Hi.

What is the peak reverse voltage being applied to each of the diodes left-to-right D1, D2, D3, D4 all 1N4007 here:

230VAC phase----------|>|---------|>|---------|>|--------|>|---------load--------------neutral

Same question if the diodes are D1=Vr=1Kv(1N4007)-----------D2=Vr=50V(1N4001)--------D3=Vr=5V(LED)------------D4=Vr=5V(LED)

 

There's no way to tell, you can only put bounds on it in terms of the sum of the reverse voltages being at most some value.

In general, what really matters is the reverse current that is allowed to flow, as this is what causes the damage in most diodes. For rectifier diodes, the data sheet general assumes that the application provided no meaningful way to limit reverse currents to tolerable levels, and so they specify that minimum reverse voltage they can withstand before breaking down to the point where that maximum current will flow (the minimum they can withstand is then the maximum you can apply).

The 1N4001 and 1N4007 datasheet specifies a max reverse current of 5 µA (at room temp, 50 µA at 100°C) at their rated reverse blocking voltage.

If the LED can handle that reverse current, you should be okay. Many basic LEDs (for example) specify a maximum reverse current of 100 µA at 5 V reverse voltage, so putting the other diode will provide adequate protection in that case.

 

Since the diodes are all in series, the same current flows in all of them. Each diode has a voltage that is a function of the current. Think of the current starting at zero, then increase it slowly. When the sum of the voltages reaches the applied voltage, the current stops increasing. (I am not saying this is what actually happens, only that it allows you to predict the voltages.)

 

Since the leakage current of the diodes is not equal, the reverse voltage across them will not be equal.

The higher the leakage, the lower the voltage.

 

Since the leakage current of the diodes is not equal, the reverse voltage across them will not be equal.

And since that is true, some of the diodes could exceed their reverse voltage rating and break down.
To put diodes in series you can connect a resistor across each diode of value to carry the maximum reverse current at their rated voltage, which should then prevent any diode from exceeding its rating.

 

Since the leakage current of the diodes is not equal, the reverse voltage across them will not be equal.

The higher the leakage, the lower the voltage.

With 4 diodes in series, the current must be the same in all of them. Both the reverse and forward voltage readings may still vary.

 

It also depends on whether the load has a smoothing capacitor.
If there is a smoothing capacitor, then the inverse voltage across the diode string is twice as much as it would be for a purely resistive load.