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Hi, I really want to buy a power supply instead of using batteries to practice and debug electronic projects. Is there a cheap power supply that provide like up to 12 V that I can buy?
Hi, I really want to buy a power supply instead of using batteries to practice and debug electronic projects. Is there a cheap power supply that provide like up to 12 V that I can buy?
There are a number of ways to do this. As already suggested your local electronics store might be a good resource - depending on where you live. Jameco, among others, have kits. Some kits are quite complete yet others require that you put together the transformer, enclosure, etc.
You might consider the "wall wart" approach - the low cost black cubes that plug directly into the wall outlet. Quite often they serve as AC adapters for telephones, radios, etc. Note that some will be regulated and some will not. Usually they are fixed voltage though some can be had with variable voltages. Jameco has both regulated and unregulated. Leads will be long, relatively speaking so you need to add a capacitor or two at the point of use, to take out spikes, noise, etc.
You might use a higher voltage wall wart as a place to begin. An example might be one that delivers 24 vdc at 500 ma. You'd build a very simple regulator out of an LM317, a resistor and a pot - all easily obtained. That combination might deliver 2 to 21 volts or so at 1/2 amp - enough for many things. I'd add a fuse to limit the current. This approach takes high voltage mains work out of your hands. Many of these supplies are protected internally.
I did notice that Jameco has a wall wart type of 24 vac supply (part no 540744) that is rated at 40 VA (about 1.5 amps). This seems like a nice, safe beginning for someone who wants to build most of the supply but would prefer not to deal with mains voltages. You start with 24 volts.
My personal favorite for building a bench power supply is to hack up an old PC power supply. You can do it with either older AT or newer ATX power supplies. It's not much work, you just have to add in a switch, some binding posts, and a power resistor to keep the supply on.
I have been using my hacked bench supply for several years now and never had trouble with it. I even added an LM317 variable regulator with a pot so I have a variable voltage output as well (although limited to around 1A)
here's a guide I found with a quick google search, there are plenty of others if you look around:
Some comments on the computer power supply:
1. Good idea in that they are often quite available.
2. If you do it right the mains wiring is done for you so you can stay out of that kind of construction.
3. If the max output of the supply is 12 volts and you want to adjust from let's say 3 volts up to 12 volts there will be a dead spot because of the voltage drop of the regulator. For a 317 I think it's 3 volts - and let's just say it is for discussion. You'd have a nice smooth supply from 3 to 9 volts and would then have to switch to straight thru for the 12 volts. A low dropout regulator would reduce the margin but there will still be a dead spot. Maybe there's a way to boost the 12 volts to 15 so you end up with 12 after the regulator. This doesn't make it a bad idea and it may serve you adequately.
I have a few of these powersupplies left. They were created for experimenters. If anyone is interested in them, I am selling them for what it cost to build them. If you want one send me a private message and we can arrange the details.
These are the specifications:
This powersupply was designed for the experimenter, as it supplys 4 regulated voltages at one time. It operates from 120V 60 cycle AC, primary power. The maximum combined currents without overheating the power transformer is 600mA. The output voltages are +12V,+9V,+5V and -5V. They all share a common ground connection.
The +5V and ther -5V requlators are mounted on heatsinks and can each supply indivually 600mA on a continous basis.
The +9V can supply 300mA on an intermittent basis, but can supply 75mA on a continous basis as the regulator is not mounted on a heatsink.
The +12V can supply 125mA on a continous basis.
With this power supply most op amp circuits, digital, and speaker amplifier circuits can be powered at the same time, as all voltages are availible at the same time.
In addition to the power supply as shown, it comes with a two conductor power cord with a molded plug on one end and pigtail leads on the other end. The power supply is supplied with 1/2” threaded spacers, but the spacers will be removed from the circuit board for shipment.
Not shown in the picture is an LED power on indicator connected to the circuit board, and a on/off switch.
They also come with a schematic.
Whatever your aproach (build yr own, buy one), the one piece of advice I would give is ensure you get one with variable current limit as well as variable voltage control.
Whatever your aproach (build yr own, buy one), the one piece of advice I would give is ensure you get one with variable current limit as well as variable voltage control.
I agree. Unfortunately I don't have that on my homemade hacked ATX bench supply, and it's come back to haunt me a few times when a loose wire has strayed and made a nice spark or blown up some parts :lol: If you are designing your own switching supply, it's not as hard to do, since you know the schematic and you have access to all the various lines. If you are using a pre-made power supply like I have, it's not nearly as easy because unless you reverse-engineer it a bit, you won't have access to the necessary lines to add a proper current-sense circuit to the output/feedback loop.
You could potentially still add a very low-value current-sense resistor to the output, however the supply would be regulating to the point before the resistor so you would get some voltage drop across it that might be undesirable. With a normal ATX supply, the output voltage tolerance isn't extremely tight (I think the ATX specification is +/- 0.5v on the 12v rail, and I've seen it vary within that whole range with different loads) so the additional voltage drop might not be a big problem. The next problem is the actual limiting. With a proper design, you would simply drop the output voltage accordingly to clamp the output current at its limit. Without access to the feedback loop to the switching controller (or other over-current input on it) I don't know if you could really do it properly, and you might have to do something as crude as just disconnecting the output by latching on a relay until reset (basically making the whole thing a variable current limit circuit breaker)
Does anyone have any clever ideas that I have overlooked for adding a decent current-limiting feature to the output of an existing switching PSU, without having to reverse-engineer and hack the actual PSU circuit?
Look in the LM317 applications data and you'll see a current limiting feature that can be added on. I do not know how much voltage drop you'd see. It's probably more suited to delivering constant current but my thought was that if you were delivering less current that it would not be in limiting mode. I probably will give it a try myself sometime.
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