# Completed ProjectCan a 12V automotive fuse be used in a 6V circuit?

#### Rahulk70

Joined Dec 16, 2016
498
I have a 6V circuit that draws around 4.2A. The 6V power supply is a Lead Acid Battery. So for safety reasons I want to protect the circuit with an external fuse block. Can I use a 12V 5A fuse? Or is it the total power of the circuit that matters(here in this case around 26 watts)? If that is the case then I'll need a 12V 3A fuse I guess?

#### Alec_t

Joined Sep 17, 2013
11,130
It's the current that's important here. So the 5A fuse is the one to use. A 3A fuse would blow almost immediately with a 4.2A current.

#### Rahulk70

Joined Dec 16, 2016
498
It's the current that's important here. So the 5A fuse is the one to use. A 3A fuse would blow almost immediately with a 4.2A current.
Thanks. So I guess all that matters is the current rating of the fuse, not its voltage.

#### crutschow

Joined Mar 14, 2008
24,709
Thanks. So I guess all that matters is the current rating of the fuse, not its voltage.
Not quite.
The fuse voltage rating should be no lower than the circuit voltage but it can be higher.

#### Rahulk70

Joined Dec 16, 2016
498
Not quite.
The fuse voltage rating should be no lower than the circuit voltage but it can be higher.
So you mean the watts of the fuse as well as the circuit should match?

#### #12

Joined Nov 30, 2010
18,210
So you mean the watts of the fuse as well as the circuit should match?
That's like saying the doors in my house are 78 inches tall so only people that are 6'6" wide should enter.
The voltage rating on a fuse is the highest voltage it can stop when necessary.
The fuse current rating is the highest current it can allow.
The wattage, as the current multiplied by the voltage rating, is useless.
You can't use a 12 volt fuse rated at 30 amps on a 120 volt circuit which uses 3 amps and you can't use a 120 volt fuse rated at 3 amps on a 12 volt circuit that uses 30 amps.

Each limit is separate and must be considered separately.

Last edited:

Joined Jul 18, 2013
20,405
Usually the maximum I/V rating should be observed, for automotive fuses they are intended for use on low voltage DC, one reason is the exposed test pad on the top.
The popular 3Ag style of glass fuse can be used up to 250v.
See the Littlefuse site and others for details.
http://www.littelfuse.com/products/fuses/cartridge-fuses/3ab_3ag-6_3x32mm-fuses/312.aspx
Once you get into higher voltages, the insulation and sweep through rate is important at high voltage.
Max.

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#### crutschow

Joined Mar 14, 2008
24,709
So you mean the watts of the fuse as well as the circuit should match?
No. Watts has nothing to do with it.

The fuse voltage rating is the highest circuit voltage that should be used with that fuse.
If you use a fuse at higher than its voltage rating, it may arc over and not properly clear the circuit when its current rating is exceeded.

The fuse current rating is not affected by the voltage.

#### BR-549

Joined Sep 22, 2013
4,938
yes.

#### Kermit2

Joined Feb 5, 2010
4,162
Heat is created in a resistance when current flows.

The heat loss is also known as I(eye)squaredR

Meaning current, represented by the "I" in ohms law, in amperes is squared and then multiplied by the resistance value, represented by R, in ohms.

All that because this stupid phone can't put in superscript and I'm to lazy to seek another way.

So voltage has little to do with the actual physical red hot, melting apart operation and more to do with the physical structure of the fuse itself.

#### BR-549

Joined Sep 22, 2013
4,938
Ouch....I'm seeing double now.

#### KeepItSimpleStupid

Joined Mar 4, 2014
3,994
There's a voltage rating of the fuse BECAUSE, at below that rating, there is a possibility that the circuit can complete through air or an arc inside the fuse. So a 250 V rated fuse is good for any operating VOLTAGE below 250 V. The automotive fuses are usually rated for 32 V or so.

There are lots of other parameters such as interrupting capacity and delay. For that information, you have to look at the datasheets.

The smallest cross sectional area for the most part determines the current rating.

#### Kermit2

Joined Feb 5, 2010
4,162
Ouch....I'm seeing double now.
If the condition lasts for 4 hours or more, see a physician immediately, as this could be a symptom of a more serious life threatening medical emergency.

#### Rahulk70

Joined Dec 16, 2016
498
That's like saying the doors in my house are 78 inches tall so only people that are 6'6" wide should enter.
The voltage rating on a fuse is the highest voltage it can stop when necessary.
The fuse current rating is the highest current it can allow.
The wattage, as the current multiplied by the voltage rating, is useless.
You can't use a 12 volt fuse rated at 30 amps on a 120 volt circuit which uses 3 amps and you can't use a 120 volt fuse rated at 3 amps on a 12 volt circuit that uses 30 amps.

Each limit is separate and must be considered separately.
So I guess its okay to use a 12V fuse in my circuit. I'm planning to use a 7.5A fuse since 5A is very close to 4.2A. I'll just try short-circuiting a 7.5A fuse and see if it blows.

Joined Jul 18, 2013
20,405
. I'll just try short-circuiting a 7.5A fuse and see if it blows.
That is a real good way to test a fuse.!
Max.

#### MrChips

Joined Oct 2, 2009
20,867
That is a real good way to test a fuse.!
Max.
A better method would be to test the fuse at 5A, 5.5A, 6A, 6.5A, 7A, 7.5A, ... etc. until the fuse blows.
Then you throw away the no-fuse and get a new one.
(Called destructive testing.)

#### Rahulk70

Joined Dec 16, 2016
498
Guys! So I tried the short-circuit destructive testing with 3A,5A,7.5A,10A,15A auto blade fuses & a 20A cartridge fuse. I didn't think a 6V 4.5Ah battery could give such large currents on short-circuits.Checked the manufacturer datasheet just now to get an idea and its capable of 50A for 5 secs and around 150+Amps on short circuit. All the fuses where like BAAM!! I couldn't see the the smaller ones melt except the 15A was like glowing red hot and it deformed the plastic before breaking the link. While the 20A glass cartridge fuses also blew with a blue colored flash in the center and the wire inside split into two.

#### Rahulk70

Joined Dec 16, 2016
498
Oh forgot to ask this. In the above circuit the continuous current draw (4.2A) is directly with a universal 3 position 3 wire ignition key switch. The switch model is Minda D-08. But I couldn't find any datasheet or specs about it(its almost 2 decades old but an unused one). So is this setup fine or should I run the device through a relay? Can the ignition switch handle this current continuously?

Thanks.

#### BR-549

Joined Sep 22, 2013
4,938
If you can get 4.5 amps for an hour.........how many amps could you get in a sec?