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Topic: How to calculate the needed psu for my project. (Read 1 time) previous topic - next topic

dc42

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.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Feffe76

@dc42

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?

Thanks.

Grumpy_Mike

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:-
http://www.thebox.myzen.co.uk/Tutorial/Power.html

dc42

#8
Apr 29, 2012, 01:41 pm Last Edit: Apr 29, 2012, 01:48 pm by dc42 Reason: 1

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.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Feffe76

@Grumpy_Mike

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?

Thanks.

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