Arduino getting hot

Hi, I tried to search the forum but did not find anythig useful yet. I'm kind of a newby with electronics, so please forgive me if what I'm asking is not so smart.

I have built and started using a controller for my acquarium. I use:

Arduino mega 2560 LCD shield + keypad RTC 1307 8 channel Relay module Thermistor Distance sensor External wall mount transformer 9v 2A

All is working properly but Arduino voltage regulator is getting hot.

I suppose the relays are using too much power: they have optoisolation and the pin themselves are rated at 20mA current, so not much. What I fear is that the power for the whole relay bank (I connected it to +5V pin of the arduino) is sucking too much. When the arduino is powered via USB, when more than three/four relays are open, the RTC clock does not work well, so I suppose the 1000mA I am supplying are not enough... The relay module has 10 inputs: GND, VCC, 8 signal pins. Do you think it is possible to supply an external power +5V to the relay module?

thank you

How hot ? Its probably normal and within spec - the LCD backlight and relay coils will be taking say 300mA at most between them, and ATmega perhaps 40mA, so I'd expect the regulator to perhaps be dissipating 1W or a bit more, it won't be cool...

Also are you sure your supply is actually a regulated 9V (if unregulated it might be a fair bit higher).

Do you power the relays from the Arduino's 5V pin? If all relays are switched on at the same time and the LCD's backlight is on, the current consumption may even more than the 300mA MarkT suggested. The LCD datasheet should tell how much current backlight draws. The relay's datasheet tells the coil resistance. When you know the coil resistance, you can calculate how much current the relays' coils draw when energized.

Do you think it is possible to supply an external power +5V to the relay module?

It's certainly possible and maybe necessary.

Can you point to the relay board you have? If it is like this one: then it uses Songle SRD-05DC-SL-C relays that have relay coil resistances of about 70 ohms.

I=E/R = 5/70 = about 70 mA (These relays are often quoted as 80mA) . 8 relays on = 560 mA or more. This is definitely too much for the normal Arduino 5V regulator.

Read the information about the relays on that link (IF those are the relays you have)..

Let us know what your results are!

Check the Arduino IC installed correctly!!!

Ah, synchronicity! Graynomad just recently posted Thermal images of a Mega, which points to a quite useful article about heat. Here's part of it:

Please refer to for official detailed information, as there is actually total limits per Pin Bank that need to be observed as well.

Not saying that's specifically your problem, if you even have one. But it's good reading for sure.

Hi, I'll be posting some information on other Arduino power schemes over on that Thermal Image page linked to above.

i have a fix for this but you need some skills in working with regulators and ptc resettable fuses

this might be of interest:,106232.0.html

was originally posted in bar sports by graynomad

fkeel: this might be of interest:,106232.0.html

was originally posted in bar sports by graynomad

Wow, even moar synchronicity! :P

ups. sorry about that. :stuck_out_tongue:
(am I the only one who consistantly reads his name as Grany Moad ?)

this 5 volt reg should provide more than enough bang for whatever your doing it is a simple switching reg that has up to 3 amps output at 5 volts i will post a picture of my pre reg host board using this part shortly