DC power supply....Build or buy?

[phantomcow2]

I have isolated all of my new problems for today to my power supply. This so called 24vDC power supply is giving a whopping 5volts as an output. Obviously this wont cut it.
SO now I am faced with getting a new power supply to run those stepper motors. HobbyCNC has a schematic for a power supply. In fact as i look at the photos section of their Yahoo group, most people seem to use this schematic.
I could do this, or I could just go to mpja or some place of that nature and buy a premade supply. Is there any reason to build it as opposed to just buy a premade one?

[DerekZahn]

> Is there any reason to build it as opposed to just buy a premade one?

The same reasons as for any other piece of the puzzle: Because you think it's fun, because you can't get exactly what you want (voltage, amperage) pre-made, or to save money.

The big decision if you buy one is whether to get a switching supply or not. Different people have different opinions about whether switching supplies work well for CNC applications.

[phantomcow2]

I dont entirely understanding this whole switching thing. Is there an advantage to it?
If those are the only reasons, i think i am better off trying to find a pre made that fits what I need

[Don Peterson]

The difference between a "switching" power supply and an "analog" power supply is rather simple. Think of a pipe carrying running water. If you open a valve a little bit, you get a small stream of water. This puts a back pressure on the valve between the input and the output while the water is flowing.

A "switching" power supply has a "valve" that is either all the way open, or all the way closed. By opening and closing that valve extremely fast (several thousand times per second), you can get the same small stream of water. The amount of water you get is determined by the "duty cycle" of how long the valve is open vs. closed. The advantage here is that there is little power dissipated accross the valve. When the valve is open "all the way" there is no pressure drop. When the valve is closed, there is no water flow.

It's the same with electronics. Power is equal to Voltage times Current. A power device dissipates very little power when fully on or when fully off. By adjusting the same duty cycle, you can control the voltage output.

The advantage of switching power supplies is that they tend to be more efficient and cheaper. One of the downsides is that they can be noisey.

Hope this helps....

Don

[ViperTX]

Well....why is the 24 VDC supply you have now only putting out 5 VDC?

I suspect that the HobbyCNC supply is not regulated. The MPJA one is probably regulated. It really doesn't make a difference for this application....base your decision on time spent acquiring / building the supply versus just buying one.

[JerryFlyGuy]

Don, is this what a person would call a PWM style supply? or is that something else all together?

JerryFlyGuy

[Don Peterson]

Yes. PWM (Pulse width Modulation) is used to vary the duty cycle of something. This could be used in a regulator circuit, motor controller, or even a form of D/A (Digital to Analog) converter. Some really early sound circuits used PWM for output of sound.

There are two kinds of PWM. One is where the overall frequency never changes but the duty cycle between the "on" and "off" change. The other is where one of the states (on or off) stays a constant time, and the other state changes in time. This has the added effect of changing the overall frequency because the overall period (sum of both states) changes. If you hear that "whining" sound coming from your motors change in pitch, that means that the frequency of the PWM is changing. This assumes that you have a PWM driver for either the speed control or the chopper. Both methods have their advantages and dis-advantages.

[philba]

If the power supply is just for stepper motors, then you don't need (much) regulation so a very basic PS will do fine. transformer, rectifier and capacitor. There is a long thread around here that goes through this in extreme detail - search for power supply.