DIY Bench Power model: PP[date] - Page 1
Forum section: electronics / projects

author: m k
author_rank: Super Contributor
timestamp: November 02, 2020, 06:15:13 pm
content: It seems that I've forgotten too much, so viewer discretion is recommended.I don't mind eighter if even not so stupid things are questioned.Anyway, I'm missing some voltages, currently available are only 2*18V adjustable China goods.Goal is to have adjustable 2*30V, very low voltages are not nesessary, every other corner seems to have it's own saved black box mains brick.So I've recently scrapped and cannibalised four UPSes with outputs as 600W or more.Salvaged transformers are 2 + 1 + 1, where two are identical, one has different wire colors and one has center tap.All can produce over 20V rectified, so three is needed and luckily one has that center tap.The center tapped one had a fuse of 2*30A, I'm not going to go there, the whole room of its destination is behind C10.First question, generally,if all three outputs are floating by default and connected only when max voltage is needed, should the connection be internal?Meaning that then only far ends and center tap being available for the user.Second question,for a size of a shoe box or two, what is a practical cooling limit?If that combined transformer triplet would give around 1,8kW and say 5V is taken out the rest needs a potato pot and then some.Also even that subwoofer and car jump start cables are guite affordable it doesn't feel very practical to have a local extra heater under the table.Since there are already myriads of these great tools maybe few extras make this more meaningful.Those transformers have primary side taps and salvaged parts include couple dozen of small relays.Adding also few of those in the corner saved small mains brick transformers an independent adjustment section can be added.Balanced also, so one for back and one for front.Obviously this thing is strictly linear.That triplet can also be a bit less gentle for the mains so soft start it should have.Salvaged relays have only one swing, that is a bit limited but they are many so relay flip-flop is still possible.Means that there are on and reconnect delays.Since all user parts are obviously front side those adjustment sections must be connected.And to maintain safety that must be made by optoisolators.Is that all for primary side?I have two unknowns.First is switching primary winding from full length to side tapped and leaving the side floating.Other is the same winding change but a fault situation where tap connections are not happening symmetrically and whole triplet is in use.Intuition says that neighter is a problem if zero point switching is used.For secondary I think that 10A must be enough, my guess is that even that the number plate stated 600W it can actually be something else also for real and during that short time the small battery is emptied.Also for close to max current situation if center tapped section can handle the +/- voltage it must be a primary target, means that the triplet is not going to go much lower than can be taken from the sides and doublet is not going to go much lower than can be taken from the center.That way waste heat can be handled more easily.For regulators I thought an adjustable regulator stack or BJTs controlled by them.Salvaged parts include something also but they are left aside for now and since single cell charger is another device this one can use pretty common stuff.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 02, 2020, 06:48:22 pm
content: Three large transformers may need some kind of soft start, especially if they a toroidal type. With a classical EI type it may still work without, but not sure. Usually there is not very much range switching than can be done on the primary, especially when in a 230 V country (one could with 115 V mains, but this would still be an odd solution).With transformers from an UPS, one point to check upfront is whether they offer sufficient isolation: for a 24 V to 230 V step up the requirements may be lower. At least it may be designed for the low voltage side to be grounded.Usually for a DIY supply it is a good idea to first start small - a low less magic smoke from a lower power circuit.Usually the combination of a 3 pin regulator like LM317 and additional power transistors is not making a good lab supply - maybe an acceptable regulator for a fixed well behaved fixed load. It starts with a higher drop out voltage and some real difficulty to add a current limit. Those voltage regulators are not really made to work with external power devices and they lack separate sense and drive pins.

author: m k
author_rank: Super Contributor
timestamp: November 03, 2020, 04:53:50 pm
content: Transformers are 080-48183-01 and 41-040005-00G.All windings are floating, or at least the meter is not seeing anything.I should have an insulation tester somewhere.Laminated EI cores are without any coverings, weighting a bit over 1,7kg and core being around 200cm3.Short side wire thickness is unknown, maybe 1,5mm, but effective cross section of a core is 12,9cm2.My guess is that it's a standard 3x2,5x2 inch core, for what ever that means.And orientation now is of course reversed from the original.Idle current for 080-thing is 300mA at full wall AC.I have few 30A China goods shunts, probably not very good for low currency measurements.What if there are say 4*LM338K and all have their own shunts and four channel MCU ADC is reading them.That way one regulator load for 10A total is still guite light.Is that possibly enough for current limiting?I have no idea how fast it should be.It seems that I've lost a bag of not so new stuff, TO-3 power transistors, so right now can't check the variables at hand.Should include at least 2N3055, 2N5302 and 2N5686, with them at least component count would go down a bit for 3 floating parts.Seems that this is not exactly a cost reduced version to start with empty handed.Speaking of common components,I tried the find an easy way for load testings and old sewing machine pedal came up.That then led to a carbon pile regulator.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 03, 2020, 05:36:10 pm
content: For more than about 1 A, I would forget about LM317, LM338 or similar and more think about LM723 + power transistors like 2N3055 or reference (e.g. LM336, Tl431) + OPs + power transistors. I would first start with a small version with 1 polarity for maybe 1 A. If this works well (and still 2N3055 left) one can add more power transistors and build the other side.Wanting a +- supply with a common ground essentially needs the shunts on the high side. It works but can be tricky.A point to decide would be if the current reading / control needs to be accurate or could tolerate some error (e.g. from the base current).

author: m k
author_rank: Super Contributor
timestamp: November 04, 2020, 02:51:10 pm
content: It seems that you are not so enthusiasted(?) by that power part stacking.I guess this can start from robust end and being polished later.Like if every regulator section is completely separated then changing something in there is quite trivial.It seems also that this open delta - open wye inspiration is already fading away.There is still that fourth transformer waiting.If that is added then is there any real reasons to keep that one center tap alive?Transformers are now insulation tested.https://infinispark.com.au/how-to-perform-an-insulation-resistance-test-on-a-transformer/All passed.Parts are also partially found, means that 2N3055 is not among them, others are though.Also some founded parts are stuff I didn't know I have and are so not so essentially ordered again, like the next set of 78xx regulators.And nuts, 2mm plastic nuts, with open pouch, and itchy pitchy runaway.So do not put stuff in atypical places.For four transformer setup the system is still under 10kg light and idling around 300W.Obviously all transformers will get individual power switches.Combined idling voltage is around 100V so no problems there, a single regulated output can then also top where it is topping.Is there something fundamentally wrong with say 2N5686 or MJ802, like being too stiff or something?Some of those 2N5302 not so shiny ones are like from a village blacksmith and at least one have an assumed year stamp of '79.Alternatives, less than four2SC1413, 2SD551, both only one2SC2837, MJ3001, both 10AStack needed2N5426, 2N5428, 2N3767other 2S stuffBDX53C, BDW93C, BU406, BU806Is it seeking troubles too much if all regulators are different?Standalone is of course fine but connected +/- configuration can have at least some ground fluctuations, I guess.On the other hand, if something must be ordered then something else also.

author: m k
author_rank: Super Contributor
timestamp: November 05, 2020, 10:11:53 am
content: How I'm gonna test anything, I don't have a picture maker.So next must be a regulator test jig and audio jack probe.Idea for jig is 7805 and LM317 voltage references, BC546 differential amplifiers, relay and diode controlled input voltage change, mechanically over sized shunt and relay controlled power resistors.Then probing points are plenty and the jig should at least ring.And then already a limiting thing, before anything has happened.Audio probe must of course be a surround sound coded for adequate channel count.On the other hand, ebay has stuff, any recommendations?AD7606 and ADS1256 modules seems to be popular.And what is this PCF8591 thingy?Seems to do it both ways with a lollipop.I think I'll change that audio jack to a simple Arduino approach.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 05, 2020, 09:36:17 pm
content: I would think about the digital part later. First get the analog half running with a classical pot for the set voltage.I would prefer to have 2 completely separate units. This allows to have the shunt on the low side, which makes it a little easier to combine the current and voltage signal, it something like a lab supply (with adjustable current) is wanted. For an only crude current limit (more for protection than real regulation) the shunt on the high side may be OK.The 2N5302 is a powerful transistor. However the SOA is a little limited at higher voltage. Without input tab switching one would need up to some 35-40 V worst case to get 30 V out. The SOA limit for this voltage is at around 2 A per transistor, even with very good cooling. I doubt mechanical tap switching would be fast enough to make much difference.For the start one transistor and maybe 1 A can be OK and worry about higher current later.Adding a current limit can be tricky if one starts with something like LM317 / 780x. The regulator could still be something like a reference.

author: m k
author_rank: Super Contributor
timestamp: November 11, 2020, 11:08:52 am
content: It seems that for starters the surroundings are much more difficult than actual electronics, like the base of the prototype.One usually have wood around, I'm lucky and have some other stuff also but those are eighter too wide or too tall.And then finally wood is only one that can be molded and constructed easily.Anyway, I got two donating units more and dismantled one.So now I have 5 equal max. output transformers, where 2 have center taps.If I now split the construction in two and give both parts a floating and center tapped outputs I have two center taps on the same table and the wall is Schuko.Is there a some sort of a loop connection possibility that should be precautioned somehow?Are Secondary fuses a regular thing?This "new" transformer is also "hi-pot" tested, it passed.Tester is also tested, it showed 1M and 468V over 1M resistor.All devices have also a small sort of an ac-line wattage transformer, those are also tested, they also passed.Estimated (600/240) withstanding is something like >2,5A with primary as 3 rounds andsecondaries(ac-C-R-L(Tr)-ac) like 63, 65 and 2*68 ohms, where 65 has a different part number.Fifth was measured in circuit and gave 70ohms and had third part number, same series as the 65 one, both most likely from same type of device.Most likely the measurement wasn't very accurate but so it seems to be also with those compenents.These center tapped devices have four relays, two input and two output.Should I ditch those input ones right away, simply because they have been practically on the whole usage time?Other model have 5 identical relays that are dumped together but can be separated by date codes and/or colors.Is there an easy way to test what can be reused and what should not?E,obviously those UPS devices are actually IPS devices.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 11, 2020, 11:39:44 am
content: To get some 30 V out something like 40 V no load after the rectifier would be OK. As far as I understand this would be 2 transformers in series. The point between the 2 transformers could be used as a tap for lower output voltage. Tap switching could be with relays or alternatively electronic, more like 2 power transistors and a diode in series. One could also use the center tap of one transformer to get additional steps, especially when using relays.If the transformers are rather large and especially in toroid shape, one may need some kind of soft start to avoid the inrush current to blow the fuses. This could use 1 relay.If build as a full lab supply with adjustable current regulation, one could make use of 1 smaller transformer for the control part in the usual floating regulator configuration.