Local/Front End PSU Project - Page 1
Forum section: electronics / projects

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 06:21:46 pm
content: I've been thinking about building a small front end regulator/PSU to be used on the desk near any project I'm working on.Something like this:I think that would be nice, I wouldn't mind having 2-3 (or maybe even 5) of these.Requirements:Input: 5-30VOutput: 0V to near input voltage.Up to 3A (depending on input and output delta and cooling)Voltage regulation and Current limiting.Precision: 5-10mV and 5-10mA.Constant current and trip.Reasonably small.Show set Voltage and Current limit plus actual Voltage and current together with power in Watt.Unit runs off input voltage, no other wires needed.Easy enable/disable output (dedicated button)Nice to have:Programmable control and monitoring, e.g. USB or perhaps BluetoothOptional remote sensing.Remember last settings.Thermal protection.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 06:27:39 pm
content: I was thinking about the good old LM317, but it's just got too many limitations.So I have been playing around with a simple MOSFET based regulator circuit:This is stable and does a nice job of regulation (ignore the giant capacitors, that was what I had)The tiny heatsink is just for testing, a larger one will be needed.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 06:36:40 pm
content: A few switch on measurements with different loads (don't want any overshoot)No Load:500mA Load1000mA LoadCould be a bit faster, but no overshooting.

author: SeanB
author_rank: Super Contributor
timestamp: August 31, 2013, 06:47:52 pm
content: Very nice, build a few into a small aluminium case so you have a long brick on the bench and then you can have a few outputs in an easy to use form factor.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 07:03:31 pm
content: Quote from: SeanB on August 31, 2013, 06:47:52 pmThanksI think I will keep them in separate enclosures just for flexibility, but yeah you could do 3 in a row, that would cover most uses. Maybe something clever to make them stick together, perhaps a couple of magnets.I am considering adding a switchable capacitor multiplier on the input, then they can be used with e.g. a 24V switchmode psu and still provide a reasonably clean output. It will of course cost a couple of volts, but worth it for a cleaner output.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 07:28:42 pm
content: Added a larger heatsink for higher current testing and getting a bit more involved

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 07:43:41 pm
content: Made a couple of small PCBs for the regulator circuit and the high side current sensor. The large heatsink is just temporary, I a planning to use something smaller with a couple of 40mm fans.It's a bit tricky to use a general purpose opamp in differential configuration and get something fairly accurateAnyway, this gives me 500mV per 1000mA over a 100mOhm shunt. That should work fine with my requirements, I might need to add a zener and a schottky to protect the ADC input.

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 31, 2013, 07:50:41 pm
content: Have you got an example schematic for the "switchable capacitor multiplier"? I'm not sure what you mean by that.

author: tszaboo
author_rank: Super Contributor
timestamp: August 31, 2013, 07:58:19 pm
content: I am considering adding a switchable capacitor multiplier on the input, then they can be used with e.g. a 24V switchmode psu and still provide a reasonably clean output. It will of course cost a couple of volts, but worth it for a cleaner output.Beware, making a high current capacitor doubler is not a trivial task. Capacitor ripple current are also big, and there is only few ICs out there, handling some 200-500mA currents.I would rather use inductors. If you succeed anyway, let us know.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 08:00:28 pm
content: Quote from: TerminalJack505 on August 31, 2013, 07:50:41 pmI just mean a capacitor multiplier that can be bypassed with a switch. E.g. when you have a clean input, it's not needed and you can get a higher output voltage, but when using e.g. a switch mode supply it might be worth a few volts less to get a cleaner output.A capacitor multiplier is very simple, just a couple of capacitors, a transistor (or a couple in darlington) and a resistor. It can effectively turn a 1000uF capacitor into a 1F capacitor at the cost of a voltage drop.

author: SeanB
author_rank: Super Contributor
timestamp: August 31, 2013, 08:04:24 pm
content: I did make one years ago using a TDA2009 audio amplifier. Worked, but ran rather warm at any load, and topped out at about 1A. Was bigger than a SMPS using a 3524 chip and inductors as well.What you are describing is a passive preregulator to reduce ripple and hum. Better will be to use a LC filter permanently in the supply line, using a high current toroid and a 1000uF 35V low ESR capacitor after it, preferably with a common mode choke as well before it and a 100n 100V disc ceramic cap before and after it.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 08:11:12 pm
content: Quote from: NANDBlog on August 31, 2013, 07:58:19 pmI believe you are thinking about a voltage doubler, that is not what I had in mind.. a capacitance multiplier is a way to make a small capacitor act like a huge capacitor at the expense of a voltage drop.The circuit looks something like this:

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 31, 2013, 08:12:13 pm
content: Quote from: jaxbird on August 31, 2013, 08:00:28 pmOkay. I understand. I guess I was thinking as "switchable" in some other sense. I thought maybe you were going to use multiple capacitors and switch them in some way so as to reduce the voltage to a point that the linear regulator would have it's Vin - Vout minimized.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 08:27:28 pm
content: Quote from: SeanB on August 31, 2013, 08:04:24 pmSure, you have to size your transistor(s) and base current (resistor) according to the expected load current and max ripple for best results. But the total dissipation will be the same when combined with a linear regulator. Just a matter of whether it's the transistor or the regulator (MOSFET) dissipating the power.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 08:44:11 pm
content: Quote from: SeanB on August 31, 2013, 08:04:24 pmA capacitance multiplier is not a great choice for a class AB amplifier, it will have too many current spikes. It works much better with a pure class A amplifier where the current is more stable. For class AB, real capacitors are much more effective.

author: Spikee
author_rank: Frequent Contributor
timestamp: August 31, 2013, 09:01:02 pm
content: Wouldn't it be much easier and less expensive to build a adjustable switching supply that has those things ?With decent output filtering it would hardly be noticeable that you even use one. The TI power architect on their webench platform would be a good place to start. You could search for a adjustable switching supply that also has an adjustable current limit using the TI application and have a working design in 10 minutes. After that you could evaluate and improve the values of the passives.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 09:21:47 pm
content: Very happy with my E-load build for this project:Details here (https://www.eevblog.com/forum/projects/electronic-load-build-log/)

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 09:30:36 pm
content: Quote from: Spikee on August 31, 2013, 09:01:02 pmSure you could go that route, and it would be much more efficient, but you will end up with considerably more noise on the outputs or if you have enough capacitance/filtering on the output you'd probably have pretty poor current limiting/regulation.There is usually a reason why lowly 200W lab PSUs are giant heavy beasts.

author: Marco
author_rank: Super Contributor
timestamp: August 31, 2013, 10:18:41 pm
content: Quote from: jaxbird on August 31, 2013, 07:03:31 pmIt wouldn't accomplish anything more than just putting two voltage regulators in series AFAICS.If wideband noise can couple through the voltage regulator it can couple through the capacitor multiplier.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 10:30:31 pm
content: Quote from: Marco on August 31, 2013, 10:18:41 pmIt's not a magic circuit.. but it will remove the rough ripples from switching regulation or a transformer like a huge capacitor would.A choke before and after surely improves the outputYou can read more about the theory here:https://en.wikipedia.org/wiki/Capacitance_multiplier

author: Marco
author_rank: Super Contributor
timestamp: August 31, 2013, 11:22:33 pm
content: Quote from: jaxbird on August 31, 2013, 10:30:31 pmA voltage regulator with the same input capacitor and the same voltage drop at Vsmin will remove far more ripple.An idealized voltage regulator is a capacitor multiplier too ... except it multiplies by infinity.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 31, 2013, 11:34:09 pm
content: Quote from: Marco on August 31, 2013, 11:22:33 pmYou will need to provide some general proof for that statement.Thanks

author: Marco
author_rank: Super Contributor
timestamp: September 01, 2013, 10:22:59 pm
content: Just simulate the figure 2 circuit fromthis page, without the rectifier and 4700uf capacitor and using a DC+AC-ripple input ... then put a zener in parallel to the capacitor connected to the base of the transistor to turn it into a poor man's linear regulator and compare the results.The reason the capacitor multiplier makes sense for the application in question is precisely because they don't really want a regulated voltage, they just want a steady voltage ... with a voltage regulator you have to be able to satisfy the drop out for the worst case mains voltage, which in non worst cases could deprive you from extra voltage headroom (and you can use less power in the power supply, but I find that a poor argument ... the power is going to be burned somewhere in the amplifier regardless). When you do need a regulated voltage though the approach makes no sense.

author: Zbig
author_rank: Frequent Contributor
timestamp: September 03, 2013, 06:41:31 pm
content: Nice idea but another way to achieve similar goal would be to build small and handy desktop "remote control unit" talking SCPI via USB to remote programmable lab PSU. Use Raspberry Pi (with some LCD, perhaps) and you're basically there. Not terribly elegant and a bit of an overkill probably, but customization and programming/scripting possibilities would be virtually unlimited.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 04, 2013, 06:42:32 am
content: Quote from: Marco on September 01, 2013, 10:22:59 pmFor the capacitance multiplier to be most effective you need to use a darlington, and a couple of resistor, capacitor stages to smooth out the control current. In that configuration there is no difference in the noise on the output between a decent regulator and a capacitor multiplier.For this project a fixed pre-regulator is not a good choice as one of the main requirements is that it should work with a wide range of input voltages. (see 1st post for details) That is one of the reasons I want to have a go at the capacitance multiplier.Cheers

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 04, 2013, 07:04:03 am
content: Quote from: Zbig on September 03, 2013, 06:41:31 pmThanks, you could do that, it would probably be easier just to write some code for an Android tablet than building a dedicated device for a remote control.It would be nice too, but a different kind of project.With these my plan more towards having the flexibility to e.g. do multiple voltages or individually controlled current limiting from a single rail. Plus you will not have to run really long supply leads, so unless you have (and use) remote sensing, this is likely to give a more accurate supply voltage. And you could run these from any supply (30V or less), battery, 24V switchmode etc.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 04, 2013, 07:14:03 am
content: Made a new board, this will hopefully be working with regulation, current sensing, limiting etc:I have not had time to populate it yet, but I hope it does the whole analog portion. Still need to add controls, micro, display, temperature monitoring, fan control...

author: Jebnor
author_rank: Regular Contributor
timestamp: September 04, 2013, 11:32:59 pm
content: Quote from: jaxbird on September 04, 2013, 07:14:03 amVery nice etch. Is your copper flood your groundplane? Is it connected anywhere?

author: Dave
author_rank: Super Contributor
timestamp: September 05, 2013, 12:53:35 am
content: That board doesn't look like it was etched, but rather milled. I may be wrong, though.You might want to avoid floating copper islands, because they can lead to EMC issues. The same goes for copper peninsulas (peninsulae?), they act like antennas and pick up garbage.Of course, if this really is a milled board, you are going to have quite a challenge to avoid these issues.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 06, 2013, 01:51:55 am
content: Quote from: Jebnor on September 04, 2013, 11:32:59 pmQuote from: Dave on September 05, 2013, 12:53:35 amThanks, yeah, Dave is right, I used milling. Much prefeer that to etching.It's a valid argument, but this is just a quick prototype.. It takes a lot longer to remove all the extra copper, so for quick prototypes I usually just do 3 runs to get decent gaps and make the soldering a bit easier. That way it only takes 20-30 minutes including drilling.I forgot to enlarge a few of the pads too, but still much better to work with than a vero/strip board

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 06, 2013, 02:01:28 am
content: Got most of the test board populated:Since it's single sided, it's going to have a few air wires for the grounds and supplies to various parts.The tiny heatsink is just for initial testing.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 06, 2013, 04:54:31 pm
content: Analog test board fully assembled and running:I did run into a couple of minor issues, first I only had 1 shunt resistor mounted for initial testing, and for some reason I was lucky enough to get a faulty resistor that was around 50K instead of 0.5RA brand new faulty resistor is not something you see every day. Anyway, I also managed to swap the +/- inputs on the differential amp, so it would go negative instead of positive when a load was applied, easily fixed.Besides those, it appears to be working.Still need to do a number of measurements to see what the performance is like.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 09, 2013, 05:48:26 am
content: Ran a few tests on the analog prototype board.Here switch on into a 12V/20W light bulb with the current limited at 1000mA (bulb resistance changes during heat up)The constant voltage looks good too, no overshooting, looks the same as the captures I posted earlier.But I have noticed a bit of oscillation during constant current, it's only about 25mV P-Pwhen I current limit the output to a few volts, into a load, while the voltage control is set to max. (the oscillation gets smaller and disapears as the set voltage gets closer to the current limited output)25mV peak to peak max (using a 12V supply) is not a big deal, but I'll try see if I can get rid of it with a few subtle changes in capacitors and resistors.I would like to do a few pulse tests, but while I did add a few components on my load build to have this feature, I didn't implement the software for it yet

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 12, 2013, 08:42:08 am
content: Got the oscillation fixed with a capacitor change, also found some much greater oscialltion when connecting a capacitor and a resistor on the output during constant current. Anyway, seems to be fixed now.So I have moved on to the digital control, just want a simple display with one rotary encoder, a button and an LED to indicated when the output is enabled:I did find another problem though, there is something like 2.5V on the outputs for a short period during power on, oops.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 12, 2013, 09:49:24 am
content: Fixed the 2.5V output for 600ms during power on, using a diode. Well, it's not perfect, I always have 60mV on the output when it's off. I suspect it's due to a minor offset between the micro's PWM output and the analog board, probably some better wiring will improve that. Anyway, I'm not aiming for high precision, but always nice to have something decent.Switching on the output is a bit slower with the PWM and low pass filter rise time, but nice and clean with no overshoot even without load:I have a couple of trimmers on the analog board to fine tune the differential amp for current measurements. It's just offset and gain, then I can get away with standard 1% resistors and an LM358 opamp.It does seem to be fairly accurate: (Measured current and set current on the display)Meter agrees:And another value just to confirm that I've not just been lucky at 1AMeter agrees again:My goal is only 10mV and 10mA precision, but it seems I should be able to go for near 1mA on the current. Voltage is probably best just left at 10mV as it's a 20V range on both PWM and ADC, but current is only up to 3A.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 12, 2013, 10:57:16 am
content: Made another little board, this is for micro and display supply, plus fan control:I didn't want to use LM317 for the main regulator, but seems I now end up with 3 of them for other purposesI will try this little heat sink with a couple of 40mm fans and see if it will be enough, I suspect I might need something slightly larger

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 12, 2013, 11:26:45 am
content: New supply for digital soldered in and grounding improved:This is much better, but still a small voltage on the output when it should off, but as low as 20-30mV now.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 12, 2013, 12:41:39 pm
content: I've been thinking a bit about the case design.I think something that will work both as stackable and side by side is the most flexible. I did orginally think about display and controls on the top, but I think better try fit everything on the front and inputs at the back, something like this:It will be about 120mm wide, 100mm deep and 50mm tall.Any inputs are most welcome.

author: dr.diesel
author_rank: Super Contributor
timestamp: September 12, 2013, 12:47:04 pm
content: Quote from: jaxbird on September 06, 2013, 01:51:55 amVery nice! Which PCB mill are you using?

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 12, 2013, 12:56:38 pm
content: Quote from: dr.diesel on September 12, 2013, 12:47:04 pmThanks, it's a DIY thing, I've built a few different CNC machines, this one is quite small (around 200mm x 160mm work area), but very rigid for good performance. But my spindle is not so great, needs a bit of experience with it to get good results with minimum runout. I'll post a picture of it.Not sure what the actual precision is, but it's using acme screws, not ball screws, so that is probably the biggest weakness. And it's not very fast, max 800mm/min without risk of loosing steps.

author: dr.diesel
author_rank: Super Contributor
timestamp: September 12, 2013, 01:05:02 pm
content: I despise chemicals and have been thinking about either buying or building one myself. I need to do some additional reading, I believe there are several units available for purchase for < $1000 these days.Look forward to the picts!

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 12, 2013, 01:21:45 pm
content: Quote from: dr.diesel on September 12, 2013, 01:05:02 pmAgree 100%,I used to etch and it's a mess, also never felt comfortable disposing of the old chemicals, plus drilling holes was always a pain, but sure, if only doing smd that is not much of problem.Just keep in mind that machining FR4 fiber boards isn't 100% healthy, lots of fiber dust that I'm sure isn't healthy so I've enclosed mine in a small box with a window to minimize spread and dampen noise. Plus of course routing does take some machine dependent experience for good results.It's not hard to build one, but I've found that using at least 1/2" / 12mm alu stock/plates is needed to make it sufficiently rigid for good accuracy.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 14, 2013, 05:24:58 am
content: Designed the base part of the little case for the PCBs I have now, did end up a bit complex, but I will attempt to print it and see how it comes out:Plan is to complete it with the current mess of boards and wires, so I can use it for some time and see what works well and what I want to change.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 14, 2013, 06:39:32 am
content: Printing base of the enclosure is done, a bit messy. I had the temperature slightly too high, forgot to adjust it after filament change.Anyway it will do fine for the prototype and it seems to fit nicely:

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 14, 2013, 07:28:31 am
content: I wanted the front to be something like this:But there is no way I can print that with usable results. It would require support material.I will drop the tilted display and go with this instead:

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 14, 2013, 08:00:33 am
content: Fresh out of the printer, but unfortunately the display doesn't fitThat happens a lot with 3D printing, usually a mix of printer making holes smaller and designer making wrong measurementsMinor corrections and a new one is on the way.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 14, 2013, 09:09:18 am
content: 2nd attempt is much better (but hard to take pictures of these displays)Now I just need a lid and a button for the rotary encoder.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 14, 2013, 09:34:24 am
content: Will need some wire work before I can fit a lid on though

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 15, 2013, 11:10:02 am
content: Cleaned up the wiring and put a lid on it, it's now fully functional:Not so pretty, but at least I know it looks worse inside2 x 40 mm fans controlled by micro and a temperature sensor.At 20W dissipation it settles at ambient +35C.

author: jaxbird
author_rank: Frequent Contributor
timestamp: September 15, 2013, 11:37:30 am
content: I decided to use the 2x16 display like this:The voltage will show set voltage when output is disabled and actual voltage when enabled, but if changing voltage while enabled it will show the set voltage while it's being set and then after a couple of seconds switch back to actual voltage.Under the voltage it rotates between output power in Watt, temperature, input voltage and how much it's dissipating in watt. Changes about every 2-3 seconds.