Our UK domestic supplies tap off the same cable that supplies industry with three phase power.
The current capacity of these supplies is enormous.
I suspect that the mains impedance is more likely to be ten times lower than suggested.
minimum impedance ~ 0.05 to 0.1ohms.
My Merlin Gerin (B6 to B40) MCBs are rated to 10000A breaking capacity.
The Dorman Smith B06A is rated EN60898 10000, EN60947-2 15kA
These breaking capacity ratings are required due to the possibility of peak fault currents approaching these levels on a 240Vac supply. These in turn can only become that high if the total impedance including the house wiring is much lower than 0.3ohms. IEC 898 obviously recognises this possibility.
AndrewT said:
British/UK wiring has indeed potential for much higher short-circuit currents because of rather strange outlet ring-connection with one thick wire running around the house.
Rest of the world uses "star wiring"
AFAIK for example in Finland 230v outlets are usually 10A(16A) fused and wired with 1.5mm2(2.5mm2) cable.
20meter(40meters total L+N) run of 1.5mm2 cable has about 0.7ohms resistance. For long installations in bigger homes 2.5mm2 would be reguired to guarantee fuse's action.
and what if the first star connected socket is on a short length of twin and earth?
Here the electrician is supposed to calculate the size of cable required to ensure fast rupture of the fuses in event of fault and that during the kA fault period that the voltage on the safety earth never exceeds the set limit (I think we use <=50V).
However, our electricians rarely if ever do the calculations, they always assume that the emprically derived 2.5mm^2 does for various short lengths of power ring and adopt 4mm^2 for longer rings (which supplies my audio room).
But, Kstr said the mains impedance is unlikely to be below 0.3Ohms. Now, you are adding in the house wiring to the mains impedance.
Here the electrician is supposed to calculate the size of cable required to ensure fast rupture of the fuses in event of fault and that during the kA fault period that the voltage on the safety earth never exceeds the set limit (I think we use <=50V).
However, our electricians rarely if ever do the calculations, they always assume that the emprically derived 2.5mm^2 does for various short lengths of power ring and adopt 4mm^2 for longer rings (which supplies my audio room).
But, Kstr said the mains impedance is unlikely to be below 0.3Ohms. Now, you are adding in the house wiring to the mains impedance.
Mains impedance
I should be quick to point out that my 0.3 Ohms typical min. value referred to a typical german domestic situation. I have measured mains impedance at household outlets on various occasions and at various locations over the years and never came accross values below 0.3 Ohms. Typical, not worst case. Seems to differ very much from country to country. I have seen graphs for the US with similar values. Of course, and that point should be stressed, actual and reliable values can only be obtained from the power company in charge at your location, backed up by professional measurements of an accredited electrician.
Actually I just measured things at the nearest outlet from my distribution panel, fused with a 16A breaker:
Mains Voltage, unloaded : 220.6V
Two hairdryers, drawing 2400W@230V nominal (8.9A, measured) : 216.2V
Zmains@50Hz = 4.4V/8.9A = 0.49Ohms.
Same procedure, at the closest outlet to the main power entry in the whole rather big building (25 appartments), in the distribution room:
Mains, unloaded: 221.3V, loaded: 217.2V
Zmains@50Hz = 4.1V/8.9A = 0.46Ohms.
Regards, Klaus
I should be quick to point out that my 0.3 Ohms typical min. value referred to a typical german domestic situation. I have measured mains impedance at household outlets on various occasions and at various locations over the years and never came accross values below 0.3 Ohms. Typical, not worst case. Seems to differ very much from country to country. I have seen graphs for the US with similar values. Of course, and that point should be stressed, actual and reliable values can only be obtained from the power company in charge at your location, backed up by professional measurements of an accredited electrician.
Actually I just measured things at the nearest outlet from my distribution panel, fused with a 16A breaker:
Mains Voltage, unloaded : 220.6V
Two hairdryers, drawing 2400W@230V nominal (8.9A, measured) : 216.2V
Zmains@50Hz = 4.4V/8.9A = 0.49Ohms.
Same procedure, at the closest outlet to the main power entry in the whole rather big building (25 appartments), in the distribution room:
Mains, unloaded: 221.3V, loaded: 217.2V
Zmains@50Hz = 4.1V/8.9A = 0.46Ohms.
Regards, Klaus
Also in my very limited experience with mains wiring I haven't seen oulet impedance below 0.3ohms.
For worst-case:
Someone could dig up I^2*t fuse clearing ratings for example for BS 1362 (UK) fuse and compare it to http://www.alldatasheet.com/datasheet-pdf/pdf/111085/IRF/GBPC3506A.html ratings. Some ratings for smaller crappy fuses that I found indicated the bridge rectifier as a survivor.
Diode bridges fail usually short-circuit so they still would protect caps from overvoltage in case of extreme over-current.
For worst-case:
Someone could dig up I^2*t fuse clearing ratings for example for BS 1362 (UK) fuse and compare it to http://www.alldatasheet.com/datasheet-pdf/pdf/111085/IRF/GBPC3506A.html ratings. Some ratings for smaller crappy fuses that I found indicated the bridge rectifier as a survivor.
Diode bridges fail usually short-circuit so they still would protect caps from overvoltage in case of extreme over-current.
Mains Impedance
Mains Impedance (were we measure) will vary somewhat dependant on what other loads is present at the time for our measurements.
Mains Impedance (were we measure) will vary somewhat dependant on what other loads is present at the time for our measurements.
Well, I hooked up my amp which I thought was having problems with DC on the line, but now it works fine... 
I would still like to build a blocker, though... Well, two, actually- one for my subwoofer and another for my power amp, receiver, CDP, DVDP, TV... So, basically, each one needs to be able to handle about 1500-2000W, preferably 2000W. What kind of cap value would I need for this? 40,000uF?
I found two other circuits, this one recommended by Nelson Pass:
and this one:
Would one of these be better than the Bryston? I am kind of impartial to Nelson's suggestion, since it was his suggestion...
I have been thinking about making a power conditioner of sorts and including an LC filter in there as well with a voltage and current meter on the front panel so I can see what is going on (I am curious how much amperage I am drawing at any given moment). Is this an undertaking that should be avoided?
Thanks all...
I would still like to build a blocker, though... Well, two, actually- one for my subwoofer and another for my power amp, receiver, CDP, DVDP, TV... So, basically, each one needs to be able to handle about 1500-2000W, preferably 2000W. What kind of cap value would I need for this? 40,000uF?
I found two other circuits, this one recommended by Nelson Pass:
and this one:
Would one of these be better than the Bryston? I am kind of impartial to Nelson's suggestion, since it was his suggestion...
I have been thinking about making a power conditioner of sorts and including an LC filter in there as well with a voltage and current meter on the front panel so I can see what is going on (I am curious how much amperage I am drawing at any given moment). Is this an undertaking that should be avoided?
Thanks all...
Why not just rectify the AC with a bridge rectifier (no capacitor or very small capacitor) and then run a MOSFET H bridge from it? Then simply have the H bridge flip every zero crossing. Then add a few parts to absorb inductive spikes.
The DC will then become AC.
The DC will then become AC.
Hi Need,
the Pass circuit has an effective 2350uF in series with the transformer.
This will need to be increased to at least 20mF + 20mF for 1kW(220Vac) on one diode drop.
On two diode drops (in each direction) you can get about twice the power through it. Try not to get the diodes conducting in normal operation. They will get hot if you repeatedly pulse them on. I would rather run them cold for improved reliability (look where they are located).
The second circuit is potentially very dangerous, if you go inside when powered up.
One polarity is always connected to Live and the other polarity is always connected to Neutral. This is the worst condition.
Either insert the DC blocker in the Live line and accept it MUST be protected from poking fingers. Or fit it in the Neutral line and again protect it. The disadvantage of fitting in the neutral line is that if the blocker becomes opencircuit, the whole amplifier primary side becomes LIVE even though the amp appears dead. TAKE CARE.
the Pass circuit has an effective 2350uF in series with the transformer.
This will need to be increased to at least 20mF + 20mF for 1kW(220Vac) on one diode drop.
On two diode drops (in each direction) you can get about twice the power through it. Try not to get the diodes conducting in normal operation. They will get hot if you repeatedly pulse them on. I would rather run them cold for improved reliability (look where they are located).
The second circuit is potentially very dangerous, if you go inside when powered up.
One polarity is always connected to Live and the other polarity is always connected to Neutral. This is the worst condition.
Either insert the DC blocker in the Live line and accept it MUST be protected from poking fingers. Or fit it in the Neutral line and again protect it. The disadvantage of fitting in the neutral line is that if the blocker becomes opencircuit, the whole amplifier primary side becomes LIVE even though the amp appears dead. TAKE CARE.
Hi folks.
I want to build the Bryston 'DC Trap' for my recently completed dynaco st-70 amplifier, which is suffering from really annoying transformer buzz. I'm in the UK and using the Triode Electronics PA-521-S 240V transformer by the way.
I've ordered an overkill 35A 1000V bridge rectifier from www.rswww.com (part no. 395-4427).
I have loads of various caps lying around in my parts bin, and was wondering what an 'ideal' duo of caps would be JUST for my Dynaco ST-70? Would 2 x 2200uF/16V Panasonic FC do the job, or would I be better of using my 2 x 10,000uF / 63V elna cerafines? (probably wayyyy too much overkill there).
Thank you.
- John
I want to build the Bryston 'DC Trap' for my recently completed dynaco st-70 amplifier, which is suffering from really annoying transformer buzz. I'm in the UK and using the Triode Electronics PA-521-S 240V transformer by the way.
I've ordered an overkill 35A 1000V bridge rectifier from www.rswww.com (part no. 395-4427).
I have loads of various caps lying around in my parts bin, and was wondering what an 'ideal' duo of caps would be JUST for my Dynaco ST-70? Would 2 x 2200uF/16V Panasonic FC do the job, or would I be better of using my 2 x 10,000uF / 63V elna cerafines? (probably wayyyy too much overkill there).
Thank you.
- John
neither,
use 4700uF 10V or 10mF 10V.
The larger will drop less voltage and need to be bypassed by the diodes less often.
use 4700uF 10V or 10mF 10V.
The larger will drop less voltage and need to be bypassed by the diodes less often.
Well I built the Bryston-blocker and... it works! I must admit for some reason I was dubious about it having any effect at all. But the humming transformer in my new Dynaco ST70 has now reduced by about 70-75%. Very pleased. Seems to run alot cooler as well.
Of course now I'm curious how much DC my build is actually getting rid of, and if I could make the transformer totally quiet by using more capacitance?...
I'm currently (ha!) using a 35A bridge rectifier and 2 x 10,000uF 35V caps.
For anyone interested here's the critter:
Cheers,
- John
Of course now I'm curious how much DC my build is actually getting rid of, and if I could make the transformer totally quiet by using more capacitance?...
I'm currently (ha!) using a 35A bridge rectifier and 2 x 10,000uF 35V caps.
For anyone interested here's the critter:
An externally hosted image should be here but it was not working when we last tested it.
Cheers,
- John
Hi John,
the capacitors are in inverse parallel. Is that the way Bryson installs then?
They usually go in back to back, series connected, i.e. red to red.
the capacitors are in inverse parallel. Is that the way Bryson installs then?
They usually go in back to back, series connected, i.e. red to red.
Can you measure any DC voltage across the bridge. I am a bit surprised to see them connected that way as well, you don't really want any reverse polarising voltage across electroylitics long term.
It's good to know it actually works (very neat job as well), as this was a problem I had once with a large 550 va toroid, and something I would probably build into any future design.
Adding more capacitance will lower the effective reactance or "resistance" that the transformer sees in the primary winding.
A few years ago the HiFi press was advocating such things as direct connection to the consumer unit and dedicated spurs for the HiFi. I think they would have had a direct connection to the power station if they could. And here we all are 😉 adding extra impedance. 😀 😀 Funny old world.
It's good to know it actually works (very neat job as well), as this was a problem I had once with a large 550 va toroid, and something I would probably build into any future design.
Adding more capacitance will lower the effective reactance or "resistance" that the transformer sees in the primary winding.
A few years ago the HiFi press was advocating such things as direct connection to the consumer unit and dedicated spurs for the HiFi. I think they would have had a direct connection to the power station if they could. And here we all are 😉 adding extra impedance. 😀 😀 Funny old world.
I agree with KSTR here.KSTR said:
In our view back to back connection of the polar electrolytic caps is mandatory.
johnm said:
You should insulate those caps from the board beneath (maybe you have, hard to see in the picture how they are mounted), the plastic cover of the caps is pretty thin and not supposed to be depended on for insulation. Especially not enough for insulation between a mains connected circuit and ground.