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Agathon
17-06-2017, 10:26 PM
I'm running 3 Leadshine AM822 drivers with 2 at more or less their maximum current (5.86A RMS) and my 36v switch-mode power supply just isn't up to it.

I've been recommended to buy an unregulated supply of around 60-70v. I'm guessing that a 10A supply will be enough to drive all three motors (the Z axis requires very little power). Can anyone recommend a supplier or a method of construction.

While I currently have just 3 axes but might want to increase to 5 although the 4th and 5th will need lower power motors.

Clive S
18-06-2017, 12:17 AM
I'm running 3 Leadshine AM822 drivers with 2 at more or less their maximum current (5.86A RMS) and my 36v switch-mode power supply just isn't up to it.

I've been recommended to buy an unregulated supply of around 60-70v. I'm guessing that a 10A supply will be enough to drive all three motors (the Z axis requires very little power). Can anyone recommend a supplier or a method of construction.

While I currently have just 3 axes but might want to increase to 5 although the 4th and 5th will need lower power motors.

Ok I am in Spain after a bit of vino But here is a link http://www.zappautomation.co.uk/ps806-12-linear-power-supply.html but you could build your own for nearly half the price with bits from Rapid on line. https://www.rapidonline.com/electronic-components

Agathon
18-06-2017, 12:25 AM
Thanks Clive - I bet it's hot there!

I've seen those rectifier/smoothing capacitor boards on most home-built PSUs are they available from eBay and Aliexpress? Not quite sure what to search for.

Enjoy you holiday!

David

Agathon
18-06-2017, 01:38 AM
Having thought about it, I only need to run 2 of the AM822s from a 60-72v PSU. If they are set at their maximum current what sort of VA do I need from the transformer? I'm guessing this will partly depend on the specification of the smoothing capacitors.

Clive S
18-06-2017, 08:27 AM
Thanks Clive - I bet it's hot there!

I've seen those rectifier/smoothing capacitor boards on most home-built PSUs are they available from eBay and Aliexpress? Not quite sure what to search for.

Enjoy you holiday!

David

The boards I think you are referring to are not suitable as they have very small caps on them.

routercnc
18-06-2017, 08:45 AM
Have a look at Joe's recent build:
http://www.mycncuk.com/threads/4513-3-Axis-CNC-router/page19
post #373 onwards

And mine from a while ago:
http://www.mycncuk.com/threads/6988-routercnc_MK3/page3

Clive S
18-06-2017, 08:48 AM
Having thought about it, I only need to run 2 of the AM822s from a 60-72v PSU. If they are set at their maximum current what sort of VA do I need from the transformer? I'm guessing this will partly depend on the specification of the smoothing capacitors.

Some links here.
https://www.rapidonline.com/50a-bridge-rectifier-metal-case-519318

https://www.rapidonline.com/samwha-hc2a478m35040ha-4700uf-100v-85deg-hc-snap-in-capacitor-11-3121

https://www.rapidonline.com/lcr-ep0882-p-35mm-nylon-capacitor-mounting-clamp-11-3004

This has secondary of 25-25v some people like 24 - 24V
https://www.rapidonline.com/vigortronix-vtx-146-500-125-500va-toroidal-transformer-230v-primary-2-x-25v-88-0005

And a vid by Joe. He used 4 caps but three is fine.
https://www.youtube.com/watch?v=4OoQQg76ar4
https://www.youtube.com/watch?v=WISDqx-e23M

Agathon
18-06-2017, 02:03 PM
Many thanks.

What do you think about power output. I'm running the 2 AM822s at 5.8A RMS (actually, I think that once I have the interface cable I'll programme the drives to 5.5A RMS (2.8 x 1.96) - but that's splitting hairs) so that's total RMS current of 11.6A. The AM822 manual says that 50-70% of the motor current will be sufficient.

So 2 x 5.5A = 11.0A. 50-70% = 5.5-7.7A x 70v = 385-539VA. So a 500VA transformer should be OK?

Does that sound right or should I be working with the AC output voltage of 50v?

I see that Joe used a massive 750VA transformer.

Clive S
18-06-2017, 02:38 PM
Is there a reason you are using just two motors like it is a lathe?

I run 4 nema 23 on my router without problems and 3 on the mill both with 500va transformers the next one up is 625va.

Remember that the motors rarely all run at peak current together.

GND
18-06-2017, 03:01 PM
And your calculations should use the AC rather than the DC voltage as the factor into the VA rating - so your figures immediately fall by a factor of 1.4! The DC output is essentially the peak value of the AC voltage, having been rectified and filtered, whereas the VA rating is RMS.

Hope that helps!
Graeme

Agathon
18-06-2017, 08:46 PM
Is there a reason you are using just two motors like it is a lathe?

I run 4 nema 23 on my router without problems and 3 on the mill both with 500va transformers the next one up is 625va.

Remember that the motors rarely all run at peak current together.

It's a 3 axis mill Clive but I initially thought that I'd run the Z axis (which is just a quill with 150mm travel) with the 36v SMPS as it needs very little power. However, now that Graeme has put me right about using the AC voltage I think a 500VA supply will comfortably run all three axes. I can use the SMPS to run the 4th and 5th axes when I get round to them - happy days.

I have another problem to overcome with this machine since it has a quick-change tooling system with the tool ejected when the quill is fully retracted. There are three microswitches two associated with the normal Z- Z+ with the third setting the limit for tool ejection. I'd like to do this automatically in Mach3 rather than having to manually jog up beyond the limit switch. I'll start a new thread on this.

Thanks for all your help. Much appreciated.

I'll post some photos and maybe a video or two once I've tidied things up a bit.

JoeHarris
18-06-2017, 09:27 PM
I see that Joe used a massive 750VA transformer.

Hi, for the record mine is a 625VA tx.

johngoodrich
19-06-2017, 08:22 PM
hi. this is what jazz sent to me for my psu. i'm running 2 4nm nema 23's and a 8nm nema 34 and it runs absolutely fine

You'll need:
Transformer with 2 x 25V secondery coils with 625Va output.
1 x 35A 300V or higher Bridge rectifier
3 x 4700uf 100Vdc capacitors with holders.

This transformer will work.
http://www.airlinktransformers.com/c...nge/CM0625224/

This Bridge rectifier.
http://uk.rs-online.com/web/p/bridge...fiers/6296320/

Capacitors
http://uk.rs-online.com/web/p/alumin...itors/8712748/

Brackets:
http://uk.rs-online.com/web/p/capaci...clips/2035698/

if you search for it there is also a wiring diagram for it on here somewhere. all you need is very basic soldering skills and its easy to do

john swift
19-06-2017, 11:56 PM
Hi John

the link to the transformer does not work
a web search for the part number cm0625224 links to

https://airlinktransformers.com/product/chassis-mounting-toroidal-transformer-standard-range-cm0625224

power supply circuit
21911
21910

I notice that data sheet for the transformer does not specify the transformer regulation (change in output from no load to full load)

the 24V output from each secondry is when fully loaded
I expect off load it could be 5 or 6% higher

in my part of the UK, depending on the time of day
the mains supply is 243 to 248 V RMS

when the mains supply is 248V
the DC power supply output can be about 77V DC off load
or 73V DC fully loaded

John

PS
with my mains supply of up to 248V
If I decided to build a power supply
I may opt for CM0625222 with two 22V outputs wired for 44V
to give me 71V DC off load

Neale
20-06-2017, 12:27 AM
John - for the reasons you suggest (load regulation/mains variation), I went with a 2x45V transformer with the secondaries wired in parallel. I used a 625VA Vigortronics toroidal from Rapid Online. You could get the other components from Rapid as well and maybe get free postage. I'm using EM806 drives that are good for max of 80V and nothing's gone pop yet!

Judy a thought.

JoeHarris
20-06-2017, 12:37 AM
John - for the reasons you suggest (load regulation/mains variation), I went with a 2x45V transformer with the secondaries wired in parallel. I used a 625VA Vigortronics toroidal from Rapid Online. You could get the other components from Rapid as well and maybe get free postage. I'm using EM806 drives that are good for max of 80V and nothing's gone pop yet!

Judy a thought.

2x45v? woocha!!


Sent from my iPhone using Tapatalk

john swift
20-06-2017, 12:48 AM
Hi Neale

The other variable is when an axis decelerates and the motors becomes generators returning energy back to the power supply capacitors

the voltage at power supply terminals could be increased beyond the drivers limit
if the decelerating axis returns a lot more energy than other axis use and the power supply capacitors can absorb

John

PS
Hi Joe
the great thing with the 625VA transformers with Bifilar wound 45V secondaries is they can be connected in parallel to give you 45V at 13.88A
you just get the phasing of the winding correct !!!

Agathon
20-06-2017, 01:04 AM
Hi Neale

The other variable is when an axis decelerates and the motors becomes generators returning energy back to the power supply capacitors

the voltage at power supply terminals could be increased beyond the drivers limit

John

Plenty to think about here for the electronically semi-literate like myself. I think keeping the voltage around 70v seems sensible. Is there any disadvantage to using a transformer with a pair of 45v secondaries in parallel? Conversely, any advantage to using a 24v in series?

Rapid Electronics seems like a good place to buy. Is there anything I need to look out for when buying capacitors? I believe there are differences in quality from one manufacturer to another - I don't want to spoil the ship for a ha'porth of tar.

john swift
20-06-2017, 01:45 AM
Hi Agathon

in power supplies its worth checking the ripple current rating of your capacitors is OK for your circuit
and if available buy 105C high temperature capacitors that can run hotter than the 85C standard electrolytic capacitor

John

for ripple current
http://www.skillbank.co.uk/psu/ripple.htm

Neale
20-06-2017, 10:54 AM
One reason I went with 2x45V in parallel rather than 2x24V or 2x25V in series is that it gives a few volts headroom. As has been said already, these transformers are specifically wound in a way that allows parallel connection safely but you shouldn't parallel-connect arbitrary transformer windings. Phasing is important; get it wrong with series-connection and you get zero output but get it wrong with parallel-connection and you are going to have a very hot transformer very quickly (before the fuse blows...) I'm using 625VA, but from some power measurements I have done on my machine, I reckon that 500VA is plenty. However, the bigger transformer was only a few quid more and it fitted the space available, so I went with the over-specced option. If I were building these things commercially, I would go 500VA or maybe a touch less and still be confident that I was going to run within ratings. I have a habit of over-building my own kit...

This link (http://www.mycncuk.com/threads/10500-help-please-with-toroidal-transformers-selection?p=90834#post90834) gives some results I measured on my own power supply. Yes, there is still a lot of mains frequency ripple on the DC side but the stepper drivers are very voltage-variation-tolerant. After all, they are typically specced to run between 24-80V absolute max, something like 28-70V in practice. While it is true that there will be some back EMF when the motor is decelerating, I didn't see any obvious evidence of large spikes on the oscilloscope amongst the general high-frequency switching noise (although I accept that this kind of thing will be difficult to see) but you have to remember that there are the smoothing capacitors that are going to take the sting out of these voltage spikes.

Overall, and even though I'm not the kind of guy who generally accepts blind "do this and you'll be OK" kind of advice without a bit more analysis, the answer is that everyone who has used 2x24 or2x45 transformers with 3x4700uF or 2x6800uF capacitors with the AM882/EM806 class drivers has been very happy with the results and you are unlikely to do better than this.

Agathon
20-06-2017, 12:09 PM
One reason I went with 2x45V in parallel rather than 2x24V or 2x25V in series is that it gives a few volts headroom. As has been said already, these transformers are specifically wound in a way that allows parallel connection safely but you shouldn't parallel-connect arbitrary transformer windings. Phasing is important; get it wrong with series-connection and you get zero output but get it wrong with parallel-connection and you are going to have a very hot transformer very quickly (before the fuse blows...) I'm using 625VA, but from some power measurements I have done on my machine, I reckon that 500VA is plenty. However, the bigger transformer was only a few quid more and it fitted the space available, so I went with the over-specced option. If I were building these things commercially, I would go 500VA or maybe a touch less and still be confident that I was going to run within ratings. I have a habit of over-building my own kit...

This link (http://www.mycncuk.com/threads/10500-help-please-with-toroidal-transformers-selection?p=90834#post90834) gives some results I measured on my own power supply. Yes, there is still a lot of mains frequency ripple on the DC side but the stepper drivers are very voltage-variation-tolerant. After all, they are typically specced to run between 24-80V absolute max, something like 28-70V in practice. While it is true that there will be some back EMF when the motor is decelerating, I didn't see any obvious evidence of large spikes on the oscilloscope amongst the general high-frequency switching noise (although I accept that this kind of thing will be difficult to see) but you have to remember that there are the smoothing capacitors that are going to take the sting out of these voltage spikes.

Overall, and even though I'm not the kind of guy who generally accepts blind "do this and you'll be OK" kind of advice without a bit more analysis, the answer is that everyone who has used 2x24 or2x45 transformers with 3x4700uF or 2x6800uF capacitors with the AM882/EM806 class drivers has been very happy with the results and you are unlikely to do better than this.

Many thanks Neale. I'm interested in a great deal of things and am a dedicated life-long learner. However, with some things I just want to grasp the basics, do as instructed and get on with it so this is all very good advice. I am fortunate in have a very competent electronic engineer friend who can always be called upon for advice if I'm a bit unsure about how to proceed.

Best regards

David

Agathon
20-06-2017, 01:57 PM
I just want to check that the symptoms I'm getting with my set-up is down to an inadequate power supply. Here they are:


Losing steps on rapid travel
Drives trip "sensorless stall detection"
I also get an occasional message from Mach3 saying something like "one or more of the drives exceeded the 25000 kernal setting".



I've got the current settings at maximum.

Neale
20-06-2017, 02:46 PM
I just want to check that the symptoms I'm getting with my set-up is down to an inadequate power supply. Here they are:


Losing steps on rapid travel
Drives trip "sensorless stall detection"
I also get an occasional message from Mach3 saying something like "one or more of the drives exceeded the 25000 kernal setting".



I've got the current settings at maximum.

Point 1 - could be a number of things. On rapid, I would expect the stall detect to come in, but I suspect that it's not just the motor being overloaded. Could be tied in with point 3 below?

Point 2 - I get this if I try to work the motors too hard (mainly, that just means trying to go too fast. In my case, the motor speed is getting close to the stepper motor's corner speed, the point where torque starts dropping rapidly). I found that increasing current helped (torque depends on current) and I have ignored the actual numbers; I've turned up the current to the point that the motors run just a bit too hot to touch which is fine for steppers. Increasing voltage helps, as it allows the motor current to rise more rapidly. The driver will stop the motor being overloaded, but the higher voltage gives the motor that bit more of a kick at the start of each pulse.

Point 3 - sounds like you are trying to drive the motors with too many microsteps. Using my machine's settings as an example - 5000mm/min with 2005 ballscrews means 1000RPM. I'm using x8 microstepping, so 1600 pulses per rev. That works out at just under 27K pulses per sec. The Mach3 kernel speed is tied up with its ability to generate accurately timed pulses, and maybe you are pushing its limits. I'm using Mach3 but with an external motion controller which takes over the pulse generation and frees you from some of these Mach3 parallel port issues.

I may have made some false assumptions in that bit of reasoning so don't take this as gospel! First thoughts based on what you have said, though.

Clive S
20-06-2017, 04:18 PM
I just want to check that the symptoms I'm getting with my set-up is down to an inadequate power supply. Here they are:


Losing steps on rapid travel
Drives trip "sensorless stall detection"
I also get an occasional message from Mach3 saying something like "one or more of the drives exceeded the 25000 kernal setting".



I've got the current settings at maximum.

I believe you are using about 36V smps at present it will be night and day difference on 70V toroidal

Agathon
20-06-2017, 05:09 PM
I believe you are using about 36V smps at present it will be night and day difference on 70V toroidal

Just want I want to hear Clive.

I'm just being cautious and will look into all that Neale mentions too.

Many thanks again, David

Clive S
20-06-2017, 05:51 PM
Just want I want to hear Clive.

I'm just being cautious and will look into all that Neale mentions too.

Many thanks again, David

I have given you the list in post #7 just build it any probs just ask. Its all been done before many times tried and tested. Good luck.

Agathon
01-07-2017, 01:17 AM
OK, so I've built the power supply (625 VA). Ignore the overlong DC wires in the pic - I've left them long as I'm not sure where I'm going to permanently mount a fuse holder. On that topic, what size fuse should I fit for the output? The temporary set-up has a 20A fuse, but if I go with a 20mm fuse holder the largest fuse is 10A. Would a time-delay fuse work given the short peak over-current demand?
2203722038

Clive S
01-07-2017, 10:55 AM
You have gone to town on that. Have you fired it up to see if you are getting around 68V DC if so fine. Are you going to mount the tx on a ali back plate? Re the fuse if you are going to make a control box use din rail fuses and fuse to each drive

m_c
01-07-2017, 11:42 AM
Only the input to the transformer should be fused.

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

Agathon
01-07-2017, 12:09 PM
You have gone to town on that. Have you fired it up to see if you are getting around 68V DC if so fine. Are you going to mount the tx on a ali back plate? Re the fuse if you are going to make a control box use din rail fuses and fuse to each drive

I'm getting 72v DC. I wanted to mount the whole lot on an ali or steel plate, but just couldn't find anything suitable in my stock. Do you think the transformer needs to be on a metal plate? It has a rubber washer bottom and top that would frustrate any heat transfer. Most of the old Swiss machines that I deal in for a living have low voltage control systems (low current-draw of course) and they usually use Paxolin to mount all the electrics. I did think about using this as I have a large sheet, but it was just easier to use the MDF.


Only the input to the transformer should be fused.

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

Thanks, that saves me a lot of trouble.

JoeHarris
04-07-2017, 02:47 PM
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
05-07-2017, 12:23 AM
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
05-07-2017, 12:58 AM
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
05-07-2017, 11:58 AM
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

22087


John

PS

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

Lee Roberts
05-07-2017, 09:22 PM
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
05-07-2017, 09:35 PM
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
05-07-2017, 10:06 PM
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
05-07-2017, 10:54 PM
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
05-07-2017, 11:39 PM
Hi Joe

I would solder the diode across the fuseholders terminals

22089


John

JoeHarris
05-07-2017, 11:44 PM
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!


Sent from my iPhone using Tapatalk

Agathon
06-07-2017, 02:04 AM
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!


Sent from my iPhone using Tapatalk

That sounds like you're talking about the AC PSU input fuse. You definitely don't want any diodes in that position!

Clive S
06-07-2017, 07:47 AM
That sounds like you're talking about the AC PSU input fuse. You definitely don't want any diodes in that position!

Have you read the post? The diode is to go across the DC fuse output of the power supply to the drivers to give a circuit for the bemf to be able to be dumped into the caps. in case the fuse blows

Agathon
06-07-2017, 11:32 AM
Have you read the post? The diode is to go across the DC fuse output of the power supply to the drivers to give a circuit for the bemf to be able to be dumped into the caps. in case the fuse blows

I have been following carefully. Joe mentioned a single "flip down type" fuse holder which I interpreted as possibly being an MCB - his video shows one on the 240vac side. Just wanted to make sure he wasn't confusing the two lots of fuses with disastrous consequences!

Clive S
06-07-2017, 11:42 AM
I have been following carefully. Joe mentioned a single "flip down type" fuse holder which I interpreted as possibly being an MCB - his video shows one on the 240vac side. Just wanted to make sure he wasn't confusing the two lots of fuses with disastrous consequences!

No Prob. But in #39 John show a diagram showing the fuse holder clearly showing the fuse connected to the drive +ve

JoeHarris
06-07-2017, 11:43 AM
No Prob. But in #39 John show a diagram showing the fuse holder clearly showing the fuse connected to the drive +ve

Fear not on this occasion I wasn't confused!!

john swift
06-07-2017, 12:29 PM
Hi Agathon

the circuit from post 34
22096

when every thing is working
the fuse short circuits the diode its connected to
so the power supplies DC output is connected to the driver

when a fuse fails power from the DC power supply is blocked
and now the diode provides a path any for excessive voltage
generated by the motor to be returned to the power supplies reservoir capacitor(s) and used by any other axis driver

if the FET's have become short circuit then a large fault current will blow the drivers supply fuse and the motor will be short circuited so no generated current can be returned via the diode


inside a stepper driver the FET's that form the H bridges
have built in diodes connected across the drain & source terminals
22098
the diodes form a bridge rectifier that converts the AC voltage generated in a mechanicaly driven motor into DC that adds to the external DC supply that powers the stepper driver



John

PS
as an aside
one of many DIY hand cranked torches using a stepper motor as a generator

https://www.eeweb.com/blog/extreme_circuits/stepper-motor-generator
22097

Agathon
06-07-2017, 12:54 PM
No Prob. But in #39 John show a diagram showing the fuse holder clearly showing the fuse connected to the drive +ve


Yes, I saw that.


Hi Agathon

the circuit from post 34
22096

when every thing is working
the fuse short circuits the diode its connected to
so the power supplies DC output is connected to the driver

when a fuse fails power from the DC power supply is blocked
and now the diode provides a path any for excessive voltage
generated by the motor to be returned to the power supplies reservoir capacitor(s) and used by any other axis driver

if the FET's have become short circuit then a large fault current will blow the drivers supply fuse and the motor will be short circuited so no generated current can be returned via the diode


inside a stepper driver the FET's that form the H bridges
have built in diodes connected across the drain & source terminals
22098
the diodes form a bridge rectifier that converts the AC voltage generated in a mechanicaly driven motor into DC that adds to the external DC supply that powers the stepper driver



John

PS
as an aside
one of many DIY hand cranked torches using a stepper motor as a generator

https://www.eeweb.com/blog/extreme_circuits/stepper-motor-generator
22097

Sorry about this, I seem to have sowed some confusion about my own clarity. My A-level physics knowledge is a bit rusty but I understand the back emf issue and the role and function of the diodes - excellent and clear explanation though.