1.2-30V 1A split power supply design - Page 1
Forum section: electronics / projects

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 26, 2013, 03:31:36 pm
content: hello, i have been recently designing a power supply that can supply +-1.2V to +-30V .i belive there to be some errors in there as this will be my first attempt of a power supply that is fed from AC power and i dont want any bad to happen.currently this is what i have:230V 50Hz (UK mains) is fed threw an initial power switch and then passes threw a 200mA time delay fuse threw a transformer and then back to neutral. the earth is connected to a 1M resistor which is then connected to an earth pin on the front of the supply.the other side of the transformer outputs 24-0-24 RMS voltage at 52VA which is fed into a bridge rectifier to give approximately 33.9V (not counting diode voltage drop) .for the rectifyer i am thinking of using 4 Schottky diodes to minimise voltage drop to get the full 30V range but please let me know if i should not do thatthe +V and -V are then fed into a lm317 and lm337 respectivly to regulate the voltage according to the dual 10k pot (im searching for a dual 10 turn 10k pot so if any one knows where i can get them please let me know)there are several filter and smoothing caps in certain places to prevent ripple and stuff like that, at the end of the supply there is 1A fuses on all outputs/inputs.there is also a feed coming off of the +V rail which is then split in half using the potential divider which is then regulated to 9V.the 9V is used to control the main power relays which are normally closed. the relays are activated according to both of the temperature sensors (bottom block of components). there is a thermistor each attached to the heat sink of both the lm317 and lm337 to measure the temperature and if it rises over 100 degrees Celsius it will cut the power and turn a warning led on. the reasoning for this is because its going to be in a confined space and there may not be room for a big heat sink.what do you think of the design and do you see any problems or improvements which could help?

author: c4757p
author_rank: Super Contributor
timestamp: May 26, 2013, 03:48:19 pm
content: - The Schottky diodes can give off a bit more high frequency noise as they switch off, because they're faster. I don't think it will be much of an issue, though. You can put 10-100nF capacitors across them to dampen that if you like. Also, you might not save much. The peak current through them will be quite high, so VFmay well be high as well. What diodes are you using?- How about some MOV protection on the transformer? Both primary and secondary. Mains is rather unpredictable.- Have you calculated the value of the rectifier output capacitors? They look a bit small to me, but I didn't calculate.- What are you doing about heat? You'll have a lot of it at lower voltages. I see the thermal cutoff, which is good, but how much current can you draw at lower voltages without tripping it?- If you want to play around with analog stuff, try making a variable current limit that tracks the maximum current this can give out as the voltage changes, so you don't overheat anything.- Use a thermal switch. Much cheaper than the op amp/thermistor/relay solution.

author: mariush
author_rank: Super Contributor
timestamp: May 26, 2013, 04:00:27 pm
content: why make another thread, what was wrong with the old one ?The shottky diodes and voltage drop on the bridge rectifier is the least of your problems. At peak current, you'll have more voltage drop (fluctuation) on the capacitors.You seem to ignore all I said about power dissipation and the problem with trying to get regulated low voltages (1.2v-10v) with a high input voltage. You just can't do it... you have 15-25w of power dissipation to play with, which is the voltage difference times the current (30v - 1.2v ) x 1A = 28.8w ... not possible.There's no point in using a relay to cut the ground.A relay uses about 20-50mA. It's not a good idea to use voltage dividers for that, you're going to toast those resistors. Assuming each relay needs 30mA, we're talking about 100mA to keep 3 relays active. Power dissipated over those 10k resistors will be something like 15v x 100mA = 1.5w so unless i have a brainfart now you'd need 3-5w resistors. Then you'd still have 15v to 9v regulation so you'll waste another 6x0.1 = 0.6 watts on the 9v linear regulator.what's the point of f4 ?maybe add a diode to prevent reverse voltage .. and/or diodes from input to output across the regulators or to protect the adjust voltage path.

author: c4757p
author_rank: Super Contributor
timestamp: May 26, 2013, 04:09:56 pm
content: Quote from: mariush on May 26, 2013, 04:00:27 pmI've come to accept that "power supply design" is a weekly topic on this forum.QuoteI know beginners always shy away from switching designs, but they're seriously not that hard. I think you ought to investigate a switching solution. Voltage-mode feedback (significantly easier) with a linear postregulator to clean up the increased ripple of a voltage-mode system. 1A isn't that much.QuoteYes, I forgot to mention that - LM317 pretty muchrequiresa diode from output to input if the output capacitor is much above a few uF. It doesn't like the peak discharge current of that capacitor. And I'd definitely stick a big 1N5400 or so in antiparallel with the outputs.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 26, 2013, 04:14:15 pm
content: Quote from: c4757p on May 26, 2013, 04:09:56 pmhow would i go about doing a switching supply?

author: c4757p
author_rank: Super Contributor
timestamp: May 26, 2013, 04:31:37 pm
content: Dave did a video on designing a switching supply using an MC34063 - watch it. It won't do 1A, but you can find a similar chip that can (maybe I'll dig around a bit). Usually, you just hook it up the way the datasheet says. Making it adjustable is similar to making an LM317 adjustable, actually.

author: c4757p
author_rank: Super Contributor
timestamp: May 26, 2013, 04:33:59 pm
content: The adjustable version ofLM2576looks promising. "Simple Switcher". Up to 40V input, up to 3A output, and down to 1.23V output. The slightly tricky part will be choosing components to make it properly variable, but even that should be just a matter of finding the minimum inductor and minimum capacitor over the range.

author: mariush
author_rank: Super Contributor
timestamp: May 26, 2013, 04:34:45 pm
content: Dave did a tutorial with mc34063 in the boost configuration (from low input voltage, get higher voltage)Formulas are about the same for the buck configuration (get lower voltage from higher voltage)There are online calculators for the components needed :http://dics.voicecontrol.ro/tutorials/mc34063/http://www.nomad.ee/micros/mc34063a/This MC34063 is cheap and easy for beginners but it peaks at 1.25A and not sure if it can really do more than 1A across the output from 30v. Maybe about 0.75A is the peak.Better ICs are LM2596, LM2576, or LM259x/LM257x etc etc these can do 2-3 amps, maybe even more.You can use one of these as pre-regulator, like.. configuring it in such a way that it's output voltage is always 3-4v above the linear regulator's requirements (so that it will have 2-2.5v it needs to stay stable and some buffer for the switching regulator's output ripple - you can only tune the parts for a particular voltage, for a wide range of output voltages you'll get more ripple so you need to consider that)The DC output will still not be as ripple free as using a linear regulator directly from a transformer but it's better than giving the regulator 30v directly. Ideally, you'd use transformers with multiple taps or several transformers with small secondary taps like I said already several times.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 26, 2013, 04:37:11 pm
content: i was about to search for the video thanks for finding it for mei shall have a look once i have finished Dave's latest one (another 15 mins

author: Paul Price
author_rank: Super Contributor
timestamp: May 26, 2013, 04:48:25 pm
content: It only takes very few components to make a well working, overload protected switcher.Your output voltage will depend on the input voltage from your transformer or PC laptop power supply. If you put 18V in at 3.5A, you can get 15V out at 3Amps with just a tiny heatsink. You don't need the HV version of the chip.If you build two of these, with separate transformers, isolated from each other, you can then have two identical positive and negative supplies capable of 3A with a minimum of parts, reliable, small in size, easy to build, cheap, and kooooool.It is easy enough to add a pre-built digital Voltage module for monitoring of the output with a Digital Voltage module available from ebay for just a few dollars. Or your next project could be to make the digital voltage display.It also only takes a few pennies more to add two LM7805 regulators to provide a fixed 5V and -5V fixed outputs to the same. Or if you are not satisfied with that, you can also add LM7812's and LM7815's to give you ultra smooth low noise, low ripple, current limited and thermally protected, fixed + - 5, 12 and 15 fixed outputs. At that point it as easy to only adding banana jacks and meters onto the front panel for your custom fixed voltage outputs. Try finding a pre-built new power supply like that new for less than a king's ransom.Why should an electronics beginner pay a small fortune for some ready-built supply that would just teach you nothing about the most useful basic elements of electronics, and about building stuff?If you do it yourself, it's not that complicated, you can have your own custom koooooolist power supply that you can say you've built on your own, for a small fraction of a price you would pay for a new one.

author: Paul Price
author_rank: Super Contributor
timestamp: May 26, 2013, 04:51:03 pm
content: In your original design, eliminate the three mechanical switches, they will be quickly be damaged by turn on current surges.Try to think if it is really important to have switches at all, or using some semiconductor type of switch? A simple solution does exist.If you want to have switches where you've have them now in your circuit, they must be able to handle high surge currents over many many times of use, not so cheap or easy to find and they would have to be rather big. Contacts in switches are easily pitted or eroded or can even weld themselves shut by high surge currents created when they are directly connected to large capacitors.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 26, 2013, 05:41:56 pm
content: just finshed two of daves switch mode videos so i know the difference between a linear and switch mode supplyim going to ahve a look further into it and pull up a designi shall post it up later

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 26, 2013, 10:21:52 pm
content: ok when using this equasion to work out V.uS what do i take Vout as?E.T = (Vin - Vout - Vsat) . ((Vout + Vd) / (Vin - Vsat + Vd)) . (1000/150)should Vout be 30 as that will be my maximum voltage output?page 13:http://www.ti.com/lit/ds/symlink/lm2596.pdf

author: c4757p
author_rank: Super Contributor
timestamp: May 26, 2013, 10:49:32 pm
content: Well, you're trying to find the minimum inductor you can use. Run the calculation at both the minimum and the maximum voltage and see which number is larger - that's your minimum.When you get to the next row: when they say "low ESR" they're not kidding. At full output current you just might blow your capacitor if you use a cheap junk box cap. In a pinch you can parallel a bunch of regular caps for testing, but that's hardly an elegant final build.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 26, 2013, 10:57:57 pm
content: lm317 and lm337 have built in temperature cut off. It's in the data sheet. Also why a bridge rectifier on a centre tap transformer?

author: c4757p
author_rank: Super Contributor
timestamp: May 26, 2013, 11:01:49 pm
content: Quote from: Strada916 on May 26, 2013, 10:57:57 pmBipolar output? Nothing wrong with that.

author: c4757p
author_rank: Super Contributor
timestamp: May 26, 2013, 11:05:50 pm
content: And a comment on that note: I don't think you'll find a negative-input switching converter to match your LM2596, though I could be proven wrong. It's nottoohard to design yourself but I wouldn't recommend it to a beginner (you'd need to both be familiar with buck converters and with working on a positive-ground circuit). I'd recommend either keeping the negative rail linear, with a reduction in output power rating on that rail (when was the last time you used 1A on a negative rail?), or using two completely floating positive regulators (you'll need a different transformer, with two full secondaries instead of a center-tapped one), then joining them at the regulated output.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 27, 2013, 05:49:10 am
content: Because its still got an ac waveform? And it needs to be rectified to become dc?

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 05:58:21 am
content: Huh? I'm not sure what you're asking.

author: Unknown
author_rank: Unknown
timestamp: May 27, 2013, 07:30:17 am
content: Probably a response to Strada916' s post, and a failure to use the quote button.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 27, 2013, 07:49:55 am
content: Quote from: c4757p on May 26, 2013, 11:01:49 pmSorry was thinking of single voltage not bi polar.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 27, 2013, 12:17:50 pm
content: Quote from: alm on May 27, 2013, 07:30:17 amIm on my mobile an ive only just found the quote button, also i did not realise that someone else had posted.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 27, 2013, 01:14:44 pm
content: ok i have done some calculations according to the lm2596 datasheet and have come up with this schematic.I am unsure what value input capacitor to use but from the datasheet it recommends a tantalum capacitor, i am also unsure if i want to add a post ripple filter which might be a good ideai have increased the transformers voltage to 35V rms so it will give approximately 50V output.so what do we think?

author: mariush
author_rank: Super Contributor
timestamp: May 27, 2013, 02:00:05 pm
content: LM2596 has absolute maximum input voltage 45v - 30-32v you'd get from a 24v rms transformer is already high enough.Forget about tantalum capacitors on the input, get a low esr 100-470uF 50v capacitor and you should be fine (with max 30-40 input voltage).I'm not sure the SB560 is adequate for the task, get some better shottky diodesThe datasheet shows you an adjustable version of this switching regulator in Figure 20, page 9:http://www.ti.com/lit/ds/symlink/lm2596.pdf15uH for inductor is seriously undersized for an adjustable (pre-tracking) switching regulator, the datasheet recommends 68uH, I'd probably use 100-220uH or whatever's more common/cheap for the uH and resistance and peak current. It would get lower efficiency maybe but whatever.Use a slightly higher capacitor on the output, just to get lower esr. You'll need about 47-100uF at least.Read the datasheet carefully and the application information...If you want the switching regulator to be a sort of pre-regulator (as in take 30-35v in and output 3-4v above the desired output voltage so that you would then use a LM317 to further regulate) you have to do the math and adjust the switching regulator to work right on the whole output range.. inductor values change depending on output voltage, input and output capacitor values change, you have to make a compromise between functionality and efficiency through the output range.Texas Instruments has a software app called Switchers Made Simple :http://www.ti.com/tool/switchersmadesimple-swYou can input there input voltage range, output voltage and current and it suggests what LM2*** to use and shows a typical design along with parts and alternatives.. you can then check the specs of those parts and buy something with specs close to the ones in the design shown.Run through several output voltages and derive from there what parts you should use.

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 02:31:00 pm
content: Yup... what he said.Comment on the diode: I don't know if you're aware, but you can get them in TO-220 for bolting to a heat sink, with current ratings upwards of 30A, for US$1 or so. Look at those. At 3A out, you want the diode rated higher than that for best reliability. A 3A 1N5822 or something will run quite hot.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 27, 2013, 02:55:08 pm
content: there we go I have changed the inductor, input cap, diode (to-220 case).i tried that program which I did notice was referenced in the datasheet but i am unsure how to use it for a variable supply

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 03:18:23 pm
content: As I've said, for the variable supply you just need to know the largest minimum value at any output voltage. Run it twice, once for the minimum voltage and once for the maximum, and take the biggest parts it ever recommends.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 27, 2013, 03:36:06 pm
content: I am still unsure why the inductor is larger then what i worked out. I worked out my E.T to be 10 for 30v and 5 for 1.235V. Then using the graph they both wanted 15uH for 3A.Am i doing soming wrong?

author: Paul Price
author_rank: Super Contributor
timestamp: May 27, 2013, 09:05:54 pm
content: If you are doing the math right, maybe you are instead doing something right.

author: mariush
author_rank: Super Contributor
timestamp: May 27, 2013, 09:48:18 pm
content: Quote from: carbon dude oxide on May 27, 2013, 03:36:06 pmLet's go at this logically. Open the datasheet for this IC:http://www.ti.com/lit/ds/symlink/lm2596.pdfGo to page 9 and check both the fixed version and adjustable version designs and ask yourself : If the 15uH inductor value was right, why are they using a 68uH in both examples?If you want to use this as a pre-regulator in front of a linear regulator, you want it to output anything from about 4v to 30v, because LM317 needs about 2v above output voltage to output a proper voltage. So 1.25v + 2v = 3.25v - let's say you output a minimum of 3.3v +/- ~ 5% from the pre-regulator.Now scroll down at page 11 and you have there : Table 1. LM2596 Fixed Voltage Quick Design Component Selection TableYour input voltage will be about 24-30v, and to output 1A to the linear regulator, your pre-regulator should be able to output more than 1A, so you would "tune" it for 2A or 3A... now look at that quick table and see what inductors they are recommending for >20v , 2-3A... so you see any 15uH value in that table?Your pre-regulator has to work in the worst case possible ( output 3.3v , 2A, maximum input voltage 40v) therefore it's recommended by that table to use a 47uH inductor.Your pre-regulator has to work in the best case possible (output about 32v , 1-2A, maximum input voltage 40v) but the quick table doesn't go that far... it only shows 12v output, 2A, 40v input and for that it already needs 150uH inductor.So logically, you would assume the higher the output voltage, the more inductance is required for the regulator to do more than 1A at such big output voltages but that may not always be the case - the example for adjustable version of the chip further down the page shows 28v in, 20v out @ 3A and they come up with 47uH inductor..Either way, it should be fairly obvious that 15uH just isn't a right value.Go through the steps to determine the components, from page 12... but those steps are for determining the somewhat optimum components or minimum requiredfor the output voltage and current you do the math for- they're not for 3.3v to whatever voltage you do math for.

author: madires
author_rank: Super Contributor
timestamp: May 27, 2013, 10:42:04 pm
content: What about adding a LC output ripple filter, like 20µH and 100µF?

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 11:16:10 pm
content: Quote from: madires on May 27, 2013, 10:42:04 pmCart before the horse?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 06:33:11 am
content: I remember someone on this thread saying that this IC cannot regulate negative voltages, how would i go about doing negative voltages?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 06:47:45 am
content: Or could i find a IC controler that can handle a max of 70 volts and get it to regulate the voltage between v+ and v- then just turn it into a split supply afterwards where i would then use the linear regulators to further regulate it all

author: Unknown
author_rank: Unknown
timestamp: May 28, 2013, 07:31:58 am
content: How would you split it? Assuming you are drawing 1A from the +30V rail, what are you going to do with that other 30V?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 08:18:04 am
content: Hmmm good pointSo how would i regulate the negative rail?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 12:20:02 pm
content: What if i had a transformer with two seperate output windings both at 24v rms i could have 1 smc puting out a positive voltage and the other putting out a negative voltage, the. Have both feeding into a linear positive and negative regulator respectively.Would that work?

author: madires
author_rank: Super Contributor
timestamp: May 28, 2013, 01:03:06 pm
content: Quote from: carbon dude oxide on May 28, 2013, 12:20:02 pmYes, you could use a LM317 and a LM337. But you could also keep both isolated and use the LM2596 for each secondary. Since both are isolated you can simply connect the +V output of #1 with the Gnd of #2, i.e. Gnd of #1 the becomes the -V output. AFAIR, someone mentioned that already.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 01:44:39 pm
content: also would it be worth having this controlled by a micro controller? that way i can use a single 10 turn pot instead of attempting to find a dual 10 turn potthat way both rails can be regulated to the same + and - values using only 1 pot.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 02:13:33 pm
content: Quote from: madires on May 28, 2013, 01:03:06 pmIm affraid i dont fully follow what you said :/I have a picture of how i think it would work but im probably wrong ^.^I dont think it will be the bittom one but i put it there as it had a little bit of logic behind that idea

author: madires
author_rank: Super Contributor
timestamp: May 28, 2013, 02:23:00 pm
content: Quote from: carbon dude oxide on May 28, 2013, 01:44:39 pmA dual 10 turn pot (or two pots linked by small gears) with additional adjustment pots seems to be the most simple way. With a MCU you need a digital pot for each rail and have to take care about the voltage differences.

author: madires
author_rank: Super Contributor
timestamp: May 28, 2013, 02:31:48 pm
content: Quote from: carbon dude oxide on May 28, 2013, 02:13:33 pmThe bottom one parallels the secondaries, i.e. you're doubling the current. That way you don't get a split PSU. The top is nearly correct for the isolated solution (two LM2596 or LM317), just remove the earth symbols (don't connect both grounds). For the LM317-LM337 solution connect both secondaries in the middle (that will be the ground) and use one bridge rectifier.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 02:59:31 pm
content: Like so?This way the ground of the 1st smc is the +v (-v) of the second smc or should it be the other way around?

author: madires
author_rank: Super Contributor
timestamp: May 28, 2013, 03:20:41 pm
content: Quote from: carbon dude oxide on May 28, 2013, 02:59:31 pmIf you remove "+V to #2" and change GAD into "-V to #2" you'll get the correct setup for the LM317-LM337 based circuit. If you're aiming at the LM2596 keep the top circuit from the drawing before and remove the earth symbols on both grounds. The earth symbols imply that both gounds would be connected to the chassis and mains earth.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 04:45:46 pm
content: Quote from: madires on May 28, 2013, 03:20:41 pmif i use the top circuit would both gnd be connected? or would they both be isolated?if there both isolated how would i have a common ground at output of the power supply?

author: fridrik
author_rank: Newbie
timestamp: May 28, 2013, 05:06:22 pm
content: If the supply is supposed to be dual tracking then you do not need a dual pot. You just use the pot for the positive side and sense the positive output voltage, invert it and feed it back into the negative supply. there are schematic diagrams of this configuration on the web somewhere using LM317 and LM337. However if you are going to have two isolated transformer taps you might as well keep the supplies completely isolated and independent, using two pots to control the voltage of each one.I don't quite understand people are recommending the switch mode route, If this is your first supply then surely a linear supply will do fine. Yes power dissipation will be huge at low voltages and high current, but this has been done many, many times and with a big heatsink you are fine. If you cannot fit the heatsink then just lower the specs a little bit.

author: madires
author_rank: Super Contributor
timestamp: May 28, 2013, 05:07:59 pm
content: Quote from: carbon dude oxide on May 28, 2013, 04:45:46 pmThey need to be isolated.QuoteBehind the LM2596s you would connect one +V output to Gnd of the other and that connection is going to be your common ground. The Gnd of the first one becomes then -V, because it's +V below the common ground. It's like you would place two 9V batteries in series with a center tap as common ground. The negtive side of bottom battery becomes -9V, the connection between both batteries (positive side of bottom battery and negative side of top battery) is the common gound and the postive side of the top battery will be +9V.

author: madires
author_rank: Super Contributor
timestamp: May 28, 2013, 05:19:54 pm
content: Quote from: fridrik on May 28, 2013, 05:06:22 pmI fully agree! But it's nice to discuss both solutions, because buck converters aren't more complicated than a classic linear voltage regulator anymore. Some datasheets of buck converters explain the calculation of the inductor very well, even with some examples.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 05:26:54 pm
content: this is not the first supply i have done, i wanted to make a higher voltage supply with a large range. I had several people say that a linear supply is not good for this due to the large gap between input and output voltages when i set the output to low voltages. From memory there can be an 8 volt difference which is what one person said (forgot who it was sorry.to combat this we went threw a few designs such as a multi tap transformer and some relays to switch the voltage to lower ones when i want a low voltage output. Then i was suggested a smps so it has developed from there. I have learned quite alot in the possess of going through the design.

author: fridrik
author_rank: Newbie
timestamp: May 28, 2013, 06:14:04 pm
content: Great, sorry for disrupting the discussion then.Do you plan on putting a linear regulator on the output of the buck or just use a LC output filter?

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 07:48:17 pm
content: I am thinking of putting a linear regulator on the end and using the smps to drop the voltages down so that the linear regulators are not bathing in magma

author: madires
author_rank: Super Contributor
timestamp: May 28, 2013, 07:56:27 pm
content: Quote from: carbon dude oxide on May 28, 2013, 07:48:17 pmThen you have to synchronize the buck converters and linear regulators and keep the voltage drop of the latter in mind.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 08:45:52 pm
content: Yes i did have that though of making sure that the voltage comming out of the smps is at least 2v higher then the linear regulatorsThis is why i thought it might be better with a micro controler so that i can program it all but a potentiometer solution will probably be easyer to use

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 11:24:44 pm
content: Ok so if we ignore the transformer for now would this be a working setup?

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 11:38:06 pm
content: No.1) MC34063 has a maximumpeakswitch current of 1.5A. For a buck converter, that gives you only 750mA output, even less for inverting.2) MC34063's maximum voltage is 40V. In your inverting configuration, you have +35V on Vcc and V- on gnd. That means it has 35+|V-| applied across it, with is sure to be more than 40V.Quote from: carbon dude oxide on May 28, 2013, 08:45:52 pmThat offset can be done easily with an op amp.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 28, 2013, 11:43:44 pm
content: I dont know why i put that part number on there :/ but in theory if there is a ic that can do that would it work?

author: c4757p
author_rank: Super Contributor
timestamp: May 28, 2013, 11:51:22 pm
content: Quote from: carbon dude oxide on May 28, 2013, 11:43:44 pmOr that part itself! You've used it correctly, except for the voltage and current limits.But yes, it looks fine to me.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: May 29, 2013, 10:00:25 am
content: What controlers allow for inverting from +35 volts down to -30 ?

author: Paul Price
author_rank: Super Contributor
timestamp: May 29, 2013, 12:40:13 pm
content: Nunofdem.You build two isolated supplies, each identical with each having a separate power transformer to make two power supplies and one is your positive and the other your negative 3A supply.Please see what I've said in an earlier reply.The negative supply is no more than the same positive power supply circuit standing on it's head.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: June 07, 2013, 05:23:41 pm
content: Ok would it be possible to do the centre tap idea but have multiple linear voltage regulator stages seperated by 7-8V and switch between the outputs of them? That way i can have the positive and begative rail and not go over 8v between input and output. It may only be 1.5A but i do have a beasty heatsink at home just sitting there which i could use.

author: Paul Price
author_rank: Super Contributor
timestamp: June 08, 2013, 01:23:32 pm
content: Draw a schematic. Sounds like it might work. Details please.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: June 08, 2013, 09:36:26 pm
content: ive had alot of thought on my last idea but i have decided it will not work very well :/ one of the main reasons is when i initially thought of it i was thinking of some nice ideal lm317/lm337 regulators with no internal voltage drop T.Ti have however come up with another idea in the prossess which expands on another idea someone else has posted about having a multi tap transformer. unfortunately i cannot find many multi taps so im maby thinking of and idea of having multiple transformers along with a microcontroler controlled input so when it comes to switching from one transformer to another it can match the output voltage just before switching rails.i will draw a schematic out of how i think it might work

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: June 08, 2013, 10:35:33 pm
content: Ive done a bit more research and have come accross this which uses an LM723 as the regulator bit it only does te positive side voltage regulationhttp://elect.wikispaces.com/file/view/N0006D05082009.pdf/97924473/N0006D05082009.pdf

author: madires
author_rank: Super Contributor
timestamp: June 09, 2013, 10:11:33 am
content: Quote from: carbon dude oxide on June 08, 2013, 10:35:33 pmPlease read the datasheet of the LM723, it supports negative voltage regulation too. BTW, I'd use a BD135/137/139 for Q1.