# Zener diode to compensate for voltage spikes so a lower voltage Capacitor can be used for filtering

#### jlawley1969

Joined Feb 22, 2021
97
The should hopefully explain the concept considering how long it is.

but as an example I am rectifying 110VAC and want to filter it and so I should chose something like 250V. The reason I dont is because of spikes in voltage so I should choose something around 400V to be safe.
Why couldnt I use a zener diode in parallel with the capacitor to account for the spikes (have the zener breakdown at like 200-240V) so that I could use a 250V capacitor.

I assume there is a reason havent seen it in the circuits I have taken apart so far.

#### Papabravo

Joined Feb 24, 2006
21,256
The should hopefully explain the concept considering how long it is.

but as an example I am rectifying 110VAC and want to filter it and so I should chose something like 250V. The reason I dont is because of spikes in voltage so I should choose something around 400V to be safe.
Why couldnt I use a zener diode in parallel with the capacitor to account for the spikes (have the zener breakdown at like 200-240V) so that I could use a 250V capacitor.

I assume there is a reason havent seen it in the circuits I have taken apart so far.
I'm pretty sure I've never seen a 250 Volt Zener Diode.

#### Ian0

Joined Aug 7, 2020
9,998
Why couldnt I use a zener diode in parallel with the capacitor to account for the spikes (have the zener breakdown at like 200-240V) so that I could use a 250V capacitor.
You don't need to, because there is only a small amount of energy in the spikes as they are only very short-lived.
If you're worried about them, add a small amount of series inductance.

#### dl324

Joined Mar 30, 2015
16,988
I assume there is a reason havent seen it in the circuits I have taken apart so far.
That's because MOVs are used.

#### sparky 1

Joined Nov 3, 2018
768
The TVS diode was developed to handle spikes

#### jlawley1969

Joined Feb 22, 2021
97
SO if everyone is saying tvs then why do so many (atleast cheap) wall outlet to usb charger not use smaller capacitors(as in NOT 400V) and TVS diodes? cost? use for various markets?(Im in the USA)

#### Ian0

Joined Aug 7, 2020
9,998
Because, unlike semiconductors, electrolytic capacitors don’t fail on a brief overvoltage transient. How many power supplies have you ever found with failed capacitors? Nearly all the faults are failed MOSFETs and failed secondary diodes.

#### sparky 1

Joined Nov 3, 2018
768
Yes looking at simple designs having fewer parts count lower labor less pcb machinery cost can be cheap and noisy for the application; usb charger.
Some usb charger designs are less noisy. The consumer awareness now idoes'nt prefer cleaner battery charger for the cost but more battery.
Most of my small wattage power supplies will use USB C in the future and they have TVS. Just typical inexpensive switchmode rated little higher than the cheap USB PS. In stepping up to higher output involves better handling. The batteries are getting bigger so the old cheap chargers are just not enough.
On the initial startup the inrush into smoothing capacitors is more the problem the cheap smaller wattage supplies just cannot be pushed that far.
https://lygte-info.dk/info/usbPowerSupplyTest UK.html

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#### DickCappels

Joined Aug 21, 2008
10,226
SO if everyone is saying tvs then why do so many (atleast cheap) wall outlet to usb charger not use smaller capacitors(as in NOT 400V) and TVS diodes? cost? use for various markets?(Im in the USA)
Because many of them are specified to operate at 240 VAC which has a peak voltage of 339 volts. A 400 volt capacitor is the lowest standard voltage that meets the 339 volt requirement with adequate derating (safety margin).