How do you figure out what size fuse to use?

I will be setting up an Airwire G2 and Airwire G3 with 14.4 battery packs, how do I figure out what amp fuse to use.

Thanks in advance.

Dave S

I almost always use a 10 amp fuse. None have ever blown except one fellow who derailed his engins and kepr it running. The motor went out and blew the fuse.

i have used self reset 6 amp. and 10 amp. auto add on. do not use any more than airwire says

Richard

I use 5-amp fast-blow fuses. I install them on the positive lead between the power switch and the electronics.

Later,

K

I use 4 or 5 amp fast-blow in line fuses in all my airwire installs. In 5 years of doing installs for myself and others I have only had 1 blown fuse. One install I did is an airwire g2 that runs 2 phoenix p8 cards and two aristo FA1 locos. I used a 4 amp fast blow for that one.

Terry

For a fuse to be effective, it must be less than the max current limit of the power source.

If a battery can output 5 amps, then a 10 amp fuse may never open, therefore it is completely useless.

So, a 2.2 amp battery pack using a 10 amp fuse is not a good idea, but if the battery can do 3 amps for a short time, then a 2.5 amp fast blow would be great.

Batteries are rated for continuous current, but do have a surge/peak current. The fuse needs to be less than the surge/peak for it to protect your electronics.

Also, note that if a fuse can carry 10 amps, it takes much more to make it blow.

Poly-switch fuses are a bit slow for use as electronic protection. They are just fine as overall circuit overload protection.
You can make a 3 amp (for example) Poly-switch trip much faster by replacing it with 3 x 1 amp Poly-switches. They will hold the 3 amp load but once just one has tripped, the others will follow almost instantaneously.

(Yes, I am aware i am reviving a thread that is almost three years old!

but it makes more sense to continue this conversation, rather than start a new thread…)

I am building a battery/sound car with these components:

Tenergy battery, Li-ion 14.8V 2200mAh

Phonenix P8 sound board

Airwire G3 decoder

more info on the car im building:

http://forums.mylargescale.com/40-rc-battery-operations/71994-building-battery-sound-car.html

I was googling for info on what size fuse to use, and came across this thread.

Airwire, the manufacturer of the G3 decoder, says:

"Make sure the fuse is rated at 10 Amps and it should be a slo-blow style. "

in their manual:

http://www.cvpusa.com/doc_center/web_r15_G3_Decoder_Manual_44_pg%20.pdf

But…all the conventional wisdom, including this thread, says to use a smaller fuse…

Normally, when in doubt, I would simply do what the manufacturer says! since they know their product best…and Airwire says 10amp fuse! So…why is Airwire suggesting a 10amp fuse when they know perfectly well their product is being used with 2200mAh or 4400mAh batteries? Which is 2.2 or 4.4 amps. I have never heard of anyone using a battery pack of more than 10 amps! So…I am believing both sides of this…I have no reason to doubt Airwire, yet all the comments in this thread also make perfect sense…So, who’s right?

thanks,

Scot

Scott,

Based on a recent project at work, I learned this tid-bit about fuses. ALWAYS make certain that the VOLTAGE RATING for the fuse is higher than the voltage you will be using. Said another way, automotive fuses should not be used for our garden railroad power circuits as they usually exceed the voltage ratings of automotive fuses.

Bob C.

The fuse is there for protection, obviously. Like has been stated, a 10 amp fuse in a 3 amp circuit will probably never do its job, that is to open up (blow) to stop an over current condition.

My question would be; what is the measured normal draw from the battery? What is the measured peak draw under normal conditions? Then I would look for a fuse that is 1.5 times normal draw, as long as that number is above peak draw. Or 1.1 to 1.2 times peak draw, if peak draw is higher then the previous calculated number. That way if something does go wrong, the fuse will step in and protect the electronics, like its supposed to.

Good advice on making sure you have voltage rating of fuse correct.

But auto fuses are normally rated at 32v, so great for use by us: http://www.littelfuse.com/~/media/automotive/catalogs/littelfuse_fuseology.pdf

Typical rating for “normal blow” fuses is blow at 200% rating right away, blow at 100% after a few seconds.

5 amp is a good choice, since motors can temporarily have a pretty high surge from a stop, even if running current is 2 amps or less.

Greg

Scot Lawrence said:

So…why is Airwire suggesting a 10amp fuse when they know perfectly well their product is being used with 2200mAh or 4400mAh batteries? Which is 2.2 or 4.4 amps.

Scot, that rating is just a measure of the power capacity of the pack. It has no bearing on the amount of current it can deliver. A 2200mAh battery pack will (theoretically) deliver 2200 milliamps for one hour, one amp (1000 mA) for 2.2 hours, or 10 amps for about 15 minutes. An R/C car motor will drain a 2000mAh pack in about 10 minutes, with the motor drawing 20 amps. (The Li-Ion packs we use are not well-suited for R/C car applications.)

The packs I use have built-in protection boards which limit the current output to 6 amps. What happens, though, is that when there’s a short (and current exceeds that 6 amps in the event of a short) the board’s definition of “protection” is essentially suicide. It sacrifices itself to protect the pack. All well and good, but at $10 to replace the board, it gets expensive to cover accidental “oopses.” That’s why I use 5 amp fuses in most installations. They’ll blow before the board’s protection kicks in, saving me the time and expense of replacing the protection circuit. When you’ve got upwards of $300+ in control electronics, a $1 fuse is pretty cheap insurance.

Later,

K

Kevin Strong said:

Scot Lawrence said:

So…why is Airwire suggesting a 10amp fuse when they know perfectly well their product is being used with 2200mAh or 4400mAh batteries? Which is 2.2 or 4.4 amps.

Scot, that rating is just a measure of the power capacity of the pack. It has no bearing on the amount of current it can deliver. A 2200mAh battery pack will (theoretically) deliver 2200 milliamps for one hour, one amp (1000 mA) for 2.2 hours, or 10 amps for about 15 minutes. An R/C car motor will drain a 2000mAh pack in about 10 minutes, with the motor drawing 20 amps. (The Li-Ion packs we use are not well-suited for R/C car applications.)

ah! excellent, thanks Kevin, that’s the bit of understanding that I was missing…I was thinking the battery packs would only deliver 2.2 or 4.4 amps, maximum, ever…I didn’t know they could deliver more than that! ok then, now this all makes sense.

thanks!

Scot

Good thread guys!

Do not forget the other rating of a fuse… fast blow and slow blow vs neither.

Slow blow will tolerate a high current for a short time, fast blow will not. Fuse is marked SLO for slow blow.

So a large surge current can blow a fast blow fuse but not the others.

If airwire used a 10 amp slow blow fuse, it may never blow, but a fast blow could.

Yes, and again, the manufacturer’s sites will tell you the specifications on the fast and slow blow.

It’s normally such and such percent of rating in a certain number of seconds.

I don’t normally recommend slo blo fuses since most people don’t know where to get them. The automotive blade fuses, both the normal and the mini size are good, and you can buy holders easily from a car parts store.

Glass fuses are too much trouble for most people, and the holders are harder to find, nice weatherproof ones that is.

Greg

I agree with Greg on using automotive blade fuses. I like the small ones. However I don’t like the holders you can buy as they are just too clunky for tight installs. I make my own (not waterproof) with some spade connectors and heat shrink…

Wonder what the original poster used for fuses.

I personally use automotive ATO fuses in my builds with 22 gauge wire and 4.5 amp fuse. However if a friend brings his train over to your layout that runs totally unfiltered track power a 10 amp ATO fuse will work fine if his rolling stock has a dead short.

I forgot what the “original poster” was protecting with the fuse?

this post has been edited by: Rooster

You can also find smaller female spade crimp terminals that will match the mini fuses. the regular fuses use the easily available 1/4" spade connectors.

Greg

Yes you can Greg, but I had those on-hand and also used the other side as a removable connector…