U3V70F12 to activate mercury relay

I am trying to activate the coil on a mercury relay using the U3V70F12. Coil data listed below

  • Coil Data
    • 28 Ω
    • 433 mA
    • 5.2 VA
    • 5.2 Watts

3.3v input and when connected to the relay coil it only outputs 4.4 v 180 mA.

When I connect it to a solid state relay it outputs 12.2v 40mA.

Is there anything I can do to get it to activate the mercury relay?

Could you post more information about your relay, preferably including a link to the specifications or product page?

What are you using as your 3.3V input voltage source, and more specifically, how much current can it source? Have you tried measuring the input voltage when your system is powered to see if it stays steady at 3.3V or drops (which would indicate an inadequate power source)?

Also, could you post some pictures of your setup that show all of your connections?


The largest relay I would use is listed here:

I currently have this setup on a RAK WISblock 19007 Baseboard:

I am using a 5v 3A USB-C wall charger to power the baseboard.

When powered on, the voltage is a consistent 3.302 v at the VIN & 12.26 v
at the VOUT on the U3V70F12. When I attach the mercury relay, the voltage drops to 3.1v at VIN, and the output voltage when reading the coil on the relay it fluctuates from 3.8v to 3.9v


Thank you for the additional information. Could you monitor the 3.3V source with a scope and see what it is doing? I did not see any indication of how much current the 3.3V output from that board can handle from your link, but with a 433mA load on the output of the U3V70F12 regulator, your 3.3V source would need to be able to handle around 1.6A. You might try powering the U3V70F12 regulator from your 5V 3A source instead to see if that makes any difference.

By the way, you specified that the relay you linked to is the largest you would use; do you have other smaller ones you could try?


I switched out the usb-c wall charger for a Meanwell power supply dual (5V 8A) and connected it directly to the U3V70F12. I was getting around 4V on the input side and the correct 12V on the output with around 402mA. This was able to activate the mercury switch. I believe it is either the RAK WISblock or the usb-c pigtail that I currently am using but I am banking on it being the RAK baseboard not being able to provide enough current.

Can I power it externally and continue to use the RAK board to control this regulator to switch it on and off?

I am glad that seems to have fixed the issue. As long as you have a common ground between the regulator and your control board, it should be fine to enable/disable it while it is powered from a separate source.


That worked! I have the RAK WISblock w/ ESP 32 turning the mercury relay on and off. Thanks for your help with this.

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