DCT-03 The DC trap, DC filter, high-end style, heavy duty

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Do you have an amplifier with a (sometimes) buzzing toroidal transformer? This DC trap or DC filter removes or at least lowers the buzzing.

Interesting features:

  1. Room for up to 6 x 68000 uF snap-in high performance caps.
  2. Four DC blocking voltages.
  3. Very thick copper traces, virtually impossible to burn off.
  4. Gold plated pads.

The background

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Why does the transformer hum

Why does especially the toroidal transformer hum, sometimes hard? The iron core is very efficient and the magnetic flux is high, very near magnetic saturation of the core. This gives very little room for "extra" flux which DC can cause. DC on AC mains is simply unsymmetrical waveform of the sinus wave. What happens if you have a hair drier with half speed? This is accomplished with a diode and halfway rectification, causing the positive peak voltage become lower than the negative. This creates a small DC level which some apparatus don't like, especially toroids. An another reason for DC on the mains is switching transients in large networks.

The DC trap makes sure that no DC slips through. The primary winding is AC coupled with help from six big electrolytic capacitors. The inrush current will go through the diodes when the capacitors have been charged over 1 or 4 volts and the normal current will only go through the capacitors.


The schematics

Click on the picture to get a larger view. The picture shows the schematics of the amp. Of course you can't use it for anything except for an overview. Please download the pdf-file instead if you want to see the details.


WARNING - CAUTION

As you can see at the picture the pcb will be at mains potential and therefore lethal. You must insure that this pcb is well protected against unintentional touching. All parts are dangerous to touch when the mains is switched on.


Circuit description

This design is very simple, just a couple of diodes and capacitors. The purpose of the diodes is to limit the DC voltage across the caps, especially at high load and at power on. The pcb contains of 16 diodes where there are two in parallel for each direction. This is for increasing the current capacity to the double. Normally you should not connect diodes in parallel in they not are on the same die but in this case I think it will work fine since they will have the same temperature and will most likely not continuously carry any larger currents. I recommend that you solder all 16 diodes. The only disadvantage is a slightly higher cost. Don't try to desolder. You will only damage the pcb.

The capacitor impedance determined by:

Z = 1/(2*pi*f*C)

where

f = mains frequency, 50 or 60 Hz

C = capacitance in Farads

Maximum current without passing the diodes:

Max Irms = 0.7*U*2*pi*f*C

where

U = the voltage when current is flowing through the diodes which also can be tranzils. In my case 0.7, 1.4, 2.1 or 2.8 Volts

C = capacitance. If six 68000 uF are used the capacitance will be 102000 uF.

f = mains frequency, 50 or 60 Hz

X1 and X4 are 2-pole just because they are separated and the connector has two poles. No need for the extra pole really.

I have added room for three wires in and three wires out. Neutral and Protective Earth can be connected also just as a joint between incoming mains cable and the internal wiring.

Terminal Pin Name Incoming power To transformer Notes
 X1  1 Phase in X -  
X1 2 Phase in - - Not used, only for mechanical stability.
X2 1 Neutral in X -  
X2 2 Neutral out - X  
X3 1 Protective earth in X -  
X3 2 Protective earth out - X Connected to chassis. The incoming wire could also be connected directly to the chassis.
X4 1 Phase out - X  
X4  2 Phase out  -  - Not used, only for mechanical stability.

 

 


Build directions

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This design is very easy to build, just mount and solder everything. Decide for yourself if you want sixteen diodes or less. The pcb requires much heat due to the thick copper traces. Don't be scared to really "burn" the solder joints. Make sure that the tin is really melting and form nice and shiny looking joints.


Test

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----- Please notice this first!-----------


WARNING - CAUTION

As you can see at the picture the pcb will be at mains potential and therefore lethal. You must insure that this pcb is well protected against unintentional touching. All parts are dangerous to touch when the mains is switched on.


If you can, use a 0-5 V DC source to test the diode function. Connect to Phase in and Phase out. Make sure you have a limited current from the power supply. If it works with let's say 50-100 mA you should have 0.7, 1.4, 2.1 or 2.8 V depending of how many diodes you use. If you change polarity you should get the same result. With this you'll see that the diodes work. You could also use a diode tester in order to see that the diodes are working.

If it works all right with low DC voltage then test with 230 VAC and notice if the tranformer hum will disappear or not.

The DC trap is ready.


The PCB layout

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Please download the pdf file if you want to study the pcb layout.


Technical data

Operating voltage: 0-240 VAC, 50-60 Hz
Max DC voltage: 1, 2, 3 or 4 V
Max inrush current: Approx 120-200 A peak depending of chosen diode type
Max continuous current: See datasheet for used capacitors
Dimensions: 81.3 (3.2") x 192.4 (7,58") mm
Click on the picture to see a preview of the pcb.