How often will the capacitor go above 450V due to mains transients?

Hello, please consider the following capacitor/diode being placed across the output of a mains rectifier. (no other circuitry involved)
How often would you expect this capacitor’s voltage to be taken above 450V? (I am speaking about connection to the mains in UK or Europe or USA or Australia or New Zealand)

Expect never to happens. But - do you live at a location where nearby powerlines is exposed to thunder weather conditions? You should know if you ask your local electric company.

Probably never. Typical transients are far too short to charge a capacitor of that value through the resistance of the diodes (I didn't check but I assume they have sufficient PIV rating) and the inductance of the wiring and impedance of the capacitor itself. It depends upon individual circumstances of course but transients are just that, irregular short bursts normally lasting no more than a few uS.

I think you need to invest in, or build, a transient monitor to check in real life but based on your previous posts, I think you are too quick to blame transients for failures when other factors are more likely to be the cause.

Brian.

Thanks, do you know how low the capacitor's Faradic value could be made and still gaurantee it not being taken above 450V by a mains transient?

Again it depends on the duration and voltage of the transient and other factors but as a pure guess, maybe 1nF.

Brian.

Again it depends on the duration and voltage of the transient and other factors but as a pure guess, maybe 1nF.

Click to expand...

Thankyou....in that case, we will use a 1nF capacitor and a diode as our transient protection in our product.
However, it seems unlikely that it could be effective......i mean, surely if it were effective, then nobody would ever buy MOVs and TVS's....They'd instead just use the circuit of the top post with a 1nF capacitor?

Hello treez,
The only danger of a massive surge is due to lightening strikes close to the equipment, (say, just in front of the house) or a car crashing into
a power pole and causing the distribution lines of low and high voltages to short together.
Both of the above instances is fairly rare.
But as already stated, the chances of the 450WV capacitor exceeding its rated voltage is extremely minimal.
In all the domestic repair instances I've had, myself and other colleagues in the field have never encountered
a main filter capacitor exploding due to transients and mains over-voltage situations.
Majority of damage to equipment caused by lightening strikes and such have damaged components in unlikely
areas of the appliance. In many instances its bypassed various appliances and hit unlikely ones.
eg. A TV, DVD player, a VCR and a digital clock were on a wall unit, and after the lightning strike the only thing
not working was the digital clock. On inspection, the fuse was violently blown and many tracks in the primary
side of the PSU were vaporized.
Lightning strikes can be very unpredictable in regards to what it want's to damage.
Regards,
Relayer

It is an unusual situation but where I live, the most common fault other than complete black-outs is failure of the neutral line in a TP+N distribution network. It has unusual consequences, it leaves properties connected in series across two of the phases, effectively sharing a 440V feed. The voltage to one property depends on the load drawn by the other. I've been called out to investigate 'strange' occurrences on several occasions, usually someone switching an appliance on makes the neighbors lights turn on instead!

As I said, it is very unusual and a consequence of poor maintenance of the supply which is particular to my location. It means the supply can theoretically rise to a peak AC voltage of about 600V and stay there for prolonged periods. Inevitably, the result is massive destruction of appliances, especially as the waveform is no longer sine either.

Brian.

The 10th page of this
http://cds.linear.com/docs/en/datasheet/1249fbs.pdf
…states how sudden load removal on the mains is the cause of abnormally large mains transients.
I am thinking the only way to deal with this is to put “small” transient protection in the products, and just have a separate transient protector on the mains phase…consisting of the rectifier capacitor of the first post, and then loads of paralleled TVS’s to absorb the surge. Do you agree?