Electronics > Projects, Designs, and Technical Stuff
Caps across chokes trick
Ysjoelfir:
--- Quote from: 001 on October 12, 2019, 08:58:49 am ---
I googled an Italian site about it!
http://www.sugardas.lt/~igoramps/article47.htm
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cool! but sadly that isn't italian (which is actually moderately acceptable translated by google translator) but lithuanian or russian (which is absolutely horribly translated by google) and I doubt many of us here are capable of understanding what is written there. do you mind translating the relevant parts?[/b][/size]
since I am interested in "the old techniques" I would be intrigued to learn more about it.
001:
--- Quote from: Ysjoelfir on October 12, 2019, 09:25:02 am ---
cool! but sadly that isn't italian (which is actually moderately acceptable translated by google translator) but lithuanian or russian (which is absolutely horribly translated by google) and I doubt many of us here are capable of understanding what is written there. do you mind translating the relevant parts?
--- End quote ---
I`m using google translate too. It is strange a little to read it but author tune LC to resonance at ripple f and put it to an amp
but no any formulas exept muddy graph
Google say:
--- Quote ---The adjustment is made by selecting the capacitor capacitance, connected in parallel with the inductor. Do this in advance, even before installing the throttle on the chassis. The setup technique is very simple. It is necessary to assemble the measuring circuit shown in Fig. 1, connect a sound generator to its input, and an oscilloscope or an AC voltmeter to the output.
If you know (at least approximately) the inductance of the inductor, then to reduce the time it takes to adjust along the curve in Fig. 2 it is necessary to determine the order of the magnitude of the capacitance required for the resonance and start the adjustment of the throttle with such a capacitance.
By connecting a capacitor to the inductor coil and setting the generator output at a frequency of 100 Hz so that the voltmeter needle is closer to the middle of the scale, the generator frequency is changed to a small extent, observing the instrument reading. In one case, the output voltage will decrease, in the other - increase
Continuing to change the frequency of the generator in the direction of increasing the output voltage, they reach the point at which the voltage becomes maximum. This point corresponds to the actual resonant frequency of the choke-capacitor system. If it turned out to be above 100 Hz, the capacitance of the capacitor should be increased, which can be done by connecting an additional capacitor in parallel with the existing capacitor. If the actual resonant frequency is below 100 Hz, instead of the existing capacitor, you need to put another, smaller capacity.
Thus, by gradually selecting the capacitance, the inductor is tuned exactly to a frequency of 100 Hz. Note that an inaccuracy of tuning by only 10 Hz reduces the filter efficiency by more than half.
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MagicSmoker:
--- Quote from: 001 on October 12, 2019, 09:29:40 am ---...author tune LC to resonance at ripple f and put it to an amp but no any formulas exept muddy graph
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The formula to find the shunt capacitor of a parallel resonant "trap" when you know the inductor and ripple frequency is just an algebraic rearrangement of the resonance equation:
Cres = 1 / [(2pi)2 * f2 * L]
Where C is in F, f is in Hz and L is in H.
Note that putting a cap across the choke in a choke-input filter improves the attentuation at the parallel resonant frequency but worsens attenuation for frequencies above it because it turns the LC filter into a capacitive divider, basically. This may sound irrelevant when you are dealing with mains ripple, but rectifiers produce a burst of noise that extend well into the RF region every time they undergo reverse recovery. That's why in a lot of audio(phile/phool) gear you'll see little pF-range capacitors across each bridge diode (technically a resistor should be included to make it proper RC damper...).
As others have already noted using parallel resonant traps isn't common these days, but one place I do see it used quite often is to notch out specific harmonics in 3-phase AC systems - the 5th and 7th seem to be most popular. In these cases it really doesn't matter that the choke (and capacitors) are massive because, well, so is everything else.
001:
--- Quote from: MagicSmoker on October 12, 2019, 09:58:52 am ---The formula to find the shunt capacitor of a parallel resonant "trap" when you know the inductor and ripple frequency is just an algebraic rearrangement of the resonance equation:
Cres = 1 / [(2pi)2 * f2 * L]
Where C is in F, f is in Hz and L is in H.
--- End quote ---
Thanx a lot! Sorry for stupid queston but what about choke DC resistance? Is it critical for Q?
--- Quote from: MagicSmoker on October 12, 2019, 09:58:52 am --- That's why in a lot of audio(phile/phool) gear you'll see little pF-range capacitors across each bridge diode (technically a resistor should be included to make it proper RC damper...).
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This is strange too
Someone use an ultrafast diode but shunt it with cap :-//
Berni:
Yep a high Q is needed if you want a really sharp and deep notch. But as others have brought out you may not want that in the first place because inductors with feromagnetic cores will generally loose inductance as higher DC current is passed trough them and this moves the filters center frequency.
As for diodes. Yes capacitors across diodes are a common trick. Keep in mind that fast diodes are only fast for the turning off part. A normal diode turns on almost as fast as a fast diode, but takes much longer to turn off. So you get less power loss by using fast ones, yet you still get some switching noise with normal slow ones. This is where the capacitor comes in. It smooths over the switching point of the diode so that its not as sharp, this causes the switching edge to have less harmonics in it, so you get less of the harder to deal with higher frequency noise propagating on into the circuit. It works, but don't expect to hear a instant improvement in sound by strapping diodes into the bridge of your audio amplifier. Properly designed audio equipment is designed to tolerate some mains noise anyway.
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