60-70v unregulated power supply recommendations

[JoeHarris]

Only the input to the transformer should be fused.

Fusing the output is more likely to blow up your drivers, than protect anything.

Is this true!? If so I may be heading in the wrong direction! I already have fuse holders for my drives - could put Neutral Links into them I suppose and convert them into switches! or just take them off... I have seen both sides of this argument and haven't got a clue which is right...

Also when selecting fuses for the VFD and the 24v supply I'm not sure what I should be getting in terms of current or speed. Apologies - clueless.

[m_c]

When modern drivers decelerate motors, the extra energy gets returned back into the power supply. With an unregulated power supply, the energy gets absorbed by the smoothing capacitor(s), which under even hard deceleration of your typical machine during normal use, might lead to a few volts increase.

If you add a fuse, and that fuse blows for any reason while the motor is moving, there is now nothing to absorb the return energy, which leads to a voltage spike on the drivers input, which can be more than enough to cause the drive to go overvoltage and release magic smoke.

Plus fuses are not likely to protect the drivers. By the time enough current has flown to blow a fuse, the chances are the electronics in the drive have already failed. Fuse blowing times are measured in milliseconds upwards. Electronic failure will happen in microseconds.

Fuse selection is generally best to see what manufacturers recommend. For anything with a high start up surge, you really want to use time delay fuses.

[Agathon]

When modern drivers decelerate motors, the extra energy gets returned back into the power supply. With an unregulated power supply, the energy gets absorbed by the smoothing capacitor(s), which under even hard deceleration of your typical machine during normal use, might lead to a few volts increase.

If you add a fuse, and that fuse blows for any reason while the motor is moving, there is now nothing to absorb the return energy, which leads to a voltage spike on the drivers input, which can be more than enough to cause the drive to go overvoltage and release magic smoke.

Plus fuses are not likely to protect the drivers. By the time enough current has flown to blow a fuse, the chances are the electronics in the drive have already failed. Fuse blowing times are measured in milliseconds upwards. Electronic failure will happen in microseconds.

Fuse selection is generally best to see what manufacturers recommend. For anything with a high start up surge, you really want to use time delay fuses.

Great explanation - it's obvious once you think about the BEMF issue.

[john swift]

you can have fuses in the DC suppy wiring to stepper drivers provided you have diodes wired across the fuse holder to maintain the path for the returned energy




John

PS

the Gecko G540 has an internal 10A fuse in the positive supply line !

[Lee Roberts]

John,

What's the formula to workout what size Diodes should be used ?

I find this one intresting because surly the diodes need to be rated such that they dont negate the use of a fuse in the first instance but can also still handle what may get thrown at them?

Any clarification is appreciated.

[Clive S]

John,

What's the formula to workout what size Diodes should be used ?

I find this one intresting because surly the diodes need to be rated such that they dont negate the use of a fuse in the first instance but can also still handle what may get thrown at them?

Any clarification is appreciated.

Lee the diodes will not conduct in normal use but if the fuse does blow and the motors is decelerating it would conduct to dump the BEMF into the caps and it would only happen for a few m/secs a diode handling 1-2A would probably be ok

[john swift]

Hi Lee

Clive is correct , when a fuse blows the diode will only conduct
while the motor is generating a higher voltage than the power supplies output

just because its what I have in my spares box I would use
a 1N5408 - 1000V PIV 3A diode
( I think I used them to rectify the 250V mains )

with a 70 to 80V DC power supply a 1N5401 - PIV 100V 3A diode would be OK

John

[JoeHarris]

Hi Lee

Clive is correct , when a fuse blows the diode will only conduct
while the motor is generating a higher voltage than the power supplies output

just because its what I have in my spares box I would use
a 1N5408 - 1000V PIV 3A diode
( I think I used them to rectify the 250V mains )

with a 70 to 80V DC power supply a 1N5401 - PIV 100V 3A diode would be OK

John

How would you implement this in practice? Solder the diode across the ferrules on the cartridge (I think it would fit back into the holder)?

[john swift]

Hi Joe

I would solder the diode across the fuseholders terminals




John

[JoeHarris]

Thanks John, I have the din mounted flip down type fuse holder so not sure without looking at it if this would be possible but I understand the principal and I'm sure there would be a way!


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