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Power Supply.


Powersupplies for CNC tables contain high voltage and can be dangerous if not constructed carefully. The details show here is for your information only and does not take into account your local voltage or safety requirements. I have no control over your skills as an electrician and therefore offer no guarantee of safely. I recommend ......

  • If in doubt you contact a qualified electrician to check your wiring before applying power.
  • Keep children and pets away from exposed power supply components.
  • Follow your local rules and regulations for main voltage wiring.
  • BE CAREFUL. Think twice. Keep one hand behind your back when testing.

Stepper motors need a good meaty power supply capable of supplying several amps. My original supply was a transformer from a discarded UPS. UPS's are a great source of power supply electronics, they usually contain a rugged transformer and some heavy duty power mosfets. Although the transformers in these UPS are designed to run in step up mode ( 12 volts to 330 volts as an example ), they will run the other way around as a step down transformer. I had a transformer from a 24v UPS, and when I connected this back to front and fed 240VAC into the secondary, I had 16VAC on the secondary. After connecting a full wave rectifier and filter capacitor I ended up with about 22VDC. This power supply could supply several amps and it did work ok considering it cost very little to make, mostly spare parts from the jump pile. If I were to build another powersupply based around a transformer, I would spend the money and buy a good toriod transformer, they run cooler and are more efficient.

You can pick up a new cheap battery charger for under $40, and they come with a transformer capable of up to 5 amps. For safety add a double pole single throw switch ( DPST) and fuse. The capacitors should be rated at twice the output voltage. To work out DC out from AC in, based on a full wave rectifier, you multiply the AC voltage by 1.4 to get the DC voltage, ie 16vac times 1.4 equals 22.4vdc.

While my 22vdc supply worked, I knew I needed a higher voltage to get the most out of my stepper motors. The higher the voltage the better, so long as its within the capabilities of the stepper controllers. My stepper controllers had a maximum rated voltage of 24VDC, so I really needed a power supply capable of supplying close to 24VDC at several amps. Unfortunately I didn't have any power transformers in this range, so I looked at another option, PC Power Supplies ( or PS for short ).

The humble old PC PS is a great little bugger. They are tough, in that they can supply lots of amps with short circuit protection, they have their own cooling fan, and are dirt cheap for what they are. There are two main types of PC power supply, the AT and ATX. The older AT supply used a true power switch, the ATX used a power soft-start provided by the PC's mother board and used a different motherboard connector. You can use either type for your CNC project, but you need to make a small modification to fool the PS's into thinking they are powering a PC.


Useful Links  
>> PC Powersupply Pin Connections
>> How to make a bench power supply with a PC power supply

AT Modification. Connect a 47ohm 1 watt resistor across the 5VDC output ( Red and Black Wires ). This provides a small load to the power supply to make it operate correctly.

ATX Modification. As well as the 47ohm resistor across the 5VDC output, you need to fool the soft start function into turning on the power supply. To do this, connect pin14 and pin15 together on the mainboard connector.

I had a couple of the older AT type PS's, these could supply 8 amps on their 12VDC output. If I connected these two PS's in series, by connecting the yellow wire from the first PS to the black lead of the 2nd PS, I would have 24VDC at 8 amps. But before we get carried away connecing power supplies in series, we need to make one important modification.

The 0 volts ( black wire ) on these PC PS's is connected the the metal case, its earthed as it were. This is fine for a single power supply in a PC, but if we want to connect PS's in series we need to isolate the metal case from the 0 volt wire. If we dont, and the metal cases touch, then we will be shorting out one powersupply and it will shut down ( or may be damaged ). Even if we manage to keep the metal cases isolated, the 240VAC power lead will still earth out both cases. Now dont go disconnecting the mains lead earth pin, thats not only illegal but dangerous. Instead we just need to isolate the 0 volt lead in the 2nd power supply. The physical connection is usually under one of the screw that holds the PS circuit board in place. You could either cut the copper track leading to this pad, to use a insulated washer under the pad and leave the screw out. Either way, make sure you test there is no electrical connection between the black wires and the PS's metal case with an ohm meter

Final note, the Kill Switch, or E-Stop. Every CNC table should have a kill switch of some shape or size. Basically, the kill switch is a easy to access, press to dissconnect switch that will remove all power from the machine.





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