Repairing traces on a circuit PCB

[shockpoint]

Hi there,

I am repairing traces on a dyson AM04 hot and cold fan.

The traces that have burned are the traces attached to Line and Neutral from the power cord. You may see this in the attached image.

The width of these traces (the burned up segments) are about 1-1.2mm in width. I do not know the depth.
The current draw - on cold fan, it is 40W over 230V. With the heater I do not know exactly but the power draw can go up to 1900-2000W on full heating.

the failure mode seems to be an overvoltage (I dont know the details) causing the Metal Oxide Varistor to fail short circuit. This has allowed L and N traces to short and burn up.

my question is are there any guidelines for selecting the wire gauge to use as a repair for these considering the potential current draw?

[forrestc]

If they're only a couple of mm in width, you're probably looking at around 4A through those traces, absolute max, ever, even with thicker copper.

I'd just use something like 18AWG wire to repair them which is far in excess of what you need.  You mentioned that these went to the MOV which burned up.  One other option is to just use the leads from the replacement MOV (assuming through hole here) to connect to the appropriate pads.  That is, don't trim the leads, just bend them over to attach to the input pads.

[inse]

The photo doesn’t reveal too much detail, but if you can measure the width of the former trace you can calculate the cross section and select a wire accordingly or simply use whatever is at hand.
In case the trace was made as fusible link, you will never match the original behaviour.
It’s botch work anyway, but that’s ok if you keep it in mind

[shockpoint]

If they're only a couple of mm in width, you're probably looking at around 4A through those traces, absolute max, ever, even with thicker copper.

I'd just use something like 18AWG wire to repair them which is far in excess of what you need.  You mentioned that these went to the MOV which burned up.  One other option is to just use the leads from the replacement MOV (assuming through hole here) to connect to the appropriate pads.  That is, don't trim the leads, just bend them over to attach to the input pads.

@forrestc thanks for your reply.
Yes the MOV is through hole.
I see what you mean, that does make sense. Would I need to put UV-cure green insulation on the leads after soldering them onto the input pads?

The photo doesn’t reveal too much detail, but if you can measure the width of the former trace you can calculate the cross section and select a wire accordingly or simply use whatever is at hand.
In case the trace was made as fusible link, you will never match the original behaviour.
It’s botch work anyway, but that’s ok if you keep it in mind

@inse thanks for your response.
I have been thinking about whether it was made to be a fusible link, just strikes me as rather cheap of Dyson to do it that way if that were the case.
I've inspected the board and the wiring from the AC input, and unlike its UK counterpart, there is no fuse located on the 3-prong wall plug or at any point along the cable.
So I think you might be right as I thought it was quite unusual for the L line not to have a fuse coming in from the wall.
I would like to make a modification to this to add a fuse into the L line wire.
I have been considering the fuse ratings - do you have any tips on how to select a fuse rating for what is essentially a heater?
I have some trouble reconciling the numbers as I don't have much experience in calculating heating element currents.
40W at 230V is for the usual fan function
The data sheet suggests 1900W for the heating element at 230V. I am not sure if this is appropriate or not but just plugging the numbers into P = V*I I get about 8.69 amps (assuming worst case 2000W usage). Plugging that back into P = I^2*R produces an R = 26.5Ohms, which is within the range of what I would expect for a heating element.
So should I get a fuse rated for 230V and 10A?

[forrestc]

Yes the MOV is through hole.
I see what you mean, that does make sense. Would I need to put UV-cure green insulation on the leads after soldering them onto the input pads?

No on the green insulation - or more specifically solder mask.  I would pay attention to assembly and make sure that you don't end up contacting some metal or similar.   But as long as you don't have to worry about shorting out to some metal in the enclosure you should be ok.

[shockpoint]

Thank you. I will report back with results once I've done everything.

[shockpoint]

Update

So what I did was the following
- repaired the traces (likely fusible links) with wire
- installed a modular fuse with a fuse holder
- replace the MOV

The fan is working again
The fuse was specced to 10A, so hopefully it will protect the traces from catching fire in the event of another MOV going into short mode to protect the electronics.

[inse]

Sounds like a job well done