# Relay switching small capacitive load

I have a relay as an additional safe torque-off switch for a motor driver, that supplies the power to the MOSFETs. Now the main capacitive load is behind the relay, otherwise it will be destroyed after the first use, but to reduce some ringing when switching the high-side, a 10 - 22 μF capacitor (ceramic) should be added and this will be seen as a capacitive load by the relay.

The relay is the following: https://docs.rs-online.com/5098/0900766b80fa75dd.pdf

How can I decide, what the capacitive load limit of the relay is? The supply voltage is 48 VDC, the energy by the capacitors is 0.5CU2 = 25.3 mJ, which seems not a big problem on a first sight, but I'm no sure. As a final solution I see to use a resistor of around 22 kΩ next to the relay contacts, which should load the relay to around 36 V in 0.5 s.

• Ic=Vc/ESR of cap which means the contacts burn with energy dissipated in contact resistance and Cap total ESR. You must show all variables in a schematic with specs for desired limits of 1A resistive load absolute max. At 48V. Then derate for longer life. May 29, 2021 at 15:22
• What I don‘t see is how the energy plays a roll. I mean when I use a 1nF cap with 10mOhm ESR I don‘t expect the relay to stick, since the energy is too small. So somehow not only the maximum current must play a roll, but also the energy from the inrush current, and that threshold should somehow be refered to the capacitive load and the switching voltage May 29, 2021 at 20:25