Help me choose correct power supply please

My son and I are using the Tom McGuire Step Cheap driver for a home-built CNC router. I have one axis wired-up and it is working so I am currently adding the other two axis.

Our testing to verify it works just involved using a 9v battery which worked but obviously it was not very powerful. I then tried a bank of rechargable AA batteries which put out about 10.7v and are rated at 2.2A. This made the motor run with a lot of torque so obviously much better. Now we are ready to get a real power supply for this thing. The driver supports from 12v up (to 24 or 36v I think.) The stepper motors we are using are rated at 5.16v, 1.2A. (Japan Servo Co KH42JM six-wire in bipolar series config... is this the best way to wire it???)

Will a "wall wart" 12v-2A power supply be sufficient or ... ???

Also, somewhat related, using the 10.7v-2.2A battery pack the motor gets pretty hot, as in if I hold onto it for a few seconds I need to take my hand off hot. The circuit itself however generates zero heat at all in any of the components. Is this an issue we should be concerned about?

as a basic answer the more voltage you have the faster the motors will run ( given your upper limit of your stepper driver )
then the question of torque this is governed by the max amperage available and the rated amperage of the stepper motor .

now the question of why your stepper motor becomes hot , stepper motors are energised when the motor is stationary , which is why they keep position when stationary , however this is also the point of when it uses maximum current , stepper motors have a maximum current rating and this should not be exceeded , modern stepper drivers are
current limited for this reason and can resonably look after themselves .

stepper motors do in fact run hot , and so long as the current is within range they will be happy , although you feel they are hot to touch , this is normal.
this is why great care needs to be exercised to maintain the max current limit , too much current and you will burn out the motor , this is why you have the current limit resistor in most diy circuits ( the light bulb in the circuit mentioned is for this ) if as you say they are too hot then increase the bulb load or wirewound resistor .

dependant on the machine you are building , i would suggest a more modern circuit with current limiting
as to power supply you need to be able to supply current to cover the situation of all motors running , so for example 3 motors with 1.2a per winding( 2 windings per motor) is 1.2 * 2 *3 = 7.2 A , but as steppers use slightly less current when running , you can be under this quick and dirty way of estimating , as all motors wont we running flat out all the time so a 7a - 8a supply should be ok , or for example use seperate supply for each motor a 12- 24v supply is a good happy medium for a small machine .

and set your current to say be around the 6.5A point should give you some margin , as you dont want the motors max'd out all the time

hope this helps , iv'e try'd to keep this basic rather than go into all the theory behind it all .
but the main point is to keep the current under control and within the bounds of the stepper motor .

for example for small machines some people use old pc power supplys using the 12v feed

cncbasher wrote:

hope this helps , iv'e try'd to keep this basic rather than go into all the theory behind it all .
but the main point is to keep the current under control and within the bounds of the stepper motor .

Yeah it was helpful! Thanks a bunch. I'm not afraid of a little theory though... I have a bachelor's degree is in mathematics and my son is the process of getting one of those himself. But simple answers the question too!

So, 12v to 24v, around 7 to 9-ish amps limited to around 6.5A.

But... it leads to another question... I think we need regulated/linear based on what I've read elsewhere. But I've also seen where unregulated might have advantages in the backlash limiting department. So, regulated or unregulated?

Also, did I wire the steppers correctly by going bipolar/series? The only way I've seen to do bi-polar/parallel is with an 8-wire config but I think bi-polar is the way to go.

andypugh wrote:

Just to add something to what cncbasher said, you can completely ignore the stepper motor voltage spec. It is irrelevant. I don't know why they even mention it.

That's good to know. So the voltage should be as close to the max of the driver circuit as possible for best results is what I take from that then. Thanks!

cncbasher wrote:

... stepper motors have a maximum current rating and this should not be exceeded , modern stepper drivers are
current limited for this reason and can resonably look after themselves .

OK so, after thinking a bit about this part of your reply, does this circuit qualify as a "modern stepper driver" that can self-limit the current? It will supply up to 2A so it is within spec of my motors. I need to look at the datasheets for the voltage they can handle though (...oh it's 36v right there on the schematic...) I have a bunch of L298N chips laying around that I bought to make some H-bridges for some robotics projects so I would just need to get the L297's and the right diodes and I could easily build that.

This also looks like it will micro-step which would work to my advantage.

I'll read through the datasheets of those two ICs too... maybe I will find the answer to my own question...

i just tend to use straight forward power supplys for steppers i.e transformer , rectifier and capacitor . rather than switched mode

as motors dont need anything other than basic regulation theirs no benefit , but if you have them to hand then use them .

to answer your question over " the circuit " my answer is no . however it is basic and it works . it does not have anything other than full step or half step switchable step, and basic regulation .

it all depends if you want to build your stepper drives or purchase some , the costs these days sometimes quite suprising for modern drives , can cost less or very close to the actual cost of components to make your own , but of course you don't have the fun of letting the smoke out in the process .

what type of machine are you looking to build and for what purpose ? , it may be that simple is best , it's all part of the learning process.
sometimes it's easier for us to forget where we started as young boys with Valves and a bunch of wires on the kitchen table .

i now think if only i had not played with them on the kitchen table , where would i be now ! .

cncbasher wrote:

it all depends if you want to build your stepper drives or purchase some , the costs these days sometimes quite suprising for modern drives , can cost less or very close to the actual cost of components to make your own , but of course you don't have the fun of letting the smoke out in the process .

what type of machine are you looking to build and for what purpose ? , it may be that simple is best , it's all part of the learning process.
sometimes it's easier for us to forget where we started as young boys with Valves and a bunch of wires on the kitchen table .

i now think if only i had not played with them on the kitchen table , where would i be now ! .

This captures what we are trying to do... enjoy the experience of making something uniquely our own. The machine will be able to do about a 12x12 carving area and hopefully will be built with a modular z-axis design that has the flexibility to swap-in an extruder and heated bed to convert it to a 3D printer or a laser for simple etching. We are not going to do anything commercial with it at all - just for fun and the occasional personalized gift, maybe do a custom PCB for a robotics project or two. My son wants to dabble in woodburning art also so we might experiment with mounting a soldering iron with an etching tip on there as well.

We are mainly taking design inspirations from instructables.com and the Zen Toolworks machines, as well as other simple designs we have seen online.

So far we've had a great time building the base and soldering-up the Step Cheap driver. For me, it is also somewhat therapeutic because I am still recovering from a serious illness I had 18 months ago and I am still not able to work full-time. So this keeps me busy and keeps my mind engaged when I am too exhausted to teach.

And, we were doing this on the kitchen table until my wife said enough to that and sent us to IKEA to buy a nice new workstation for the computer room/office!