Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: comet on March 11, 2012, 12:21:59 PM

Title: switch mode or linear?
Post by: comet on March 11, 2012, 12:21:59 PM
Hi I have always built my power supplies with toroidal transformers and whacking great caps,2 reasons firstly because you can suck more instant power out of a toroid and secondly ive been told that switch modes can give of some frequencies that can mess with servo drives ect. But, swich modes are very competitivly priced if your looking for 24 or 36 volts. Can I just put a whacking great cap on the out put of one to make it more suitable?

Title: Re: switch mode or linear?
Post by: Tweakie.CNC on March 11, 2012, 01:18:36 PM
Hi Tony,

You would need to check with the makers of the switch mode power supply to be certain but it is my understanding that it is not recommended to do this.

Title: Re: switch mode or linear?
Post by: HimyKabibble on March 11, 2012, 01:31:17 PM
Putting a large capacitive load on a switcher is likely to degrade it's performance, and won't provide any real benefit on a well-designed switcher.  Personally, I would stick to linear supplies for steppers or servos, but switchers can be used.  and "whacking great caps" are not always a good idea.  They should be big enough to provide an acceptable level of ripple, but no more.  The Gecko site has lots of good advice for *properly* sizing and spec'ing power supplies for both steppers and servos.

Ray L.
Title: Re: switch mode or linear?
Post by: comet on March 11, 2012, 01:35:29 PM
Yes i read putting a large cap across the output draws to much inrush current and can lead to the switchmode kicking out.
Title: Re: switch mode or linear?
Post by: Jeff_Birt on March 11, 2012, 08:08:07 PM
I've never been sure why some folks think using a switch mode power supply is a bad idea. I have a 20 year old bench top mill that is still using the same switch type power supply as when it was new. I have also been building G540 based stepper drives for about three years now using a good quality switch mode supply with zero problems.

I suspect the root of the issue is that there are a lot of cheap/crappy switch type power supplies out there. There are several companies in the east that make MeanWell knockoffs. These look like the MeanWell but are usually full of cheap parts and typically will not have the longevity of a good quality unit.

For small machines a good quality switch mode supply will cost about the same as building up an unregulated supply but the switch mode will be smaller and more efficient. On larger machines it can be a significant savings to use a simple unregulated supply. (Well upfront cost might be lower but long term it can turn out to be less expensive to operate the more efficient switch mode supply.)
Title: Re: switch mode or linear?
Post by: GytarrMann19 on March 13, 2012, 02:41:49 PM
The servo or stepper drive that you're powering should spec acceptable AC ripple. Usually it seems to be around 10%. Easy to check if you have a digital multimeter- just set it to AC volts and check across your DC supply. I.E., if supplying 90vDC to a drive, you could have say 9vAC present. Be sure to check both loaded and unloaded. As the current draw increases, the filter cap will start to deliver more AC ripple.

Title: Re: switch mode or linear?
Post by: titchener on March 14, 2012, 06:05:59 PM
I use a 800 watt 48 volt switching supply to power the 3 size 42 stepper motors on my BP clone mill.

I put a large cap on its output as I was concerned about too much voltage rise from the "back driving" of the supply that occurs when the motors are decelerating.

Its been working fine for over 6 years now. I don't believe that putting a large cap on a switcher will cause a huge input current inrush as most switchers have a maximum output current they liimit themselves to at high load, and while the cap charges the supply will be at this max current but its not a huge current spike, in my case its likely the current required to supply the 800 watts the supply is capable of generating.

I guess its possible that a really cheap switcher might have problems with a big cap but I doubt that any half decent switcher will have this problem.

Paul T.