Electronics > Beginners

Getting started with a 240VAC PCB

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mariush:
I don't see any freewheeling diodes for the relays.. I suppose they could be on the other board.

Layout wise, the two left most relays would be "weaker" due to thinner traces , and I'd be a bit concerned about pcb flexing and bending due to too much cutouts and the weight of the relays.


I'd also consider using 2  2x10 headers instead of a less commonly used 2x16, could also help with rigidity.

Yeah, the resistors should be at the very least further away from the pcb, maybe use some ferrite rings or some ceramic/plastic spacers to have resistor sitting above the pcb?

John B:
You can't tell by the image, but I duplicated the traces of the common 240V input on the bottom layer as well to use the space and lower the resistance. I can remove the top trace running under the resistors and try to fill out the bottom plane more. What kind of clearances are safe on the mains side, between the input, switched loads and snubber networks? If I'm reading the tables right, I'm only seeing a clearance of around 2.5mm for external exposed conductors. Traces covered in solder mask seem to be an order of magnitude smaller clearance, but I don't even need to push it that low.  I could set the pad clearances to around 5mm and still fill out more copper on the bottom plane.

All the freewheeling diodes are on the driver board. I deliberately kept this relay board to the bare minimum of parts as I'll end up with 5x PCBs and would like the ability to change the driver board.

John B:
Plus I can use a wider footprint for the resistors, which will give a bit more space to add copper, if necessary. Lastly would it be a good idea to go for a 2mm PCB rather than 1.6mm due to the cutouts? The engineering fee is a fair bit more just to go for 2mm PCBs. Alternatively I can add a few more mounting points, I can add them wherever there's space as I'll have to mount the PCB to another mounting board where I can drill the holes as required.

Siwastaja:
Regarding coil freewheeling, just use bidirectional TVS, with working voltage just above relay supply voltage, and maximum clamping voltage well below the transistor maximum voltage (Vds_max), it's the simplest and most effective solution by far. For example with 12V relay, SMAJ15CA and 30Vds_max MOSFET does the job.

A diode slows down the contact release too much, causing excessive sparking and wear of contacts so the relay performs worse than its datasheet lifetime ratings. And any other solution than bidirectional TVS is a multi-component solution and/or requires some calculation.

John B:
I was going to PWM the coils to save on power, down to approx 70% of their full current. At 490 or 960Hz. As such I was using schottky diodes. I imagine a TVS would be unsuitable for this?

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