When connecting a 4 channel relay to a Arduino Pro mini, there is a noticeable power drain from the board when relays are active. This is clearly visible as the 16x2 LCD dims. I remember reading that this could damage the Arduino, is this true?

In any case, since the entire project will be powered by a rechargeable 12V battery, I bought some buck converters to lower the voltage from 12V to 4.66V (this is what I identified the output to be from the Arduino, I even tried increasing it to 4.8V), so I can power the relay switch directly instead of going through the Arduino.

However, the relay does not power on and I have no idea why. Can someone help me understand? I'm new to this.

Connections specs



I found this answer but I do not know if it applies and, if it does, what changes I would need to make with my current connections.

  • 2
    Maybe you never connected the grounds? Please show your wiring.
    – Majenko
    Jul 30, 2022 at 20:43
  • The 5V is from the buck converter to the relay, then the GRND goes back to the battery through the buck converter again. Jul 30, 2022 at 20:44
  • What about to the Arduino?
    – Majenko
    Jul 30, 2022 at 21:03
  • @Majenko sorry for the crude drawing, but this is the connections I have. Jul 30, 2022 at 21:05
  • That's far less crude than many people post... Those connections look fine to me.
    – Majenko
    Jul 30, 2022 at 21:06

2 Answers 2


I'm not a double-E but I'll take a stab at it.

Arduinos' on-board voltage regulators are not particularly efficient; they sink to ground whatever the board doesn't draw, dissipating it as heat. And they usually are not provided with heat sinks, meaning the regulator is heat-limited. You're powering the Arduino with 12v, so whatever current the Arduino does draw, that same current times 7 volts (the voltage drop from 12v to 5v) is waste heat. Driving four relay coils draws (using the specs on the Amazon page) adds 80mA (times 7v) to that heat dissipation.

It looks like at least some Pro Minis (yours?) use a Micrel 2095 voltage regulator which has internal current limiting which may include thermal protection(?) -- shutting off your Arduino when the regulator gets too hot.

Despite that some of the above is speculative on my part, it does suggest an experiment. You've got a regulated 5v source - instead of powering the Arduino at Vin from the 12v source, try powering it's +5v pin with +5, which would significantly reduce the regulator's power dissipation (and therefore, its temperature).

  • I don't see relay coils powered from the Mini in the question
    – Juraj
    Aug 1, 2022 at 12:33
  • The product description I referred to in my answer says: "5V 4-Channel Relay interface board, and each one needs 15-20mA Driver Current. " [emphasis mine]
    – JRobert
    Aug 1, 2022 at 12:52
  • but the coils are powered from the 5 V pin of the relay board and that is not connected to Micro
    – Juraj
    Aug 1, 2022 at 15:08
  • 1
    I think this is not an answer, but rather an extension to one of the points in mine answer or one of the points in askers questions. It would be better as just a coment. I wont downwote as its interesting addition, but neither upwote, as it does not contain the actual answer in additional info.
    – Tomas
    Aug 1, 2022 at 21:44


  1. You should not ever use inductive load from voltage regulator shared with MCU.
  2. Relays are massive eaters, and the 4.6 volts were actually caused by arduinos NTC kicking in to protect arduinos supply, the relays on your picture are designed for power around 5DC so you should design your circuit in that regards. I am impressed by the fact that little regulator on pro miny even survived it, meaning its likely to be reasonably made board. The Pro miny voltage regulator is nowhere near as powerful as thatone on Nano or UNO and only barely manages to run anything more then the MCU itself.
  3. Watch your current. I am not sure whats the max current of your buck converter. Amazon specifications usually do not concern themselves with petty things like reality. SO it might not be strong enough to run all 4 relays. Though Colour is matching more often then wrong
  4. Again relays are massive power eaters and the lower voltage, the higher current so you can often lower your consumption by using relays made for higher voltage.
  5. Next batteries don't store that much power, get influenced by external influences and their capacity further decreases as the life goes. you can save a lot of power by using transistors/mosfets where possible.

It takes considerable additional design steps to make project run on a battery and replacing relays for bigger transformers or mosfets is often part of it.

Set any of the relays on, in arduino code and measure its voltage against ground. If its 5V then arduino is sending signal fine.

If it is not, then check the resistance between its and arduinos ground. if its just few ohms look for issues on the arduino. if resistance between grounds is big or even unmeasurable, then connect the arduino ground with relay board ground. If you can measure 3-5V (Dependant on a pro mini version) between arduinos output and the ground then you don't have enough power on the relay modules supply.

Extra note:Make sure the arduino is not powered from buck converter as volage regulator has minimum voltage diference it can drop and could not be powered via regulator from a supply of less then 6.something Volts

As stated in Jroberts answer once everithing is working, you might want to disconnect the suplly to arduino from 12 volt source to Vin; and connect your arduino to your buck converter set at 5V (4.95ish-5.05ish) to 5V pin adding 10uF capacitor between the 5V and GND on arduino. The buck converters are more efficient in dropping voltage, then voltage regulators, so that would extend your battery life. (this isnt majourisue as compared to the arduinos consuption is tiny compared t the consumption of the relays

  • It feels weired when Edgar Bonet stopped edited all mine answers. I was newer meant to stop him, it was just an observation...
    – Tomas
    Jul 31, 2022 at 13:19
  • I don't see relay coils powered from the Mini in the question
    – Juraj
    Aug 1, 2022 at 12:34
  • 1
    @juraj: That is reference to the original setup where asker describes to power it via arduino and that the voltage output reduced and LCD dimmed, this according to the asker made asker believe that the voltage he measured was a correct voltage to output from buck converter, supplying too low voltage in his second (new) setup
    – Tomas
    Aug 1, 2022 at 21:29

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