Multiple power rails query - Page 1
Forum section: electronics / beginners

author: seb1982
author_rank: Regular Contributor
timestamp: March 05, 2013, 12:12:26 pm
content: Hi guysI'm an absolute beginner, I'm afraid - I've not done any electronics since simple 555 circuits at school 15 years ago, and I'm currently trying to get back into the hobby - I used to love it!I'm currently making the ubiquitous newbie's power supply (cue everyone rolling their eyes!). I've more or less got my circuit for this sorted, but then I thought - I know, why not complicate it with a digital lcd readout (hey, the blue and white ones are cool, OK? O0)I was thinking about using an ATMega 328 for this, as I can program that relatively easily with my limited skills and equipment at the moment.Obviously, that's going to be running off 5v and my AC-AC transformer for the input to my PS is rated at 24v 2A.My query is what the best way to get the 5v? Rather than bodge it, I thought I should find out what the best way is: should I just do a simple additional rectified 5v voltage regulator circuit in parallel with the power supply circuit using the same 24v transformer output, or is there a better way to do it? What if I wanted a 12v rail to run a small temp-controlled cooling fan off, too? I was thinking that neither of these applications is going to be drawing a massive current, and I'm only designing the power supply for a 1A max load at 30V, so it wouldappearthat the transformer is more than capable of it, but then I'm an idiot, and not to be trusted around electricity yet!Thanks ever so much for your time and knowledge!

author: Gall
author_rank: Frequent Contributor
timestamp: March 05, 2013, 12:44:20 pm
content: If your transformer has only one output, go on and use it. Just use two step-down regulators, one 24V->12V and another one 12v->5V. Low cost switch mode regulator like LM2596 will do pig. The 12V->5V stage may be just linear 7805.

author: mariush
author_rank: Super Contributor
timestamp: March 05, 2013, 01:51:16 pm
content: Rectify the AC voltage first, which will get you 24-30v DC when there's no load connected to the transformer.A linear regulator like a classic 7812 would work to get 12v, but keep in mind how much power will be dissipated as heat in the regulator. Generally, a fan will use 0.1-0.2 A, so even if you're using a very efficient fan, you're going to waste ( 24v-12v ) x 0.1 = 1.2 watts. In worst case, it will dissipate 2-3 watts, which is A LOT, the regulator will be hot.It might be worth testing a MC34063 or one made by another company (just look for 34063, lots of companies make it) to get about 8-12v. A fan won't care about ripple and other things and will run at less than 12v but at slower speed, so you might be able to get rid of fan speed adjusting and make your circuit simpler by simply powering the fan directly with less voltage.You just want this to output about 8v or more, so that you can use then a simple linear regulator like 7805 to get smooth 5v for the microcontroller. Cheap linear regulators need at least 2v above the output voltage to work properly.LM2596 would do the same thing as a MC34063 but it's more expensive as it's rated for 3A of current, but you don't need that much, you fan will only use 0.1-0.25A, and you'll need about 0.2-0.5A for the microcontroller, so if you do the formulas for MC34063 to do 0.5A at 12v, you'll be fine.MC34063 is really cheap, comes in DIP format (so you can put it on breadboard for testing but to work well it has to be on pcb at later time like any switcher chip) and Dave even made a video about it where he goes through the formulas - just search for it on youtube.

author: KuchateK
author_rank: Regular Contributor
timestamp: March 05, 2013, 02:53:17 pm
content: You can get cheap LM2596 based DC-DC converters on ebay. They work pretty good and whole adjustable module costs almost as little as TO220 regulator. They are even easier to use, you don't have to worry about capacitors or cooling.

author: hammil
author_rank: Regular Contributor
timestamp: March 05, 2013, 03:22:04 pm
content: Well, since you're a beginner, I'll do my best to explain a couple of thingsIn general, there are two types of DC supplies: Linear and switch-mode.Linear supplies are dead easy. You have a mains transformer, bridge rectifier, output filter, regulator. As you go into higher currents, your filtering will become more complex (you might want to use two capacitors and an inductor - known as an LC-Pi filter). Your regulators can be your standard 78xx series, or other adjustable regulators, but a little circuit I like to use is this:http://falstad.com/circuit/#%24+1+5.0E-6+10.20027730826997+50+5.0+50%0Aa+240+208+336+208+1+15.0+-15.0+1000000.0%0A174+192+160+240+224+0+1000.0+0.401+Resistance%0Af+336+208+384+208+0+1.5%0Aw+384+224+384+256+0%0Aw+384+256+240+256+0%0Aw+240+256+240+224+0%0Ar+384+256+384+336+0+100.0%0Ag+384+336+384+352+0%0AO+384+256+464+256+0%0AR+384+192+384+96+0+1+100.0+2.0+20.0+0.0+0.5%0AR+192+160+192+112+0+0+40.0+12.0+0.0+0.0+0.5%0Ag+192+224+192+240+0%0Ao+8+64+0+34+20.0+9.765625E-5+0+-1%0AIf you have what's called a 'rail-to-rail' op-amp, you can use a small 12V regulator that won't supply much current, then connect your filter output to the FET's drain pin directly. The feedback loop keeps the output stable.ANYWAY. Sorry if I went off-track.Switch-mode supplies and converters are a little more complex, but they all work on the same principle: Turning the input voltage on and off rapidly, then smoothing the output. Since the input voltage is either on or off, you can get a very high efficiency, upwards of 90%, with these supplies.The drawback is that there's a lot of noise compared to a linear supply, which is very clean.

author: seb1982
author_rank: Regular Contributor
timestamp: March 05, 2013, 03:31:57 pm
content: Thanks for the replies folks, and as ever that's a fantastically informative video from Dave!So, would it be OK to put these (whether I end up using switch mode or linear) on the output of the transformer in parallel with the variable PSU circuit, or should I get a transformer with dual (or more) secondary windings to use for these other voltage rails?

author: hammil
author_rank: Regular Contributor
timestamp: March 05, 2013, 09:56:24 pm
content: Depends if you want each rail to be isolated or not. So, would you want the +5 and the +12 rails to have their own ground terminals that are not electrically connected (This would be useful if you wanted to connect the +5v jack to the ground of the +12v, then you would effectively have -5V at the +5v ground), or would you be fine with a common ground terminal for everything?For the former, you'd need multiple transformer windings. For the latter, you can just put regulators in parallel no problems.