Practical Difference Between Linear and Switching Power Supply

[Brenck]

Hi all,

I know the difference of switching and a linear power supply, but in therms of cnc whats the practical difference between them? Is there any of them that is better to drive the motors? If so why?

Lets take kelinginc PS:

Switching

KL-350-48 48V/7.3A
110V/230V $59.95

Linear

KL- 4875 : $99/pcs Specification:
Unregulated Power Supply 350W, 48DC/7.5A , Input: 120VAC
or 230VAC

Those are basically the same specs the 200mA difference between them is nothing.

Why would you want to pay more for the unregulated one?

Thank you all

Gustavo

[Al_The_Man]

They will both work to virtually the same extent in spite of their obvious differences, but the reason I personally use linear is the fact they appear more rugged in practice and are much easier to fix, should they fail at all.
Often the parts used in S.P.S. are either house numbered or unobtainable, if a fix is attempted.
It may cost more initially, but you only have to buy one complete replacement and this is now costlier.
Al.

[Brenck]

I heard that S.P.S have more chance of causing interference to the stepper cables is that true?

Is it going to be a big deal if I use 48V/7.5 or 48v/7.3 to control 4 - 3amps motor with a G540?

With the right calculation, according to gecko's, I was suppose to have 8A but 7.5 is pretty close and the 4th axis will be a rotary axis that I wont use much... Most of all I'm just making the 4th axis to see how it works, all the 3 machines I built were 3 axis, I want something new...

Thank you Al

[vger]

As long as the SPS is working properly there should be no problem with interference. Just make sure you ground the case on the switcher. I prefer a good linear for the same reasons that Al stated.

A fourth axis huh? I like mine... I usually keep it in the C axis orientation but ocassionally move it to A axis with a tailstock.

Enjoy
Steve

[neutronics]

I work for a company that manufactures high-power RF amplifiers. We use Meanwell switchers in most of our equipment and RF interference has never been an issue. The only time we have used linear supplies at work is for extreme current (over 100 amps).

I use a no-name switcher for my ham radio equipment at home and it does make a lot of noise. So I understand where you're coming from in that regard. I believe the noise mainly eminates from the power output cables - the noise originates within the power supply and is "transmitted" via the output cables. I haven't seen a schematic for a Meanwell but I suspect they use an RFI filter feedthru to prevent noise from making it to the output cables.

At the office we've never had problems servicing switching supplies because most of the issues stemmed from common solid state components. However, we have had problems servicing linear supplies in cases where the transformer was bad.

[Andre' B]

Originally Posted by neutronics The only time we have used linear supplies at work is for extreme current (over 100 amps).

Are those regulated supplies?

I would think that the main difference between regulated switching and linear supplies would be effiecency and at higher currents the heat sinks on the linear supply are going to get very big.

[lgalla]

Gecko recomends not using a regulated switching power supply.If used one must put a large cap on the output,10,000 uf or so.

[eldata]

Originally Posted by Brenck Why would you want to pay more for the unregulated one? Gustavo

SPS advantages: more compact, lower cost and essentially constant voltage operation.

SPS disadvantages: Some cannot handle back EMF voltage well, some have shutdown or hiccup overload protection which would be undesirable for CNC operation. Not easy to repair.

Linear unregulated advantages: Easy to repair, more robust - no issues with back EMF or transient overcurrent scenarios.

Linear unregulated disadvantages: Less compact, higher cost (transformer alone can cost more than a comparable SPS), noload voltage is typically 10V or so higher than full load voltage.

Keling's SPSs handle back EMF well and use constant current limiting for overload protection. It would be my choice here.

[Al_The_Man]

One other advantage to linear unregulated, especially if Toroidal transformer is used is the ease of which you can tailor the secondary by reducing or adding turns, or if you need a small auxilliary supply of 5v~24v it can easily be added by overwind.
Al.

[eldata]

Originally Posted by Al_The_Man One other advantage to linear unregulated, especially if Toroidal transformer is used is the ease of which you can tailor the secondary by reducing or adding turns Al.

It may be easier to modify a linear unregulated PS, however, if the objective is to fine tune PS voltage then any unregulated PS is at a disadvantage compared to a regulated one (linear or switching) in this regard. Example, just about all the popular SPSs below 50V offer an exterior user adjustable trimmer pot for voltage adjustment. The adjustment range is typically 8 to 10V even for those stating plus/minus 10%.

Originally Posted by Al_The_Man or if you need a small auxilliary supply of 5v~24v it can easily be added by overwind. Al.

I have three homemade Toroidal based linear unregulated PSs here and could never justify having to modify the transformer. One of the following approaches has always made more sense to me;

Digi-Key - LM317TFS-ND (Manufacturer - LM317T)

Digi-Key - 296-20687-5-ND (Manufacturer - TL783CKCSE3)

Digi-Key - T983-P5P-ND (Manufacturer - EPS120050-P5P)

[Al_The_Man]

Originally Posted by eldata I have three homemade Toroidal based linear unregulated PSs here and could never justify having to modify the transformer. One of the following approaches has always made more sense to me; [/url]

Agreed they would work in some cases, but if you want to come down from say 80v to 5~24 they are power wasters also you do not have isolation if you needed it, many times I just need the 24vdc for control circuitry where regulation is not required.
It depends on your application.
Al.

[eldata]

Originally Posted by Al_The_Man Agreed they would work in some cases, but if you want to come down from say 80v to 5~24 they are power wasters also you do not have isolation if you needed it, many times I just need the 24vdc for control circuitry where regulation is not required. It depends on your application. Al.

Gecko uses the TL783 to come down from 80V in their G320 rather than have the user supply another voltage level for electronics. This probably applies to any drive requiring a single voltage so the waste you mentioned is often the norm.

Personally, I would use the wall adapter approach or buy another small transformer from a surplus store rather than get into calculations and winding modifications if isolation is needed (assuming a suitable Toroidal with a low level control winding was unavailable). So personal preference seems to be a factor as well.

But I'm glad you mentioned 80V since for voltages above 50V the linear unregulated PS seems to be the only sensible game in town. Below 50V and the SPS starts making more sense IMO.