SMT diode and resistors for mains

[trevwhite]

Hi all.

I have a mains circuit that uses 1N4007 diodes and standard through hole resistors. I want to replace these components with smt diodes and resistors. I thought I would need at least 3mm clearance between the diode pads. Anyone recommend a smt diode?

For the resistors is it safe just to put two 1206 in series?

Thanks

Trev

[SeanB]

I would go for 3 or more SMT resistors, depending on power dissipation. Do your calculations on the assumption mains is at max allowed level plus 10V and one resistor is shorted and the other 2 are at lowest end of tolerance band, and derate 50% from this. Then they will have a long life.

[larry42]

I would not use 1206 ( any amount) on the mains...

Remember mains is 230V rms nominal. Surges up to a kV or two are common (Cat II)

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[mikeselectricstuff]

Resistors have voltage ratings - read the datasheet, and use series combinations where necessary.

[Neilm]

I would not use 1206 ( any amount) on the mains...

Remember mains is 230V rms nominal. Surges up to a kV or two are common (Cat II)

I have used 1206 resistors on mains circuits - it was measuring a 600V CATIV system. I just used a lot of them in a nice long straight line. The resistors I found had very high pulse ratings, so as long as the power wasn't exceeded everything was fine.

It passed with flying colours when we tested it with the 8kV spikes required.

Neil

[trevwhite]

Thanks for the replies.

I was thinking more about clearances between pads than actual power ratings.

[Niklas]

I have used diodes with SMC package to handle voltages up to 350 Vdc. Check for S3F or S3G or something similar. Some manufacturers add an extra E in the beginning, ie ES3F.

The creepage distance between the pads is dependent of the PCB material and also of the pollution degree inside the enclosure. You can add distance by putting a milling line, with a minimum width of 1 mm, between the pads and let it extend 1.5-2 mm out from the diode package.

One other option is to add some conformal coating to the PCB after the diode is soldered. We used that on a board with 450 Vdc and some SMB package diodes. Some coatings are possible to inspect with UV light to make sure that all areas are covered.

1206 can be used if connected in series. Check voltage and power rating, as some other forum members already noted. Also note the creepage distance between the pads and add as many resistors you need to get the desired creepage, power and voltage rating. If the distance between the pads on a single resistor is 1.5 mm, then you need, at least, 3 of them to get more than 3.5 mm.

[Rerouter]

just remember that mains has repetitive peaks of 340V (240*1.414 for sine) on a normal day, with the limit on 375V (265V RMS) say when your product is plugged in where solar inverters are saturating an substation,