A video about one small but important capacitor

A quick overview of what the video is about.

Switch-mode power supplies are very convenient, but they have their quirks too.
Every switch-mode power supply has a transformer, and every transformer has parasitic interwinding capacitance. This capacitance affects the level of output noise from the power supply.
The thing is, because of how a switch-mode power supply works, high-frequency ripple is constantly present on the primary winding.

In the diagram, I’ve marked the parasitic capacitance in red. It lets ripple from the winding’s “hot” terminal sneak through to the output.
To reduce the noise, a special noise-suppression capacitor is installed; it’s shown in blue in the diagram.


If you look at the schematic of a typical power supply, this capacitor is easy to find too. I’ve marked it in red here.


Most often, this capacitor is installed near the transformer.


The correct capacitor must be a safety-rated Y1 or Y2 type.


But sometimes manufacturers cut corners and install ordinary high-voltage capacitors rated for 2 kV, or even 1 kV, which is quite dangerous.


The point is that Y-type capacitors are safe: if they fail, they fail open rather than short-circuiting, so you won’t get an electric shock.
A capacitor can fail due to voltage spikes caused by lightning-induced surges, as well as ordinary static electricity.
I’ve come across failed high-voltage capacitors in this circuit many times, and sometimes they burned out with quite a show.

The photo below shows which capacitors are suitable and which aren’t.


Now, to answer your questions.
1. Which type should I choose, Y1 or Y2?
It doesn’t really matter. Y1 withstands impulse voltages up to 8000 V, while Y2 withstands up to 5000 V, but both are safe.

2. Do they differ in rated voltage? How do I make sure I choose the right one?
Don’t worry, there’s no such thing as a capacitor with the “wrong” voltage rating here—all Y-class capacitors are “universal.”

3. I don’t know what capacitance to use; there are so many different values.
Choose a capacitance in the range of 1–2.2 nF. If you use a lower value, it will make more “noise”; if you use a higher value, you may feel a tingle from the current. If you’re not sure, use 1.5 nF :)

4. Some capacitors are marked X1 as well. What does that mean?
In general, any Y-class capacitor can be used in place of an X-class capacitor, which is connected across the power supply, but never the other way around.

And of course, here’s the video :)


That’s about it. As always, I’ll be glad to hear your questions. I hope this was helpful.