# Finding correct power supply



## badprenup (Jun 5, 2010)

I'm trying to build a basic circuit, but I do not know what power supply I need. The circuit is powering 3 things:

Object 1:12V, .2 amps
Object 2:12V, .24 amps
Object 3:12V, .54 amps

I can't remember any of the math from high school, and don't know if they need to be in any particular order. The are all the same type of object so the order doesn't matter.


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## DonaldG (Aug 23, 2007)

Hi & welcome to TSF :wave:

First of all, I presume that it is a DC voltage rather than an AC voltage...

As each item needs the same voltage, 12V you only need one power supply unit (PSU). So, to work out the total current demand of the three items, all you need to do is add the current together: 0.94 amp... Lets call it 1 amp.

Therefore the power needs of the circuit is 12V at 1amp....*BUT*...

you should ALWAYS use a PSU that will deliver at least 50% ~ 100% more than the calculated demand. Therefore you need 12V at 1.5 amp or GREATER....

It does not matter which order the items are placed....

-------------------------
Many folk get a little confused at current (amperage) demands of a circuit - perhaps the following will help to explain...

Note that I use the words 'demand' & 'deliver'. 
The needs of a *circuit* is what it *demands*, whereas:
The *PSU* can only deliver *up to* the maximum current that the PSU was built to deliver....

If the circuit 'demands' 1 amp and the PSU can only deliver 1 amp, then it is running at maximum stress and will get very hot and be a potential fire risk. That is why you should add the extra.

As a side note, it is quite safe to use a PSU capable of delivering 12v @ 12 amp to power up the circuit that only needs 1 amp. this is because the circuit will only draw (demand) 1 amp and that the PSU will only supply (deliver) 1 amp even though it is capable of delivering much, much more!


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## Done_Fishin (Oct 10, 2006)

Nicely put Donald .. even I could understand .. :grin:

if there is any chance that the three circuits might be running at the same time and the way they work could interfere (by drawing current in bursts) with each or one another, you could also add 3 schottky diodes (one to each of the positive rail of your circuits) plus a large capacitor (perhaps 1000uF/16V) at the cathode to ground, that would help buffer changes due to load variations. 

The schottky diodes only waste about 0.3 volt so would be ideal in splitting your main supply without losing too much voltage.


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