Questions about Power Supply design - Page 1
Forum section: electronics / beginners

author: Karamel
author_rank: Regular Contributor
timestamp: March 08, 2016, 12:22:19 am
content: Hi,I am EEE student from Turkey. Briefly, i need voltage source when i am trying to work my experimental or personal circuits which need +-12v or more. I am, therefore, trying to understand how power supplies works and which power supply is good for me because, I decided to make own power supply. I've read something on internet. Regarding this, some people said use transformer, some people said use cheap smps for input and then, go it in to LDO or regulation circuit. Actually, I cannot uderstand them which one is better for me?SMPS + regulation circuit?SMPS + LDO?Transformer + rectifer + filter + regulation circuit?P.S: I need a symetrical voltage feeding. Such as, +-15, +-24, +-30 etcBest Regards,Karamel

author: uncle_bob
author_rank: Unknown
timestamp: March 08, 2016, 02:12:21 am
content: HiThe most basic question is really one step back from where you started:Why not buy a power supply that already does what you need to do? You can buy them both new and used. I can easily design and build a power supply myself. I find it cheaper to buy a used one than to buy all of the parts that go into building one. By the time you put one in a proper box and add meters, a home built supply can get very expensive.So let's say you have a good reason and already realize you will spend more money than you need to:Basic power supply spec has information like this:Adjustable voltage range from +/-3V to +/35V.Maximum output current 1AAdjustable over-current (short circuit) protectionRegulation < +/- 10 mV for no load to full loadNoise on the outputs is < 1 mv rms over 10 Hz to 100 KHz.Outputs track each other to within 1%Outputs can float relative to ground up to 100V.Both outputs have independent metering (you can read volts on one and amps on the other).=====I'm very sure that what you are trying to do will have different numbers than the ones I have guessed at above. Not every supply needs to have every number filled in. You *do* need to have a pretty good idea of what you are going to need the supply to do:Do you need 30W out of the supply at +/-3V *and* later also need 30W at +/- 30V? That drives your design in a specific direction.Do you instead need 1A out of the supply regardless of output voltage? That may drive the design in a slightly different direction.Yes, there is a lot more to this. Power supply design is not at all simple. Building a supply that will take normal lab bench abuse takes a lot of care.Bob

author: nour
author_rank: Frequent Contributor
timestamp: March 08, 2016, 04:08:34 am
content: Just have some 78XX and some 79XX and 15-0-15 2 A transformer and they will do the job just fine as long as you don't exceed the rated currentif you want a variable voltage use LM317 and you will have power supply with less that 25$ which will do most of your workif you want more than 1 Amp you should consider buying ready made power supply with current and voltage control

author: Simon
author_rank: Unknown
timestamp: March 08, 2016, 07:36:04 am
content: the first question is how many amps do you need ? and the second is of course why can't you buy a comercial unit ?

author: Kappes Buur
author_rank: Regular Contributor
timestamp: March 08, 2016, 07:36:23 am
content: https://www.eevblog.com/forum/beginners/neewbee-wants-a-power-supply/msg349346/#msg349346

author: Karamel
author_rank: Regular Contributor
timestamp: March 08, 2016, 11:44:17 am
content: Hi, thanks for all replying.-Why am i trying to make it myself?There are two reason.First, if i make it, i can learn how power supplies works.Second, in Turkey, lab power supplies are too expensive. I can make it cheaper than buying.So, first of all, i want to learn how they works and i would like to make it. Just, i need some information about power supplies.Best regards,Karamel

author: uncle_bob
author_rank: Unknown
timestamp: March 08, 2016, 12:42:37 pm
content: Quote from: Karamel on March 08, 2016, 11:44:17 amHiI would be very careful about the "to expensive here" part of the argument. The same rules / taxes / shipping / hassle that make a $20 used lab supply expensive also run up the cost of getting *all* of the right parts.=====The first part in any design is to decide what you want to end up with. That certainly includes voltage and current. It also includes issues like the enclosure, heatsinks, fans, fuses, metering, and RFI filtering.Next step is to take a rough look at this and evaluate a couple of design approaches. This lets you evaluate your decisions in the first section and likely you find gaps (problems) with your specifications.After that you loop back to the start and adjust your specifications a bit to line up better with what a practical design can accomplish. Then you head back through the rough design phase and see how that stacks up. You may loop several times.Once you have that part fairly well thought out, you can start looking at pc board layout software and the various tools for analyzing a design. You may also start investigating surface mount construction techniques and the test gear needed to check out your design. All of these details are as much part of the design as anything else above.Note that there is no money spent so far.You should have enough information at this point to make a guess at what all the stuff needed to do this will cost. Tools to fabricate sheet metal, reflow soldering gear, DVM's, a scope probably all are part of it. They are not directly a cost of the supply, they are a cost of the project. You may or may not be able to guess how long it all will take. Try to do so and then keep track of how you do against your estimate. It will help you a *lot* in the future when you have to estimate how long a job will take.This is the point you actually have a decision to make. So far it's all been talk and emails. You now will be spending money. That's not a reversible thing. Check over all of your assumptions. Re-check your design. Get in the first (small) group of parts. Get enough gear to check them out. Validate a small portion of the design, see how it works, make sure you have it right. Move on to the next section.Once you have all of the prototype assemblies worked out, the result will be a giant bunch of wires all over your bench and a few parts buried under them. The result is sort of a power supply, but not anything you can use. You then take what you have learned on each little assembly and roll it up into a final design. You do the various pc boards, you start on the sheet metal. You now order the final parts.Once all the parts come in, you put it all together and see how it works. If you are like me, it will be <90% correct. The decision then is if you do another pass at the finished product.Bob

author: Karamel
author_rank: Regular Contributor
timestamp: March 08, 2016, 06:53:45 pm
content: Thanks for replying BobIs there anyone who can share any schematics in order to analyze and because I am a freshman at electronics so, I can not understand everything quickly.Best Regards,Karamel

author: botcrusher
author_rank: Regular Contributor
timestamp: March 08, 2016, 10:44:24 pm
content: The cheapest possible solution could be an ATX power supply hooked up to a DC-DC converter and a:please don't kill methis cheap DC-DC booster i got off ebay *seems* to work pretty damn well.www.ebay.ca DC DC boost converter Constant Current Mobile Power supply 250W 10A LED DriverCaveat emptor, i have not used this for heavy loads yet, just some fans and random small motor in seriesIf you want to make it from complete scratch... I'm probably not the best person to answer that.

author: Karamel
author_rank: Regular Contributor
timestamp: March 08, 2016, 11:05:41 pm
content: Quote from: botcrusher on March 08, 2016, 10:44:24 pmHi,DC-DC Booster? I am trying 220v mains voltage to +-0-30v adjustable dc range?I guess, there is any mistake because in your given link, description says it must be a whole new ball game?Input voltage: DC8.5V-48VOutput voltage: 10-50V continuously adjustableBest Regars,Karamel

author: Karamel
author_rank: Regular Contributor
timestamp: March 08, 2016, 11:16:45 pm
content: Hi,I have found a DC-DC buck step down converter which has input range DC 7-32V and output voltage range DC 0.8-28V. Is it useful for me? I can quickly get in rectifier and take full wave signal then, I can filter it with capacitors? Is it good idea?http://www.ebay.ca/itm/DC-DC-CC-CV-Buck-Converter-Step-down-Power-Supply-Module-7-32V-to-0-8-28V-12A-HC-/261997095157?hash=item3d003e68f5:g:~MIAAOSwLVZVxbOW

author: uncle_bob
author_rank: Unknown
timestamp: March 08, 2016, 11:59:27 pm
content: Quote from: Karamel on March 08, 2016, 11:16:45 pmHiThe switcher will require you to change caps as you vary the output. It will not be stable over a reasonable range of loads and voltages unless you do. It also does not take care of your tracking requirement. It is unlikely that it will survive the things that people do to a normal bench supply.====So based on the fact that we're now looking at a positive only regulator, have the specs changed? We're also looking at a pre-built board rather than do it yourself.=====As you dig around on eBay, are a lot of plug in the wall DC supplies. They also are quite cheap and come in a variety of useful output voltages. They also show up in dark coroners of your home, attached to junk electronics.Bob