Why do we need ac. Cap betw. Neutral and Live?

Hi all, I would like ask why do we need an AC cap between the Live and Neutral?

In addition to that, why do we need the 51 Ohm resistor?

My final question is this, when i connect both end of the oscilloscope to the ac live and neutral, one of the 51 Ohm resistor will blow up. I do not understand why. I experimented it twice, and the same result happens. Prior to connecting the probe, the circuitry is running fine. The probe itself is ok in my opinion, we use it to measure direct 240Vac and everything is fine. So where do we make the mistake?

Thks.

Rgds,
chicken_feet

Some technical 'guess'

Resistor maybe to limit inrush current when first turning on the circuit.
Cap may be to limit high frequency noise generated when diode bridge switches on-off.
Maybe you are hooking the 'ground' side of the probe to the 'hot' side of the line, thus causing great current to flow through the resistor. Maybe when you hook it directly, you hook ground side of probe to the neutral side of the line.

I would first like to know what is the probe impedance i,e if you are using a 50Ω or 1MΩ probe, hopefully if you are using a 1MΩ probe you might want to check your what is the impedance setting on the oscilloscope impedance (50Ω or 1MΩ).
The most likely explanation for blowing the 51Ω resistor would be, the setting on the of oscilloscope would be 50Ω, so now the total resistance between the Hot and neutral will be 150Ω, hence the current flowing thru the path is 240/150 Amps. I assume that the resistor are SMD which are rated for 1/8 W, which would not be able to handle current more than 1 amp, hence blowinng the resistor.
I wonder what kind of power supply design uses 51Ω, unless it is a RF circuit, I have never seen a power supply terminated in series with 51Ω, so you might better recheck the design.

Ground connection of probe must be connected to NEUTRAL because oscilloscope power cord internally connects neutral to chassis and oscilloscope ground. You must not connect oscilloscope GND to LIVE conductor. That is why 51ohm resistor blows.

Hi all,

Thanks for your reply, I have checked the impedance of the Probe and oscilloscope. The probe impedance is 1M Ohm while the oscilloscope is 1M Ohm too. The funny thing here is that, we are probing at the 240VAC, that is before the 51Ohm resistor.

We are in the process of trying out connecting Live to the probe and connecting the neutral to the ground of the probe. Technically, we don't understand why this would be any difference, as there should be isolation internally. Besides that, we have disconnected the Earth from the Oscilloscope.

Thks.

chicken_feet

// added after 1 min.

We have tested and found out that if we connect the live to the probe and the neutral to the probe ground, the resistor did not blow. I am still digesting the theory behind it. Thanks all !!!