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Old 24th April 2007, 08:47 PM   #1
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Default Question about caps on transformer primary

I have seen on a few power supply designs small capacitors (film types I believe) placed in parallel with the transformer primaries. What is the purpose of these? What's a typical value, voltage rating and type? Thanks!
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Old 24th April 2007, 09:59 PM   #2
Tweeker is offline Tweeker  United States
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Very small caps are there as EMI filters. They must be specially rated for the task. X is for line to neutral, Y for line or neutral to ground. The caps must withstand large line transients (ei 2,000V) and if not, fail gracefully.

Metalized paper is the typical choice. It has the best self healing.
Be sure your foil hat has a good low impedance ground.
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Old 24th April 2007, 10:17 PM   #3
Tolik is offline Tolik  Israel
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This caps are for filtration of RF noise, catched by mains wires from different EMI sourses such as radio transmitters, static atmosfere electr. discharges, etc. The vaules : voltage rating - >/= 330 V , capacitance - 1000pF - 100nF, film or ceramic capacitors. To improve protection you can to add ferrite beads on mins wire, or in case of low power PSU ( for preamp , DAC, Xover) possible to use small filtering coils.
Sometimes you will see the caps are connected in parallel with diodes in bridge recitifer . Purpose - to kill switching voltage splashes . This occurs with all regular(standart/slow recovery time) diodes. Best idea here - to use HEXFRED diodes with extremly short recovery time, or if you rady to pay for best result - HEXFRED with soft recovery diodes.
They are expencive but doing the job better than any other. HEXFRED are goog for all projects, HEXFRED SR - I highly reccomend to use in projects where high current utilised during operation time (like solid state class A amps) , or very sensetive parts used ( tube preamps ).

Hope its was helpful
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