Hi thanks for reading. A lot of people use a capacitor and resistor across a transformer to snub and reduce spikes from the power supply. But is this the best place to put the snubbing circuit.
What about placing it after the rectifer, so it snubs the transformer and rectifier. Or is better to place it after the big power supply capacitors, or any benefit to leaving it out of the power supply and placing the snubber on each of the amp boards.
Which is the best place to put the snubbing circuit to get the lowest noise.
And is there any benefit to using more than one snubbing circuit to possibly get lower noise..
Thanks
What about placing it after the rectifer, so it snubs the transformer and rectifier. Or is better to place it after the big power supply capacitors, or any benefit to leaving it out of the power supply and placing the snubber on each of the amp boards.
Which is the best place to put the snubbing circuit to get the lowest noise.
And is there any benefit to using more than one snubbing circuit to possibly get lower noise..
Thanks
I'm going to say that the answer lies in knowing what you want to improve, measuring the problem and seeing which solution or combination of solutions gives the best result. Each case is unique because of wiring and PCB parasitics.
Diode rectifiers can cause ringing in the transformer as the diodes come into and out of conduction. This ringing can extend into the RF frequency range.
A good wideband scope can show much of this and the effect of a snubber across the secondary gauged.
Without any tests I would place a snubber (series R/C network) across each secondary and possibly similar across each diode in the bridge. That will get you 99% of the way there.
Diode rectifiers can cause ringing in the transformer as the diodes come into and out of conduction. This ringing can extend into the RF frequency range.
A good wideband scope can show much of this and the effect of a snubber across the secondary gauged.
Without any tests I would place a snubber (series R/C network) across each secondary and possibly similar across each diode in the bridge. That will get you 99% of the way there.
Question - If you are paralleling dual secondaries to one diode bridge for double the current - will one R/C network before the bridge suffice ?
You snub the undesired transient as close as possible to the source of the transient. Circuits have the potential for multiple sources of transient energy release, so it should not surprise you to realize two things:
a. I should expect the possibility for more than one RC snubber.
b. It is unlikely that one snubber will optimally tackle multiple sources simultaneously
Case in point, the largest energy transient will likely be from the transformer, so you snub directly across the winding(s). The characteristics of the transformer determine the optimal snubber values, as opposed to load characteristics.
A smaller transient source will be from rectifier switching, which is highly dependent on the type of rectifier you use (again, as opposed to load characteristics). You snub that device directly across the device terminals.
The transformer and rectifier have vastly different parameters, so the values with which you snub each will be different.
As Mooly said, you need to understand what/why you are snubbing. Solutions are quite standard and easy to implement with brute force and a scope.
a. I should expect the possibility for more than one RC snubber.
b. It is unlikely that one snubber will optimally tackle multiple sources simultaneously
Case in point, the largest energy transient will likely be from the transformer, so you snub directly across the winding(s). The characteristics of the transformer determine the optimal snubber values, as opposed to load characteristics.
A smaller transient source will be from rectifier switching, which is highly dependent on the type of rectifier you use (again, as opposed to load characteristics). You snub that device directly across the device terminals.
The transformer and rectifier have vastly different parameters, so the values with which you snub each will be different.
As Mooly said, you need to understand what/why you are snubbing. Solutions are quite standard and easy to implement with brute force and a scope.
If you parallel the windings directly then there is no other way to do it. They become as one as far as snubbing them is concerned.
Maybe scan/read a few dozen posts in Mark Johnson's QuasiModo and CheapoModo project threads for clear insights. There you'll find plenty of 'scope traces and explanations.
+1 Mooly on the '.. 99% of the way there.'
Cheers
+1 Mooly on the '.. 99% of the way there.'
Cheers
I don't have a scope. From reading snubbing tutorials people are saying it is not the capacitor that removes the spikes it is the resistor.
From this it appears the big smoothing caps found in power supplies just shifts the noise away to a different frequency rather than remove it.
I have found tutorials on how to snub transformers, rectifiers and make crc filters, but nothing on snubbing the actual amp boards to apply that final polish finish. Do you have any links to this.
From this it appears the big smoothing caps found in power supplies just shifts the noise away to a different frequency rather than remove it.
I have found tutorials on how to snub transformers, rectifiers and make crc filters, but nothing on snubbing the actual amp boards to apply that final polish finish. Do you have any links to this.
As frequency goes up, capacitive reactance (Xc) comes down... the cap behaves more a like a short circuit to HF. We want that in many ways.
Working alongside that is inductive reactance of the cap and that goes up with increasing frequency working in opposition to Xc. That means the cap behaves poorly at high frequency. We don't want that.
So a small film or ceramic cap performs better than a big electrolytic (it has lower self inductance to begin with) and the low value resistor makes it all much more predictable with regard to unknows in what you are connecting it to.
If you don't add the resistor you might find in some cases that addition of a cap alone can make things worse as it resonates with parasitic circuit elements. Add the resistor and it all becomes very predictable and will just work in the majority of scenarios.
Working alongside that is inductive reactance of the cap and that goes up with increasing frequency working in opposition to Xc. That means the cap behaves poorly at high frequency. We don't want that.
So a small film or ceramic cap performs better than a big electrolytic (it has lower self inductance to begin with) and the low value resistor makes it all much more predictable with regard to unknows in what you are connecting it to.
If you don't add the resistor you might find in some cases that addition of a cap alone can make things worse as it resonates with parasitic circuit elements. Add the resistor and it all becomes very predictable and will just work in the majority of scenarios.
You are snubbing the leakage inductance of the transformer, so the snubber goes there, not after the diodes. The resistor is what turns the energy to heat and thus 'snubs' the spikes away, but you must have the cap so the resistor only gets the high frequency spike it is meant to snub. Without the cap the resistor burns up quiet fast.
Carbon comps are the best snubber resistors, and they may need to absorb some energy, I have seen many > 100C even tho the spike is very short.
Without a scope you can probably get away with a typical value, it's likely better than no snubbing, but check for how hot the resistor gets.
Carbon comps are the best snubber resistors, and they may need to absorb some energy, I have seen many > 100C even tho the spike is very short.
Without a scope you can probably get away with a typical value, it's likely better than no snubbing, but check for how hot the resistor gets.
Snubbers? Rude little circuits! I usually place one across the primary of the power transformer to stop the backwash from causing arcing across the power switch contacts.
The energy of the transient is supplied by the leakage inductance of the transformer and the wiring, but this energy is released by the switching element.
A transformer feeding a linear load, reactive or not, will not generate transients, but a switching diode will always create transients, even if the circuit's inductance is very small.
It is therefore preferable to snub each diode, but in the case of a bridge, it would mean 4 individual circuits.
If the wiring inductance is low and RF suppression not paramount, a single snubber across the transformer can suffice
A transformer feeding a linear load, reactive or not, will not generate transients, but a switching diode will always create transients, even if the circuit's inductance is very small.
It is therefore preferable to snub each diode, but in the case of a bridge, it would mean 4 individual circuits.
If the wiring inductance is low and RF suppression not paramount, a single snubber across the transformer can suffice