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Old 24th April 2007, 09:47 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, 10:59 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.

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Old 24th April 2007, 11:17 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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