Switch-mode power supplies

I have been thinking on a power supply for the CNC. I was going to tackle a Linear power supply but that’s too much work and by time I’m finished it would be too big and heavy from a beefy transformer. So comes to mine a switch-mode power supply. If you didn’t know computers use switching power supplies. Starts out kinda Linear like but in a nut shell uses a form of PWM (Pulse Width Modulation). Lets you have more power in a smaller package and has more bells and whistles such as over current protection and so forth.

Lets have a quick rundown on a typical PC ATX switch-mode power supply. I’m not talking about a Cosair 1000watt dual 12volt rail, just a typical one you would replace when upgrading a PC.

They’re typically dual channel/rail. One rail for 12v and one for 5v. This was the standard for the much older AT style and for ATX they tossed in a 3.3v but it is tapped into the 5v rail with ether a linear voltage regulator or some sort of rectification. Over time other stuff was added such as the +5VSB (5 Volt Standby) that even when the system is completely off 5v is still surging around the power supply to ether keep CMOS in check, Wake on Lan running or for today powering USB devices such as keyboards. There is much changed in ATX 1.0 and ATX 2.0 specs other then four extra pins were added from 20pin to 24pin. If you want to know more history on it check out Wikipedia.

So if you want to convert a switch-mode power supply for a project here are some key items to look for. Even if it is a cheap power supply and doesn’t have them all isn’t lost and you might be able to add them.

MOV (Metal Oxide Varistor)
This little blue like disk suppresses voltage spikes. Pretty much the same thing used in power surge protectors. There are two of them connected to each Live and Neutral lines to Earth ground. I typically keep old ones from stuff I tear down. Sometimes a supply will have a ton of them. Before rectification and after. Sometimes for each rail/channel. You can get away from using a surge protector but I wouldn’t recommend it since they don’t handle brownouts, over current from a short circuit.

Transient Filter
It’s a filter to suppress noise from the switching transistors and noise in the mains AC line. I’ve came across a couple of PSUs where this filter was not available and did mess with other equipment such as a TV and made the video a tad snow like and added a audible tone from the speaker.

the problem with PSUs is that their switching transistors produce EMI/RFI that could seriously affect other electronic devices in the house. Also we must protect the PSU from incoming noise and voltage spikes coming out of the power grid, so the role of this stage is twofold and serves as protection in both directions.
Noise can be classified into two types, according to the conduction mode: Common Mode Noise (CMN) and Differential Mode Noise (DMN).

  1. CMN is electrical interference with reference to the ground or common wire. It consists of high frequency spikes and comes from faulty wires or from EMI/RFI of nearby devices. Common mode choke coils along with Y capacitors are used to suppress CMN.
  2. DMN represents the noise that is measured between two lines with respect to a common reference point, excluding common-mode noise. To suppress DMN, X capacitors are placed across the lines.

The EMI/Transient filter in PSUs is always placed before the bridge rectifier, because in this position it also suppresses the noise coming from the bridge’s diodes (yes, even those produce noise, especially at the moment they are turning off). The necessary parts for a proper EMI/Transient filter are two Y and two X capacitors, two coils, an MOV (Metal Oxide Varistor) and a fuse. Very briefly an MOV is a voltage-dependent resistor that protects the PSU/system from voltage spikes coming from the power grid. However, especially in low-end PSUs, manufacturers omit some components in order to save money. Usually the first to be left out is the MOV. If your PSU does not have an MOV in the EMI/Transient filter then you should always operate it along with a surge protector or a UPS, otherwise a spike could damage permanently not only the PSU but your project too. There is typically a Thermistor in the mix as well. After the EMI/Transient filter an NTC (Negative Temperature Coefficient) thermistor is usually used to protect the other components from large inrush currents. A thermistor is simply a resistor that adjusts its resistance according to its operating temperature. The resistance of a “cold” thermistor is usually 6-12 Ohms and after the start up of the PSU the thermistor heats up and lowers its resistance to approximately 0.5 to 1 Ohm.
High efficiency power supplies use a relay that bypasses the thermistor after the PSU starts up, in order for the thermistor to cool down and operate normally in a hot switch restart (off/on) of the PSU. Also by bypassing the thermistor efficiency is improved a little bit since no energy is wasted by heating the resistor.

Most decent PSUs have a fuse. However cheap ones from China like to skip this and you can’t really add a fuse to the PCB of the supply but you can add one to the Live lead or a fuse to the device you want to power. If you came across a PSU with a blown fuse I wouldn’t recommend on using it because there is a chance it will blow again when replaced since chances are there is something else dead or dieing in the PSU.

These PSUs use a ton of electrolytic capacitors. If it is a older unit check them out visually to see if they have burst. If the top of a electrolytic cap isn’t flat then it needs replacing. I prefer to use Rubycon brand caps. When re-capping make sure to use the rated size. You can go higher on the voltage but don’t go under. So if you needed to replace a 1uF cap rated at 25v you can use a 1uF cap rated at 50v. However these can be a little fussy so try your best to stay within the voltage rating.

That is what I look for when choosing a PSU for projects. If they do not meet my preferences I’ll see if I can make the changes and if not I’ll just strip them down and use the parts for other projects. Sometimes you can get some nice mosfets and voltage regulators from these, not to mention resistors, diodes and caps.

To make things clear you can’t really do much with switch-mode power supplies parts such as the transformers and a few coils because unlike a typical transformer they use different signals on the output.


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