Interfacing AC power lines

[BobEdge]

Hi,

Can anyone please tell me if i need to connect the 0V line to neutral, or earth if connecting an input to ac power lines, as described in Microchips app note AN521??

[sougata]

Hi,

Even if you reverse your phase and neutral the zero crossing detector will work. But be extremely careful while testing your circuit cause fatal voltages are there. Also to make measurements with a scope you need an isolation transformer or a scope with floating inputs (balanced probes perhaps).

[mister_e]

If it was me... i would use opto-coupler instead. For debugging and development, isolated PIC supply, opto-coupler from the main... few meters away of my face always serves me well

If your PIC supply is not isolated of the main, it's another story. If you're using a transformerless PSU (Capacitive dropper) there's few things to know first.

It will help us if you elaborate a little bit more about you app.

[sougata]

Hi,

I designed a FAN/LIGHT IR remote controller which had a tight BOM (cost) restriction so I had to use a TXR-Less PS and direct AC ZC sense. But for my other projects like thyristor controlled charger, Pro Dimmer I always use an Opto. So I would go with Steve. Infact while development and debug you can use a isolated PS (Txr based) and opto then when through with the code and hair falls convert it to dirt cheap.

[BobEdge]

Hi,

Thanks for your replies.

The application is controlling a 3 phase welder with current feedback giving between 200 & 3000 amps. I already have a working product, but on some sites the welders act strangely. I eventually thought what are the 3 phase inputs refferenced to? So I am now thinking i need to connect the pcb's 0V line to Neutral or earth. Earth would be best since most 3ph supplies dont have a neutral connected in practice.

Bob...

[sougata]

Hi,

If you are doing a phase angle control then make sure you have a EMI/RFI filter between your zero cross sensing path. Using an opto-coupler is a good choice and the LED currents are not that much sensitive to the line noise (third harmonic injection by your thyristor control module). On the photo-transistor side use a constant current source followed by a schmitt trigger buffer. You can also use a low pass filter there. This gives you a rock-solid sync. If you are not doing a phase angle control then make sure you are firing the thyristors right at the zero cross. I use a sustained fence-pulse circuit (using the HPWM module of PIC). And for low current applications I use opto-triacs (both zero cross and non zero cross depending on the application). As far as the field experience goes , phase reversal are common problems that drive the synchronized trigger algo crazy. Post your schematic if possible.

[Archangel]

Hi,

Thanks for your replies.

The application is controlling a 3 phase welder with current feedback giving between 200 & 3000 amps. I already have a working product, but on some sites the welders act strangely. I eventually thought what are the 3 phase inputs refferenced to? So I am now thinking i need to connect the pcb's 0V line to Neutral or earth. Earth would be best since most 3ph supplies dont have a neutral connected in practice.

Bob...

Hi Bob,
Your welder is producing up to 130000 watts of plasma arc (assuming a peak of 46 volts, just a guess) anyway that's a whole lot of RF floating around. You should shield your PIC in a metal box grounded to the welder. I assume you are using 4 wire 3 phase with the fourth wire being ground. Mister_E's recommendation of an opto coupler is a good idea, also shield the wires going into the box containing the pic. I had friend who raced a Midget car with a radio and it would cut the ignition when he keyed the mic due to R/F, shielding fixed it.
JS

[Ioannis]

... few meters away of my face always serves me well

I remember once I was trying to light a LED and suddenly the top of the LED was launched like a rocket and bounced 3-4 times to the ceiling and the walls before land somewhere in the lab! I was very lucky that the part missed my eye that was nearly on top of it!

Ioannis