How to figure out what size fuse to use

[jdbostrom]

Just wondering how to figure this out. I just want to know how much "bigger" the fuse should be rated than the current draw from motor, air compressor, etc. Also how many and where should I put them?

Thanks,
Jonathan

[chuck chesney]

Hi Johnathan...long time, no talky.

Hi Johnathan...long time, no talky. To answer your question, You don't want the fuse much "bigger" than the motor current draw. You want it "smaller", or equal. In other words, if the motor ( or compressor or whatever) draws for example, 5 amps, you want the the fuse to blow at 5 1/2 amps. Otherwise you have a very expensive component that will blow out and protect a fifty cent fuse. Since motors draw a considerable amount of current above the normal running current during startup, use "slow blow" fuses, which will allow the motor to sart with it's nomal surge current, then settle back down for its running current.
The fuse goes between the load(Motor) and the power source.

[jdbostrom]

Hi Chuck,
Thank you very

[color=#000000]Hi Chuck,
Thank you very much. Is that all it is then; one fuse? I assume that you can get the "slow blow" fuses anywhere]

[jdbostrom]

Ooops, I missed what you

Ooops, I missed what you said there.

The fuse goes between the load(Motor) and the power source.

So I have one between the motor and battery, the compressor and battery, and the BEC and battery?

[chuck chesney]

Hi Jon,
Yeah, it's pretty straight

Hi Jon,
Yeah, it's pretty straight forward stuff...put one between every component you that you want to protect and the power source. On the drive motor, it's a good idea to put the fuse in between the ESC and the motor. That way you won't stress the ESC if the motor/prop gets fouled by weeds or such.
On the BEC, the chip will usually shut itself down if it starts to overload until it cools off. If you have a good heat sink in it, there probably will not be any major harm done as long as it does'nt happen too often. If you decide to put a fuse on the BEC, use a FAST BLOW fuse, not a slow blow, and make sure that the fuse is rated at the same size as the maximum output of the BEC chip (usually 1 amp).
You will not have to fuse the radio or servos, just the motor driven items, and maybe the BEC(optional).
You can get the fuses at Radio Shack or almost any automobile parts store.

[Larry Kuntz]

Hi Jonathan,
All excellent information

Hi Jonathan,
All excellent information from Chuck but may I add something. Make sure any of the wiring you are using will carry the current your fusing it for. I hate to see wires acting as fuses and heating up and maybe melting their insulation. Maybe you already knew this, if so just disregard. Things like this if over-looked sure mess up a nice model in a hurry.

[jdbostrom]

Thank you both,
I went

Thank you both,
I went to Radio Shack today and picked up 2 packs each of 2 amp and 3 amp fuses; one of each was slow blow and the other was fast blow. Unfortunately my mulitimeter will not read as much current as I have here to decide which to use (in case you are wondering, the reason I got these particular fuses is because I showed my motor and compressor to the guy at Radio Shack and he told me that he would go with nothing smaller than the 2 amp, but nothing larger than 3 amp). What do you think I should use for the each the motor and the compressor, the 2 or the 3 amp fuse? I am using a Johnson 350 motor powered by a 7.2 volt 4500ma NiMh battery.

Larry-
I am using 18 gauge stranded wire. This should do, shouldn't it?

[chuck chesney]

Hi Jon,
I would go with

Hi Jon,
I would go with the three amp fuse for the Johnson motor. I think it will be OK. A safer, more conservative approach would be to start with the two amp, run your boat under safe conditions and see of the fuse holds, or blows when it should'nt. If it blows when the motor is just running in a normal fashon, then try the three amp. Without an amp meter or a multimeter that can read amps, it's a crap shoot, but I sure would'nt go much more than three or four amps on the Johnson.
A general tip on wire size...usually bigger is better, particularly for motor drive apprications. Also, DC current is MUCH harder on swithces, wire and such than AC. I have found that simple number 14 lamp cord will work fine on our models, and is inexpensive and easy to find In fourty years of building electric boats, I've never had a failure.
By the way...where did you find a source for the Jonson motor?

[jdbostrom]

Thanks Chuck,
I think I

Thanks Chuck,
I think I will do like you said, go with the two amp, and if that blows under normal conditions, put the three amp in there instead.

The motor was given to me by Ray Mason. Not sure where he got it from, but I think he said he got it out of an old power tool.

Jonathan

[Larry Kuntz]

Jonathan,
Sounds like your all

Jonathan,
Sounds like your all set. Chuck’s right, you can’t use too large a wire (except it’s physically too difficult to work with). Harbor Freight ITEM 90939-0VGA $12.99 will read 10 Amps. http://www.harborfreight.com/
They have a bunch of fairly cheap multi meters. Might be handy to have around. I have access to variable dc power supplies so I just run my motor (loaded with the prop I plan to use) or compressor to see the draw in amps.

[jdbostrom]

One more question- Do I

One more question- Do I use a slow blow or fast blow fuse for the little air compresser?

[chuck chesney]

Hi Jon,
I'd use a slow

[color=#0000FF]Hi Jon,
I'd use a slow blow on the compressor. A good general rule is this]

[jdbostrom]

OK, thanks.

Jonathan

OK, thanks.

Jonathan