Mysteries of the Y Capacitor, Revealed

I’m becoming more of a big electronics genius every day. Staying humble is not easy.

Yesterday I posted about the resurrection of my old HP 3300A function generator, and I mentioned the weird capacitors across the AC mains. Right where the three wires entered the box, there were two caps (in one package). One went from neutral to ground, and the other went from hot to ground.

One of these caps had shorted out, and that had killed the generator. I had to replace them. When a cap between hot and ground shorts out, you have a hot wire going straight to ground, and that is pretty worthless.

A commenter mentioned a different type of component which would look just like a dual capacitor. He said EMI (electromagnetic interference) filters resembled capacitors, and he said they had inductors inside them. I went back to my schematic to check, and I didn’t see any inductors, so I think all I had were caps.

I started reading up on this stuff. I could not figure out why anyone would need filters right before a big transformer and two big reservoir caps with diodes. The reservoir caps and diodes kill AC and turn it into DC, so I would think they would also kill any high frequencies in the line. My guesses are worth about what I charge for them, however.

After some Googling, I learned that what I have is a “type Y” setup. Caps like this are intended to keep interference from passing through the power connection. If it’s coming from outside, they keep it out. If it’s coming from inside your device, they keep it inside.

I can’t call the dead HP engineers who designed the machine, so I can’t ask what the big concern was. I was thinking maybe the AC wires, if not filtered at the entrance to the box, could act as antennas and shoot RF into other internal components, distorting the signal the machine provides. I don’t really know.

Here’s an important fact: it is well known that when capacitors in this configuration die, they can short, and then you can get shocked. This is bad, unless you view death as a postive outcome. You need special capacitors designated X or Y. These capacitors will resist things like power surges. Do they eventually die from old age and short anyway? Search me.

I know nothing about X capacitors, but the Y type come in two varieties: Y1 and Y2. Y1 capacitors are expensive industrial components, so people use Y2. You can find them on Ebay. I guess I’ll have to order some and replace the brand-new capacitors I just installed.

I am wondering if there is any point in putting caps like this on my guitar amps. I think the odds that the sound will improve are infinitesimal, and the chance of electric shock is appreciable, but it would still be a cool feature to brag about.

I have always assumed it was impossible to get a shock from a grounded metal box, but I am used to being proven wrong, so I may as well get the caps.

There are components made to divert power surges. I forget what they’re called. There is a list of “immortal mods” for guitar amplifiers, and it lists things you can do to an amp to make it resistant to failure. The power surge shunt things are on the list. You can look it up. I don’t know if they would work on Y caps. Haven’t checked.

I know this information is fascinating to everyone. Try not to get so engrossed you forget to look away from my blog and do whatever it is you’re being paid to do.