I'm building this circuit:

I'll be use a 12v DC power supply (same as pictured), but with a 220uF electrolytic capacitor because of the 400 ohm 12v relay I'll be using (instead of 220 ohm).

But looking through the toggle switches available, there are tons. I guess I need a SPST, or a SPDT wired to act like one? Also, it should be On-On?

My main question is on the ampere rating required. Would a 3A switch do? How can I calculate the amps required?

I tried Googling, but can't find any info. More info on the circuit itself is in my other questions.


1 Answer 1


Your diagram shows SPDT, so that's what you need. SPST isn't going to work.

The peak current through the switch will be the same as if you connected the relay coil directly to the 12V supply without the capacitor in place. (The capacitor merely causes the current to decay down to zero from this peak value.)

Therefore, if your 12V relay has a coil resistance of 400 Ω, the peak current will be 12V/400Ω = 0.030A.

With a 220 µF capacitor, the relay will activate for roughly 50-100 ms each time the toggle switch changes state. The exact value depends on the holding-current requirements of the relay.

  • \$\begingroup\$ Hi Dave, thanks for your detailed answer! Looking at RS Components' site in their toggle switch category (at the link above), they sell different ampere current rating switches. They have, for example, 0.1A, 0.5A, 3A and 5A. The 3A and 5A are the cheapest. So I'd be able to use a higher amp switch, right? Thanks again Dave! \$\endgroup\$ Commented Mar 15, 2014 at 19:46
  • \$\begingroup\$ Yes, anything higher is fine. I doubt you could find a switch that's only rated at 30 mA anyway! :-) \$\endgroup\$
    – Dave Tweed
    Commented Mar 15, 2014 at 20:52

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