Reed Switch Ringing

I am using a normally open reed switch for an LED lighting application. The lights are power through a basic AC adapter that outputs 12 volts DC and 1.5 amps. The reed switch I am using has a switching current of 1.5 amps, carry current of 3 amps, and a maximum voltage well above the scenario I am using it for. It is a littlefuse reed switch model MRPR-20-17-23.

The problem I am having is that as I bring the magnet close to the switch, the lights turn on as expected, but there is a very high pitched ringing sound being produced from the reed switch.

I’m not the most proficient in the electrical field and I’m at a loss for what is causing this sound. Could anyone potentially offer me insight as to why this is happening?

I’d be happy to provide more information as needed. I appreciate your help in advance!

The output of a lot of AC-DC converters isn’t pure DC, but contains a fair amount of “ripple” or AC content that averages out over time to yield a DC value in the ballpark of what the label says. Constant-current supplies for LED applications are notable for this.

My initial offhand guess would be that you’re seeing some sort of effect that’s causing this ripple to get translated to mechanical motion of the reed contacts, which in turn becomes an audible sound. A similar thing often happens when ceramic capacitors are used to filter power supply outputs.

If that’s the case, the thing you’d want to be concerned about is whether or not the reeds are electrically separating; they’ll arc and won’t last long if they’re opening & closing at some audible frequency. On the other hand, if they’re just vibrating together a bit without separating, it’s probably not a big deal aside from the noise. Look closely: can you see the reeds moving? If you look at it in the dark, do you see any arcs/sparks?

Switching significant current directly through the use of reed switches is generally discouraged, as the contacts really aren’t all that stout. It’s usually a better idea to employ them as signal-level components switching a few dozen mA at most, in order to control a transistor or something else that carries the load current. If there’s no visible/detectable arcing taking place and this is just a personal project rather than something going into commercial production, I’d suggest just giving it a try and seeing what happens.

1 Like

It could be that the current flowing through the relay contacts causes a magnetic field that cancels the external field such a way that the contacts are released, the current flow stopped -> removing the cancelling field, thus causing the contacts to conduct again, i.e. having a electromechanical oscillator.

Try to change the orientation or location of the external magnet to see if that is the cause.