000X)?
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