Electronic – Wired vs SMD resistor on 230V optocoupler input

mainsopto-isolatorresistors

For circuits similar to the one below, we used large wire resistors with proper power rating in our past designs.

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The question came up, if it would be a good idea to replace the resistor with SMD chip resistors (lower cost, less space, easier assembly).

Most 2512 SMD resistors for example are rated for 200V (mainly thick film material). So using two of those in series should work with respect to the voltage.

You can get wired resistors with guaranteed failure mode "open" and non-flammable. Is there some equivalent SMD resistor type available? What material/technology would I look for?

Creepage distance shouldn't be a problem, since the opto coupler properly isolates the rest of the electronic.

Are there any other considerations I missed?

Best Answer

SMD resistors are perfectly acceptable for this purpose. You need to choose your resistors for both voltage and power rating. You also have the option to use more than two resistors like you have shown: the space occupied by having two resistors in series in each leg (4 resistors total) is still tiny compared to the through-hole resistors you used previously.

Be sure to size your resistors to handle possible over-voltage conditions. You can also specify resistors that are either fusible or flame-proof if required by your certifying authority.