450V rated Linear regulator suffers high voltage spike to its input

[T3sl4co1l]

Put (22R + 220nF) in parallel.

Tim

[Zero999]

Thanks, We cannot add anything on the PCB itself , as there's no room.
We can only add stuff to the live and neutral wires that connect to the product. –Hence the X2 capacitor we added, as described in the top post.

The "500v peak" scope shot in the top post was with a 100nF X2 capacitor connected between live and neutral. There was also 25uH (10 metres) of coiled up mains cable ahead of that so that we could simulate line inductance.
When we changed the coiled up cable to 100metres of (coiled up) length, then the voltage peak goes up to 550V (as attached)…..youch! (100m of mains cable had an inductance of 73uH )
Obviously, the more “line inductance” we add, the higher the peak voltage to the LR8 is going to go….youch again!

So, the attached 550v peak scope shot is with 73uH of mains cable and then a 100nf x2 capacitor, then the product

We are wondering what is the typical maximum line inductance in the installation that our lamp will see?
It’s a lamp that gets put in pub gardens, outdoors.

That's not a very accurate test. It will probably show worse high voltage spike, than what you'd get in real life.

Were both the live and neutral coiled up together? It will have more inductance if the coil is just on the live or neutral, than both conductors inside the same sheath.

In any case, a coil of cable will have a lot more inductance, than a straight length of cable.

[Niklas]

The actual inductance is not only depending on the length of tha cable, but also on the loop size. Two separate wires will have higher inductance than a cable where the individual wires are kept together by the insulating jacket.
Sine shaped, damped oscillation at power on. Almost the same will happen with a DC step on the input with a ceramic capacitor. (Linear has an interesting appnote on this, google MLCC Linear). Compare it to a stepup regulator, where the wire is the coil and the capacitor is the switch transistor. At power on the capacitor is empty, which equals a closed switch. As the voltage over the capacitor reaches the input voltage, the switch "opens". V=L×di/dt
You would like to have a capacitor between L and N for filtering and protection of your regulator. To change the di/dt you can add a series resistor between L and the filter capacitor. The regulator sits in parallel with the C, but the 230 Vac sees an RC. The addition of a series resistor was mentioned earlier in this thread.