Fuses on CNC setup

[mtompson]

I need to get an idea for fuse values between my power supply & 3 drivers..

I have been advised to use fuses 50% higher than the peak ( once I've worked out which peak! ) the power supply I'm using is a PS407 - 40V DC at 0A and 38V DC at 7A but in mentions a 9A peak in it's pdf doc. And the drivers are MSD542 both from motion control products Ltd.

The output from the power supply is being divided into 3 so I assume I can divide the 9A peak as well? ( and then maybe 50% on top? ) I simply want to protect the drivers from any nasty surges.
thanks for any ideas..
MT

[pminmo]

Check the driver manual 1st for a recommended fuse by the manufacturer. Fast blow fuses vary based on mfg and type, but it's not uncommon to see a fast blow fuse handle double the rating for several seconds.
If the drivers doesn't have a over voltage protection circuit, you should put a diode across the fuse so that back emf can dump into the filter capacitor of the power supply. See D3 here for example: http://pminmo.com/PowerSupply/simpleps.htm

[Torchhead]

Originally Posted by mtompson I need to get an idea for fuse values between my power supply & 3 drivers.. I have been advised to use fuses 50% higher than the peak ( once I've worked out which peak! ) the power supply I'm using is a PS407 - 40V DC at 0A and 38V DC at 7A but in mentions a 9A peak in it's pdf doc. And the drivers are MSD542 both from motion control products Ltd. The output from the power supply is being divided into 3 so I assume I can divide the 9A peak as well? ( and then maybe 50% on top? ) I simply want to protect the drivers from any nasty surges. thanks for any ideas.. MT

The power supply will probably source 18A for a few seconds unless it has electronic current limit. Fuses are primarily there to keep things from catching on fire. Fuses are really slow compared to any semiconductor. To save a power FET you need things to blow in microseconds. A fuse will blow after the expensive components have let out their blue smoke (:-)

The Motors and drives draw current at whatever level they need to. The rating on the power supply has nothing to do with how much they draw...it is only a recommended limit to prevent damage to the supply itself.

You also cannot assume with three motors, each will draw 1/3 of the total current. Current draw is determined by motor load and speed to an extent. One motor could be drawing 3A and the other 1 (or less). Few steppers will draw anywhere near on AVERAGE, the current that is on the label since running them at higher than nameplate volts actually lowers the overall average current. Work is done by Watts. Watts is a function of volts * current * duty cycle. For a given watts (unit of work) if you raise the average voltage the average current is reduced in proportion.

Only thing that will save a drive is to remove power faster than current can spike in the circuit. You need an electronic fuse that shuts off power in microseconds. Since electronic fuses are fairly complex they are not an off-the-shelf item.

Fuses are recommended in the drive voltage lines more to keep a failed drive from burning up other components than to save the drive.

If the drives don't have built-in protection there is nothing (cheap) you can do to save them.

TOM CAUDLE
www.CandCNC.com

[mtompson]

many thanks for your help,
MT

[pen55]

Hey MT, its funny that i should come across this thread today.

Take a look at this link: http://www.mycncuk.com/forums/showthread.php?t=258
and you will see what i mean by the above, cheekiemonkies is about where you are right now !

[Santa Fe Al]

Originally Posted by pminmo Check the driver manual 1st for a recommended fuse by the manufacturer. Fast blow fuses vary based on mfg and type, but it's not uncommon to see a fast blow fuse handle double the rating for several seconds. If the drivers doesn't have a over voltage protection circuit, you should put a diode across the fuse so that back emf can dump into the filter capacitor of the power supply. See D3 here for example: http://pminmo.com/PowerSupply/simpleps.htm

Hi Phil,

I looked at your schematic and could not find a value for D3. I am using 3.0 amp steppers with a 28 VDC 12 Amp PS. Also, G251s.

I'm guessing I should use a 3 or 4 amp diode for D3. Please advise if I am correct.

Thanks, Al

[pminmo]

D3 is there to provide a path for motor generated emf if the fuse fails. Your motors won't generate a lot of continuous current, I would look for a schottky rectifier of 2 or 3 A. STPS2150 is one Mouser has in stock.

[Santa Fe Al]

Originally Posted by pminmo D3 is there to provide a path for motor generated emf if the fuse fails. Your motors won't generate a lot of continuous current, I would look for a schottky rectifier of 2 or 3 A. STPS2150 is one Mouser has in stock.

Thanks Phil,

Al