Use SMD components for mains voltages? - Page 1
Forum section: electronics / beginners

author: RogerClark
author_rank: Contributor
timestamp: April 02, 2016, 11:56:09 pm
content: I'm in the process of building a microprocessor controlled dimmer controller. Using the usual techniques, i.e a zero voltage crossing detector, an opto isolator 4N35 and an opto triac MOC3021 and a big triac BTA41-600Anyway.To save space on the board, I've been looking at using SMD components, but of course there are issues with isolation distances and creepage as well as voltage rating on components.On the zero voltage crossing detector, it looks like SMD components may be practical, as I'm using 4 x 100k resistors - effectively in series, with a bridge rectifier (MB6S , which is rated at 420V RMS). So each of the resistors has around (240V RMS *1.414 )/4 V across it, i.e around 85VSo it looks like 1206 SMD parts would be OK for this. (or I could use 2512 size, but these would be 1W and somewhat overkill)But I also need some 1W 390 Ohm and 1W 470 Ohm resistors for the triac drive circuit, along with some 10nF and 47nF capacitors.In theory, I seem to be able to buy SMD 3225 size 400V rated capacitors, and also 1W 2512 SMD resistors, however I'm wondering if using SMD for the high power side of the board is asking for trouble. Especially, as its much cheaper if I can source components from AliExpress - where its hard to guarantee that the components actually match the advertised specification.I suppose I've almost answered my own question, i.e probably best not to use SMD for the triac side of the board, but I'd be interested to hear if anyone disagrees with this, or whether, even using 1206 100k in series with 85V across each one could be problematic (I could use 2512 instead of 1206 here but the space saving would not be less than using through hole resistors , vertically in the board)Thanks

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2016, 11:18:23 am
content: 4 size 1206 resistors in series is not a problem. Depending on how critical the path is, one could even get away with 0805 size.With vertical mounted resistors one has to be careful with the wire not touching the lower cap of the resistor.SMD caps can be a little tricky, as man film type caps don't stand the temperatures for SMD soldering. So the choice is limited. MLCC have the tendency to fail short from mechanical stress. So it might be better use TH caps and maybe also high power resistors.

author: qno
author_rank: Frequent Contributor
timestamp: April 03, 2016, 08:59:14 pm
content: You should check the voltage rating of the resistor.Many smd resistors are rated for 50 volts or less.I have never checked smd resistors but the old tht 0.25 watt and 0.125 watt resistorsdid became lower when a high voltage is placed on them. Not nice when you are buildinghigh voltage voltage dividers.There is also a curve in The Art of Electronics second edition.I do not have the third edition (yet) so I am not shure this graph survived the update.It should all be in the datasheet

author: RogerClark
author_rank: Contributor
timestamp: April 04, 2016, 01:19:59 am
content: Thanks guysRe: Data on resistorsI find this very hard to get hold of from local suppliers (Altronics and Jaycar in Australia). This applies to standard through hole resistors as well as for SMD.I guess if I bought from Element 14 or Mouser etc, getting that data may be easier, but I think the more cost effective option is to not use SMD at all for thisThanks againRoger