Next club meeting: 7:00 PM Wednesday, November 21, 2012

[dingo]

All right see y'all at the flag

 

[asutherland]

Excellent, see you there for beer and not-so-good food

 

[Glenn-BJ74]

Fuse trivia ... blah-blah-blah

Automotive fuses are rated 32 volts, because they are used in both 12 volt and 24 volt systems.

http://www.optifuse.com/fuses-auto-blade-reg-apr.php

So a 10 amp fuse will blow at 10 amps x 32 volts = 320 watt load. A 12 volt alternator puts out 13.5 - 14.5 volts. So assuming a 14.5 volt circuit voltage, then a 10 amp automotive fuse will blow at 22 amps, which is higher than most would expect.

For general circuits a 135% fuse rating is recommended and for an electric motor circuit 175%, due to the high starting load.

The 3/4 hp Puma draws 560 watts. So a 12 volt rig with a 14.5 volt alternator - the Puma should draw about 38 amps at that voltage. At the 32 v a fuse rating - a 30 amp fuse would handle 960 watts, which exceeds the 560 watt load by 70%.

I have a 40 amp circuit breaker fro my Puma, but I also have a inverter on that 6 gauge AWG circuit.

 

[Toshi38]

Automotive fuses are rated 32 volts, because they are used in both 12 volt and 24 volt systems.

http://www.optifuse.com/fuses-auto-blade-reg-apr.php

So a 10 amp fuse will blow at 10 amps x 32 volts = 320 watt load. A 12 volt alternator puts out 13.5 - 14.5 volts. So assuming a 14.5 volt circuit voltage, then a 10 amp automotive fuse will blow at 22 amps, which is higher than most would expect.

For general circuits a 135% fuse rating is recommended and for an electric motor circuit 175%, due to the high starting load.

The 3/4 hp Puma draws 560 watts. So a 12 volt rig with a 14.5 volt alternator - the Puma should draw about 38 amps at that voltage. At the 32 v a fuse rating - a 30 amp fuse would handle 960 watts, which exceeds the 560 watt load by 70%.

I have a 40 amp circuit breaker fro my Puma, but I also have a inverter on that 6 gauge AWG circuit.

You're thinking about it wrong. Voltage doesn't really matter that much since the voltage drop over the fuse is almost negligible. Fuses are not rated for a specific wattage... they're rated for an amperage.

A 10 A fuse will blow at 10A regardless of weather its in a 12V circuit or a 30V circuit.

Edit: You are right about the power of circuits protected though. A 10A fuse protecting a 12V circuit will allow a load of 120W a 10A fuse on a 24V circuit will allow a load of 240W

If you have a 560W max that you want to blow the fuse at that wattage for a 24V circuit you'd want 560W/24V = 23.33A fuse (Prob use a 25 or safely) for a similar 12V circuit you'd want 560W/12V = 46A fuse (prob use a 50A fuse)

Also keep in mind a fuse won't limit the current supplied ... it will just blow if it goes over a certain current.

 

[slowandsteady BJ]

Good meeting.

We planned a Xmas tree run for the first weekend in Dec. Sat the 8th (I believe).
There were some exciting fuse rating debates....
Home heating solutions.....
Sand blasting in cardboard boxes.....

We might have talked a bit about Cruisers in there somewhere.

Whatever the case, I had a good time.

 

[dingo]

Good meeting.

We planned a Xmas tree run for the first weekend in Dec. Sat the 8th (I believe).
There were some exciting fuse rating debates....
Home heating solutions.....
Sand blasting in cardboard boxes.....

We might have talked a bit about Cruisers in there somewhere.

Whatever the case, I had a good time.

Agreed!

 

[Glenn-BJ74]

You're thinking about it wrong. Voltage doesn't really matter that much since the voltage drop over the fuse is almost negligible. Fuses are not rated for a specific wattage... they're rated for an amperage.

A 10 A fuse will blow at 10A regardless of weather its in a 12V circuit or a 30V circuit.

Edit: You are right about the power of circuits protected though. A 10A fuse protecting a 12V circuit will allow a load of 120W a 10A fuse on a 24V circuit will allow a load of 240W

If you have a 560W max that you want to blow the fuse at that wattage for a 24V circuit you'd want 560W/24V = 23.33A fuse (Prob use a 25 or safely) for a similar 12V circuit you'd want 560W/12V = 46A fuse (prob use a 50A fuse)

Also keep in mind a fuse won't limit the current supplied ... it will just blow if it goes over a certain current.

So are saying for fixed resistance load, voltage and current are proportional so the fuse would blow at 10 amps unchanged by voltage?
But for a given power rating voltage and current are inversely proportional, so for 24 volt motor you would want 1/2 the fuse rating for the same wattage 12 volt motor, as you say above.

 

[Toshi38]

Ya pretty much....

The fuse will blow at 10A regardless of whether circuit is 12V, 14.4V, 24V, 30V ect. (Fuses will have a given working voltage range that is based more on their resistance to arching but that is a different story)

So if you are trying to protect a known wattage then you just divide the wattage by the voltage to determine fuse size (don't forget to include the overload amount you wish to allow otherwise you will blow many fuses)

So example: I have a 500W power inverter I wish to protect it and install a fuse that will blow at 650W (this allows a bit over so I don't constantly blow fuses at 500W ect) if I have a 12V truck like mine I would calculate 650W/14V = 46A fuse I would choose a 50A fuse based on the fact I know a 500W load is likely only going to be applied in the event of a short. Most power inverters include an overload protection) if I was doing the same power inverter on a 24V truck I would choose 650W/28V = 23A so I would chose either a 25A or a 30A fuse (based on the same assumptions)

Remember the fuse should be the weak link to blow before wires/ect. If you can't find a good fuse you can always look at fusible links for larger loads.

Clear as mud?

 

[wussyPup]

Automotive fuses are rated 32 volts, because they are used in both 12 volt and 24 volt systems.

Yup, and in other systems (e.g. marine) up to 32 volts. The volt rating describes the suitability of a fuse for a system. In applications over 32 volts there is a risk that after a fuse blows, the current may arc across the gap and continue to energize the circuit. The 32 volt rating says, "Don't ever use this fuse in systems having more than 32 volts."

You're thinking about it wrong. Voltage doesn't really matter that much since the voltage drop over the fuse is almost negligible. Fuses are not rated for a specific wattage... they're rated for an amperage.

Yup, I wish I'd said that last night...

Whatever the case, I had a good time.

Yup, me too, it was great to catch up with folks...

So are saying for fixed resistance load, voltage and current are proportional so the fuse would blow at 10 amps unchanged by voltage?

Yes, I think so, but grade 9 was a long time ago for me and I'm ready for a correction...

If you increase the voltage supplied to a fixed resistance load, the current used will increase proportionately, and when that current exceeds the fuse's rating the fuse will blow.

Take a 12 volt, 60 watt bulb (easier math than a 55 watt bulb). P=VI where P is power in watts, V is volts, and I is current in amps; or 60=12I, so that 60 watt bulb draws 5 amps at 12 volts. To determine the resistance of the bulb filament, Ohm tells us I=V/R where R is resistance in ohms; or 5=12/R, so the resistance in that bulb is 2.4 ohms. Now, supply that bulb with 24 volts, the resistance hasn't changed, so with I=V/R; or I=24/2.4, the bulb is now drawing 10 amps at 24 volts, generating P=VI, or P=10*24, or 240 watts heat and will cook something; hopefully just the filament.

But for a given power rating voltage and current are inversely proportional, so for 24 volt motor you would want 1/2 the fuse rating for the same wattage 12 volt motor, as you say above.

Yes, a 24 volt version of a motor would draw half the current as an equally powerful 12 volt motor and could be safely powered with lighter wire with a lower rated fuse.

Some AC motors are safely and easily switched between 110 volts and 220 volts to provide the same power; my table saw motor draws 18 amps at 110 volts (and needs 12 gauge wire on a 20 amp breaker), but when wired for 220 volts it will draw 9 amps (and 14 gauge wire with a 15 amp breaker will be OK).

This is not the same as supplying a 12 volt motor with 24 volts without changing the motor windings. That 12 volt motor will do more work (run faster, generate more heat) when powered with 24 volts.

 

[wombat]

I knew someone would bring up ohm's law sooner or later.