AC power connection schematic explanation needed

[MEnsing]

Thanks Peter for answering my questions and posting the PCB layout.

Marco

[Nelson Pass]

Most often the source of the DC is the Triac, which is why
I don't use them.

Even if you don't use a Triac, there might be a lamp dimmer
or a hair dryer on the circuit somewhere that does, and
so DC blocking is often necessary to keep the toroidal
transformer's mechanical noise down.

You will see this more and more, as the lines appear to
be getting dirtier with time.

Bridge rectifiers in the AC line for blockage cannot be relied
upon that well, and the best solution is two of them in series,
paralleled with the back to back electrolytics.

Actually just a nice fat pair of electrolytics is up to the task.

[Peter Daniel]

That circuit was built in 1995 after the A75 article, which was using triac for switching. I was wondering if I should remove it, because someone already suggested that it may be a source of noise as well. Any comments on that? I could replace it with 110V relay. Is it worthwile?

[jleaman]

WOOOOOO more work to admire.. man i so want to learn pcb making stuff i have lots of schematics to design and build pcb's for.. i hate makeing them via felt marker..

[MEnsing]

I liked this schematic because I don't needed to use a big, fat and ugly industrial relay. After reading Nelsons comments I don't know if I am going to use this schematic. If not it would save me some work.

[Peter Daniel]

If you are concerned that the triac is the source of DC at primary, you shouldn't. I've built 4 amps using that circuit, and they are extremaly quiet when it comes to transformer behaviour (which was not always the case on my other amps without that circuit).

The only thing I might be concerned at this point is if the triac might generate some unwanted noise after initial switching on.

[mlloyd1]

Nelson's comments reminded me of a circuit I posted a while back. Rumor has it the circuit posted here does a great job of removing DC on the line. It's pretty simple, too!

I use relay shorting a 10ohms (or so) resistor in the transformer primary circuit for my power-on surge limiting. Works great for 1200VA tranny (with lots of energy storage in the supply). Originally (over 15 years ago) used the 555 to control the relay (I had LOTs and needed to get rid of some) and later (about 7 years ago) changed to the Elektor circuit. My delay time is 5-10 seconds.

mlloyd1

Originally posted by Nelson Pass
.....the best solution is two of them in series,
paralleled with the back to back electrolytics.
...
Actually just a nice fat pair of electrolytics is up to the task.

[jh6you]

Originally posted by Nelson Pass
Most often the source of the DC is the Triac, which is why
I don't use them.

Even if you don't use a Triac, there might be a lamp dimmer
or a hair dryer on the circuit somewhere that does, and
so DC blocking is often necessary to keep the toroidal
transformer's mechanical noise down.

You will see this more and more, as the lines appear to
be getting dirtier with time.

Bridge rectifiers in the AC line for blockage cannot be relied
upon that well, and the best solution is two of them in series,
paralleled with the back to back electrolytics.

Actually just a nice fat pair of electrolytics is up to the task.

A simmilar advice from another thread:

If you are experiencing mechanical hum from your
transformer, it is often caused by the presence of
DC on the line. Usually this comes from some appliance
using current asymmetrically, such as a lamp dimmer.

The hum comes usually from toroidal transformers, which
saturate easily with DC, and when they recover, they
draw an extra pulse of current, causing the noise.

You can put a pair of back-to-back electrolytics in series
with the AC power line to block this, and it works fine.
Makes sure the current rating of the electrolytics is
high enough, and the they are joined at a like polarity,
such as + to +.

I want to apply the back-to-back electrolytics as recommended, but after a bit better understanding of the background. The b-t-b will make a status of equipotential polarities of dielectric materials, and so the voltage rating of the electrolytics will be no more care. The current rating will be the only care for the use of the b-t-b. Is my understanding in the way?

Thanks for your response.

JH

[Juergen Knoop]

Hi Peter,
as you invited to this thread...the schematic confuses me.
It is from a commercial amplifier?
There are four fuses in series to protect one transformer primary and some circuit. Load current will pass them one after another.
With ideal fuses, the 8A will blow rendering the others useless.
Within real world just anyone of them will blow, but leaving the others useless as well.
Certainly I'm missing something. But what?
Regards
Jürgen

[jeapel]

bridge limit voltage 1 to 1.4 v

-in your case 10,000uF + 10,000uF = just 5000uF

-in my case 10,000uF // 10,000uF = 20,000uF

-Xc of 20,000 X my aleph 3 2A = ~ .35v and bridge
limit to 1 to 1.4 it s ok but in your case attention
to check that maybe .5v on 5000uF is max.
to be safe