Mains zero cross detector pulse issue

[Faringdon]

Hi,
Cant show it, but its a  very common discrete zero cross detector.(youll know what i mean, theyre all pretty much the same)
Has anyone ever had issues with the position of the zero cross pulse going haywire?....not all the time, just now and then.
Was the problem something to do with the 4Meg leakage in the diff probe used to  look at the zero cross pulse?
Or to do with the 60Hz AC power source used to give the 60Hz , 85-125vac?

Ours is delta three phase input, but we need it to work with one phase aswell....so thats what we're testing at the mo'

Incidentally, the same ZXD seems to work fine when we use it in the 240vac 3 phase "star" product

[mtwieg]

It's very common, and they're all the same, but you can't show it...? I've seen a lot of wonky AC cross circuits in my day.

That being said, such circuits are usually very simple and don't randomly go "haywire". They react to whatever they're connected to, so something is perturbing the system from the outside. For example, surges due to distant lightning strikes, etc.

How are you observing the circuit going "haywire"? You should try and capture the actual AC input voltage at the same time, you will likely find your root cause that way.

[Jeroen3]

If you connect your delta three phase inputs with only one phase you have reduced the voltage significantly. Your detector should work with this lower voltage.
eg: normally, with 400v, you have 230V to virtual star, with single phase this halves. Your circuit must work on a wide voltage range.

[Faringdon]

Thanks,
The zero cross circuit has a 400V, 10nF film capacitor in it. If that capacitance gets higher in capacitance with the higher mains, then that causes this issue......i wonder if this is the case?...its a 400V capacitor and it has at the most, 51V peak on it......so i wonder if this guilty capacitor is actually not the 10nF that we think it is.?..and actually it increases its capacitance when at 125vac, it has the slightly higher voltage on it...?

Actually, this guilty suspect 400V film capacitor has 38vpk on it when at 85vac, and 46vpk on it when at 100vac, and 52vpk on it when at 125VAC....i wonder if this is just too little voltage for this capacitor?

Maybe this capacitor is not giving the rated 10nF?

[floobydust]

Nope, it's not the capacitor, wrong rabbit hole.

[Faringdon]

Thanks, the reason i suspect the cap , is that this cap is the main thing that affects the position of the zero cross pulse....making this cap bigger in Farads makes the ZX pulse lead the zx by more time...and this is the fault we are seeing...ie...we are seeing th zx pulse lead the zx by more time as the AC is increased from 85vac to 125vac. (this wouldnt happen of the cap was the same value throughout...and most of the ccts dont show this fault)

...having said that, some of the ccts using this cap, which are exactly the same, are not showing this fault. But then sometimes, they do show it.

And the 240VAC circuit does not show this fault. At least not in the short intervals where i tested it.

If the fault would present itself permanenetly it woudl make things easier. Its intermittent.
I bet this is a common fault with AC cross circuits, as discussed, they are all pretty well the same, the discrete ones.

[gnif]

Locked topic.

@Faringdon, stop double posting, final warning.
You already have a topic discussing a 400V capacitor.
https://www.eevblog.com/forum/beginners/400v-film-capacitor-voltage-vs-capacitance/msg5126871/