[Joe Weldor]

I was given a Mean Well SP-300-24, It’s a 300 watt 24VDC 12.5amp switching power supply.
I am trying to power some small Johnson Electric DC motors that I would like to use on an engraving spindle. It runs the smaller motors, which I believe to be 12 volts and draw about 1.5 amps. But on the larger motors, 14 volt and 18 volt that draw 2 to 3 amps it goes into the hiccup mode. Is there anything I can add to this power supply in order to run these larger motors?

[bharbour]

It is probably a couple of things. A 14 volt 3 amp motor will pull up to about 5x the operating current during startup if you were running it on 14 volts. Since you are running it on 24 volts, that 15 amps becomes 24/14 * 15 or about 25 amps.

You could put a series resistor in line to limit the startup current to what the supply can handle and then when the motor is up to speed, short the resistor with a switch or relay. If you wanted to get creative, you could put the relay coil across the resistor and it would switch in at start and out as the motor spun up. You will have to mess with the right relay coil voltage to switch at the right time. When the job is done though, the motor will still be running on more power than it was designed for and will probably not last long.

It is probably a better idea to get motors rated for the current your supply can provide or a bigger supply. Motor operation is one place that the simple, unregulated supplies are better than a switcher.

Good Luck,
BobH

[Joe Weldor]

Hi Bob, Thanks for the reply. I guess that I had already given up on the SPS, your info has convinced me to go a different direction. My cnc hobby is a low budget operation, but I would like to ask if you know of any web sites I could check out for info on building an unregulated power supply. What size transformer, rectifier and other components along with simple wiring diagrams? To come up with a finished product that puts out 14 to 18 VDC with 4 to 6 amps.

[bharbour]

An unregulated supply is easy to build. For a 12 volt DC supply, get a transformer rated at about 9 volts AC and 1.5 x the current you want. For a 16 - 18 volt DC supply get a transformer rated at 12.6 Volts AC. Get a bridge rectifier rated at 1.5 x or more for the current you want to supply and at least the voltage you want to supply. Most bridge rectifiers will have much higher voltage ratings, this is ok.

For running a motor or other uncritical load, get a 5000uF electrolytic capacitor rated at 2 x or more of your output voltage.

Connect the transformer low voltage side (called the secondary) to the AC inputs on the rectifier bridge. Connect the capacitor across the + and - outputs of the rectifier bridge. Be very careful to connect the + side of the capacitor to the + side of the bridge and the - side of the capacitor to the - side of the bridge. Getting this wrong will cause a foul smelling explosion. The motor can be connected across the capacitor or the bridge rectifier + and - outputs.

The Line input side of the transformer (called the Primary) may be simply two wires or may be several, depending on how flexible it is.Refer to the manufacturer's data sheet for this one.

The transformer output voltage gets picked at less than the required DC voltage because the transformer AC voltage is measured as RMS volts, and the bridge rectifier converts that voltage to Peak to Peak. If you want to calculate it, the DC output voltage of the bridge rectifier will be about 1.4 x the RMS AC input voltage. The current rating of the transformer needs to be higher than the DC motor current partly for a little bit of over-rating and partly because the AC input current required to get a DC output current with a bridge rectifier is higher. I forget the conversion factor on this one, but it is probably 1.4 x.

The 1.5 x overrate on the rectifier current capacity just a safety margin. More is OK.

The capacitor voltage rating needs to be picked at 2x or more than your DC output voltage for over-rating. Capacitors are kind of an inexact science and an over-rated capacitor will last a lot longer. The capacitance value can be calculated based on the line frequency (50 or 60 Hz) and the output current, but 5000uF should give decent results.

I tend to buy new parts from reputable places, but it costs more. There are lots of places that will sell used or surplus parts at much lower prices. The risk is that you may not get what your are expecting. I like Digikey and Mouser for parts and both have fancy web catalogs. Places like Burden's Surplus Center have parts and often complete supplies at pretty good prices.

Good Luck,
BobH

[Joe Weldor]

Hi Bob
This is good info, thanks for making it easy to understand. I have a couple of transformers lying around that I think might work for this PS. I'm going to gather up the other components and see what I can come up with.
thanks again
Joe Weldor