Manyfuse

[Mathurin]

Right, so I got an auto type blade fuse in my works now.

The max amperage I expect to pass is ~45a, so a 50a fuse would be nice to have. Trouble is, the max I can find is 40a. Now, I'm pretty confident I could just use two 20a's to get 40a, but I wonder if I can use an 20a fuse + an 30a fuse together to get 50a? Somehow, I expect that when pulling the amps the 20a is gonna blow, shortly followed by the 30a.

Otherwise I could trash this auto blade fuse thing, find a 50a fuse and solder it directly in line a la Bob Mcree, saving the weight of a fuse holder. After all, if the fuse blows then something went seriously wrong somewhere.

[lemmiwinks]

Would the 45A be continuous or peak? If it's only momentary then the 40A fuse might be ok.

I've often wonered if you could parallel fuses up to increase their rating too.

[Mathurin]

Peak, going up a real steep hill at 36v it'll reach >40a if you don't pedal, but pedal any and it drops.

[lemmiwinks]

Peak, going up a real steep hill at 36v it'll reach >40a if you don't pedal, but pedal any and it drops.

In that case, buy a packet of 40A blade fuses. Insert one fuse in holder, place packet of spare fuses in pocket, go for test ride

[Mathurin]

Well, I don't want to risk having the warp core fail on me while escaping from a pack of klingons...

[Lowell]

Use two of those fuse holders in parallel and you'll be good to go.

[Mathurin]

But then I'd need 25a fuses, I only seem to find 20-30a fuses


Edit: found some 25's on the internets, unobtanium IRL?


Edit2: From Wiki, car fuses styles & ratings, I have ATO style holders.

* mini: 10.9x3.6x16.3 mm (2A, 3A, 4A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A)
* ATO: 19.1x5.1x18.5 mm (1A, 2A, 3A, 4A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A, 40A)
* maxi: 29.2x8.5x34.3 mm (20A, 30A, 40A, 50A, 60A, 70A, 80A)

I guess two 25's it will be.


Still, I'd like to know: is it ok to use two fuses of different ratings?


Edit3: While I'm at it, how does one properly rate fuses for a given system? I'm guesstimating a hair over the max I'm planning on, that is I intend to limit amps to less then 50a, but the motor hardly ever sucks over 40a anyways, so a 50a fuse with it should be good?

[Lowell]

Fuses of different ratings should work ok as the lower current fuse will have slightly higher resistance. I wouldn't do it myself though as that just means more different parts to carry as spares.

[Matt Gruber]

when i added turbo bypass, max amps went from 36 to 53. i added a 20a 3ag to the 30a blade fuse. works good.

to size a fuse, start small enough to blow(try when klingons take a nap) then go up to where it doesn't blow

[fechter]

I think two unequal fuses in parallel will work, but the actual current needed to blow them might not be exactly the sum of the values. It will be higher than either one alone.

[knoxie]

Hi

Your other option is to use Maxi blade fuses, which is the same design but bigger of course. There are 2 schools on the fuse things, some people fit them (in the phase wires I am talking here) and some people don't.
I find what blows the fuses more easily is the combination of current and voltage going through them.

If I run my motor at 48V 35A the 40A phase fuses are fine, if I run it at 72V 35A the 40A fuses blow and I have to put in 50A maxi blade fuses, its a little odd.

I ran in to the same problem though, fyi I also solder them inside the motor can as well, I didn't run fuses on the Puma motor once and I blew the controller! its a lot quicker to change a fuse in the controller than it is to change the fets that for sure, but like you say if you are blowing phase fuses regularly it is normally indicative of a motor problem somewhere.

Cheers

Knoxie

[fechter]

When you run a higher voltage, you can develop a higher motor current at low throttle due to the current multiplication effect of the controller.

If your hall signals get screwed up or you have a bad connection, that can blow things instantly.

The trick is to figure out what the largest fuse is that will still blow before the FETs.

I still think there's something sub-standard about the stock FETs, but I haven't been able to figure out exactly what. They're rated for 75A with a 70A package limitation, and a pair should survive 140A continuous. The peak current could be much higher for short durations. The maximum duty cycle is 1/3, so the average will always be much less than the peak.

Perhaps they're getting factory rejects or untested ones for cheap. I'd be surprised if the manufacturer pays more than 25 cents each for the ones they're using.

[xyster]

If I run my motor at 48V 35A the 40A phase fuses are fine, if I run it at 72V 35A the 40A fuses blow and I have to put in 50A maxi blade fuses, its a little odd.

When you run a higher voltage, you can develop a higher motor current at low throttle due to the current multiplication effect of the controller.

Makes sense. I put a regular 40 amp fuse between the battery and the 35 amp controller and it's never blown, even at 84 volts and 35 amps.