Power supply question

[Rod Fitzsimmons FreyParticipant @rodfitzsimmonsfrey57685]

Hello! I am in process of wiring my mill which uses Gecko stepper drivers, a Mesa 5i25/7i76 combo, a VFD that would like 220V.

I have a transformer to take the 220V down to 60 V and 24 V AC, and a bridge rectifier/capacitor bank to rectify it. The rectifier board has high voltage and low voltage output terminals.

My question is about isolating power sources. Can I use the 24V DC from that rectifier to both power the stepper drivers and provide power to the Mesa board and other sensors like the prox sensors? It was obviously intended for that purpose, but is it a good idea? I do have a DIN mounted 24V supply, but I'll save it if there's no problem using the one unregulated supply.

Thanks for any insights!

[Rod Fitzsimmons FreyParticipant @rodfitzsimmonsfrey57685]

[John HaineParticipant @johnhaine32865]

In principle yes but the various units you mention may well not be happy with unregulated/smoother DC. I'd suggest that the 24V needs to be at least regulated. What actual power rails do the drives etc need? Stepper drivers often need a higher voltage for the output stages, hence the 60V I assume, and typically a 5V logic level rail.

There's a standard IC, the LM7824, which outputs 24V from a minimum input of 33V. Your 24V rectified and smoothed should just about reach this (24 x root 2 = 34V). I'm sure there are other ICs that will work with a lower input – in fact this one:

**LINK**

which goes down to 27V and can supply 2A max. 73 pence each from that supplier. You could provide one for each subsystem. They will need a bit of strip board for mounting, a heatsink and probably some decoupling – see the application notes.

Edited By John Haine on 19/05/2021 07:13:14

[Joseph Noci 1Participant @josephnoci1]

So, it's LinuxCNC then …My favourite Controller to hate…

'It was obviously intended for that purpose,'

Not sure what you mean or how you concluded that, but that is not normally advisable.

The 5i25 gets power from the PC bus; the 7i76 can receive its 5v requirement from either an external supply, or from the 5i25, which is how you will set it I presume.

The 24v ( in fact, can be anything from 8v up to 24v) for the 7i76 is 'normally' isolated from the 5volt ground reference to avoid inducing rubbish into the 5v ( an the PC, 5i25, etc, etc)

In 'most' cases, esp on hobby size machines, you can have the +5v and +24v ground common – do it with a good size copper wire, NOT the machine body!

However, the Stepper/servo axis motors should not share a common ground with the rest of the supplies – the stepper current is noisy and that should be kept away from the other supplies. That said, you can still get away with it if you ground sensibly and if the machine volume is small, ie, wire runs are short and motors are small – connect the grounds for all supplies together only at one point, right at the rectifiers/smoothing caps. Run separate ground wires from that point to the sensors and the 7i76. The supply leads ( ground inc) for the stepper drives should come directly from the smoothing cap if possible.

What size steppers are you using, what size mill is it, and why is the stepper drive voltage so low (24v) ?

What is the 60V for? is there good current available from the 60V supply, and can the stepper drives handle 60v ?

I have built machines with 5i25, 7i92, 7i76 and 7i76E, mill, router and lathe(don't ask..), and in all I have common ground, with the servo/stepper supplies isolated. The router I tried with one common ground and would get spurious Z limit detections which were a little annoying during a job.

Did I say – I don't like LinuxCNC?

Joe

 

EDIT – Hah – post banged heads with John's…

         and I second Johns comments of voltage levels – if you apply the 24VAC, rectified, smoothed DC, now more than 30VDC, thats not good for the 7i76, or your sensors. Better to use the 24VDC rail mounted supply you have and no faffing with add on pcb's with regulators, etc

 

Edited By Joseph Noci 1 on 19/05/2021 07:27:45

[SillyOldDufferModerator @sillyoldduffer]

The risk of sharing is that, when a device is called on to pulse a powerful stepper motor, it might put enough noise on common wiring to discombobulate anything else in the system relying on clean power. Best practice is to separate power supplies, and to filter, shield and route wiring to minimise accidental coupling.

However there's a good chance a simple solution might work. Electronic designers are well aware of noise problems and often defend their circuits. Trouble is, the level of protection varies between nothing at all and prohibitively expensive. Looking at Printed Circuit Boards, it's common to find the designer provided for filtering but someone reduced cost by not fitting the components!

Good news. In my experience stepper motor drivers are well protected, but beware naive web forum reassurances based on small numbers of amateur results! My microcontroller operated rotary table uses the same power supply for everything apart from the Arduino being semi-isolated behind a second regulator. It works fine but although I got away with it, the result wasn't guaranteed. It was a risk. Nonetheless, given positive results on all my stepper bodges projects, I'd be inclined to try a single power supply, test thoroughly, and install the second power supply only if there's trouble.

'Test thoroughly' may be a problem: in the event of misbehaviour I have a certain amount of experience and a capable oscilloscope. Glitch errors are hard to debug without the right tools and 'common sense' is no substitute for well-equipped expertise. In which case it may be easier to throw money at it and apply 'best practice' from the get go.

Dave

[Rod Fitzsimmons FreyParticipant @rodfitzsimmonsfrey57685]

Thanks for all the advice – it was very helpful! My goal is to do this "right" rather than squeeze pennies, largely as a learning exercise – it was working before but had clearly been worked on by a number of people before with different opinions on how to do it, and it was a bit of a hodgepodge.

Joseph: The 60V is for the stepper drives and I assumed the 24 volt was for the controls since it is a bespoke rectifier board (it was made by a Canadian importer/modifier called Novakon) and all the relays etc. on the machine are 24 V. I agree, assuming anything is a poor idea, and even if it was the intent it may not be the best idea.

My modified plan from what I read here is three power supplies:

1. a PC power supply feeding the motherboard and (via the 5i25) the 7i76 5V logic
2. an unregulated supply taking 220V ac to 60V dc and feeding the stepper drivers
3. a 24V switching PS (I have a Meanwell 2.5A DIN mounted unit in hand) for the 7i76 field voltage plus the e-stop loop.

The e-stop loop controls 24V relays feeding the VFD and the 220:60 transformer.

I'm still a bit fuzzy on isolating the various grounds. All three power supplies, plus the VFD, will be fed from the same 220V mains circuit. I'd thought the mains earth will be connected to the machine chassis as well as the electronics enclosure, the ground side of the unregulated power supply output, and the VFD ground. The Meanwell site says the output common should also be connected to the chassis ground, but wouldn't that introduce ground loops? Should I leave the grounds from the switching supplies floating?

Thanks for all the help!

[Joseph Noci 1Participant @josephnoci1]

Rod,

Connect the mains ground to the machine metal body – a good connection – thats your lifeguard.

Connect the VFD ground to the same mains ground connection with a copper wire connection – preferably not via the machine body

Then in your installation I would –

Connect the +5V supply ground to a 'common ground point' ( this can be the chassis, but be sure the connection is good and cannot work loose – I prefer it to be a tag board or junction block, etc)

Connect the 24VDC negative lead to that same common ground point.( this means your 24VDC supply is no longer isolated, but it is not needed in your application)

Then take a single 20 or 18guage wire from the 'common ground point', to the mains ground connection.

So now Mains ground, +5V ground and 24VDC ground are common.

I would leave the 60VDC supply isolated – ie, don't connect its -Ve lead to ground anywhere.

If you do want to connect it to ground ( it can reduce EMI, sometimes…) then connect it from as close as possible to the smoothing cap -Ve lead to the Common Ground Point.

Ground loops are avoided by the common ground point.

DO ground the switching power supplies -Ve leads to the Common point, and make a good between the chassis of the power supply to the machine body and/or the mains ground.

The MESA 7i76 card's 24VDC input -Ve lead should come from that common ground point.

Joe

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