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I was wondering if someone can explain the purpose of the capacitor C1141 from the attached picture. I know that C1142 is a filter cap: that much is obvious to me. However, I can't seem to grasp the purpose of C1141. I observed that it is obviously not polarized so it's dealing with AC because it is placed before the rectifier diodes. My guess is that it is not involved in filtering but rather some kind of RF blocking (maybe?). I am obviously missing a term for that because I couldn't find anything online or in literature.

A little context: It is the power supply of an old Tektronix type 453 oscilloscope.enter image description here

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    \$\begingroup\$ A guess, so it's a comment, not an answer. Mains frequency rectifier diodes have a lot of charge storage, so generate a large spike when they turn off. C1141 will keep that out of the transformer and so the mains wiring. \$\endgroup\$
    – Neil_UK
    Oct 23, 2019 at 18:52
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    \$\begingroup\$ Dontcha just love those schematics drawn with such care, by hand? \$\endgroup\$
    – glen_geek
    Oct 23, 2019 at 20:50
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    \$\begingroup\$ very satisfying schematic to watch ;) \$\endgroup\$
    – Mitu Raj
    Oct 24, 2019 at 18:47

2 Answers 2

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The capacitor is to reduce RF interference.

When a diode is conducting it will create charge carriers to carry the current - this is known as "charge storage".

When the voltage reverses as part of the normal AC cycle (50/60Hz typically) the current will keep flowing through the diode in the reverse direction until the stored carriers are depleted.

When the carriers have been flushed from the diode the current will abruptly drop to zero and the voltage across the diode will suddenly rise. This can generate high frequencies up to tens or hundreds of Megahertz. This can interfere either with the equipment itself or radiate (or conduct via the power cable) to other equipment. The capacitor suppresses the effect of this interference. Often a capacitor will be put in parallel with each diode rather than a single capacitor.

Any interference will usually have modulation at twice the AC input frequency and appear as a buzz in radios or audio amplifiers.

Common power diodes can exhibit this but the effect is exploited in devices called "Snap Recovery Diodes" (Wikipedia - Snap recovery diode). To minimize the effect some diodes are designed intentionally to gradually deplete the charge - they are known as "soft recovery diodes".

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  • \$\begingroup\$ What causes that snap behaviour in a high-level sense? Because a capacitor in paraller with the diode in the same way wouldn't cause that behaviour...unless it actually just dissappeared leaving an open-circuit in it's wake. \$\endgroup\$
    – DKNguyen
    Oct 29, 2019 at 21:30
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    \$\begingroup\$ When the charge in a normal capacitor is depleted the voltage reverses and the charge starts building up in the reverse direction. In a case of a diode current keeps flowing until the charge carriers (that were created when current was flowing in the forward direction) have all recombined - the diode then reverts to a small conventional capacitor in the reverse direction, no charge carriers are created as the diode does not conduct in the reverse direction. The voltage can suddenly rise creating the sharp voltage step. \$\endgroup\$
    – Kevin White
    Oct 29, 2019 at 21:40
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It is there to reduce EMI. One name for it is a snubber capacitor.

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