Safty capacitor??

[KNielsen]

Maby a noob question, but still learning...
How do I measure with my oscilloskop if a safty capacitor is needed or a good choice at this location when a PSU is build from scratch? Or needed at all??
I know it's frequency related regarding the safty cap value, but how do I measure this?
I have a newer digital 2ch o.scope from Siglent.

Pic is just for illustration and drawn the location I'm referring to.


Best regards

 

[cademan]

Safety caps are usually used on the line side.

 

[audiodummy]

Safety caps are ensuring it does not cause electrocution if it fails, else a regular capacitor can be used at the same location as long as it can stand the voltage and current.
Capacitors are put across AC lines for noise suppression. Most of time it's difficult to measure this noise as it depends on the other circuitry to determine the noise. Need one it's large enough to dampen noise but small enough not to waste energy.

 

[gadget73]

right, safety caps are either across the power line, or from power line to chassis. You can certainly use one though.

 

[AE Bob]

Capacitors at that location (or in the primary circuit of the transformer) are usually there to limit the rate of fall of the magnetic field in the core of the transformer, when the power is disconnected. If the AC is at a peak when the power is disconnected, and the capacitor is not there, the collapse of the magnetic field in the transformer core can occur VERY fast . This causes the magnetic lines of force to cross the conductors in the coils at a much more rapid rate (flyback effect), causing a VERY high voltage spike (like the ignition coil in a gasoline engine). The capacitor limits how fast the magnetic field can collapse (by drawing current at the faster slew rate of the collapsing field), keeping the spike voltage down. A capacitor in that location will frequently have an AC voltage rating (rather than a DC voltage rating), as it has continuous AC across it, although DC rated capacitors with a voltage rating substantially greater than the AC on the winding are also used.

 

[KNielsen]

its not a safety cap

So what's the purpose of the cap shown? Saw a video on YouTube explaining it as a saftycap, so I thought so....

 

[KNielsen]

Capacitors at that location (or in the primary circuit of the transformer) are usually there to limit the rate of fall of the magnetic field in the core of the transformer, when the power is disconnected. If the AC is at a peak when the power is disconnected, and the capacitor is not there, the collapse of the magnetic field in the transformer core can occur VERY fast . This causes the magnetic lines of force to cross the conductors in the coils at a much more rapid rate (flyback effect), causing a VERY high voltage spike (like the ignition coil in a gasoline engine). The capacitor limits how fast the magnetic field can collapse (by drawing current at the faster slew rate of the collapsing field), keeping the spike voltage down. A capacitor in that location will frequently have an AC voltage rating (rather than a DC voltage rating), as it has continuous AC across it, although DC rated capacitors with a voltage rating substantially greater than the AC on the winding are also used.

Is there any values that's is "widely used"??
I could try measure with my scope set to peak detect (if it called so )... I'm just not sure if I can pickup the frequency, but it's worth a try

 

[AE Bob]

Frequently the capacitor is placed across the power switch contacts. It performs the same function there, although in that position it has the relatively low impedance of the AC line in series with it, when the switch opens.

 

[KNielsen]

Frequently the capacitor is placed across the power switch contacts. It performs the same function there, although in that position it has the relatively low impedance of the AC line in series with it, when the switch opens.

This cap I know... If the PSU is below 25vdc, is a saftycap needed??

 

[AE Bob]

If the secondary voltage is low voltage (like 25 volts), then the capacitor should be placed in the primary, or as more usual, across the power switch contacts. The spike from a freely falling field, can be quite large. The capacitor across the switch contacts also prolongs the life of the switch (prevents arcing when the switch is opened).