Capacitors??? How big?

[TINMAN265]

I use arduinos to trigger relays and often they are triggering locks. The issue I have is that some of the locks I use are fail secure 12V locks that often draw up to 3 amps... BUT only for milliseconds and it is usually at least 1-5 minutes before they retrigger. I have usually used a 12v SLA battery to meet the needs of these power hungry locks. I would like to have additional options and I thought that a nice capacitor might give just the bump needed to facilitate a clean release. What size do I need? how do I size it so I know how you came to that conclusion? I have to wire it in to the common of the relay so it is ready to go when the relay closes the appropriate path... What does that look like in a wiring diagram (which wires to which leads)? Thanks for any help!!

[Ian.M]

Q=CV

Q is charge (current multiplied by time), C is capacitance, and V is voltage, specifically change of voltage.   Therefore if you need to supply 3A for 10ms without dropping more than 2V you'd need at least 15 mF (15000uF) of capacitance.

Then you have to consider the capacitor's ESR, which directly contributes extra voltage drop by Ohm's law (V=IR), so smaller supercapacitors not intended for pulse discharge applications, probably wont be suitable for your application due to their higher ESR.

[TINMAN265]

thats pretty big... probably just end up sticking with SLA battery then. Thanks for the information!

[selcuk]

I think you need diodes instead of capacitors. Please check page 8 of this document about switching inductive loads like door locks.

https://mediac.industry.panasonic.eu/p/2020-08/ti_relay_general_application_guidelines_en.pdf