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Trace width / spacing for Mains voltage

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yanir:
Hi, I'm working a project that switches mains voltage using a relay. I also power the micro controller and relay with mains (US 115v 60hz).

I've been using a variety of trace width calculators and in general they use the same equations but I occassionally get different results. Is there a calculator or method that you would recommend?

This is a product that will undergo certification and needs to be within spec.

Also, I have seen boards that are certified and have much lower track widths than the calculators recommend (10A @ ~74mil track and ~130mil space between next mains track.)
The calculators recommend way over 100mil for 10A, at 2oz/ft^2. why such a difference?

My design is space constrained, I am currently using 98mil tracks and around 75mil spacing. I can make changes but I a limited by a connector (that I would prefer not to change). So I can't go much wider.

Any one have any ideas or direct experience with this? Are there any good resources on this topic?

Thanks

IanB:

--- Quote from: yanir on August 28, 2011, 06:35:35 pm ---Any one have any ideas or direct experience with this? Are there any good resources on this topic?
--- End quote ---
I'm sorry, but don't put currents like 10 A on a PCB. My dishwasher control board put low voltage control electronics on the same PCB as a relay switching mains power to the water heater. The PCB tracks could take it but the solder joints at the relay and connectors could not (solder is a high resistance connection once significant currents are involved). Every dishwasher with this design eventually failed with a burning smell, smoke, and a fried PCB.

I imagine you must of course be dealing appropriately with safety and isolation concerns. How do you isolate the microcontroller from mains spikes and surges that might get through the voltage regulator?

yanir:
Adequate heat sinking should control the solder from reflowing. This too is an appliance control application. I have examined several pcbs for similar products and also much higher currents ones(cooktops). They all hold up, the cooktop has massive heatsinks on two mosfets that share the load.

As for protection, right now I just have a 1.5 VA transformer and a rectifier but a fuse may be used.
 I have seen several similar design that don't use any fuse. This is a cost sensitive application, as most of these are and this may the reason for lack of additional protection.

The certification process will ensure that the device is safe. I want to use the largest track width I can, but I have to work with a specific board shape.

 

IanB:

--- Quote from: yanir on August 28, 2011, 07:10:02 pm ---Adequate heat sinking should control the solder from reflowing. This too is an appliance control application. I have examined several pcbs for similar products and also much higher currents ones(cooktops). They all hold up, the cooktop has massive heatsinks on two mosfets that share the load.
--- End quote ---
The issue with my dishwasher is that the relay controlling the heater current is through-hole soldered onto the PCB. No heat is generated that needs heat sinking, but the solder joints are a weak point. Any weakness in process control during board manufacturer can produce a marginally high resistance joint. Since the voltage is 120 V the high resistance at the joint poses no impediment to the current; the current simply keeps flowing and the joint gets hot. The hot joint starts to oxidize, and gets hotter. Eventually there is a cascade reaction and the joint burns out. When I examined my dishwasher after it failed I found the relay pin was islanded in the middle of the solder cone on the pad and surrounded by a black air gap.

The same thing does not tend to happen with low voltage applications like PC power supplies in spite of the high currents involved. At low voltages a high resistance joint simply reduces the current. At mains voltages the current keeps flowing until failure.

I recognize that many appliances are using this kind of design, but it does not make it a good design. It is merely a symptom of low cost engineering and short service life expectations by the manufacturer.

Design codes for high current mains wiring specify crimped or screwed mechanical connections for a good reason. These are more reliable and durable than soldered connections.

You will do what the job demands of course, but I'm afraid you touched a sore point with me about the quality of design in today's world. No longer are things built to last.

jahonen:

--- Quote from: IanB on August 28, 2011, 07:31:07 pm ---When I examined my dishwasher after it failed I found the relay pin was islanded in the middle of the solder cone on the pad and surrounded by a black air gap.

--- End quote ---

A single-sided PCB? They are notorious for such failures, plated-through holes are much more robust against that.

Regards,
Janne

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