Electronic – Relay contact sticking while driving a capacitive load

capacitivecontactrelay

I am using a Panasonic relay (ALQ105) rated for a resistive load of 10 amps 5A at 250V to drive LED bulbs. These LEDs have capacitive circuits in their ballasts and hence draw high inrush current when switched ON. Steady state current is quite low (less than an amp at 220 Vac).

This inrush current (and possible spark during switching) is causing contact welding in the relay. I cut open one and found the contacts stuck together. A gentle prying with my finger nails separated them together. I couldn't find any black deposits (which I suspected would be there due to sparks). The value of capacitance in the circuit is unknown and different in different cases. As such I am looking for a general method to get rid of this issue. I could think of these three possible ways:

1) Use a series resistor and possibly an inductor to limit the high inrush current – Possible negative effects would be steady state power dissipation in resistor. Also, I am not sure about what values of R and L should I choose.

2) Use NTC thermistor like this – http://www.cantherm.com/media/productPDF/MF72_JUNE_2016_1.pdf

This looks a better idea as compared to L-R circuit but I am not fully sure of the calculations that I need to do for selecting the correct part.

3) Switching to solid state devices like triacs. Since there won't be any mechanical contacts, there won't be any welding. I am currently using BTB-16-800-BW triac in other applications. (ST BTB16 Triac datasheet)

This has a steady current carrying capacity of 16A which is much more than what my requirement is. It also has a surge current limit of 160 A. However I am not sure whether 160 A is good enough for the load I am dealing with.

Please help me selecting the best feasible solution for this issue.

[Updated relay contact rating to correct value of 5A at 250VAC]

Best Answer

You might want to use an inductor after all. Besides limiting the inrush current, it will improve your circuit's power factor which right now should be reduced by the capacitive nature of the load, at least judging by your description.

Of course, you'll need to find out how much of capacitance you have to counter to calculate the right inductor value. You'll have to either obtain that information from the datasheet or measure it. If you can't, just pick a value and try it out (I'd expect something in units or tens of mH to work).