Adding safety caps to old equipment without being a know it all about it

[taste_tester]

One thing I often wonder about is how many old linear or switching power supplies i end up fixing due to corrosion, parts going bad or whatever it is. Anyway, it's often old enough stuff that it's all on single sided boards with hand drawn tracks. So, this stuff usually has a non polarized / non grounded AC cable with the AC mains going straight into the circuit, and soldered to two tabs on the power supply board. I found an example diagram online I was going to go with-  but I'm not sure how one picks the value for the safety cap to begin with?

[tggzzz]

They are not "safety caps". Their purpose is stated in your illustration's filename.

The capacitance is not critical, but the voltage and X/Y ratings are critical.

I don't know whether that makes me a "know it all".

[Siwastaja]

As said, they are not "safety caps". Such concept does not exist, you have a total fundamental misunderstanding.

The capacitors are because certain circuits needs capacitors, nothing to do with safety. Your linear circuit does not need them, so you don't need to add anything.

Now if you used "normal" capacitors arranges as in the picture, the capacitors would make the circuit dangerous. Proper safety rated capacitors must be used to eliminate this added danger. They don't protect against anything else than their own failure.

[strawberry]

safety ''rated'' capacitors
capacitors used to suppress EMI radiated or conducted noise. for example Switching power supplies(SMPS)
not needed when your devise is not sensitive to EMI nor radiate it

[TimNJ]

Called “safety” capacitors because if said capacitors were to fail they would make an unsafe situation.

So they need to be of especially safe construction to avoid this from happening.

Not to .

[andy3055]

Assuming your idea is to provide safety to the piece of equipment, the accepted method is to replace the non polarized mains cable with a modern, polarized cable, paying attention to where the hot wire is connected.

[Terry Bites]

Once you've settled on a mains voltage and the level of leakage current you can tolerate, you choose the cap size according the the garbage frequency. I dont know if there is any real science to apply. You just keep increasing the capacitance untill you meet the emissons spec. Its trial and error really. If that causes too much leakge current then you have to use other filtering/ layout/ shielding strategies as well. These caps cost nothing so buy a selection and try them out. Just try not to be a polluter.

Its unlikely that even a pro lab will have the special equipment to measure line borne (condcuted) EMI.

The capacitors are not general purpose as some fools seem to think. Class-Y safety capacitors are designed to fail open Class-Y safety capacitors are designed to fail short. They have to meet IEC 6038414 / UL 60384 14 standards. It is definitely dangerous to replace them with general purpose caps. The makers of these caps call them safety caps because they  are. Check out Vishay, Epcos, TDK, Kemet, Murata, Panasonic and all the other safety capacitors catalogs.

[BrokenYugo]

Class X goes across the line, class Y goes from line to earth or the floating case. The number after specifies the voltage ratings.

If there weren't caps there to begin with, you don't need them. The big hazard here is old gear that used crappy wax paper and foil caps in those positions (sometimes known as the "death cap"), those caps like to develop excessive leakage and/or short, posing a shock/fire hazard. In that scenario you replace with appropriate safety caps.

[mclute0]

After Terry Bites most appropriate post....

I have found that the most impactful issue to consider is the cap type/lead spacing/fit.

I love to use MKT X/Y rated caps but they tend to be a much larger footprint than similar ceramic X/Y rated caps.

Trying to fit three MKT in the needed place on an old power supply board can be a challenge.

In the end it might just be better to add metal oxide varistor with an acceptable capacitance if the device has none.

While not having the filtering rage of capacitors, they certainly fit better than MKTs and help protect from transients, IMHO.

[taste_tester]

Thank you all for your posts. I know at least enough to know a general purpose capacitor in that position would be doom 
I still don't quite understand, but I also didn't know that linear power supplies don't affect EMI,
In the end i'm glad I didn't put a bunch of Class X and Class Y all over the place around this linear power board (that would have been very know-it-all of me)

Class X goes across the line, class Y goes from line to earth or the floating case. The number after specifies the voltage ratings.

If there weren't caps there to begin with, you don't need them. The big hazard here is old gear that used crappy wax paper and foil caps in those positions (sometimes known as the "death cap"), those caps like to develop excessive leakage and/or short, posing a shock/fire hazard. In that scenario you replace with appropriate safety caps.

For now I will stick to this advice, only checking for the paper/foil caps to replace with some equivalents and installing a proper power cable which is switching the mains hot wire.
Thanks to all again and regards.

[Circlotron]

I don't know whether that makes me a "know it all".

[james_s]

Thank you all for your posts. I know at least enough to know a general purpose capacitor in that position would be doom 
I still don't quite understand, but I also didn't know that linear power supplies don't affect EMI,
In the end i'm glad I didn't put a bunch of Class X and Class Y all over the place around this linear power board (that would have been very know-it-all of me)

Switching power supplies have a high frequency oscillator that can produce a lot of electrical noise, linear supplies do not have this so they don't create electrical noise. Bottom line is if you don't understand something, don't monkey with it, assume the engineers that designed it knew what they were doing unless you have good reason to believe otherwise.

[themadhippy]

Class-Y safety capacitors are designed to fail open Class-Y safety capacitors are designed to fail short.

bloody clever them caps if they can work out which way to fail  safely

[MathWizard]

I wish more stuff had some inrush protection. I half expect some of my cheap SMPS's to blow up when I plug them in.

Most everything I ever worked on, at least had bleeder resistors, or loads, on rectified AC cap's. Some SMPS's I've opened, have had bleed resistors across the mains input filters. Most smaller AC-LVDC SMPS's, will keep running until the bulk cap voltage is way down.

I recapped an old VT radio 1 time, IDK if I even had an LCR meter back then, or if IO used salvaged caps too. Some day I'll have to re-do it.

[ejeffrey]

Class-Y safety capacitors are designed to fail open Class-Y safety capacitors are designed to fail short.

bloody clever them caps if they can work out which way to fail  safely

Technically class X capacitors are not necessarily designed to fail short, but to avoid catching fire if they fail.  They are allowed to fail short and trip thr upstream over current protection as long as they can absorb the fault energy when that happens.
 Class Y capacitors need to prevent shock hazard in case of failure, i.e. basically not to fail at all.  A capacitor can be both X and Y which basically just means class Y, but the test voltages are different so a capacitor might be X1/Y2.

[taste_tester]

Thank you all for your posts. I know at least enough to know a general purpose capacitor in that position would be doom 
I still don't quite understand, but I also didn't know that linear power supplies don't affect EMI,
In the end i'm glad I didn't put a bunch of Class X and Class Y all over the place around this linear power board (that would have been very know-it-all of me)

Switching power supplies have a high frequency oscillator that can produce a lot of electrical noise, linear supplies do not have this so they don't create electrical noise. Bottom line is if you don't understand something, don't monkey with it, assume the engineers that designed it knew what they were doing unless you have good reason to believe otherwise.

I am usually pretty careful to not touch anything I don't understand well.  Now I simply intend to fuse the hot wire in addition to making it the one that is always switched. I thought I would just use a 0.5A 250V fuse because 1) I have a bunch   and  2) The unit is marked 120V 28W, that comes out to around 0.23A mains consumption

I am thankful to all who helped since I realized there are more subjects I need a better understanding of.

[Circlotron]

Class X cap often rated at 250VAC
Class Y cap often rated at 3kV and generally much smaller value.