60-70v unregulated power supply recommendations

[Neale]

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 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]

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 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]

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]

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]

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]

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]

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]

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?