# May questions.

Discussion in 'The Projects Forum' started by Lightfire, Apr 9, 2011.

1. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Starting today, I will ask questions per group. As my teacher told me to ask anything about electronics. OK, here we go!

1. I have fuse rated as 12 volt/15 amperes DC. But for example, I have connected two bulbs in a parallel. And those bulbs have their own fuses. If for example, the one bulb got short-circuit, the other one will stay lighted or what? Um, can I use these fuses on 2 double A batteries to test how it works.

I am planning to create a fuse box for my fuse. But I really don't know how. Would somebody don't mind tell me what materials I need? My fuses were small.

Let's say I have a pack of 4 batteries. AA battery. Let's say they are producing 300mAh.

Let's say I connected the 3 batteries by a parallel connection. So total all, their current and voltage is 900mAh current and 1.5 volts.

So they are already connected them in parallel. Let's say I will connect the remaining one to this parallel batteries in a series way.

So, total all, my batteries will be 3.0 volts and how many current? I am confusing. O_O

Is there any 12 volt circuit breaker in the world that looks like the circuit breaker from the house???

Thanks!!!

2. ### wayneh Expert

Sep 9, 2010
12,958
3,774
No, a short means the same as a wire bypassing the bulb. Current will follow the path of least resistance - the shorted bulb, not the good bulb - until something burns up. If your 12v/15A fuse was in series with your bulbs, it will blow if one of the bulbs shorts. Then both bulbs will go dark and no current will be flowing.
Not really, since the AAs are not capable of blowing the fuse. It depends on what you are testing. The fuse will be exactly the same as length of wire.
You need to specify what sort of fuse you are using. You want a solid mechanical mounting for your fuse and the wires leading from it. There are many options.
Current is a bit like water flow; it will be limited by the weakest link. Whatever current a single battery can supply will be the maximum you get. You really shouldn't configure your batteries like this, though. Depending on the rest of the circuit, for example if it was a short, you would have 3 batteries forcing current through one, and that could be a problem.
Are we to imagine what your breaker looks like? How about a picture?

3. ### Wendy Moderator

Mar 24, 2008
20,874
2,654
Many times the batteries will just get hot, you won't always know when something has gone wrong. I built a circuit using a protoboard a long while back, and accidentally left a wire in that shorted my 9V battery. It got very hot.

For low voltage and low current (small battery) circuits you don't really need a fuse. It is good to learn about them though, because big batteries can have a lot of electrical violence in them.

I dropped a wrench on a very large 24VDC power supply one time. It was a good thing it had a breaker (which does the same thing as a fuse) because the whole wrench glowed a cherry red before the breaker opened the circuit. I couldn't pick this wrench up for a while, and it was basically ruined.

4. ### magnet18 Senior Member

Dec 22, 2010
1,232
125
I think he means having a fuse in series with each bulb, all parallel, so if one portion shorts it blows that fuse, and the others stay lit, which should work.

5. ### Wendy Moderator

Mar 24, 2008
20,874
2,654
House wiring and 12V wiring can be very similar. The principles are the same.

I have seen breakers for 12V. I don't remember the currents. They don't look exactly the same as house versions though. The nice thing about 12V versions is 12V is pretty safe, you can leave the wires and connectors exposed, while house wiring they try to cover everything up. I would not bother with a fuse box, but buy fuse holders that can be mounted to plywood.

Generally fuses are for lower currents. You can use a high voltage fuse (or breaker) for lower voltages, but not the other way around. This is because a breaker that is conducting is a closed switch. A switch that is turned on has no voltage across it. At this point current is all that matters.

If you have a 5 amp breaker it will blow if more than 5 amps goes through it. The voltage isn't seen by the breaker until after it has blown (opened up).

Breakers generally die a slow death. Their usual failure mode is to blow at less current than they are rated for. If that happens you get an electrician to replace it.

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Since you are working with 12volt stuff I suggest you look in an automotive spares shop. Even if you don't buy anything you may get some ideas here. Automobiles run on 6 (mostly bikes) or 12 (mostly cars) or 24 volts (mostly lorries).

There will certainly be fuses and fuse holders to look at. Perhaps a chat with the shopkeeper might also gain you some information.

7. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
I already have many fuses for 12 volts as well as their holder but I want my holder to be something like what we have in our main.

8. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Schematic of my circuit.

If you may notice at my schematic, there's wire there with a red circle.

That wire, connecting the - and + is the reason why my second bulb got short-circuit.

Also, both of them had fuses in both - and +.

Okay, my second bulb got short-circuit. Is it possible that my first bulb will get short-circuit too?

The specification of my materials were in the schematic at the same time.

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9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,109
1,069
If you google 'low voltage DC breaker'; you will find automotive type 'resettable' breakers for about \$5 or \$10, and you will find the kind shaped like AC panel breakers for \$50 to \$100. Both will work for your needs. It's YOUR money young man - spend it wisely

10. ### atferrari AAC Fanatic!

Jan 6, 2004
2,741
839
This a forum about projects. What it has to do with homework?

Nice work to have others doing the effort for you.

11. ### Markd77 Senior Member

Sep 7, 2009
2,803
595
With the short circuit one of the fuses should blow (assuming the battery can supply 15A). It's impossible to guess which one will blow. If one of the left ones blows then no lights will be on, if one of the right side fuses blows then the left bulb will still light.
It's possible that more than one fuse will blow or that the others are damaged so they may later blow at lower than 15A.

Last edited: Apr 10, 2011
12. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
What's the essence of calling a help if they will not spend some effort? ?
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13. ### magnet18 Senior Member

Dec 22, 2010
1,232
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I don't think this is homework, the lad's complaining about not having anyone to teach him this in another thread.

14. ### Wendy Moderator

Mar 24, 2008
20,874
2,654
Homework in the truest sense, learning for the sake of learning at home. I haven't seen the other thread yet. One of me, so many threads...

15. ### magnet18 Senior Member

Dec 22, 2010
1,232
125
In the schematic you posted, you are trying to set it up so that if there is a short across one light, that fuse will blow but all the others will remain lit, correct?
If so you should put a fuse in line with each bulb, I have a schematic of what I'm saying and I can post it if you want.

16. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
I am very sorry if my questions is something silly for you... Because I can't ask for any much more...

17. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
You're correct! Hurray! OK, I want to see your schematic.

18. ### magnet18 Senior Member

Dec 22, 2010
1,232
125

I think this is what you want, one fuse for each bulb.

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19. ### SgtWookie Expert

Jul 17, 2007
22,189
1,742
Your main fuse must have a lower current rating than the wire you are using. Otherwise, if there is a short, the wire will protect the fuse by melting.

The main fuse must be located as close as possible to the positive battery terminal. That way, if there is a short circuit, everything between the fuse and the negative battery terminal will be protected by the fuse.

Have a look at this chart:
http://www.powerstream.com/Wire_Size.htm
Look at the "AWG gauge", "Conductor Diameter mm", and "Maximum amps for chassis wiring" columns. As you can see, the maximum current for AWG 18 (1.02362 mm) is 16 Amperes. In case of a short, your 15 Ampere fuse would melt before the wire would.

Now look at AWG 19 (0.91186 mm) wire; its' maximum current is 14 Amperes. In this case, the wire would melt before the fuse; you do NOT want this to happen, as you may have a fire on your hands.

[eta]
Here is an example circuit, with a 15A main fuse, AWG-18/1mm or larger wire, 2.5A fuses on the lamps, and individual switches for turning on and off the lamps when needed/not needed:

• ###### Lightfire Low-Voltage Lighting.PNG
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Last edited: Apr 10, 2011
20. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Ahhh, only one fuses needed???