Gallery, Projects and General > How to's
How to Make a Power Supply
Darren:
As promised I am putting together a post on how to make a variable power supply suitable for many Madmodder projects.
In particular this one is being built as half a unit for my Valve amplifier which requires 10V at 10A. But along this journey I shall be showing how to make a supply for many projects including machine power using low voltage motors.
Such motors include 12V car windscreen units, stepper motors, 24v Garage door openers which is the type shown in this tutorial.
One of the hardest specifications to find in off the shelf power supplies is higher current outputs as most commercial supplies are very limited in this regard. That is what we shall be dealing with here. Commercially you would need to have very deep pockets for a 10A supply in the order of 100's.
The 24V garage motor chosen for the powering of the mill bed is quite efficient at less than one amp current draw, my car windscreen motor is at least double this if not more.
But, if you are going to the trouble to build from scratch then you might as well build a good-un. This supply will eventually be able to handle 25A but for now we'll concentrate on around 4A output (the limit of this transformer) with a voltage range of 1.5-30V.
So lets get on with it and see the basic components required
In this first picture we have the transformer at the top, this is a 12/24V type at 50VA. ( that's volts x amps =50, so at 12V it has roughly 4amps and at 24V it has 2 amps)
Middle row from the left:
a 5KOhm pot or potentiometer (same as a volume control on your stereo) This one is a fancy 10 turn type (ten turns from zero to full instead of the normal one turn)
Just use a cheap one turn type. less than a quid.
Next is the voltage regulator chip No' LM338T, this is a 1.2V-37V variable type
3rd is a standard resistor, in this case 100 Ohm, 120 would be better but I don't have one. The value is not that critical for a variable voltage supply, something close will suffice. If you were making a fixed voltage output then this would need to be exact.
Next we have the rectifier, this one is four diodes in one package, we could use four individual diodes to do the same job. I find this tidier and easier. less to think about...
Finally at the bottom is something to build it all on. You could use almost anything but beware of strip boards, their thin conduction strips will not be able to carry the currents in this application.
Believe it or not this is all you need to build this supply as I am about to demonstrate....
More coming, the supply is built and tested :thumbup:
Some links
LM388T Voltage regulator, I bought mine for about a quid each off ebay
http://www.national.com/mpf/LM/LM338.html
Rectifier, you can find these on Ebay easily enough, mine is 25A but 6A would suffice for the mill drive.
http://www.datasheetarchive.com/D25XB60-datasheet.html
Darren:
Lets start putting this together,
Place the voltage regulator and rectifier as shown
Then with a pair of long nose pliers bend the leads of the resistor similar to this. Beware, do not bend the leads close to the body as they usually fall off if you do !!
It goes here between the left adjust and middle output leg of the regulator. The resistor itself has no orientation.
Then add a link from the positive leg of the rectifier to the input of the regulator. I used a wire from some household twin and earth cable.
Note: the middle two legs of the rectifier are for the AC input from the transformer. The outer two legs are + and -, but yours may be swapped around in relation to mine. Just be aware and connect accordingly.
Things will be getting a bit wobbly by now, so time to solder some parts in place. Although the regulator is a fairly tough device, try not to heat it up too much with the soldering iron. In other words get in, melt the solder and get out.
Prepare a pair of wires to connect to the transformer. I find with multi-strand wires it's often easier to tin them first. (pre-solder)
Solder these wires to the board to the AC connections on the rectifier. Again there is no orientation.
Add a wire link on the left as shown. This is the DC negative rail.
The grey wires shown here are connected to the pot and the black and red go to the motor.
I'mm missing a picture, I'll be back....hold on..... :doh:
Brass_Machine:
Awesome Darren! I was going to PM you this week because I have to build a power supply. I have to build one for THIS. Can you assist with the design? Or will this work (your thread).
Eric
:nrocks:
Darren:
Eric, This supply is perfect for your needs. As I said this is a very handy supply to have around as it has so many uses :thumbup:
Darren:
The grey wires connected to the pot, only two wires used. On a normal pot (unlike this one shown as it's the only 5K I had to hand) use the center and one outer connection. If the pot works the wrong way around use the other outer connection.
One grey wire goes here, the other to the negative rail.
The red wire goes to the motor, the other motor wire also goes to the negative rail
All you need to add now is a heat sink, I used some alluminuim strip, black as it's anodized but there is no requirement for this. You could use steel, brass etc, just don't make it too small. A propper finned heat sink would be better. Computer heat sinks or one from a salvage hifi amp would work well.
Be aware that the casing of the regulator is at full positive output voltage. Do not short this to earth. I have used an insulation kit to isolate the regulator it from the heatsink. The rectifier is already insulated so no need to worry about that. Some heat transfer paste would also be a good idea, it's the white stuff between the components that you can see oozing out.
I have attached a voltmeter and a temperature probe to keep an eye on things whilst testing.
The highest heat generated is at the lowest voltage output as the regulator gets rid of the unwanted voltage in the form of heat. As you turn the voltage up to increase the motor speed for example, more voltage/energy goes to the motor and everything runs cooler.
If you find you use the lower voltages all the time and have no need for the upper end then use a 12V transformer. Gives the regulator less work to do.
However, I found that even a stalling speed of the motor for over an hour the regulators temperature never rose above 90 deg C. it's rated at 125C.
The reg has a safety feature in that it will shut down to protect itself if it's pushed too hard or gets too hot (same thing as far as the reg is concerned) It will operate again when cooled a little.
So there we are, that is all that is required to make a variable voltage power supply of 1.2-30V, or up to 37V if the transformer had a higher output.
If you want just 12 or 18V then just use a lower voltage transformer, no circuit changes are required.
Here it is at 1.3V during the punishment test
At 4.3V
30V
You can by all means use this circuit as is, but we can make it better. In the next installment I'll be showing how to make the output smoother and how to protect the chip from short circuit accidents. You might not think you need this cos you are going to be careful right :thumbup:
Why do you think I had to order some new regulators to make this post :doh: Yep I popped a chip.
So if you bear with me I'll be showing you how to add two small and cheap components to prevent this :thumbup:
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