How to calculate the needed psu for my project.

Hi guys,

First i'd like to apologize for this post, there are propably several hundres of answers.

I need to be able to calculate the PSU needed for my projects.

My project consists of: 1 x Arduino Uno 1 x TC35 ver. 3 1 x 8 relays board 1 x Smoke sensor

Setup: 1 x 12v 2.5a psu 1 x 12v 1a psu Uno and TC35 shares the 2.5a psu The 8 relays uses the 1a with a LM7805 voltage regulator for power only. Uses 8 Uno pins to control. This setup seems only to run 5 relays HIGH, stable. Would like all 8.

Is there a formula or application that i can use? Please notice that i'm very new to electricity.

Any help would be greatly appreciated.



What about a nice Computer PSU , they'll have 5v for the arduino even the small ones are at least 1.5a @ 12v bigger , or you could divide it out among 2 PSU ,, and ther pretty cheap, especially if you have some junk/old/unwanter compters hanging around :) Just a thought

Is there a formula or application that i can use?

Yes just add up all the current of the individual parts. However you have not described in enough detail what you are trying to do to make a real calculation.

What about a nice Computer PSU

They are often very unforgiving of miss wiring, shorts cause tracks to be burn't out.


Do you mean like:

1 x UNO 7v 1 x TC35 12v 1 x 8 relays 5v + (8 x 5v control pins)

7+12+45 = 64v

1 x UNO 50ma 8 pins (relay control) 8 x 40ma TC35 2 pins (rx / tx) 2 x 40ma TC35 1a

50+320+80+1000 = 1450ma

Do i add say 10% or something to make sure? Not voltage of course.



Thought of that too but would like to know how it's really done.


It sounds as if the 8 relays together take more than 1A. Unfortunately, that ebay page doesn't say how much current the relays take, and the picture of the board does not have enough resolution for me to read the part numbers. There is no need to use a regulated supply for powering relays, so I would ditch the 5v relay board and the 7805, and buy the 12v version of the relay board instead. The 12v relays should take less than half the current of the 5v versions, but ask the supplier how much current they take, just to be sure.

The other possibility is that the 8 relays do take no more than 1A between them, but you don't have a heatsink on the 7805 so it is going into thermal shutdown. At 1A and 7v voltage drop, the 7805 will dissipate 7W, so it definitely needs a good heatsink.


Could be the amps are to low but the heat sink you've mentioned, i think that might be atleast a part of the problem. The regulator is getting very hot. So i've ordered some heat sinks. I will ofcourse check out the 12v board. The relays are Songle SRD-05VDC-SL-C.

A while after power up the relay indicator lights begins to fade and setting the sixth relay high, the board starts humming. The relay board is running 5v 1a for board power only, it feels like it's enough. The 8 control pins are powered from arduino i believe. If so the 2.5a psu is too low to power UNO + TC35?


A heat sink is not always a solution, you have the thermal resistance of the case to consider. The limiting factor is when you have an infinite heat sink and you can't get bigger than infinity. See:-

The relays are Songle SRD-05VDC-SL-C.

The only data on those relays I can find says “Coil power: 0.36W, 0.45W”. So the coil current @ 5v should be no more than 73mA, and 8 of them would take less than 600mA when all on. The power dissipation in the 7805 assuming a 12v supply will be around 4W if it is only feeding the relays. This is well within the capability of a 7805 in a TO220 case if you give it a reasonable heatsink.

I know from experience that when a regulator goes into thermal shutdown, ripple on the input is passed to the output, so this could explain the humming that you hear. The other possibility is that the power supply is not able to supply the 600mA current without producing a very large ripple. However if the power supply is rated at 1A then that should not be the case. It might be worth measuring the output voltage of the power supply when all 8 relays are on.


Defenately not the preferred way and thanks for the link. I need things like that so if you have more like those, you might think a newbie could get good use of, then do please post them.

Regarding my attempt to calculate the power consumption in one of my earlier posts. Was it close or way out there?


I need things like that so if you have more like those, you might think a newbie could get good use of, then do please post them.

There are lots of links on that site. Go to htp:// and look at the workshop, tutorial and hardware sections of the site.

Was it close or way out there?

Well I am worried about the calculation for the TC35, how are you powering this? Do you have a battery on it that you are charging with the 12V, in which case the data sheet says you need 500mA to charge it. However if there is no battery the data sheet says you will have to provide a peak current of 2A.


600ma sounds about right. But i’m not sure if the 1a actually controls the relays in that the board has a 5v input, i think this only powers the indicator lights and such. This 5v input is the only thing the 1a is connected to. So if the relays aren’t powered by it’s own 5v input then the 600ma for the actual relays, comes through the UNO’s 8 digital ports controlling them, meaning the 2.5a looses another 600ma. Wich could count for my loss of sms control through TC35 when too many relays are high. Does this give any meaning to you?

Regarding the humming and thermal shutdown of the regulator. I’m digging for a 6v 1a that should be the end of the heat problem, atleast the dimming of indicator lights should take longer and reveal if it’s low amps or/and the regulator.

Regarding the 12v relay board. If the digital ports on UNO outputs 5v do i need to put a capasitator (i think they are called) between?


I'm digging for a 6v 1a that should be the end of the heat problem

You need about 7.5V to go through the regulator, not 6V.


Thanks alot! I'll check it out.

Think you just solved my problem. The supplier said 12v 1a. But a 2a peak explains my loss of sms control when to many relays are high. The 2.5a is obviously not enough. But i'd still like to be able to foresee theese kinds of problems a mile away. So not to spend 20GBP on a "useless" psu :)

7.5v for the regulator, got it. Think i got a 9v somewhere.


Hi all.

Just wanted to tie up this post.

A 12v Relay board did the trick.