DCT-02 The DC trap, DC filter, high-end style
Click on the pictures to get a larger view.
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:
- Room for up to 2 x 68000 uF snap-in high performance caps.
- Two DC blocking voltages.
- Very thick copper traces, virtually impossible to burn off.
- Gold plated pads.
The background
Click on the pictures to get a larger view.
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 two big electrolytic capacitors. The inrush current will go through the diodes when the capacitors have been charged over 1 or 3 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. I have made room for two options in terms of DC level. Use only D1 and D2 if this is sufficient. Max voltage across the cap will be 0.7 volts. If you have no opportunity or won't risk anything, go for D3-D8 right away. The only disadvantage is a slightly higher cost. If you of some reason want to go from two diodes to six, just cut away D1 and D2. 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 or 2.1 Volts
C = capacitance. If two 10000 uF are used the capacitance will be 5000 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
Click on the picture to get a larger view.
This design is very easy to build, just mount and solder everything. Decide for yourself if you want two or six diodes. 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
Click on the picture to get a larger view.
----- 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 or 2.1 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.
The DC trap is ready.
The PCB layout
Click on the picture to get a larger view.
Please download the pdf file if you want to study the pcb layout.
Technical data
|
|
|||||||||||
| Click on the picture to see a preview of the pcb. |
||||||||||||