Another? Homebuild lab powersupply? - Page 1
Forum section: electronics / beginners

author: 97hilfel
author_rank: Regular Contributor
timestamp: February 02, 2017, 02:47:07 pm
content: So I currently try to do two things at the same time:- Upgrade my PC and- get stuff for a electronics labSo I thaugt, maybe I get away a couple of bucks cheaper, and a whole lot of expirence richer by building my own lab power supply.BUT I dont want a default one with segen segment displays... I want to build one like the Rigol once and a completly linear regulation.My Idea was to use a big toroidal transformer, actually tree of them, two for galvanicaly separated, independed channels, one for generic stuff, like the main driver.I wanted to use synchrounous rectifyers, and multiple transformer steps, which I can exchange as soon as the output voltage passes some margins like the output voltage goes down from 15V to 5V I will switch between a 20V AC output transformer to a 10VAC transformer. And my Insagne step: I want to watercool the regulation mosfets, to reduce the volume I have to use for solid aluminium heatsinks and getting away with using silent fans that never really run loud.So do you guys think this make at least a little bit of sense or is it just Insagne and I should go to farnell or similar and order a normal supply?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2017, 04:18:05 pm
content: Water cooling is unusual and not really needed of you use transformer tap switching. For learning project I would go for lower power anyway - the theory is the same and parts to blow up are cheaper and less dangerous. Usually multi channel power supplies are essentially separate units anyway - so one can start with a single channel. Commercial units could use a shared custom made transformer with many separate turns. More common transformer usually only work for a single channel, one might already need a second one for the control part.Synchronous rectifiers make some sense a low voltage and very high current, not that much at something like 12 V and higher.If it is for the costs - the cheaper way is to get a Chinese supply specified for a 3 A current and reduce the max current limit to 2 A. You usually can't get the parts for the price of these cheap supplies. If the supply is too bad - you still have the case,transformer and terminals. So it is more about learning - this also works on a 20 V 1 A supply. This can be done even without a fan.

author: Warhawk
author_rank: Frequent Contributor
timestamp: February 02, 2017, 05:28:13 pm
content: There are many designs out there. Some of them are good, some of them are bad. Most of them however share the same problem - decent mechanical construction. Start with mechanical construction analysis first. You will be surprised how much an decent enclosure or a heat sink cost. Over and above it is not worth to get it from China because of shipping costs.PS: I always wanted to build my own lab PS but I gave up. I rather use my spare time for new design ideas than walking the same way the thousands before me did.At the end you will purchase some professional from ebay. I could get 6622A for shipping costs or a broken TTI's one for 50 Eur (fixing took one weekend).

author: 97hilfel
author_rank: Regular Contributor
timestamp: February 02, 2017, 06:14:03 pm
content: Quote from: Kleinstein on February 02, 2017, 04:18:05 pmI was going to around 24V 3A supply on both channelsQuote from: Warhawk on February 02, 2017, 05:28:13 pmI already know about the enclosure stuff, I am going to let mine be cosutm build by a manufacture in germany, its "cheaper" than getting an off the shelf one and I can completly customize it. As far as I thought until now, I am gonna use something like an off the shelf watercooling loop, due to its capabilities in terms of powerdissipation, I can get away with a 240mm radiator wherever I want in my enclosure and dissipate up to 100- 150 Watts with full speed running fans, so cooling is a minor problem (but its expensive)I wanted to use synchrounous rectifyers because I wanted to try em out and dont wast the power on my rectifyers when I already have linear regulation.And I have some concernes about ripple, getting a precise and accurate voltage/current measurement and about the switching from one transformer tap to another. Thema transformer: I will get the transformers costum build. (I can afford the supply when I get some pieces every month so cost isnt the biggest issue)

author: JPortici
author_rank: Super Contributor
timestamp: February 02, 2017, 06:40:43 pm
content: my two cents: go on ebay us, get one of the remaining used GWI GPD-3303S.auction + shipping + duties will be about the same money you would invest to design/build what you are aiming at... and it will be better.what's wrong with seven segments displays?

author: 97hilfel
author_rank: Regular Contributor
timestamp: February 02, 2017, 10:36:37 pm
content: Quote from: JPortici on February 02, 2017, 06:40:43 pmI am not going to do that, I had GWI powersupplies and oscilloscopes at my old school, and they were terrible...And seven segments have a rood readability but are not good at showing more than just a numerical value

author: james_s
author_rank: Super Contributor
timestamp: February 03, 2017, 12:44:02 am
content: I built one a few years ago based on a design from a late 80s Radio Electronics magazine. Two channels of 0-50V at 0-5A, plus a fixed 5V 3A output. Linear regulators each with a pair of big TO-3 pass transistors and a huge heatsink, finding that was the hardest part. I wound my own transformer on a large toroid core from Alphacore, that's the part I'm most proud of, it looks just like a storebought transformer and works perfectly. The original PCB pattern was too blurry so I did my own in KiCad and etched it using toner transfer. I can post the schematic and layouts I used if you want.

author: tautech
author_rank: Super Contributor
timestamp: February 03, 2017, 01:18:19 am
content: While you will have great satisfaction from a PSU you build yourself (and I have), for more than something quite simple there's many better options now available.These are super quiet at low output levels as they are smart fan cooled.http://www.siglenteu.com/pdxx.aspx?id=1116&T=2&tid=17The X-E model is quite a bit cheaper than the X 1mV and 1mA model.

author: karoru
author_rank: Regular Contributor
timestamp: February 03, 2017, 02:24:33 am
content: Quote from: 97hilfel on February 02, 2017, 02:47:07 pmOnce you start searching for proper transformer to meet your design requirements, "couple of bucks cheaper" magically disappears, unless you're willing to buy cheapest transformer with required primary winding current rating, scrap secondary and wind new one yourself.If you want to go with plethora of big toroids mind that finding a premade case that will 1) fit these 2) won't bend during moving the unit 3) won't cost bazillion monies will be also a joy;)

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: February 03, 2017, 03:55:11 am
content: If you want to make alinearpower supply I would not use mosfets as the power pass element. Many beginners gravitate to mosfets but I don't understand why. Common bipolar transistors like STM D44H11 (npn) and D45H11 (pnp) are cheap and naturally suited for linear operation, as opposed to most of the newest mosfets which are optimized for low RDS(on) and intended to be efficiently driven hard on/off.I also find that sourcing a large multi-tap transformer in a single or low quantity difficult, so am in the process of rewinding a salvaged torriod right now. I am stuck on this as I don't fully have a handle on all the details of how to do a proper and safe job, and don't have the necessary wire yet. Tips or tutorials on transformer winding from any member would be appreciated! I want to make two 320VA units, with 120 Vac primary and 0,8,16,24,32 Volt secondary taps with a shield between primary and secondary to reduce capacitive coupling. Shipping costs for any heavy lump of metal to my house cost more than the transformer. It more than doubled the cost of the two torriod cores I have in hand.

author: james_s
author_rank: Super Contributor
timestamp: February 03, 2017, 04:31:42 am
content: Winding the transformer was a bit tedious but there is no magic about it. I looked at the specs on Alphacore for the optional primary windings that you can order on the cores, then wound one myself following the same specs. Then I covered it with several layers of special tape (polyester?) that I think I got from tapecase.com. Then I wound the secondaries, calculating the required turns from the number of primary turns and the desired ratios and calculated the I2R losses for the max current and the calculated length of wire and chose a wire gauge that made sense. Overall it was an educational project and one of the more satisfying things I had built.

author: 97hilfel
author_rank: Regular Contributor
timestamp: February 03, 2017, 06:35:46 am
content: Quote from: james_s on February 03, 2017, 12:44:02 amWell the heatsink problem I have already a solution in mind: PC CPU Coolers with a cosum mounting bracket, I know a company which machines 3D modelled stuff out of aluminium, brass or copper.And I know I company that makes costum made transformers (optional with potting) that arent that much overpriced.If you have some schematics I would really like to see them, but I am probably going for a digital regulation.Quote from: chickenHeadKnob on February 03, 2017, 03:55:11 amWell I never really got warm with bipolar transistors, or I should say THEY got too warm when I was using them. And for better heat handling I plan on using a setup with multiple n-channel mosfets since they are available I unreasonable high power types which also includes a big package that transfers heat easyer than small packeges, when it comes to TO3 transistors, I can only say the dont fit even closly into my system design because I dont know how to implement them.When it comes to winding a transformer, I already had to do that in highschool and they told us, if you want a "good" highpower transformer get it from a professional manufacture and 320VA is a LOT of power when you want to handle itself.

author: james_s
author_rank: Super Contributor
timestamp: February 03, 2017, 09:06:00 am
content: Here's the schematic in pdf form, you're welcome to the PCB files but it sounds like you're going to want to make enough changes that those won't be of much value.The heat produced by the pass device is determined by the current drawn through it and the voltage dropped across it, makes no difference whatsoever if it's a MOSFET or BJT in terms of heat dissipating. The worst case situation is high mains voltage, low output voltage and high current. The lower the output voltage, the higher the drop has to be across the pass transistor(s).Not sure what you mean by not knowing how to implement TO3 transistors. TO3 is just the case style, using them is identical to using any other transistor except they only have two wire pins, the third pin is the case.

author: 97hilfel
author_rank: Regular Contributor
timestamp: February 03, 2017, 10:45:02 am
content: Quote from: james_s on February 03, 2017, 09:06:00 amThank you verry much,With the TO3 transistors I mean that its hard to get them fit into my mechanical design.The mainsvoltage is european default, with 230V 50Hz normal operation and I am going to step that down to serval steps, it doesn't make much difference if the transformer has 5 taps or 7, at least when it comes down to cost.My biggest concern is about the switching time between the two transformers to make that as smooth as possible.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2017, 11:01:48 am
content: The main advantage of using a MOSFETs is that one does not have to care about the base current when measuring the output current. Otherwise MOSFETs tend to have more disadvantages in linear operation: difficult choice with good SOA, need more current to reach sufficient speed, nonlinear operation, high drive voltage and difficult to parallel for higher power.For switching the transformer taps, there are two main options:1) Switching on AC level with relays2) Switching at DC, behind separate filter caps. So this is more like a class H amplifier.

author: 97hilfel
author_rank: Regular Contributor
timestamp: February 03, 2017, 11:20:48 am
content: Quote from: Kleinstein on February 03, 2017, 11:01:48 amI taught about swithcing after the rectifyers and the filtering (probably taks a lot of space) but ising mosfets which I can switch in/out with pwm.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2017, 12:02:16 pm
content: The main disadvantage of switching after the filtering caps is that one needs separate filter caps (this adds costs and size) and in the simple version there is not really switching between taps but much of the power loss is also moved to another transistor / MOSFET. So usually this version is mainly practical with just two taps. The advantage is, that the transition is very fast and smooth. So change over can be instant, e.g. within 100 Hz ripple.Switching before the filter caps (e.g. AC side with relays) is relatively slow and introduces quite some current peaks to the transformer / rectifier. So one has to switch already if the drop is below something like 5 V and have quite some hysteresis, to prevent too frequent switching. To limit the current spikes there should be more steps, so more like 4 voltages. Many supplies use 2 relays to chose from 4 possible transformer voltages.Using a kind of PWM is tricky and would go toward a pre regulation. It can be done, but adds possible extra ripple. Here are mainly two versions:1) a switched mode regulator (e.g. buck regulator), so a rather high frequency.2) line synchronous switching and this more like dimmer, charging the filter cap only to a certain level.

author: 97hilfel
author_rank: Regular Contributor
timestamp: February 03, 2017, 12:21:45 pm
content: Quote from: Kleinstein on February 03, 2017, 12:02:16 pm#I was thinking about doing it this way:- From high voltage to lower voltage:Switch the high line down to the voltage where the lower voltage is, then turming the lower one up and then synchrounosly turning one completly down and the otherone up. Switching completed, let the mosfets cool down.- From low voltage to high voltage:Switch the high line in until the voltages match and then switch the lower voltage out and turn the upper one completly up. Again switching completet let these mosfets cool down.This would probably be a preregulation, and needs to be balanced out verry well in software to work properly and the switching mosfets need to be beefy enough to handle this with a low Rds on so they dont heat up while in on state. The switching PWM frequency would probably be over 1kHz or higher (if possible) since I want to do this switching within a couple of ms

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2017, 12:54:30 pm
content: PWM switching only makes sense with an filtering inductor. So this will end up as a switched mode regulator. This is possible, but but not that simple and ripple / "noise" can be a problem. Using just PWM with no inductor will essentially loose the extra voltage somewhere in the R_on, capacitors ESR or elsewhere - so one combined the noise of a switched mode version with the efficiency of linear regulation.The more realistic version is electronic "switch" over after the filter caps. The low voltage come through a diode, the high voltage through a MOSFET / transistor as source / emitter follower trying to deliver just enough voltage for the output stage to work properly. If the low voltage is already enough, the high voltage will not be used with no extra control circuit needed, except from driving the gate / emitter to the level needed for the output stage. When using an BJT there is an alternative option too, that uses a transistor that can withstand a high base current.

author: 97hilfel
author_rank: Regular Contributor
timestamp: February 03, 2017, 01:51:35 pm
content: Quote from: Kleinstein on February 03, 2017, 12:54:30 pmI am not going to convert this to a switchmode regulator, the scope of this project was to find out about fintunig of voltage sensing and linear regulation and not building a switchmode power supply, and I dont think that I am going to lose a lot of power on the rds on of the switch in/out mosfets when its down in the mOhm region

author: grifftech
author_rank: Frequent Contributor
timestamp: February 03, 2017, 09:27:45 pm
content: Quote from: 97hilfel on February 02, 2017, 02:47:07 pm