Lab Power Supply - Page 1
Forum section: electronics / projects

author: wigman27
author_rank: Regular Contributor
timestamp: July 03, 2013, 10:44:06 pm
content: Hey everyone,I am currently designing a lab power supply, I originally started off with a computer supply and I was searching for the best way to convert it into a lab power supply. That's when I stumbled upon the EEVBlog and Dave's lab power supply. It was exactly what I needed. So now what I would like to do is combine Dave's lab power supply with mine.I will effectively have 4 outputs, 3.3v, 5v, 12v and variable output controlled by Dave's design. The 3 fixed voltage outputs will be powered directly from the computer power supply and are capable of upto 16 amps.I am wondering what your thoughts are on the best way to measure the current output of the three voltages from the computer power supply reasonably accurately. I have looked at using current shunt resistors and Hall effect sensors. I don't really want to run the 3 voltages through the PCB as there is no really need, so I looked at using chassis mount shunt resistors which are accurate but they can be fairly expensive. Then I looked at Hall effect and they don't seem to be as accurate. So I am thinking I will have current sense resistors on the board but I am concerned about the space requirements for 3 sets of sense resistors plus the one for Dave's design.I would really appreciate your thoughts on how you would work around this.Thanks everyone.Lee

author: IanB
author_rank: Super Contributor
timestamp: July 03, 2013, 11:00:20 pm
content: What's your plan with the current? Do you just want to indicate it, or do you want to control it (limit it)?If you just want to indicate it you can look for a standard digital panel meter and fit an appropriate shunt across the meter terminals. Most meters have a 200 mV input sensitivity, so to measure up to 20 A you would need a 100 milliohm shunt. Adjust for different current ranges accordingly.You could buy a proper four terminal current shunt, or you could improvise with an appropriate length of constantan wire or manganin wire.Bear in mind that the current shunt will be after the voltage sense feedback, so at high load currents your output voltage will droop a bit.

author: wigman27
author_rank: Regular Contributor
timestamp: July 03, 2013, 11:10:27 pm
content: Hi Iain :-)Just monitor it for the three main outputs. I am going to have a microcontroller and lcd anyway so I really just need to supply the adc with the volt drop.I did forget to mention that, that was another thing I was concerned about using a current shunt was the volt drop across it on the load. I think the key is to use a low resistance shunt and use a current sense amp with a high gain. Something similar to the INA219 Dave uses in his rev c schematic.ThanksLee

author: IanB
author_rank: Super Contributor
timestamp: July 03, 2013, 11:37:45 pm
content: Well, for many reasons a computer power supply is not an ideal base for a lab power supply, so I would not try to go too overboard with the design. With the typical quality of voltage regulation on such a supply, a 200 mV drop across the current shunt at maximum current is not going to be of much concern. Of course if you put something like Dave's circuit downstream of the PC supply then you have much more control over the operating characteristics.But when you have finished learning you will either build a linear supply to a well established design using a transformer, or you will simply buy a lab grade linear supply...

author: Paul Price
author_rank: Super Contributor
timestamp: July 04, 2013, 03:33:58 am
content: Wigman27,With a simple amplifier (see attached), you only have .01 V of drop across a shunt for each amp, so your regulation would be excellent.Accurate and cheap shunt resistors are not a problem. A surface mount 1-W .005 or .01 resistor just costs a few cents.You can amplify the current sensed by these small-ohm shunts to drive a meter or feed into the A2D input of a MCU.I use an ATX power supply to work with many of my projects and it is an excellent source of +3.3, +5 and +12 at about as much current as I can usually need for any project.When I first needed a power supply for my electronic work I looked at the prices and then soon discovered I could use a discarded ATX power supply and get an excellent bench supply for almost nothing rather than buy one that would cost hundreds of US bucks for a built unit. It has been part of my lab equipment now for many years.See the attachments about how to make a very simple amplifier for low valued shunts so as to drive a panel meter or feed into your MCU. Fore each 1amp of current the amplifier output gives you 1.01V and so you can add a resistor to a mechanical ammeter if to calibrate the output. Use your DVM ammeter to calibrate the current adj. pot in series with the output to a meter you use to display current.You can buy quan 1 of a precision shunt for 38 cents..hope that is not too expensive, see the attachment from Farnell.comWith the money you save making your own power supply you might even find you can afford to buy quite a few of them.

author: wigman27
author_rank: Regular Contributor
timestamp: July 04, 2013, 04:15:18 am
content: Quote from: Paul Price on July 04, 2013, 03:33:58 amThat was my thinking :-) I had one here and I thinks it's a good way to learn analogue electronics is to build one.My initial plans were to use a stand alone shunt and just run the sense wires into the pcb. I was concerned about finding connectors and running upto 20a through a pcb. Does it cause many problems running that current through?Lee

author: ddavidebor
author_rank: Super Contributor
timestamp: July 04, 2013, 05:26:32 am
content: A computer power suppy is not a good lab one.It's earth referenced, common ground and without current limiting.It's still useful, if you've something that alredy work, you can power it on.But first a "serious" ones should be better, even a chinese crap power supply.

author: poorchava
author_rank: Super Contributor
timestamp: July 04, 2013, 06:16:31 am
content: I actually thought about using an ATX supply as a high voltage, high power supply. Modern, cheap PSU's have an absurdly powerful 12V rail(s). Something like that can be converted to something like 100V @ 5A or even more. May prove useful.On the other hand it's rather hard to get a dedicated boost controller for voltages above ~60V, so a custom pwm cpntroller / mosfet driver may be necessary.I also run my LED lighting from an ATX supply.

author: Paul Price
author_rank: Super Contributor
timestamp: July 04, 2013, 06:37:46 am
content: Not only is an ATX power supply a very good power supply, but the combination of outputs gives you about all you need to work with any circuit. The outputs are short-circuit protected.The important thing to do in constructing the current-measuring amplifying circuit is to locate the shunt as close as possible to the 1k input resistors of the op-amp because because there is a possibility of and noise pickup oscillation if you don't.If you invest in a better op-amp than the US 40-cent LM358 (from Farnell), something like it but with less offset voltage and noise you can actually change the 1k resistors in my circuit to 100-ohm and you can use an .001 -0hm resistor for a shunt.That means that at 1 amp out of your power supply, only 1mV of voltage is adding any error to the output voltage.A one-watt shunt .01 ohm shunts means you can measure 10amps without overloading the shunt. If you decide to use a x1000 amp with a .001 ohm resistor, you should substitute 100-ohm resistors for the 1k resistors and parallel the 100k resistor with a 1k resistor in series with a .01uF capacitor to cut down noise. Remember again, It is especially important to locate the 100-ohm or else the 1k resistors directly onto the shunt ahead of the ATX return wires and also try to position the op-amp as close as possible( yet make it neat looking) to the shunt as well.A .001 ohm 1 watt shunt will theoretically allow you to measure up to 100-ampsy, but that is a lot more current than you will usually want to play around with as a beginner, but the very small voltage drop is a genuine plus.With any power supply you must exercise caution. Always connect the ground connections (returning to the ATX (and going to the shunt) first to the circuit you are working on.I bought a medium size project box with a metal cover and mounted thumbscrew banana jacks on the front along with a 3-position switch to switch between three different shunts (.01-ohm that each have their own amplifier) so I can easily see the 3.3V current or the 5V current or the 12V current. I also mounted a small LED for power light.The ATX power supply works so well that I never bought a factory-built unit.For most of my projects, I simply use the 12V output connecting to a small 7805 T0-220 regulator on my breadboards or sometimes I use a 3.3V regulator for ultra-clean MCU power and the ATX 12V power is used on the breadboard to power op-amps and comparators and other circuits. Sometimes I mount the 7805 on the breadboard upon a small heatsink if I am going to use any small (>100mA) of power in my test circuit. If I need a variable output voltage, I use a T0-220 cased LM317 mounted on a small heat sink right on the breadboard with a small pot for voltage adjust from 1.2 to approx 9V running off the 12V supply to the breadboard.With the money I saved not buying a power supply ready-made, it was much quicker for me to save enough money to buy an oscilloscope.This week I am adding a switchmode 5-Amp max regulator to my ATX supply. I will run it off the ATX 12 V power supply that has been boosted to to approx 28V by a simple boost convertor. The 28V then powers the switchmode buck regulator for 0 to 24V very clean output to add to the versatility of my custom made power supply so that I can also have variable voltage output.

author: Paul Price
author_rank: Super Contributor
timestamp: July 04, 2013, 06:50:15 am
content: Wigman27, you say " I was concerned about finding connectors and running upto 20a through a pcb. Does it cause many problems running that current through?"It is no problem at all, just make sure the high-current traces going to and fro the shunt resistor on the PCB are at least approx. 1cm (1/4-in wide). Connect the current-amp input resistors (1K resistors for .01 ohm shunt) directly to the pins of the shunt.

author: Paul Price
author_rank: Super Contributor
timestamp: July 04, 2013, 07:11:33 am
content: Poorchava, it is actually quite easy to build a HV power supply at some considerable current using the ATX +12V output as the basis to power it. All it takes is an inductor (the size of which must be of course proportional to the current you need), some magnet wire (the size again proportional to the current you need) , a HV fast rectifier, HV electrolytic cap, and a cheap HV N-Chan MOSFET and just a simple 555 timer and two transistors to make a powerful boost converter at just about any high voltage you want (ok, approx 15 KV is practically a limit with an added voltage multiplier you build and requires some HV small nF valued ceramic capacitors). Of course the 555 requires several resistors and a small valued electrolytic and 4 small valued ceramic caps as well.You can also make a little less circuit sprawl by using a ULN3844 SMPS controller which has just about the same component count, but since the 555 circuit I designed only requires just two transistors, it is a close competition. The transistors are required for current sense and voltage regulation with the 555 operating as a current-mode switcher with a fixed on-time and 500KHz switching freq.And what is nice about it, if you take apart a discarded ATX power supply, you will find the large toroid cores you need to use, as well as nice 450V electrolytic caps, so you can easily make your HV power supply almost for free.

author: wigman27
author_rank: Regular Contributor
timestamp: July 08, 2013, 02:20:43 am
content: Hi Again,I have decided to use the ATX and also incorporate the battery powered option when its not plugged in. The problem I am facing with this is I want to be able to turn off the battery supply when the ATX is plugged in.This is really easy with a relay but I wanted to use a semi conductor if I could. The way I had thought of doing this is to use a P channel depletion mosfet That acts as a "normally closed switch" in the main line from the battery. Then I would send a 12V from the atx to the gate and "open the switch" when the ATX was powered. Something like thisThis is the first time I have ever used MOSFETS and I am having trouble finding one, There doesn't seem to be too many P channel depletion MOSFETS around. I also looked at a P type JFET but they only seem to be low current and high resistance.If someone could please help with the MOSFET selection or advise of a better way to do this I would really appreciate it :-)Thanks Lee

author: C
author_rank: Super Contributor
timestamp: July 08, 2013, 02:40:58 am
content: You could use a solid state relayhttp://www.vishay.com/solid-state-relays/form A is normally openone example of form Ahttp://www.vishay.com/docs/83842/lh1556aa.pdfform B is normally closedone example of form Bhttp://www.vishay.com/docs/83822/lh1523bb.pdfnote also the mosfet drivers on first linkC

author: Paul Price
author_rank: Super Contributor
timestamp: July 08, 2013, 03:07:59 am
content: Use a P-Chan MOSFET with a low RDSon, not so hard to find.

author: wigman27
author_rank: Regular Contributor
timestamp: July 08, 2013, 03:30:56 am
content: Hi,I like the sound if the solid state relay but they have a high Ron.Paul,Please correct me if I am wrong but that p type MOSFET is an enhancement type and won't conduct without the 12v gate voltage, which I need it to conduct without gate voltage and not conduct with gate voltage. Is that what the transistor over comes? Sorry for the dumb questions, I'm just not sure what the circuit is doing in your picture.

author: C
author_rank: Super Contributor
timestamp: July 08, 2013, 03:59:56 am
content: Just use the normally on solid state relay to hold the gate of a P mosfet to ground.The P mosfet when on connects the battery to the power supply input when the relay off and a battery is connected with a voltage greater then the P mosfet's gate turn on voltage.when the ATX supply is ready for use, turn on the relay which in turn turns off the P mosfet which disconnects the battery.The higher ron of the solid state relay is in the gate circuit not the load, no problem.C

author: wigman27
author_rank: Regular Contributor
timestamp: July 08, 2013, 04:44:32 am
content: OK,Sorry.. I was having a moment.. I thought that you still required a positive voltage or negative voltage (greater or less than 0) to turn it on.. I have done a bit more reading now and i think i have finally got it!! The P type is effectively on when it is grounded and off when the gate voltage is applied.. Is that right?? So sorry about that.If I have to drive the gate with relay or a transistor anyway then I really would just be better off using a relay only. What do you think?Am I better off with a MOSFET or solid state relay? Sorry again about the brain fart in the relay resistances..Lee

author: Paul Price
author_rank: Super Contributor
timestamp: July 08, 2013, 12:17:52 pm
content: You got it backwards, the P-chan is turned off when the gate voltage approaches the source voltage. When the power supply is turned on it turns off the P-chan and the p-chan is turned on by the resistor on the gate connected to ground when the ATX is off.

author: wigman27
author_rank: Regular Contributor
timestamp: July 08, 2013, 12:22:49 pm
content: I think I have it now!My brain is fried!Thanks Mate!Lee

author: wigman27
author_rank: Regular Contributor
timestamp: July 20, 2013, 04:05:38 am
content: Hey again,I know that I am probably going to open up a can of worms here BUT I really would like to know your thoughts on what type of system is best for a fixed voltage / fixed current power supply? As you probably know, I was originally inspired to build this power supply from Dave's lab power supply but due to the issues with the LT3080 I am re thinking how to do it. I would like to put out 24v and up to maybe 2 or 3 amps. I would like to supply it using down to 5V (battery powered) so I am going to need to boost the voltage at some stage. So here is the question, What type of voltage regulator system do you think is best for this purpose? The ones I know of so far and am considering are as followsStraight Linear RegulatorLDO RegulatorDC-DC Switching RegulatorUse a combination of DC-DC Switching Regulator and linear (like Dave's Rev C)Make my own using a power op amp and transistorI have read quite a few posts on here saying that LM317 isn't a good choice for a power supply, I am just wondering why? Is it because its a general purpose device and not as accurate as it could be?At the moment I am thinking a boost switching regulator followed by a linear regulator may be the go but I have also had it suggested that I would be much better building my own using a power op amp. To be honest, I am worried about making it stable enough if I build it myself (as this is my first real attempt at an analogue circuit) and is it better to use an off the shelf device that has the bugs already ironed out.I know there are going to be many different opinions on this but I would love to hear them all, conversely if you know of this being discussed already that I can read then please let me know that as well.Is there any word on how Dave is going with REV D and the new regulator or if its even still on the drawing board?Thanks againLee

author: Rerouter
author_rank: Super Contributor
timestamp: July 20, 2013, 05:21:21 am
content: wigman it really comes down to how stable a regulation you can handle, if you need something whisper quiet, e.g. supplying core voltage to a large FPGA then you will want a linear stage be it the main single stage or following a switcher,however if most of your stuff could happily run off +-5% and 10's to low 100's of mV ripple then a dirt cheap switch-mode might be for you, if you want both you either get fancy with the switch-mode and filter the buggery out of it, or follow it with a linear stage like rev c,as far as a rev d showing up, dave has put the bench model on hold while he pursues the usb model,probably the hardest part i have found more recently is trying to implement stability for varying loads, when you get into the gritty details most current limit approaches aren't exactly stable, unless you slow it down quite a bit, best tip i can provide as a quick fix is add time constants you both your set points, that way the loop can gradually correct as opposed to a large jump.

author: wigman27
author_rank: Regular Contributor
timestamp: July 20, 2013, 06:44:55 am
content: Hey Rerouter,Thanks for the replyMy basic need is a lab power supply, to do everything from supplying stepper motors to control gear for power monitoring so, I guess I would need it to be as "quiet" as I can without going silly.If I use a good switching regulator and filter it a lot, I guess I run into drama when it enters current limit stages and keeps feeding the load, I guess it doesn't matter too much but could be an issue couldn't it?I do think I like the idea of of a linear following the switcher but I don't know if its overkill for what I want.What would you suggest is a good middle range linear reg capable of putting out 24V at 3A?QuoteThat's a fair point to, which method of current limiting have you found to be the most successful?Thanks Lee

author: wigman27
author_rank: Regular Contributor
timestamp: July 20, 2013, 11:49:51 am
content: Also,While we are on the can of worm questions, it would be great to know what features you consider make a GOOD lab power supplyThanks in advance!!Lee

author: Paul Price
author_rank: Super Contributor
timestamp: July 20, 2013, 12:33:08 pm
content: A good lab power supply should be:Cheap to construct or buyHave various voltage outputsGood voltage and constant current regulation...hmmm sounds a little like a ATX power supply with a little modification.To get the custom regulation you need, I often place a LM317 with a small heatsink on the project breadboard I am working with and it then provides the best regulation with some auxiliary power supply such as my ATX power supply or (if I need higher voltages) use a PC laptop power supply at 18-21V to feed it. I also mount a 7805 on each breadboard with also a small heat sink if I am working with 5V logic or MCU's,This way I don't have the voltage drop of the long wires, don't have so many power supply wires coming from a "Ideal Bench Power Supply."

author: wigman27
author_rank: Regular Contributor
timestamp: July 22, 2013, 12:22:20 pm
content: Hey,I have been doing a little bit of looking around at regulators for me to replace the LT3080 with from the Rev C schematic and checked out the LT3081 and saw the thread on that and the 15% is hopeless I agree an the LT3081 still doesn't seem to be suited to control with an MPU or best suited to the task.Having a bit of a look around, would Micrel'sMIC29152be better suited and a better choice than the LM317? I am still going to have a go at creating the regulator myself using FETS and op amps, but want to use this to get me started.Is the MIC29152 a good choice? also am I able to provide a negative voltage of -1.25v on the set pin to put out 0V on this chip? What am I looking for in the datasheet to tell me I can or can't? or is it generally OK to do with these types of voltage regs?ThanksLee

author: ptricks
author_rank: Frequent Contributor
timestamp: July 23, 2013, 12:03:32 pm
content: ATX supplies are a decent and cheap power source, but please put current limiting resistors on the outputs. The good/bad with ATX is the current they can supply. Putting some power resistors in series with the outputs can save you a lot of grief later and power resistors are so cheap it doesn't make sense not to use them.

author: Bored@Work
author_rank: Super Contributor
timestamp: July 23, 2013, 05:30:25 pm
content: Quote from: ptricks on July 23, 2013, 12:03:32 pmNo, they are not. They are crappy firestarters. I know this won't prevent brain-dead "makers" from suggesting them as if they are the best thing since sliced bread.

author: Unknown
author_rank: Unknown
timestamp: July 23, 2013, 06:48:00 pm
content: Everyone will have different requirements, here are mine:- Low voltage and current noise (max a few mV/mA rms).- Constant voltage mode from 0 V to at least 15 V (how many modern circuits need more than 15 V?) with a resolution better than 0.1 V. Why does it have to go down to 0 V? If I sink 1 A in eg. a PCB to find a short, how much voltage drop will there be over the PCB traces?- Constant current mode from a few mA to a few A or so.- Excellent stability, regardless of the load, except that I do not expect a normal bench supply to be able to sink current.- Good transient response.- Low output capacitance: if I set the current limit to a few mA, then I don't want some big caps dumping a few amps into my circuit when they get shorted.- Floating operation, to prevent ground loops and so I can stack power supplies and generate multiple positive and/or negative voltages as required.Now how many of these boxes would be ticked by an ATX supply, and how many components would you need to add to satisfy all?

author: ptricks
author_rank: Frequent Contributor
timestamp: July 23, 2013, 08:10:54 pm
content: Quote from: Bored@Work on July 23, 2013, 05:30:25 pmAll those switching supplies in tv, computers, laptops are crappy firestarters ? There is nothing different in an ATX than a supply in other devices, they are all switching supplies. The only difference is ATX power supplies tend to be high current and a resistor solves that issue.

author: ptricks
author_rank: Frequent Contributor
timestamp: July 23, 2013, 08:22:05 pm
content: Quote from: alm on July 23, 2013, 06:48:00 pmNeed to be more specific than that , if you want things like .01% it will not be cheap. Most people will be fine with a 5% supply.QuoteThat can be added with either a switching or linear regulator. You would not put 1A on a PCB to find a short, that might damage other circuits as well.QuoteLM317 could be used , add a pass transistor for higher current.Quote

author: Unknown
author_rank: Unknown
timestamp: July 23, 2013, 11:55:49 pm
content: Quote from: ptricks on July 23, 2013, 08:10:54 pmAn ATX can easily deliver tens of amps at 12 V. Delivered in the right load, this can easily start a fire. The difference between bench supplies and a TV is that a TV power supply has a well defined, well behaved load. Not a dodgy breadboard circuit.Quote from: ptricks on July 23, 2013, 08:10:54 pmWhich screws up load regulation. The output impedance of a good voltage source is in the mOhms (ideally: 0 Ohms). Guess what your resistor does to the output impedance?Quote from: ptricks on July 23, 2013, 08:22:05 pmI think < 2 mVrms measured with 20 MHz bandwidth is a fairly typical spec for a good bench supply. 5% noise sounds quite terrible in my opinion, that would be over half a volt noise at 12 V. Even the ATX spec only allows for 1 % ripple + noise worst case.Quote from: ptricks on July 23, 2013, 08:22:05 pmIf I wanted a 1 mA test current, I would use a DMM. Say you have a dead short (like in the Lecroy scope that Dave troubleshooted a while ago) of << 0.1 Ohm. Unless you go down to the uV, 1 A is a decent current to get a voltage drop you can measure (millivolts). I'm not very worried about ground planes or PCB traces burning up at 1A (unless you have extremely tiny traces), and what do I care about a shorted component releasing some magic smoke? It was shorted anyhow, and it leads me to the culprit. The voltage could be limited to 0.5 V to prevent diode junctions from turning on. This is just one example where operation down to 0 V is useful.Quote from: ptricks on July 23, 2013, 08:22:05 pmTwo LM317s (or an LM317 and an op-amp) for voltage/current regulation. A negative supply for operation down to 0 V. An external pass transistor and heat sink (you need to get rid of 12 V * max current worst case). Sounds like an ATX power supply with little modification. And this gives you a voltage range from 0 V to 9 V or so. You can't put them in series because they're not floating. And you've essentially built a lab supply from scratch apart from transformer, rectifier and big filter caps.Quote from: ptricks on July 23, 2013, 08:22:05 pmExcept that good transient response and low output capacitance are hard to combine.Quote from: ptricks on July 23, 2013, 08:22:05 pmTransformers are a fair amount better than ATX supplies for floating operation. A transformer-based power supply lacks the solid trace that connects the output of an ATX power supply to mains earth, limiting the maximum float voltage to approximately a millivolt. Floating a transformer's outputs won't compromise EMI or safety.It's not impossible to built a decent (not great in the noise or stability department, but decent) switching bench supply. It doesn't start from an ATX supply, however.

author: wigman27
author_rank: Regular Contributor
timestamp: July 24, 2013, 12:08:54 am
content: QuoteYep, the power supply will have 4 outputs, 3 fixed voltages supplied direct from the ATX and the other will be a fixed voltage fixed current variable outlet."C" has been helping a lot with a number of things and I am monitoring the current in the three outlets from the ATX with the ACS710 which has a variable overcurrent latch that will drive a MOSFET. So I have variable current protection on those circuits.QuoteI really appreciate your help, I really do, but saying stuff like this doesn't help anybody. ATX power supplies are used throughout many many devices.This power supply is my first one and using it as a learning curve. The next one I build will have isolated outputs supplied by a transformer directly. I had an ATX and was a good opportunity to learn the ELV side of things without having to worry about 240V as well. I understand its not a perfect solution but its perfectly fine for my needs.Thanks very much to everyone else for you thoughts on a good power supply. Your help is very much appreciated!!

author: ptricks
author_rank: Frequent Contributor
timestamp: July 24, 2013, 02:25:39 am
content: Quote from: alm on July 23, 2013, 11:55:49 pmPut a 5Watt 25ohm resistor on the 3.3V line of an ATX supply then short it and wait for it to catch fire, reply back when it does.QuoteOutput impedance doesn't matter either in this situation. The 3.3V supply passing through a 25 ohm resistor to power the avr or pic chip on the breadboard isn't a problem.QuoteIt doesn't matter in this situation. We are not talking about replicating a $1000 bench supply. ATX is 5% for regulation and ripple isn't an issue with these supplies and hobbyist projects.QuoteCostly way to troubleshoot. You do not load a circuit that is shorted to find a short, you isolate the parts by breaking current paths to prevent any further damage or at worst provide an alternate lower resistance load to consume the current. Loading a circuit , especially on sensitive circuitry can cause thousands of dollars in damages. Fields like aviation electronics would fire you in a minute for attempting something like that.QuoteThis isn't about building an Agilent class power supply, it is about hobbyist wanting something to get started without breaking the bank. You have to start somewhere and for most people that is with what they already have laying around. This is why so many people don't approach engineers with questions because they are often talked down to or discouraged vs encouraged.Is an ATX supply going to replace Lab supplies , of course not, but it is more than good enough to run breadboard projects and lots of other stuff hobbyist may make. A lot of the circuits that hobbyist make are the same types that you might find on desktop pc, interface chips, usb devices, LED, stepper motors, etc.

author: Unknown
author_rank: Unknown
timestamp: July 24, 2013, 10:56:36 am
content: Quote from: ptricks on July 24, 2013, 02:25:39 amUse an LCD that might use say 100 mA for its backlight, not exactly something unusual for your average hobbyist. Will this LCD work on 0.8 V? Even a modest 10 mA draw will drop the voltage by ~10%, which could easily be out of spec for some 3.3V parts. You can lower the resistance, but this increases the short circuit current. Resistors are a lousy way to limit current, which is why they're rarely used except for very well defined loads.Quote from: ptricks on July 24, 2013, 02:25:39 amI listed the features that:Quote from: wigman27 on July 20, 2013, 11:49:51 amAn ATX supply will not make a good lab power supply, or even a decent one. It might make an okay supply for some high-current 12V projects.Quote from: ptricks on July 24, 2013, 02:25:39 amCostly? Few components (except very low value, low power resistors) are going to die from a few hundred mV, especially on the power pins, apart from those already shorted. How else would you troubleshoot a short between power planes, like in the Lecroy scope that Dave was trying to fix. You can desolder all parts on that rail, but that's hardly any less invasive. Good luck isolating parts that are connected to the same power planes. Shoving a significant current through there makes for a very sensitive four-terminal measurement, since you can easily detect milliOhms, even with a cheap DMM, without being sensitive to noise or contact resistance.

author: wigman27
author_rank: Regular Contributor
timestamp: July 27, 2013, 06:47:32 am
content: Hi Again,I have been having a play with a few ways to replace the LT3080 and I have started to experiment with the LM675 as a direct output. The circuit seems to work quite well, until I start to limit down to 200mA or below, The output voltage just won't drop low enough. There is quite a difference between the input of the LM675 which I thought was quite strange.This is the first time I have used LTspice, would someone with a little more experience please give me an idea to what might be going on here, is it a simulation issue or a design issue? I am using a third party library for the LM358 and the LM675 (which I have included). I'm not sure if that's the issue either..The location for the Sense resistor isn't ideal but has to be there for simulation I guess.I would really appreciate your thoughts on the design and simulation and the use of the LM675.ThanksLee

author: WarSim
author_rank: Frequent Contributor
timestamp: July 27, 2013, 07:33:15 am
content: Be carefully don't get into the weeds.A while ago (years) I designed my dream lab supply. I worked on it for months. Used every compensation method I knew in endless improvement consolidation cycles.I meet all my specs. 1us response, 500uv ripple, 1.25A, +-60v, 5v ,3.3v. Once I met my goals I stepped back and reviewed it. It was a monster. Very complex and allot more parts than planed.In the end it never made it to final assembly, I rationalized my expectations and bought one that was good enough.Don't over spec your project. I suggest you find a lab supply that would be in your budget. Go through the specs and make a only few changes to what you need. Then design one for your self. Don't learn the hard way, why what you want is not for sale.

author: wigman27
author_rank: Regular Contributor
timestamp: July 27, 2013, 07:39:55 am
content: Hi WarSim,Yes, your right, its probably much easier to go and by one but I am having a lot of fun doing this, I haven't done anything with analogue electronics before and I am finding a great learning experience.Again, its probably not the ideal first project, but in the end I hope to get something out of this I can use.ThanksLee

author: ptricks
author_rank: Frequent Contributor
timestamp: July 28, 2013, 11:54:03 am
content: I don't know if you have read it before but there is an old agilent handbook on power supplies that has a lot of good informationhttp://cp.literature.agilent.com/litweb/pdf/5952-4020.pdf

author: wigman27
author_rank: Regular Contributor
timestamp: July 28, 2013, 12:15:50 pm
content: Hi ptricks!No I haven't seen that, it looks really good! I will read it before I go any further.Any more feedback on the existing ideas from anyone? anything at all would be good.Lee

author: robrenz
author_rank: Super Contributor
timestamp: July 28, 2013, 12:51:44 pm
content: Download thePower Designs power supply manualsand study those schematics and theory of operation. Very simple, very quiet, very stable.