First Project - Build a power supply

Other Hardware General Electronics
1

I have been playing around with the Attiny2313 for a while now and have been doing some fun things. Been doing things like driving a unipolar stepper, flashing leds and messing around with a motion detector. Over all its been a blast.

A couple of weeks ago, I got tired of running to Radioshack for one thing or another so I made a post on my empoyers website looking for old printers that people wanted to get rid. I ended up getting 4 hp printers and salvaged a bunch of various ICs, capacitors, dc motors and stepper motors. One of the interesting things is that all the steppers were 24 volt steppers. I dont have a 24 volt power supply and I was using 2 6 volt batteries in series for 12 volts for some other things. Since batteries are kind of expensive and I am getting tired of playing with 9volt batteries, I started thinking about getting a power supply. As I was sitting there staring at a large pile of destrowed HP printer plastic that I didnt want to clean up, I noticed the brick power supply for the printer. I flipped it over and read the info. It its output is 32volts and 1100 mA. It sure seems like that would be more than adequite for my needs.

So, I decided to try and make a useable power supply for my projects out of it.

I am a computer programmer by trade and dont know jack about circuits and what may be good or bad. I am considering building a box and mounting a big pc tower case fan on it. Then I was planning to wire up 4 voltage regulators to the output of the brick. I would wire up a 5 volt regulator, a 9 volt regulator, a 12 volt regulator and a 24 volt regulator and then mount them on something that would hold them in the airflow from the fan. The output from each of the regulators would be wired to a terminal that I would plug my verious experiments into. I would wire the fan to the output of which ever regulator matches the voltage it needs.

This leads me to a question. Is just wiring up the 4 regulators to the same source going to work? The last voltage regulator I put together, I based off a howto online and there were some capacitors between the supply and ground on both sides of the power regulator. I guess I am curious if there will some funkyness having the 4 requlators wired to the same source or will there need to be some additional components added to prevent some undesireable behavior.

I salvaged 2 power supplys. 1 from a newer hp All in on printer (not a single stepper in the whole darn thing either) and another power supply from a older hp printer. The power supply from the older one is an internal supply and does 32 volts and 900 mA. Its not a brick type power supply. Just a circuit board with some various things on it.

I have read some things about Linear vs Switched power supplies so I am also wondering if I will run into some problems if they are switched power supplies. I am not really sure if these are linear or switched. If they are switched, I was hoping that the pc case fan would provide enough load to prevent problems.

Once I get the power supply situation resolved, I going to be building some fun Halloween projects where I am using steppers and motion detectors to run some various animated Halloween related critters.

I will post some photos of what I am dinking with if anyone is interested.

So, what do you think? will these printer power supplies work well for a small bench power supplies. If not, I will try the ATX power supply to bench power supply conversion.

2

Is just wiring up the 4 regulators to the same source going to work?

Yes.

However, using linear regulators, dropping 32 volts to 5 volts, even at low currents, is going to get hot.

You would be much much better off investigating switched mode regulation. For the 5V you can just re-appropriate a car USB phone charger plug. For the other voltages you'll need to build your own circuits, or use pre-made all-in-one switched mode voltage regulators.

3

Could I cascade the 4 regulators? Tie the output of the 24 volt regulator to an output terminal and to the input of the 12 volt. The 12 volt to a terminal and the 9. Finally the 9 to the 5 . Spread drops across all 4. Would that help with heat? Would that introduce other problems?

4

It will spread the heat out, but not reduce it.

5

The OP was right completely but he missed something 1100 Ma @ 32V is 35 watts of available power. With switching regulators you can probably use about 95% of it... So it is a little weak, especially for motors or steppers/servo's. I have two suggestions... this time... 1. Find an old XBOX PSU, you should find several different voltages there 5V and +12V @ 10A (120W) and since the XboX has such a terrible history (Red Ring of Death due to poor heat scavenging/cooling) there are lots of them around, I found 2 free. Now you have some power to work with. 2. Switchers are available from Ebay for usually less money than a LM7805 costs at Radio Shack (or it's European equivalent). They are available in two modes Buck (makes 12V into smaller voltages 1.2 to 10V typ @ 2A typ) and Boost Mode which makes 12V into bigger voltages 13 to 32V typ @ typ 2A and all are adjustable and very easy to use. There is a large untapped resource in cellular chargers too. Mains to 3 to 6V and 12V to 3 to 6V out are real common and frequently thrown away because of bad connectors or in the case of the car chargers a blown fuse. If replacing the fuse doesn't work there still are some... usable parts left (if nothing else once the board is reworked a little you have a fused cigarette lighter adapter. I found an old Motorola mains charger and it had a 2.5mm coaxial barrel connector and I added a 2.1 mm 3' pigtail for a sweet little 5V 1A supply.
Scavenging parts and coming to understand them and their operation will be a large part of your theoretical electronics education, Fun too as you have discovered. Your 32V PSU, likely from an old HP Printer is small enough to make a single supply and still big enough to hide a switcher inside... ONE BIG CAUTION however. It IS A MAINS operated device so connect to NOTHING EXCEPT the OUTPUT + and - connections
If you look closely at the PCB inside you will see a 2 - 3 mm division in the pcb ground plane, one side will connect to the output ground, the other side is dangerous as it is the MAINS RETURN...If the Power plug is reversed in the wall or the plug wiring is backwards there is an electrocution hazard so always use a meter and measure both the negative and positive leads of the PSU to both leads of the Mains connection (unplugged) for continuity any resistance less than a megohm is likely to have issues that might be 'unhealthy'. Perhaps usable in an isolated situation but not advisable for a beginner. In my experience. There is also the option of buying 1A 12V switching power supplies that are great and with a little switcher make a great PSU for most smaller projects and can be assembled for usually about 7 - 10 dollars US. IMO
Hope this is useful stuff...

Doc

Doc

6

building a power supply is no easy task and even more critical for low voltage electronics.
I am an industrial electrician so i know it quite well
here is a site where you can get them at low cost http://www.automationdirect.com/adc/Overview/Catalog/Power_Products_(Electrical)/DC_Power_Supplies?source=google&keyword=24%20volt%20power%20supplies&type=search&gclid=COmJsNL9v7ACFYeo4AodHTkAWw

or you could look for one on ebay but I would recommend the above site

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Also about 4 X the cost of Ebay too... Deep pockets?, I save my money for things that do something, I have built and own more PSU's than I can use now... Think I paid for one of them... On Ebay, 12V 3A linear type for $10.00, primarily for nostalgia's sake. Most were scrounged, are thoroughly tested and very functional. I buy a lot of those switcher modules as they are Perfect Regulators both up and down. Don't get hot either. Scrounging parts is one of the Joys here. I would really like to hack a Wi-fi router sometime, reload the synthesizer and re-use it as a comm device some day, haven't so far because the modules are so very cheap... But I might get bored some day... Oh yeah and the hardest part of building a great linear supply is in finding a case for it, Priced any 'hobby' boxes recently?. Linears or switchers are very simple devices, Require a transformer... or not, doesn't matter. Any task is only as hard as you perceive it to be, Most people with abilities fail simply because they Believe they "Can't" do it, once you believe that you will never give it (whatever) a serious try... Simply because you BELIEVE...
This has been my experience in my life, If I don't believe I can I will never try... U?

Doc

8

I should have had a different title because I am not interested in building a power supply from scratch. I am trying to take some existing power DC power supply units and create an enclosure for them and use the output to provide power sources at different voltages so while I am sitting at the table working, I am not constantly fiddling with 9 volt batteries. My first project was a practical joke that I am taking camping. I built a suspended frame with 2 blue Leds. The suspended frame can be moved up and down by a 12 volt stepper motor. The intent is for the Leds to look like eyes. I also have a photo resistor that I use to sense when its dark and a the little recording module that Radio Shack sells. I have the 2313 which has the photo transistor hooked up to a pin. When the pins input changes from low to high, I trigger the voice recorder to play. (using a transistor as the switch). I have the sound of a ocelot growling recorded on the recorder. Every 5 minutes, I trigger the growl. Eventually I use the stepper to move the Led eyes to the top position and blink them on. It goes into a loop where it moves the eyes up and down, turns them on and off and growls. The plan is when I go camping with our friends I am going to set it in the weeds near the campsite. The purpose I bring this up is that while I was developing this I needed 1 9volt battery hooked up to a 5 volt regulator, 1 9 volt battery hooked up to the sound player and 2 6 volt batteries wired in series to provide the 12 volts for the stepper. I had this mess of gator clips wires and batteries and invariably I would forget to unhook a battery and it would get drained. It was a pain to clean up and a pain to keep track of everything. I was hoping to concoct a power supply to provide 5 volts for my target board (the 2313 Target Board from evil mad science) and then 9-12-24 volts for the things I would be driving.

Sounds like the idea I was going to try would work but I will have a limited amount of devices that I am going to be able to drive.

So, if I dont touch the innards of the power supply and cut the end of the wire that went to the printer off, shouldnt there be 2 wires? Docedison, I am a little confused about the what your saying about the Mains. The 1100 mA powersupply is one of the black brick power supplies (from 2006 time frame I think) and the 900 mA power supply has a connector with a black and red wire attached to it. Will I be dealing with the 120 v side at all?

-Steve

9

You shouldn't but measure as I advised and find an XboX green PSU with a couple of those Regulators I mentioned. If you like I can post the links to what I bought, that way you have Hard 12V and the little boards Will supply in excess of an ampere. With switchers it's the Watts - the Efficiency so if you need 5V @ 5A you are still in the useful power area of your brick but it is about 2.8 - 9 A @ 12V. I didn't do these calculations before... or I would have advised you differently. Double check for a current path to the mains and verify it is a meg or more
Worst case is 470 uA (4.68085 'Clipped for brevity' e-4) @ 220V ac and 470K (did I miss a decimal point???) so it won't really be an issue. My little boards work well with a 12V 1A 110V mains switcher, no issues with ground loops tween my scope, PC and PSU And Thats The Truth...

Doc

10

These are all good suggestions.

Another good solution is to use a PC power supply. A recent ATX supply will give you 5V, 12V (Your most used voltages), and maybe 3.3V, and -12V.

The next "Nice thing to have" is a variable voltage supply. You can use an LM317T type regulator with a potentiometer. It will give you an output of 1.6 volts up to the voltage you are supplying to it (minus a few volts drop).. So your HP 32 volt supplies would be fine for that. There are also small switching supplies that can be made variable. I'll try to point to one..

Main thing is do it.. you will learn stuff, and only have a couple of dollars of risk...

WARNING: Terry is always selling stuff:

You could play with these: http://goo.gl/2ZeK7

Just remember the bypass capacitors on the outputs...

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Hell of a thing, hadn't thought of it myself. But with the math I think him better off with 2 buck and 1 boost module all the parts probably cost about $20.00 - $25.00. That Sir would be a nice, nice project, 3 pots and 3 small DPM's (4.50 ea) and there is a really nice bench supply for an Arduino work bench. Might 'borrow' power from a PC too, disk connector has 5 & 12V virtually for free as you are using it not just powering it up for power only... but I would Hate to shoot my Computer in the foot when I 'accidently' just did so with my project and all that Magic smoke... <BFG>...

Doc