# Circuit Breaker selection

Discussion in 'General Electronics Chat' started by emilj726, Jul 16, 2014.

1. ### emilj726 Thread Starter Active Member

Oct 1, 2010
52
2
I have a 24V power supply can deliver up to 1.1A according to its tag. For the input it says 100 to 240VAC.
I am trying to figure out what circuit breaker size I need for a input of 110VAC.

Can someone help?

2. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
1/5 amp or less.

3. ### emilj726 Thread Starter Active Member

Oct 1, 2010
52
2
How did you come up with that number?

4. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,003
745
What's the input Ac current of the psu?

5. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
current and voltage ratiol may not be exact, since efficiency of psu is not known, but a fair aproximation. voltage in 5 times that of voltage out, current of voltage in 1/5 of voltage out aprox.

6. ### soda Active Member

Dec 7, 2008
174
13
Hi

Get the VA of your secondary 24v * 1,1 = 26,4 and divide it by the primary voltage = 0,11amp fuse

7. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,395
1,607
The voltage rating comes from the voltage it must protect from, the voltage it is guaranteed to show an open circuit when tripped. That comes from the line voltage, so if only connected to 110 VAC then any rating above that is fine. If it may also connect back to the 240 VAC then you want something more than that.

Current is trickier. It can output 24 V @ 1.1 A so it supplies 26.4 watts. At the input that is 26.4 W / 110 VAC or .24 A. That assumes 100% efficiency, and makes no allowance for any momentary inrush current when first turned on.

I would sleep soundly with a 1 A breaker. It’s a standard value, and probably not going to trip out every time you turn it on, and still gives a fairly low value of danger current for faults.

If you start to get false trips at turn on, try 2A.

8. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,771
971
You typically size it at a min of 125% of the max continuous draw to avoid nuisance tripping.

And you choose a fuse with an appropriate time curve to take care of inrush..
(slow blow/fast blow,etc...)

But yes I'd put a 1A on it and call it a day.

9. ### soda Active Member

Dec 7, 2008
174
13
What you just said sounds correct. In SA the mains is 240vac and i got the answer by dividing the wattage by 240vac, so sorry for my mistake.

On the other hand you never use a circuit breaker on a dc power supply. The circuit breaker is usually in the distribution box. It's there for your plug outlets and it's rated at 15Amp each. You plug your power supply into the wall socket and then need a fuse to protect your power supply. If you gonna use a 1Amp fuse on the primary side with 240vac, your transformer will burn if there's a short.

With a 115 vac input to the transformer, you gonna need a 250mA fuse on the primary side of your transformer