# noob question on current rating

Discussion in 'General Electronics Discussion' started by Gwill, Nov 5, 2012.

1. ### Gwill

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Nov 5, 2012
hi everyone, i'm currently working on a project to make a mini egg incubator. It would be using a 12v DC thermostat and a few 12v dc bulbs. My question is, for the power supply, must the current rating be equal to the current used by the thermostat and bulbs? or can it be higher/lower?

2. ### duke37

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769
Jan 9, 2011
The current drawn will be determined by the bulbs.
The thermostat and power supply must be rated for more than this.

Bulbs can run on AC or DC, perhaps your thermostat can also.

For bulb long life, you could run them on a slightly lower voltage or put a small resistor in series to drop the voltage at the bulb.

3. ### Gwill

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Nov 5, 2012
what do you mean by this?

and another question, if i am using DC, can a car battery be the power supply for this project since the car battery is 12v?

4. ### duke37

5,364
769
Jan 9, 2011
If you have a 12V 36W bulb, then the current drawn on 12V will be 36/12 = 3 Amp.
So a 5 amp thermostat and a 5amp power supply will be just loafing along.

A 12V battery will be good but consider putting it on contiuous charge with a proper regulated charger, then you will have safety if the mains goes off.

5. ### Gwill

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Nov 5, 2012
ok this is what i want to know, if the bulbs draw 3amps and the thermostat 5amps, then should the power supply be 8amps or can it be more/less?
btw what do you mean loafing along?

6. ### BobK

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1,686
Jan 5, 2010
The thermostat does not draw 5 amps, it can handle switching of 5 amps. Only the bulb is drawing significant current. The power supply should be able to deliver an excess of what the bulb will draw.

bob

7. ### Gwill

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Nov 5, 2012
does it matter in excess of how much?

8. ### duke37

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Jan 9, 2011
Just gently operating without effort.

You can make the power supply as big as you can lift it!

9. ### Gwill

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Nov 5, 2012
thanks for the responses guys. really needed to clear that up