Confused about fuses.

[FLUTE HEAD]

I have a Xylotex driver and a power supply pushing 30 vdc and 12 amps. My question is where do I put a fuse in this combination? How do I calculate the size? I know that each motor is rated 6.8v @ 2 amps per phase(2-phase) and I have 3 motors. Ive seen schematics with a fuse from positive of power supply to driver. Xylotex will deliver 2.5 amps per phase. Thanks matt

[Al_The_Man]

What I use as a general rule is that if you want to protect each motor and amplifier where the amplifiers do not have over-current limit fold back, you could put a fuse in the DC to each amplifier, for your motors I would go with 4 amps per motor, or you could go with an overall protection to cover the power supply & amps and put a 12 amp fuse in the main DC supply, and go with the slo-blo fuse version. If you go too light on fusing you can get 'Nuisance' blowing.
Any fuse is better than none.
Al

[FLUTE HEAD]

Sounds good Al, I went ahead and put the fuse block upstream of the drivers positive.

[tachus42]

Putting the fuse inline with the Xylotex positive could be dangerous. When the fuse blows the inductive energy stored in the windings has to go somewhere and if the power supply capacitors are not present there will be a high voltage spike on the drivers power supply. The Xylotex data sheet has a warning about voltage spikes over 35 Volts on the supply.

Either use a fuse between the rectifier and the power supply caps or make a voltage clamp with some zener diodes across each driver powers supply connection.

[FLUTE HEAD]

Thanks for the input, Ill be sure to change that when I get home. I'll put it Inbetween the positive of the rect. and the positive on cap. If my power supply puts out 12 amps can I use 15 amp fuse?(all they had at radio shack) How fast and how high can the current rise if a fault develops?

[tachus42]

A better option i just thought of would be to keep the fuses in the drivers positive but put a reverse diode across the fuse so the current can flow back into the power supply caps if the fuse blows. Then you can have 3 smaller fuses which will give better protection.

[FLUTE HEAD]

Tachus, actually I have one output on p.s. and the driver has all 3 axis on one board, one input. So I guess you meant one fuse? thanks, neat trick.

[ViperTX]

Well okay, but it seems that if one axis goes you probably don't want the others to continue on their merry way. I don't know what the problem is....if the fuse is blowing because of instantaneous current demand....you might try a slow-blow fuse or find a power supply that will allow you to have a larger fuse....this is all assuming that your drivers are sized to your motors correctly and you don't have any other constraints.

[FLUTE HEAD]

NO BLOWING YET, haven't hooked everything up yet. Just needed info on where to place fuses and sizing. Everyone has been very helpful.

[Sanghera]

Ok. Could somebody explain this a little simpler. I don't quite understand all of the terms in this thread. Could someone clarify where the fuse should go on the Xylotex, the options that are available?
Thank you very much for your help.
I really appreciate it.