# Does voltage ripple matter to a relay driven by a transformer?

If you have a relay that accepts actuation voltages of between 9-25V does it matter if you have a relatively large amount of ripple on that line?

I ask because my design steps down 230V-15V with a transformer. I'm wondering whether a simple bridge-rectifier with a basic RC filter would suffice or whether I need to worry about proper regulation.

Two relays will be the only things powered by this loop. There is another secondary that will have a proper 3.3V switch-mode regulator on it for powering the on-board microcontroller.

• It depends entirely on the relay but I expect pretty much all relays to operate with some ripple on an unregulated line. Commented Oct 24, 2022 at 14:37
• It'll be one of these datasheetspdf.com/pdf/720554/Songle/SRD-12VDC-SL-C/1 I've just never used relays in a design before, so figured I'd best check Commented Oct 24, 2022 at 14:40

It won't matter. The relay will pull in when the applied V reaches 9 V. A higher voltage will make it pull in somewhat faster, but that doesn't matter in most applications.

You don't need an 'RC' filter on the bridge rectifier -- just a C. Ensure that the C is large enough that the voltage doesn't drop below 9 V in the middle of AC cycles.

Ripple on the coil voltage may make the relay 'hum' a little.

• Thanks, I suspected as much but wanted to be sure Commented Oct 24, 2022 at 14:49
• I assume 1mF should be sufficient? Commented Oct 24, 2022 at 15:02
• If the relay takes 100 mA, the V on the cap will drop by less than 100mA/(2*50Hz*1mF) = 1 V between zero crossings of the AC voltage (the capacitor will have charged to the peak of the AC). So as long as the peak (1.4 * RMS value) is over 10 V, you'd be OK. Commented Oct 24, 2022 at 15:13

If you have a relay that accepts actuation voltages of between 9-25V does it matter if you have a relatively large amount of ripple on that line?

Yes it matters, but it mainly matters that you keep the current through the relay above the rated threshold to keep the relay actuated. Another problem with ripple is it can interact with the inductance of the relay coil and also create problems so it's good to keep the voltage as steady as possible.

I once made in circuit to switch between 110 and 220v with the relay but the transformer that powered the circuit did not have enough power to actuate the relay so the circuit would flip between 110 and 220, I ended up having to get a bigger transformer.

• There are two relays, they have a power consumption in the region of 300mW. The transformer is rated for 5VA, so I don't think that's a concern Commented Oct 24, 2022 at 15:09
• Also be careful of resonance that could occur if there is a Q point with a capacitor. Commented Oct 25, 2022 at 6:09