Power Supply Schematic - Page 1
Forum section: electronics / beginners

author: jobog
author_rank: Regular Contributor
timestamp: March 09, 2015, 04:51:46 am
content: Attached is my plan for a power supply to use with a cheap Chinese function generator off of Ebay that requires plus and minus 12 volts and plus 5 volts. Any suggestions would be appreciated. Waiting for the regulators to arrive so I can breadboard. Also have a question about the orientation of the electrolytics, especially C2. Circuits I have researched online show this orientation. I'm assuming that ground is a virtual ground and the 7812 IC is getting the positive pulses and the 7912 IC is getting the negative pulses. Anxious to see what the scope shows after I get the regulators.

author: jobog
author_rank: Regular Contributor
timestamp: March 09, 2015, 04:53:44 am
content: Forgot to ask, do I need any kind of protection on the output side?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2015, 05:19:43 am
content: You may want a diode from negative to positive on each, to prevent any one supply from being reversed; you can also add a diode from output to input (across the regulator), though since nothing else connects to the main filter capacitors, this would only be important for the 7805.14V is kind of oddball; you may find 15-18V transformers are more common. In which case, mind the excess capacity (more VAs) and heat dissipation (in the 12V regs) required.It's too bad that 12VAC is just barely marginal, but vastly more common.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 09, 2015, 05:37:51 am
content: Hey nicely drawn, but did you forget to draw a junction between the gnd leads of the two 12V regulators and the center tap of the transformer?This connection must be made, there is no "virtual ground" here as I understand it, just the center tap of the transformer.Is 14V an AC voltage? Just to be sure. It is a 28V CT transformer, right?What is the expected current draw here? Don't forget heatsinking, and C2 is correct. Good attention to the pinout of the negative regulator.The scope should show 120Hz ripple on each regulator input, if you're drawing current, if not I'd guess you'll see something like an ~18.5V DC line.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2015, 05:53:46 am
content: Quote from: Alex Eisenhut on March 09, 2015, 05:37:51 amIt's a dual tracking regulator...Yeah, worth noting: on schematics, it's good practice to stagger junctions, so only three wires ever meet as a connection. Then four crossing (without a dot) always means a crossing and not a junction. The junction dot then becomes redundant... though unappealing when left off, if I do say so myself.Tim

author: jobog
author_rank: Regular Contributor
timestamp: March 09, 2015, 06:09:16 am
content: Transformer is a 28V center tap and yes, I did forget the dot for the ground on the 2 regulators. I was planning on heat sinking the 3 regulators. Not exactly sure what the current draw is but I think only a couple hundred milliamps so shouldn't be much heat. I will think about adding the diodes. Not clear to me yet what the diodes would do so have to think about it for awhile.Thanks for the responses so far.Joe

author: rdl
author_rank: Super Contributor
timestamp: March 09, 2015, 07:41:52 am
content: Protection diodes aren't always needed. One case where a protection diode is used is when there is a lot of capacitance on the output. When the power is cut off, that capacitance has to discharge. The diode diverts the discharge around the regulator. Without the diode the discharge path is back through the regulator, which isn't good and can cause damage. If you google "regulator protection diodes" you'll find all kinds of info.

author: picandmix
author_rank: Frequent Contributor
timestamp: March 09, 2015, 09:59:34 am
content: Do look at the datasheet for those regulators, they show that you need some small caps on the inputs as well.Notice that the case of the 7912 is not at the same potential as the 78 regulators so if sharing the same heatshink it requires an insulator.As said you need to add fuses to the transformer input and the 3 output lines to protect things, you could also use the resetable dc polyswitches on the outputs.While a selection of fixed lines are always handy , two other very useful features are a variable voltage supply and a current limiter which restricts the current to your project so protecting things if you have a serious short etc.There are loads of such circuits around but some might be a little advanced for you now, however recently on the market is this little B3603 module that does just about everything 0 to 30v, 0 to 3A all variable and for the price typically GB£10 its a steal - see links below.You will find it on ebay and this post has some great pics of it, though don't be concerned about all the11 pages of entries, they are just so amazed at what it is and how it does it for the price.Its only draw back is that it cannot be neatly mounted on a control panel as all its leds and switches are at differing height and some mods would have to be made for it to look good.https://www.eevblog.com/forum/reviews/b3603-dcdc-buck-converter-mini-review-and-how-the-set-key-could-be-fatal/http://henrysbench.capnfatz.com/henrys-bench/minghe-b3603-user-manual-table-of-contents/I recently build a psu with fixed regulators and also a variable voltage Lm338 but in use soon found that a current limiter would be a handy thing, so rather than rehash my pcb etc I just fitted one of those b 3603 modules, it works well.( you can also design the Lm338 or LM317 to act as variable voltage /current regulators but perhaps a little advanced for you at the moment, though this L200 design might suithttp://www.circuit-finder.com/categories/power-supply/dc-power-supply/219/a-current-limiting-bench-power-supply)You do not have to add any of the above to your fixed psu now, but I would certainly leave space to fit one in the future, I'm sure you will appreciate one of them as you advance.

author: JoeO
author_rank: Frequent Contributor
timestamp: March 10, 2015, 01:54:14 am
content: For someone to judge if your design will meet your needs, you have to tell us the current you want from each output and the current of the windings of the transformer.

author: Teledog
author_rank: Regular Contributor
timestamp: March 10, 2015, 06:07:00 am
content: Perhaps an old ATX PSU might be easier?Make a front panel with banana jacks ..and you have all the voltages you require and more...{KISS}

author: mariush
author_rank: Super Contributor
timestamp: March 10, 2015, 11:57:28 am
content: What's the power consumption on those +12v and -12v rails and the 5v? 7812 can only output 1-1.5A and you also have 7805 connected to it which will eat some of the current... Besides that, the 7805 would dissipate a lot of energy as heat if your generator draws 1A or something like that.You should check how much power each rail uses first.If the consumption is low, it may be worth using a transformer with a single secondary winding and create the -12v with an inverting regulator.If you put the whole psu into metal case or something that would prevent your psu from radiating noise into your signal generator, you could use a MC34063 or something similar to produce the something like -16..-14v and then use a 7912 to get a smooth -12v...If you want to stick strictly with linear regulators, the schematic looks ok. I would consider adding 0.01uF ceramic capacitors at the inputs as close as possible to the Vin pins. Diode protection is needed if you use a lot of capacitance at the output, or if you use the regulator with some applications like powering motors/fans etc ... most datasheets will tell you that.

author: Asim
author_rank: Regular Contributor
timestamp: March 10, 2015, 12:22:47 pm
content: I think he is planning to use this kithttp://www.ebay.com/itm/Latest-DDS-Function-Signal-Generator-Module-Sine-Triangle-Square-Wave-Digital-/291399023398?pt=LH_DefaultDomain_3&hash=item43d8bc3b26the 5V will power the Atmega & LCD. the +-12V will power the op amp. so the current shouldn't be high.

author: jobog
author_rank: Regular Contributor
timestamp: March 11, 2015, 12:27:08 am
content: Thanks again for all the responses.The Ebay kit is exactly what I will be powering with this so I don't expect the current requirements to be too much.I will be adding some small capacitors at the inputs and possibly the diode protection. Not sure the diodes are really necessary for my application. I did consider using a PC power supply but I don't have an old one and one would cost around $10-12. I figure I can build this for less than half that and get some experience at the same time. My biggest expense has been the transformer and I lucked out with Radio Shacks recent demise. Our local store is closing and I was able to pick up the transformer for 70% off. I had the bridge rectifier and capacitors in my parts bin and the regulators were a little over $1.00 for all 3. I will post a new schematic as soon as I have time to update it.I have been breadboarding the first part of this project while waiting for the regulators to arrive and I obviously don't understand the measurements I am getting. I have the transformer connected to the bridge and 100K resistors as the load on each side.Transformer is not 28V center tap it is only 25.2V center tap.Primary resistance is 66 ohms.Secondary resistance is 3.6 ohms so 1.8 ohms from each leg to the center.Primary current measures 22.7uASecondary current is 9.8uA on the positive side and -4.6uA on the negative side. Switching the resistors gives the same results.Secondary voltage is 29.3V and 14.7V from positive to center and -14.7V from negative to center.Voltage across the bridge after rectification is 25.1VDC and + and - 12.6VDC across the loads.All the above I think is reasonable except for the different currents on the two legs and the fact that calculations of the current don't match the measurements. I would think the current in the primary would be about 2.8A and the current across the 2 loads about 147mA. Can anyone straighten me out here?Here's where it gets really confusing to me. Using the scope I get 40V peak to peak on the bridge output and + and - 20V on each of the legs. Putting a 1000uf capacitor in parallel across the loads and I get + and - 20VDC. How can I be getting 20 volts on the scope and 12.6 volts on the meter?HELP PLEASE! What am I doing wrong here?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2015, 12:46:47 am
content: Sounds like a problem with your probing methodology... and, do you need a load resistor or ground reference to do that test properly?It's not clear exactly what you're doing, and what you're doing wrong, but that sounds generally like the difference.Tim

author: picandmix
author_rank: Frequent Contributor
timestamp: March 11, 2015, 09:46:13 am
content: When feeding AC into a bridge rectifier with capacitor the equation is Vx1.4 , so you should see about 35vdc with a light load.With no load that voltage would likely be the 40v dc you are scoping; a 100k resistor is an extremely light load.QuoteThat sounds wrong.To put a load on to your psu you need some high wattage resistors on each side, say around 200 ohms at 7W should use about 70ma, that will get warm or even hot after long use, so anything higher around the 200 - 400 ohm mark should do.That should then give you a more stable system to test with.As said earlier your C2 is the correct way around, but be aware and very careful when powering on your PSU after adding any electrolytic capactitor, if you mistakenly connect them the wrong way around they often explode, even blowing their contents onto the ceiling, so always keep well away from the first switch on, just in caseForgot the exact figures, but breadboard connections are basically for lower frequency signal level rather than psu power rails so if testing your regulators do not put heavy loads though the breadboard.

author: jobog
author_rank: Regular Contributor
timestamp: March 14, 2015, 02:49:29 am
content: I have a couple 430ohm 25W resistors so I put them in as the load and I am still getting 9.8uA on the positive side and -4.6uA on the negative side. I am using Vx1.414 for the voltage but can't figure out why the current is not changing as I change the load. I have looked at this thing so many times to make sure I have everything connected properly and to make sure I have the meter in series with the load when measuring current that I am getting cross eyed.

author: picandmix
author_rank: Frequent Contributor
timestamp: March 14, 2015, 10:40:56 am
content: Quote from: jobog on March 14, 2015, 02:49:29 amAssume you have the resistor across the power rails eg one end to the + rail and the other to the 0v rail. and similarly for the negative rail.They should get warm after a minute or two.When connected you want to measure the ac voltage out from the transformer and then the dc voltage out across the capacitor as well as the dc current though the resistor.If still a problem can you put up some pictures of your test bed, we might be able to spot something.Are you still running things through a breadboard. things like power resistors and regulators are generally a too tight a fit and can damage the breadboard sockets / loose contact.Edit - your test circuit should be like this, I have also tested it with a 500 ohm resistor on my psu and it gives a current /voltage of 14v/ 28ma to 24v /47maAlso , is your transformer giving out a voltage ? some do have inbuilt thermal cut outs, if yours is second hand it could be faulty ?

author: jobog
author_rank: Regular Contributor
timestamp: March 16, 2015, 03:17:02 am
content: My transformer is a 12.6-0-12.6. Measured output side is 29.4V from side to side and 12.6V from each side to center. Setup just as you show and for a 430ohm resistor I get 43.7mA/18.8V on the plus side and -44.1mA/18.9V on the negative. For the 100ohm resistor I get 160.1mA/17.6V on the plus and -161.0mA/17.4V on the negative so it appears that everything is working properly.One question I have is in measuring the voltage across the output of the transformer ignoring the center tap. It measures 29.4V with the meter which I calculate to be about 41.6V peak to peak. When I put the scope on it it shows 80V peak to peak. Why is the scope showing double the calculated peak to peak?Also, do you think a 300mA fuse is appropriate on the line side or do I need something larger?

author: picandmix
author_rank: Frequent Contributor
timestamp: March 16, 2015, 10:01:43 am
content: Good to hear you have got the circuit working ok.There is normally no need to keep those resistors in place, but using a led on each rail with its series dropper resistor is good as it provides a small load and good indication power is on that rail.As for your dvm /scope differences, you need to do a bit of reading up on scope use, there is a lengthy / detailed scope tutorial/talk elsewhere on this forum.Been meaning to go though it myself, but never got around to ithttps://www.eevblog.com/forum/beginners/oscilloscope-training-class-%28long%29/Very simply put, the oscilloscope shows Peak values and the slow DMM shows RMS or average values.See this link for the maths /comparison ( gets complicated !)http://en.wikipedia.org/wiki/Root_mean_squareAll transformers have a VA rating and along with the input and output voltages that determines the maximum current a transformer can give out, which in turn determines the maximum size of your input line fuse.Do you have any such VA Volt/Amp figure for your transformer, they can printed onto the transformers body or a label, but often there is no clear info; if not just the rough size of it, eg 3" square. will give some idea of its VA /Current ratingYour step down ratio is roughly 4.5 : 1 so a 300ma input fuse relates to about 1.3 amps load on the output so I would say, provided the transformer is at least 2" square or you have a VA figure, that or 300 or 250ma fuse is about right. You can protect your regulators / outputs with anything up to 1A fuses.

author: jobog
author_rank: Regular Contributor
timestamp: March 17, 2015, 02:04:13 am
content: Quote from: picandmix on March 16, 2015, 10:01:43 amThanks again for your help and explanation. I knew the meter was reading RMS and the scope was giving peak to peak. Your Wikipedia link brought back things I used to know but had forgotten. Of course Peak V times .707 gives you RMS and RMS times 1.414 gives you peak. Peak to peak has to include the part of the sine wave below zero so if my calculated peak is 40V peak to peak has to be 80V. Man the brain really starts to go when you hit 70.My transformer is a cheap Radio Shack so don't have a VA number but it is about 3" square.I will post a picture of what I hope is my final plan. It will include some small caps across the inputs and an LED to show it's on.

author: jobog
author_rank: Regular Contributor
timestamp: March 17, 2015, 03:05:35 am
content: Here's what I hope is my final schematic. Don't know how this will show up as it was done in TinyCad.

author: rdl
author_rank: Super Contributor
timestamp: March 17, 2015, 03:12:23 pm
content: I have a power supply I built 30 years ago which is almost the exact same circuit (it even uses a Radio Shack transformer). I suggest that you will have more useful power supplies if you make them separate. Instead of +/- 12 volts with common ground, make two 0-12 volts that you can connect for different uses (series, parallel, independent). Make the 5 volt as a completely different project.

author: jobog
author_rank: Regular Contributor
timestamp: March 17, 2015, 08:11:06 pm
content: Quote from: rdl on March 17, 2015, 03:12:23 pmThanks for the suggestion. I already have a bench power supply and this is being built for a specific application that requires all 3 voltages. My next problem is getting the design built on a piece of proto board and fitting it into some kind of case. The large size of the transformer is not making it easy to fit to a box.

author: rdl
author_rank: Super Contributor
timestamp: March 17, 2015, 08:49:58 pm
content: Well, duh. I knew that. I didn't realize this was that thread. I can't imagine that the thing needs more than a few hundred mA. Maybe you just need a smaller transformer. Is yours one of the big, heavy duty, metal cased versions? I've used these Tamura transformers before. They're pretty cheap and very small, not much more than a cubic inch.http://www.mouser.com/Tamura/Power/Transformers/_/N-8u9moZscv7?P=1z0zl2zZ1z0wd7d

author: jobog
author_rank: Regular Contributor
timestamp: March 19, 2015, 11:51:36 pm
content: Quote from: rdl on March 17, 2015, 08:49:58 pmThanks for the link. I was using the Radio Shack transformer because I got it really cheap with our local store going out of business. I just finished getting everything running on the breadboard and I'm measuring less than 100 mA when powering the unit so I think I will go ahead and order the 18-0-18 transformer from mouser. I've been fooling around with this thing turned on for about an hour and nothing feels even the slightest bit warm so I think I will forego the heatsinks on the regulators to trim the size down a little more. Again, thanks for the link.