Need help on my first hobby project(12V,100A linear power supply) - Page 1
Forum section: electronics / projects

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 05:12:15 am
content: Hi,I am a starting hobbyist in this area, and as my first project I choose to implement a 12v linear power supply with 100A capacity.(ripple voltage < 1%)After some study over the internet, I gathered the following circuit would probably work(definitely inefficient), and I would like to hear your thoughts on this approach.design(not original):- partA: Transformer: shift down the main voltage from 110v to around 20v Vpeak- possibly rewinding secondary coil of a microwave oven transformer ?- partB: rectifying circuit: 4 diodes back to back to convert AC to DC. put a fat capacitor to detect and hold peak voltage.- any suggestion on inexpensive diodes that can handle this much current? peak current seems to be 900+ Amp....hopefully my calculation is wrong- partC: Driver: use one NPN(10A capacity) to drive the base of an array(10+ drivers) of NPN drivers(10A capacity), so that the array of NPN drivers' emitter voltage(output) will be 12V. put a large cap to stabilize output voltage.- not much to say, I think(hope) this sub-circuit would work properly.I included the circuit diagram(a bit messy. randomly picked a diode part cuz dont know what diode to use).I'll update this thread to log my steps on this project. Hopefully in the end I will have some physically working stuff built up with proper testingAny help/suggestion/advice is appreciatedI've heard switching PSU is much more efficient,, but I still want to make a working linear PSU as my first project(mostly because I dont want to read all the switching PSU theory up)So,, please share with me your thoughts on how to improve this circuit(instead of telling me a fancy new approach)-Keith

author: Unknown
author_rank: Unknown
timestamp: January 13, 2012, 05:51:56 am
content: Well trying to build a 100A power supply as a first project IS a fancy new approach.So how about reducing the requirement to 2 Amps before you burn the house down?Here is my advice:1 - Forget 100A2 - Build an unregulated power supply with a 50 VA transformer.3 - Make it survive short circuit4 - Add a regulator ie LM317 (missing in your diagram btw.)5 - Make another power supply this time 10A6 - Make this survive short circuit7 - By this time you will know how to make a 100A power supply but that you don't need 100A anyway.

author: tbscope
author_rank: Contributor
timestamp: January 13, 2012, 06:14:39 am
content: Why do you need 100A? Even at 12V, this is a lot.A common technique used in professional power supplies is to put several smaller power supplies in parallel.And watch out for your cable dimensions.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: January 13, 2012, 06:39:05 am
content: Its quite complex first project! If you really need such power supply buy a cheap server power supply on ebay, but then again i dont know why you need it, but does it need to be regulated?

author: rr100
author_rank: Frequent Contributor
timestamp: January 13, 2012, 06:53:03 am
content: Even putting multiple PSes in parallel isn't a trivial task.If you know what you're doing is not so complicated but you need to consider many things (and if you don't the advice "stop before burning down the house" is a good one).Do you have the 1kW+++ regular transformer?Going from 20 VPP to 12VDC is quite a bit of power loss, that means hundreds of watts in your setup; added to the heating from the transformer - what do you do with all the heat?

author: IanB
author_rank: Super Contributor
timestamp: January 13, 2012, 06:53:16 am
content: By curious coincidence, I have just been working on a linear power supply project. You may find the following notes helpful.I started with a 12 V 2 A transformer. My goal is to have a variable 1.5 V to 12 V regulated output with a current up to about 1 A. The 2 A transformer provides headroom to keep it running cool with the difficult load imposed by the smoothing capacitor.On the output of the transformer I placed a 4 A bridge rectifier. After the rectifier I placed a 4700 uF electrolytic capacitor. After the capacitor I placed a test load of about 1 A (a 12 V 12 W bulb).Here is the voltage waveform I measured at the load:As you can see, I have a ripple of about 1 V and a minimum voltage of 13.9 V. This minimum voltage gives approximately 2 V down to 12 V for the drop-out voltage of an LM317 regulator that will come next.During this test, the 4 A rectifier (a fairly substantial device) got rather warm. It may possibly need a heat sink if it is to run at that load for a long time, though I will have to investigate further.Based on all of this, you can scale up by 100 and see where it gets you. For instance, you will need to find a 12 V 200 A transformer, and maybe a fork lift truck to move it. Then you will need to find a 470,000 uF capacitor (or a bank of 100 4700 uF capacitors paralleled up). Then you will need a 400 A rectifier. That might be quite big. It might need a fan to cool it. After that, you are going to need something interesting to provide the voltage regulation, which is a story for another day.

author: metalphreak
author_rank: Frequent Contributor
timestamp: January 13, 2012, 07:01:03 am
content: "ripple voltage < 1%" + "12V" + "100A"Switchmode power supply. I'm not sure what you need 100A of 12V for, but i'm sure it doesn't need the super clean low ripple (much much less than 1%) of a linear power supply.

author: BravoV
author_rank: Super Contributor
timestamp: January 13, 2012, 07:35:48 am
content: I'm quite surprise to hear that this is your "first" hobby project, just silly question, have you ever build a relatively low power dc power supply before like those generic ps using 7812 or LM317 ?Few Amperes of power supply is relatively easy, but 100 Amp is totally different class here, its like comparing a bicycle to a truck.Quote from: keiby.zhou on January 13, 2012, 05:12:15 amJust a friendly advise, if you're going to continue with these mentality like that and insisting using linear type at that current, I believe few residents here might starting to post nasty responds here.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: January 13, 2012, 09:34:18 am
content: I still have the same question, for what purpose in beginners hobby you need that power supply?

author: kripton2035
author_rank: Super Contributor
timestamp: January 13, 2012, 09:49:35 am
content: I'm still thinking that even if you dont want to learn smps theoryit will be a lot easier to make a 100A smps supply than a 100A linear power supply !!!may be you make a mistake and really want a 100VA12v supply ?that's totaly different...

author: Rerouter
author_rank: Super Contributor
timestamp: January 13, 2012, 10:03:49 am
content: if you wanted to approach it linearly, (not ideal by any real measure, crud loads of heat) you will want to use some big beefy power transistors, they will need to be rated for a large chunk of the current, with a very high beta, you can run all the pass transistors off of the one regulator, and for the 317 reg, its actually drawn up in the application notes, 317 could cope with ~1.5A, at a beta of 100, you could get away with 100 amps not that difficultly,but this would get very, very hot due to the level of wattage being burnt up (vdrop x current = wattage heat)and would need rather drastic levels of cooling, massive sink and a few high power fans,other thing to concern yourself with, is that transformers droop in voltage as you approach or exceed there rating, at 20V x 100A, thats a 2KVA rated transformer to meet your maximum, and that is one crud load of a transformer, in the 10's of KGedit: removed extra 0

author: BravoV
author_rank: Super Contributor
timestamp: January 13, 2012, 10:17:36 am
content: Quote from: Rerouter on January 13, 2012, 10:03:49 amEdit : Rerouter, I think you missed some basic math here, how that 20KVA (20.000 VA) came from ?To give you an idea what Rerouter is talking about, this pic below is my 1000 VA rated isolation transformer, for your need of which definitely bigger than mine, I think it should be easy to imagine the size and especially the weight.At the end, its all about the money, tell us how much you're willing to pay to build this 12v 100 Amps "linear" psu ? What is your budget anyway ?PS : This pic is from my thread posted while ago ->https://www.eevblog.com/forum/index.php?topic=6133.0

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: January 13, 2012, 10:24:51 am
content: I gues i wasnt listening on my theory class 7-8 years ago and cant understand how is 20 V, 100 A equal to 20 000 VA?

author: BravoV
author_rank: Super Contributor
timestamp: January 13, 2012, 10:28:32 am
content: Quote from: hacklordsniper on January 13, 2012, 10:24:51 amLol... you're right, I just quoted the post without thinking, edited my post.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: January 13, 2012, 11:06:57 am
content: Quote from: BravoV on January 13, 2012, 10:28:32 amIt is just a correction before the author of this thread orders a 20 000 VA, 20 V transformer

author: Wartex
author_rank: Frequent Contributor
timestamp: January 13, 2012, 12:15:26 pm
content: Quote from: hacklordsniper on January 13, 2012, 11:06:57 amToo late

author: Psi
author_rank: Super Contributor
timestamp: January 13, 2012, 12:21:53 pm
content: time to build a Jacobs ladder

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: January 13, 2012, 12:24:00 pm
content: Sweet, a pole pig, i sold mine year or two back

author: grumpydoc
author_rank: Super Contributor
timestamp: January 13, 2012, 12:30:54 pm
content: Sorry - bit late to comment on this thread.I'd echo pretty much everything that has already been said - no way this would be a first project.However, assuming you're serious (areyou serious BTW)What is this power supplyfor?Are you sure about 100A? and a 900A peak??Is the load purely resistive? Is it a motor or anything remotely inductive because there are going to be some "interesting" back EMF issues to deal with.As to your schematic - well, I've forgotten a lot in 20 years of not really doing electronics but even I can spot a few things.First 1N5401 rectifiers?? Those are rated at 3A are they not?Second your output transistor array isn't thermally stable - you need to look this up - but what will happen is that one of those transistors will get a bit warmer than the others which will turn it on a bit more so it will take a bit more of the load current. This, of course will heat it up more and so on until it goes pop. The others will follow suit one at a time.Also 6xMJE3055 can't handle 100ASuch a PSU will get hot, as others have pointed out - the bridge rectifier will dissipate something like 120W alone. With 2x diode drops in the driver & pass transistors you'll need at least 1.2V headroom so that's another 120W at least. You're designing a nice room heater!The driver base voltage needs to be constant - you can't just tap off the unsmoothed DC with a couple of resistors.Smoothing caps (or smoothing capacitor array as you'll never get a single one to meet your needs) need to be large enough and have an appropriate ripple rating (which I think is going to be 100A but I can't remember how to calculate that one).Have you considered a lead acid accumulator, good source of high current DC assuming you don't need 24/7 running. Or just source a good switch mode from somewhere. E.g.http://export.rsdelivers.com/product/rs/rcp-2000-12/rack-mount-power-supply-12v-100a-1200w/7332020.aspx#header

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 03:22:43 pm
content: Thanks all for your reply. I really appreciate all your advice on this. I am quite surprised seeing this many people reply to my thread over one night.I see I have some questions need to be answered. Here are my answers:1. goal/application of this 100A power supply.(potentially use it as supply for testing a quadrotor.(not really started building yet, have some basic research done))a. to power Lipo battery charger. Lipo battery charger actually draws quite a lot of current, and if I want to use multiple chargers to charge multiple batteries, it would use quite some ampere.b. (secondary, not investigated much yet) I also want to use this to drive brush-less motors directly(through controller of course). battery only holds around 10+ min of fly time. I want a more easy way of driving those 4 motors without charging batteries first.(for bench testing)c. to research building a linear power supply, i read quite some stuff over the internet(diodes,rectifiers,NPN etc), I want to put all the information into some solid practical application before I forgot all of them. Hopefully building one working PSU will help me understand more of their application and limits.2. why insist on linear PSU instead of switching mode PSUa. although I didnt research much about using PSU to directly drive brushless motors, I did read some places saying it is easier for linear PSU compared to switched mode PSU, because of the inductive load nature(This is all I know about this)b. diodes/recifiers/NPNs are quite some information for me. I want to build some thing out of those components before diving into reading all switch mode PSU info. Perhaps building this PSU would help me see the limitations of linear PSU, which might also help me understand why switched mode PSU is better in certain applications.Hopefully you are satisfied(at least to some extent) with my answer.Some clarification on my schematic:a. diode part is a random pick. I dont know what to use to handle such large current.b. I didnt draw all the NPN drivers, because the unregistered SPICE version only give me very limited space for drawing. Yeah, in the end it will be an array of around 15 driving NPN's(each rated to 10A)Some mistakes I realized from your suggestion:a. heat problem! yes, i didnt put much thought into this issue. I was only looking at the maximum voltage/current/power rating for each component, but didnt realize the sideeffect of heat. The reason I choose to step down to 20V instead of a value more close to 12V is because I can get away with a smaller capacitor at that place(can allow more ripple voltage at that place), but yeah,, i totally ignored the heat impact. I'll probably lower the voltage to around 13-14 volt and put a bigger capacitor to cope with this reduced voltage margin.b. Can't use resistor divider for the left NPN! Thanks for pointing this out! I cant believe I missed this point. Ye, that NPN basically determines the final output voltage. I need some precision voltage to feed into the base of the array of drivers. maybe I'll need to use a precision voltage regulator to replace that NPN. (In this way I know why the design doesn't work and why an IC is really necessary. If I start with some fancy design schematic, I'll probably never figure out why they were chosen in the first place). Does precision voltage regulator handle around 2A of current? (100A/50 = 2A, assuming beta=50)My specific questions:a. About transformer rating. I didnt read much about transformer. my current understanding is that the wire capacity determines the transformer handling capacity. Is it correct? If so, the current on mains side would be about 10-15A, and on the circuit side would be around 100A(would peak to around 1000A when rectifier charging that big capacitor)(note: 100A is the output current of the PSU. 1000A is, according to my calculation, the peak current that could happen in the rectifier diode when charging capacitor. Hopefully my calculation is wrong)b. About the rectifier. on the output side I can use array of drivers to share the current, but all the current goes through one diode in rectifier. It seems stupid to ask, but are there inexpensive diodes out there that can handle 1000 ampere? Are there any smart way to share current there as well(resistance might an issue there due to much higher current compared to output side)?

author: vtl
author_rank: Regular Contributor
timestamp: January 13, 2012, 03:41:11 pm
content: I don't think you understand how much 100A is. 100A is enough to make heavy gauge wire glow! Unless you are trying to do very high power stuff eg welding metal 100A is just crazy. And for this kind of power you would never be using a linear supply.You want to charge some lipo batteries and some brushless motors for a quadcopter? This is all fairly low current stuff. For example, a laptop usually has 6 nice big lipo cells and only consumes around 30 watts under charging. You probably only need like a 5-8A supply max. You also don't need a linear supply. The main reason why youd use a linear supply is for low noise which is not what your application needs. For your applcation a switchmode supply is better, you could just repurpose an ATX computer power supply and youll have hundreds of watts of power to play with.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 13, 2012, 03:52:39 pm
content: Quoteyou got me thinking what fun stuff i'm going to do with my welding machine transformer.

author: IanB
author_rank: Super Contributor
timestamp: January 13, 2012, 04:00:41 pm
content: If you need a fixed 12 V your needs might be better served by getting a 12 V lead acid accumulator (maybe a marine cycle type) and a nice float charger.

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 04:01:30 pm
content: brushless motor can draw quite some currentfor example:http://www.hobbyking.com/hobbyking/store/__14404__Turnigy_G46_Brushless_Outrunner_670kv_46_Glow_.htmlit rated at maximum of 40A. if you multiply by 4, its way over 100A

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 04:11:23 pm
content: charger also draws significant currentimagine a couple of these charging at the same time:http://www.hobbyking.com/hobbyking/store/__11597__iCharger_3010B_1000W_10s_Balance_Charger.html

author: deephaven
author_rank: Frequent Contributor
timestamp: January 13, 2012, 04:49:38 pm
content: Here's someone that made one:

author: Unknown
author_rank: Unknown
timestamp: January 13, 2012, 04:59:16 pm
content: I was just about to post the same videoYou don't want to charge your precious Li-po batteries with that thing btw.Other than that I think this forum can help you build such a thing - yes.

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 06:36:01 pm
content: Quote from: IanB on January 13, 2012, 04:00:41 pmThanks for the info. I'll research into this approach

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 06:38:19 pm
content: Quote from: HAL-42b on January 13, 2012, 04:59:16 pmYe, that video looks like something i have in my mind.On the charging Lipo issue. Is there a specific reason why it is not appropriate to use it to charge battery?(assuming output voltage is designed to be stable enough)Thanks

author: Slobodan
author_rank: Regular Contributor
timestamp: January 13, 2012, 07:09:55 pm
content: If you need that much power, just buy switch mode supply, if you can find 1200W one.EDIT: This supplies satisfy your needs:http://export.farnell.com/murata-power-solutions/d1u-w-1200-12-ha2c/psu-1200w-12v-5v/dp/1574398http://export.farnell.com/murata-power-solutions/d1u-w-1600-12-ha2c/psu-1600w-12v-5vhttp://ca.mouser.com/ProductDetail/Mean-Well/RSP-1500-12/?qs=sGAEpiMZZMuEhFEw3BeJbS07ldmo5wFtO12bSXvJsz4%3d

author: Unknown
author_rank: Unknown
timestamp: January 13, 2012, 07:14:45 pm
content: Quote from: keiby.zhou on January 13, 2012, 06:38:19 pmThe best explanation anywhere is graciously provided by none other that our own Daveand again... what you want is cool but it is not what you need...

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 07:19:56 pm
content: Quote from: HAL-42b on January 13, 2012, 07:14:45 pmThanks for the video linkI have actually watched that episode beforeAll this charging curve should be handled by the LIPO charger right? This power supply will simply supply power to the charger instead of directly charging the battery(that will probably explode)like providing power to this unit:http://www.hobbyking.com/hobbyking/store/__11597__iCharger_3010B_1000W_10s_Balance_Charger.htmlthey dont run on mains, and some of them require pretty large current supply(or if you have multiple units charging simultaneously)

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 07:22:12 pm
content: Quote from: Slobodan on January 13, 2012, 07:09:55 pmThanks for the infoI have looked into server PSU as well. ye, they provided decent power at 12V. However, I have heard switching PSU are not good for driving motors directly. Is it true? (I have 0 knowledge about switching PSU) I heard its related to the emf coming back from motors

author: Slobodan
author_rank: Regular Contributor
timestamp: January 13, 2012, 07:26:50 pm
content: I don't know about driving motors, but for powering that high-power charger it doesn't matter whether they are linear or switch mode.

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 07:32:23 pm
content: Quote from: Slobodan on January 13, 2012, 07:26:50 pmThanks, I'll look more into thisI still feel building a high current linear PSU is fun by itself (if feasible at all). It is probably(or definitely)notthe best approach for my specific application, but not too bad for a first hobby project

author: BravoV
author_rank: Super Contributor
timestamp: January 13, 2012, 07:43:16 pm
content: You still haven't answer my question, since this is important should other decided to help you build that linear power supply, what is your max budget ?

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 07:47:54 pm
content: Quote from: BravoV on January 13, 2012, 07:43:16 pmSorry, i forgot this important one. I am thinking about making one for around $20 - $50. max < $100.Of course the cheaper the better

author: BravoV
author_rank: Super Contributor
timestamp: January 13, 2012, 08:06:00 pm
content: Quote from: keiby.zhou on January 13, 2012, 07:47:54 pmGreat, this is a good start, now, have you ever checked the price of a transformer that is 1/10 rated from what you want ?Say secondary winding at 15 volt 10 Amperes, find the price, once you got it, now multiply it by ten for easy ball-park number, if its still < $100, than that is a good sign.Just wish you luck on finding it.

author: rr100
author_rank: Frequent Contributor
timestamp: January 13, 2012, 08:42:29 pm
content: Quote from: keiby.zhou on January 13, 2012, 07:47:54 pmAnd I'm thinking about making an electric car for $500.NOT. That's the price for one tenth of the batteries needed. Never mind the rest of 90% (and the car itself). Probably the cooper from a transformer like you want is worth more than 20-50$...As somebody said just get a big PC PS and you're done. Corsair Gold AX1200 has 100.4A on the 12V rail. You might have to put some load on the 5V rail to keep it stable and put together the smaller wires in some clever fashion to handle the large current but these problems are peanuts compared to what you want to do from scratch.That PS is $270 on Amazon and keep in mind it's new and "brand name". If you know what to look for and you have some luck on ebay you will find some refurb/returned PS that matches your specs and still has some warranty on it for about 100$. And the heat problem would be taken care of and probably the PS would be infinitely more stable than what you plan to build. And probably dual-voltage (if you care).

author: Unknown
author_rank: Unknown
timestamp: January 13, 2012, 08:45:39 pm
content: Hmm lets see..For 20 to 50 bucks you can build yourself a very nice Lithium battery charger that runs off a computer PSU.If you can scavenge an old UPS for a transformer, a usable enclosure and line voltage hardware, using these you can build yourself a nice 10 A power supply or charger, possibly around 50$.If you are really really really lucky you may find an old 1000 to 2000 VA UPS. If it is old enough it will have transformer(s) in it. If you can get that for free and are able to reuse the enclosure there is a small chance you can make it for around 100$ to 200$. That is if the wind is blowing in the right direction and the planets are aligned correctly with your star sign.

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 09:07:56 pm
content: Quote from: rr100 on January 13, 2012, 08:42:29 pmThanks for the reply.I actually already bought a server PSU for around $15. it gives 30+ amperes at 12V.(not dual voltage).It would be interesting to build a low voltage, high power PSU from scratch(if feasible at all)of course, budget is very limited

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 09:10:39 pm
content: As for transformer, I really dont know much about them. is the microwave oven transformer good enough? they seem to be able to handle high power.would rewinding secondary winding work?(like use heavy gauge wire)

author: Rerouter
author_rank: Super Contributor
timestamp: January 13, 2012, 09:24:45 pm
content: microwave transformer may work if wired backwards (high voltage side to mains) being most seem to be about 2KV, which is a ratio of about 1:8.5if correct, that comes to 27.6V if wired backwards (235/8.5)downside is, at 27V you would need an even higher VA rated transformer, and microwave ones tend to be about 600W perhaps 1KW if your lucky on the coil your interested in (other low voltage rail with high amperage for magnetron)so that will still come up short with your original spec of 100A, and i am not quite sure about how feasble it may be to wire them in parrellel

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 09:32:00 pm
content: Quote from: Rerouter on January 13, 2012, 09:24:45 pmwhen you say 1KW coil, are you referring to the current capacity of the coil wire? (or is it that the iron core that has the 1KW power limit)if wire is the bottleneck, can I re-wind the secondary coil using like gauge 8 wires?on using transformer backwards, I am a little worried that the high voltage side wire cant handle the 10A current that will come from the mains. cuz high voltage side are designed on KV level, I doubt they have large current capacitythanks

author: w2aew
author_rank: Super Contributor
timestamp: January 13, 2012, 09:45:25 pm
content: If you need to go cheap, and if you're application is not continuous high current, then why not consider a car battery and a float charger. You can get lots of current when you need it, and the charger will be relatively cheap - depending on how fast you need to replace the charge.

author: keiby.zhou
author_rank: Contributor
timestamp: January 13, 2012, 09:47:43 pm
content: Quote from: w2aew on January 13, 2012, 09:45:25 pmi am not familiar with float charger, but I did research car battery when i bought computer server PSU. car battery are actually quite expensive to obtain here in Toronto.(100+ dollars. maybe I didnt look hard enough)

author: keiby.zhou
author_rank: Contributor
timestamp: January 14, 2012, 02:21:07 am
content: I am trying to fix up the two mistakes I realized, and also clean up the circuit diagram:1. step down to 20V will generate too much heat. I'll try 15V instead2. use resistor voltage divider to drive the left NPN is NO-NO!. I'll use precision voltage regulator to replace that NPN instead.here I attached the circuit diagram with my clean up.The diode in the rectifier is still a random pick. I dont know which one to use.I am not familiar with spice, and I am not sure how to draw precision voltage regulator properly, and I used a DC voltage source instead.(impact should be minimum)I also attached the voltage after the rectifier vs Voutput at (0A load, 1A load, 10A load, 100A load)at 100A load the voltage drop is quite big, I suspect its because the rectifier diodes can't handle this much current to recharge the cap. Is it?The diode limitation is my biggest concern at this point, could anyone share some thoughts on this ?

author: keiby.zhou
author_rank: Contributor
timestamp: January 14, 2012, 02:30:21 am
content: hmm actually, looking at the large V drop at rectifier on high power, I think i might be able to get away with stepping down to 20V, cuz at high load, the voltage will probably drop down to 13-14 V anyways

author: keiby.zhou
author_rank: Contributor
timestamp: January 14, 2012, 04:01:09 am
content: ye, I think it is working(theoretically speaking,,, and havn't found a diode part yet)attached are the final circuit graph(diode part still random)at 1A load, the voltage is around 12.02 volt. voltage across NPN driver is 6.4V and power on the NPN(all combined) is therefore 6.4Wat 50A load, the output voltage is around 11.90 volt. voltage across NPN driver is 3.3V and power on NPN(all combined) is therefore 165Wat 100A load, the output voltage is around 11.80 volt. voltage across NPN driver is 1.7V and power on NPN(all combined) is therefore 170WThis result actually looks pretty good to me at this point. only around 0.22 voltage drop across 1A - 100A current range. ripple current should also be pretty small.attached are the circuit diagram and voltages/currents at various points at 1A, 50A, 100A load.now my biggest concern is actually the rectifier diode. any help is appreciated

author: keiby.zhou
author_rank: Contributor
timestamp: January 14, 2012, 04:04:37 am
content: btw, the read solid line is the output voltage(if you care at all)170W for all the loss on the drivers I think it should be manageable with decent heatsink, perhaps plus a fan ?

author: Unknown
author_rank: Unknown
timestamp: January 14, 2012, 04:26:48 am
content: What will happen when you short circuit the outputs?

author: keiby.zhou
author_rank: Contributor
timestamp: January 14, 2012, 04:29:51 am
content: Quote from: HAL-42b on January 14, 2012, 04:26:48 amI am thinking about using 100A fuse to protect that. is that the standard way of doing this?

author: amspire
author_rank: Super Contributor
timestamp: January 14, 2012, 04:50:04 am
content: How are you going to get a low ripple 15V at 100A.If you have a transformer with a full wave rectifier, then to get less then 1V p-p ripple, you will need about 1 Farad of capacitance across the rectifier output - that is absolutely massive. 100 x 10,000uF 25VW capacitors! Because the current in the transformer is not sinusoidal, the rectifiers and transformer need to be rated well above 100A. Something like a 3000VA transformer may be able to cope.With rectified mains, you have to allow for mains variations, and you will not know the exact transformer output voltage till you get it, so it is usual to give a few volts margin for the transformer. 20Volts is more realistic, which means you will have about 800watts of heat to get rid of, and that does not include the rectifier heat losses and transformer heat losses.To share current between transistors, you need a small resistor in the emitter legs of each transistor. Also a circuit like this just has to have current limiting- it is not really an option. If it is unacceptable for 20V to be applied to the load when one of the transistors fail, you will also need a crowbar protection across the output.How are you going to cool this? Sounds like a very expensive heatsink/fan arrangement to me. Water cooling? Convection cooling is not practical at this much power.Buying a switching power supply is really the way to go with a task like this. The switching power supply will have far greater efficiency, with power factor correction, the input current waveform will be far better then the transformer/rectifier solution, it will have current limiting.I have a strong suspicion that if your power supply is designed with properly rated components, it will cost far more then a switching power supply. How much have you budgeted for this?Richard

author: BravoV
author_rank: Super Contributor
timestamp: January 14, 2012, 04:58:41 am
content: Quote from: amspire on January 14, 2012, 04:50:04 amRichard, this suggestion already mentioned couple times in this thread and still he insisting to build this linear psu as his 1st hobbyist project, and the budget is < $100.Imo, procuring these needed components it self with this kind of budget needs a lot of lucks.

author: amspire
author_rank: Super Contributor
timestamp: January 14, 2012, 05:08:58 am
content: Quote from: BravoV on January 14, 2012, 04:58:41 am$100? I would be amazed if this could be done for $1000. I wonder what has been budgeted for the huge bank of capacitors, the transformer, the massive heatsink, the rectifiers, the fuses or breakers, and the case strong enough the 50KG + of hardware? It sounds like the concept has not looked very far past the prices of the transistors, which is almost a neglible part of the project.

author: Unknown
author_rank: Unknown
timestamp: January 14, 2012, 05:09:49 am
content: As they say, your pass transistors will protect the fuse very nicelyTransistors blow WAY too fast for any fuse to protect. You need some sort of crowbar circuit. In fact at these power levels you need all sorts of protection. High voltage side fuses, thermal fuses for the transformer and for the heatsinks, low side fuses, crowbar circuit, the whole works. You'll find all of these on 10A power supplies. Since yours is 10 times more powerful you need 10 times better protection.Also as you will power motors there will be back EMF to deal with. The inrush current will need dealing with as well.So no, a fuse alone is not enough.

author: keiby.zhou
author_rank: Contributor
timestamp: January 14, 2012, 05:27:15 am
content: Quote from: amspire on January 14, 2012, 04:50:04 amSay we stick with 20V of rectifier voltage. we can actually allow ripple voltage of 6+ volt, instead of 1 volt.about the watt. when the voltage at rectifier is around 20V, but current is very small( <1 A), this gives about 8W of heat.when the output current is around 100A, the rectifier voltage would only be around 14W, which gives about 200W of heat. If we tighten the voltage margin, we can get to around 170W of heat(see my prev post with graphs).In the end the power would be 100A * voltage_margin_at_that_load_point. if 3 volt margin is enough to capture all the mains variations, then 300W would be the power loss.yes, output protection is another point I need more research on. for now, putting a fuse is all I knowabout emitter resistance. I put resistors on the base side, and I think there is no need on emitter side(I might be wrong, but I need more explanation)about transformer rating. I have asked this question quite a few times. didn't get any answer yet. Is the rating the coil wire current rating or the iron core rating(if there is one) ?. cuz I am thinking about rewinding secondary coil of a microwave oven transformer with heavy gauge wire. this will also bring the cost of that part done a little bit.

author: keiby.zhou
author_rank: Contributor
timestamp: January 14, 2012, 05:31:51 am
content: Quote from: amspire on January 14, 2012, 05:08:58 amtransistors =~ $10wire =~ $10transformer =~ $0 (if rewinding secondary coil would work)diodes =~ unknownprecision voltage regulator =~ $5capacitor =~ $0 (if caps from some old computer PSU, old microwave etc can be used, and capacity is enough)case =~ unknownother =~ unknown(fuses etc, switches, panels etc)

author: keiby.zhou
author_rank: Contributor
timestamp: January 14, 2012, 05:37:29 am
content: Quote from: HAL-42b on January 14, 2012, 05:09:49 amThanks for the reminder, I need to start looking into those topics soon

author: amspire
author_rank: Super Contributor
timestamp: January 14, 2012, 06:46:28 am
content: Quote from: keiby.zhou on January 14, 2012, 05:31:51 amHow many hundreds of old PSU's do you have to salvage the caps from? If you are thinking you can get enough capacitors from a handful of supplies, they I hate to tell you this, but you are dreaming.You are going to rewire a 3000VA transformer? You actually have a 3000VA transformer?You realize the base current will be about 5A. So your reference supply will have to be well over that - it will have to a something like 12.8v 10a dc supply, and you are making it for $5?Heatsinks and fans will be several hundred dollars.One of the reasons people build switching power supplies, is that they aremuchcheaper then linear power supplies. Much smaller and cheaper transformers and capacitors. Your cost numbers just do not add up for a 100A linear supply. It is going to weigh something like 100kgs, and I think your current pricing is for about 1/2 kg of that 100 kgs.I think you need to build a smaller scale supply, like a 5A supply, as you need to learn about all the problems that I think you are currently ignoring.Richard

author: Stephen Hill
author_rank: Regular Contributor
timestamp: January 14, 2012, 11:42:23 am
content: I'm not sure I can take this thread seriously.... in fact, I'm very concerned for your own safety and the safety of others.Especially when you saying things like "I don't know much about...", "I miss calculated...", "Will this do?", "<$100 Budget".I'm currently designing my first lab power supply and I'm thinking 3A @ 12V is way more than I need.If you really need a 100A @ 12V power supply then please buy one:XP POWER - HPU1K5PS12 - Switching Power Supply100A @ 12VDC$795.90

author: amspire
author_rank: Super Contributor
timestamp: January 14, 2012, 12:06:49 pm
content: Stephen,I think you are right. Buying a switching power supply is the only practical solution, and $795.90 is probably a fair price.$100? It is never going to go anywhere.Richard.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 14, 2012, 01:38:55 pm
content: also agree with stephen, but unless i'm really serious for the job... $796 can be a PITA. the OP should be aware and learn what a 100A can do to a wire. and what risk he's facing. at least put one fire extinguisher next everytime during the project.

author: Slobodan
author_rank: Regular Contributor
timestamp: January 14, 2012, 03:52:48 pm
content: @keiby.zhouI think you should say what batteries do you want to charge with the charger you posted the link for. Number of cells and mAh rating, becouse, maybe you don't need charger that powerful (and a power suply for it).

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2012, 07:19:37 pm
content: If I were to build a higher powered linear regulator, I'd use an N-channel MOSFET as the output device and a Schottky diode bridge rectifier or go for synchronous rectification to keep the losses low.As mentioned there will be issues with heat, wasted power and huge capacitors. The power factor will also be so poor such a PSU will be illegal in the EU and I'm pretty certain the input current requirement will exceed the capacity of a US 15A socket or even a UK 13A socket. You may have to go for 240V or 208V in for this to stand any chance of working.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 14, 2012, 07:43:30 pm
content: You can pick up a 1200+ watt ATX power supply that can do 100 amps on the 12V rail. They run around $300 and there is no way you can build one as cheap as they make it. Most of them have multiple independent 12 V outputs (with 10-20 amps each), but a few have true 100 amp outputs. However, the wires and contacts are not rated that high. You would either need to open the case and install high current connectors and wiring or make sure no single cable exceeded its rating. The XP power unit is more expensive, but probably better engineered, more convenient, and more likely to actually meet its specifications.Anyway, it isn't an impossible project, either as a linear or switch-mode regulator. It isn't even that hard in the sense that is usually meant -- power supply circuits are quite simple by nature. However, it is expensive and unforgiving. That kind of current/power can do a lot of damage in a short period of time if you even make a small mistake. It is also at the level where simply over-engineering to avoid messy calculations gets even more expensive.

author: amspire
author_rank: Super Contributor
timestamp: January 14, 2012, 09:47:50 pm
content: I haven't gone through all the previous posts, but there was mention of inductive loads or motors or something?If that is the case, extra components will be needed to absorb any inductive transients. Computer power supply are not really designed to run motors.I suspect the solution will be to either find an old supply on ebay at a bargain price, or use a lead acid battery and a battery charger. A good heavy duty battery that can comfortably supply 100A continuously probably costs more then $100.And you are right, it is not an impossible project. With an unlimited budget and a good forklift to move the supply, it is not even that hard. A 3 phase trasformer and rectifiers greatly reduces the size the size of the filter capacitors needed so you would probably go for 3 phase power. But the moment you start doing the calculations with safe margins, the numbers do not seem practical. If you are going to use a fuse instead of current limiting for example, then instead of 100A, you probably have to design for a short term capability of 200A at least, and of course resistive losses go up with the square of the current. You have to look at what is the highest continuous current the fuse can carry before blowing and design for that. It may be 150A. You have to worry about the transformer saturating under fault conditions. If you use a transformer designed for short term saturation like a welding transformer, the regulation from the transformer will be lousy, so you will have to increase the voltage, which means even more losses.It sort of reminds me of someone who needs a new engine for their car. Why buy a petrol engine for many $1000's of dollars? Surely with parts from an old gas BBQ - the propane tank, the burner and knob to control the gas, along with an old pressure cooker pot, a steam engine could easily be built for - surely well under $100?

author: metalphreak
author_rank: Frequent Contributor
timestamp: January 14, 2012, 09:58:19 pm
content: I think he wants to run RC motors directly off the supply instead of using Lipo batteries.You can draw ridiculous current from a Lipo pack, to feed the 30-40A required, but you *never* charge batteries at that rate.For regular slow charging its something like 0.5-1.0C charge rate. If you have a 2200mAh rated pack, "1C" is 2.2A charge rate. If the pack has a 20C discharge rating, you can draw 44A from it.If you are planning on running those motors and stuff directly off a power supply so you can run the motors continuously, you'll probably just overheat and toast the motors and ESCs. They're also only peak current ratings, so building a PSU just for the 100A peak is silly, instead of just using a battery/capacitance bank.

author: keiby.zhou
author_rank: Contributor
timestamp: January 15, 2012, 04:26:52 pm
content: thanks for all your kind advice.I've bought a computer PSU, and I'll use that to power the charger.I'll keep this project as a mental exercise for now

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2012, 05:48:53 pm
content: For educational purposes, here's how you can make a high current low drop-out regulator with cheap/common parts. You'll also need a soft start and numerous MOSFETs in parallel, each should have a polyfuse or something to protect it from overcurrent.EDIT:Here are some component values:D4 = TL431D1 = bridge rectifier made with Schottky diodes for as low as voltage drop as possible.R2 = 18kR3 = 4.7kC1 = 0.22F

author: Kevin.D
author_rank: Frequent Contributor
timestamp: January 15, 2012, 06:55:55 pm
content: Does this 100 amps have to be continuous or just for short bursts. ?Like others have suggested if you want 12v @100 amps you could buy a engine starter/booster for 50 quid, but that is really just a lead acid battery that can supply 12v @ 300 amps for only short bursts, also I think there are ones that can start/boost straight from the mains at up to 100 amps continuously but they are like £150-£300 .I think if you wanted to make your own continuous 100 A supply though it would not be to difficult to modify an old arc welder which you could pick up from eBay quite cheaply, they output 100+ amps at about 15-30 volts s/c .The cheap ones are all ac out I think but high current rectifiers aren't expensive and you could make a high current 12v regulator to fix in welder case easily enough.

author: Armin_Balija
author_rank: Regular Contributor
timestamp: January 16, 2012, 06:47:15 am
content: I don't want anyone to take offense to this, but seeing as how his theory is very limited, I feel as though it's quite possible this guy will kill himself with that kind of power.EDIT: This is assuming he was ever able to procure the necessary parts. Seriously, you'll kill yourself. 1 A is enough to stop your heart. 100 A will turn you to dust.

author: Unknown
author_rank: Unknown
timestamp: January 16, 2012, 08:04:58 am
content: As you can learn from Ohm's law and the resistance of the human skin, the current through your body at 12 V is much less than 100 A. Which is why people usually don't die after touching the terminals of a car battery. The main danger would be in holding something conductive and connecting it across the output. This will get very hot and may burn you.

author: amspire
author_rank: Super Contributor
timestamp: January 16, 2012, 08:25:38 am
content: Quote from: alm on January 16, 2012, 08:04:58 amYou would have to try very hard to get electrocuted with 12V, but the amount of current can do scary things. In a company I worked for, someone decided to work on a UPS while wearing wristwatch with a metal band. He managed to short a 12V battery out with the wristwatch band. He actually managed to weld the links of the band together so it had to be cut off, and he had to go to hospital for the burns.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 16, 2012, 03:40:19 pm
content: Quote30mA according to photoinductionist. but you need at least 90V and up (based on your body impedance) to make that happen. 12V? TeraVA wont do you a harm, as long as it is 12V, and you dont short it to low impedance conductor. and again, according to photoinductionist with addition of my own flavoring.

author: Armin_Balija
author_rank: Regular Contributor
timestamp: January 16, 2012, 06:48:19 pm
content: Quote from: Mechatrommer on January 16, 2012, 03:40:19 pmYeah I completely agree with you, I was merely saying by the lack of his knowledge about this kind of power it'd be silly for him to even attempt it without even asking the proper safety techniques about handling 100A. I guess we'll have to wait and see, I hope nothing happens to him though, he means well.

author: Unknown
author_rank: Unknown
timestamp: January 16, 2012, 09:46:45 pm
content: I fell like all due warnings have been given at this point. The OP should know by now that the machine he is aspiring to build is quite dangerous, the voltages involved are deadly, there is a significant risk of fire, the complexity is a lot more than he assumed originally, the cost is ten times more than what he assumed it to be, and that the machine is not nearly as practical as he hoped. In doing that we showed that we are the ultimate buzz-killing dweebs that we are.What I hope we haven't done is to kill all of his inquisitiveness. Warnings have been given, it is hoped that they are well understood and more will be given when necessary. Until then we should be talking how to make things work, not the opposite.

author: Armin_Balija
author_rank: Regular Contributor
timestamp: January 16, 2012, 10:07:37 pm
content: Quote from: HAL-42b on January 16, 2012, 09:46:45 pm

author: keiby.zhou
author_rank: Contributor
timestamp: January 21, 2012, 04:37:01 am
content: Quote from: Hero999 on January 15, 2012, 05:48:53 pmThanks for the circuit schematic. compared to the circuit schematic I updated it seems like the basic approach is roughly the same(from my understanding) It would also suffer from a) require large capacitance to stablelize the 20v line and b) rectifier will need 300A+ capacity.however, you doubled the voltage at the transformer end, and I guess that means the precision voltage to control driver gate is now separate from the main voltage to drive the load, which means you can bring the stepped down voltage much closer to output voltage(12V), lowering the power loss on the drivers

author: keiby.zhou
author_rank: Contributor
timestamp: January 21, 2012, 05:15:24 am
content: I have given some thoughts to this design over the last a few daysThe two main issues that I had were:1) cant find cheap diode/bridge rectifier to handle 100A current(300+ peak)2) needs really large cap to hold the voltage after rectifier(0.2+F, the larger the better)(I know there are way more practical issues than those two listed, but those two were the main critical issues in "basic circuit design" stage)I think I found a solution to those problems:1) IRF1404 can handle 200A average current and 800A+ peak current. connect gate to drain is essentially making it a diode(quadratic I-V curve instead of diode's exponential I-V curve, but shouldnt' be much of concern in this case)2) super cap can be cheap and at the same time provides high capacity. the catch is that they only go up to 2.7v - 5.5v. so I am thinking about series connecting them to give 30V. maybe need to do some voltage balancing to avoid one cap gets overcharged, but need to research more to see if its necessaryHow do you guys think of this solution, would it solve those two issues. are there more issues with the "basic circuit design" itself?Other problems need to be addressed:1. need to seriously calculate exactly how much power is wasted on various components and to see if it makes sense. I think I'll borrow Hero999's idea of doubling the voltage at the transformer to supply voltage for the drivers' gates. this way i can lower the transformer's primary output voltage and make it closer to output voltage(to reduce power wasted on drivers)2. looking at IRF1404's datasheet, it seems its a much better alternative to the NPN I currently choose. it can handle 100-200A of currently while the NPN only handles 10A.......(Thanks you all for raising safety concerns over the past few weeks. I understand them well nowHowever, I havn't got to the stage of safety design yet. I dont even have a working circuit that would handle such high current. So, conceptually making the circuit work would be my top priority at this stage. After I have finished the "basic circuit design" stage I surely will get to learn and address safety issues)new circuit schematic with diode replaced by IRF1404 is attached

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 21, 2012, 06:32:42 am
content: add shunt resistor 0.1ohm 120W in series with each mje3055 as current balancing to avoid thermal runaway. have you checked the transformer size, weight and price? also as an advice, prepare a wheely for this psu

author: amspire
author_rank: Super Contributor
timestamp: January 21, 2012, 06:46:52 am
content: Not sure where you got the idea from that the IRF1404 could handle 200A average current working as diodes - they cannot. Not even close. You either need a mosfet based synchronous rectifier, or schottky rectifiers.All those current numbers you quoted are for the mosfet acting as a mosfet, and not as a diode.The 333W power rating is with a case temperature of 25 deg C, which is exceedingly hard to do as the case to heatsink thermal resistance is 0.5 deg/W.At 333W, you would need a heatsink that is colder then -140 degC. That means you would need a big supply of liquid nitrogen to run your supply.Realistically, you will need a great heatsink to even get 100W dissipation. With all the other heat generating devices, 50W is much more realistic.

author: amspire
author_rank: Super Contributor
timestamp: January 21, 2012, 06:52:55 am
content: keiby.zhou, I am just interested. How big do you imagine this supply is going to be? Are you seeing this as a box small enough to fit on a desk?

author: bfritz
author_rank: Regular Contributor
timestamp: January 21, 2012, 06:54:00 am
content: So,I don't think this is a good first project, and actually think that a linear 12V @ 100A output supply is not a good idea, as at this power level the losses will get very high, and the cost for the supply will get expensive, for reasons will will run into as I discuss this.If we want an output of 12V @ 100A, that is an output power of 12 * 100 = 1200 WA linear output regulator will need a minimum of 0.5V before going into dropout. (Dropout is a term for no longer regulating, and providing poor noise immunity, and noise immunity would be the reason to design a linear supply.) So, 0.5V @ 100A = 0.5 * 100 = 50W of dissipation just to meet dropout requirement.To calculate the voltage we can live with at the input, or how much C we need at the full wave rectifier output, we use the following:I = C * (dV / dt) Assuming the line frequency is 60Hz, (you are in Canada), dt = 1/60, if we assume a dV or change in voltage of 4V is ok (output of transformer 12+.5+4 max, trailing to 12+.5 at the end of a 60 Hz cycle), then solving for C = i * dt * 1/dV = 100 * (1/60) * (1/4) = .4166666F or 416,667uF. So, a really friggin big cap! LOLSo, even with a really friggin huge cap, we have 4V or ripple voltage, so on average the final regulator is dissipating an extra 2V on top of the earlier 0.5V. So this power is 2V * 100A = 200W.Then, we have the rectifier, which at 100A, we can simplify this, and assume the diodes will drop 0.5V each. So this is a drop of 1V @100A for each phase of rectification or 1V * 100A = 100W.So far we have a total power required from the wall socket (mains) of:Output power = 1200WLinear dissipation to stay out of dropout = 50WLinear dissipation due to 4V ripple voltage (assume huge cap) = 200WDiode full wave rectifier = 100WSo, assuming the transformer is 100% efficient,and there is no wiring resistance (this becomes at problem at 100A output current),and the capacitors have no ESR,You are going to need a transformer capable of 1550W.You will need to be able to dissipate at least 350W.This is getting dangerously close to the typical 15A circuit limit of 1800W.With so much capacitance, inrush current limiting will be a problem during startup, and a scheme to not trip the breaker will be needed.Although this at first seems like it is the easier design, the issues with making this work with a standard 120V circuit will become an issue, and solving those will likely make this design as difficult as a switching regulator design.I have yet to hear why this current is even needed. You talked about charging some Li-Poly batteries, and running some motors. Motors designed for power tools have peak currents of about 100A, but they are average currents in the 20-25A range. (In my last job I designed protection and fuel gauging circuits for battery packs, including power tools.)The parts for something like this will be things like stud mount rectifiers (so they can be screwed into a heat sink), and are not going to be inexpensive. If I remember you think a budget of $100. I think you will be challenged to find the transformer alone for that price, as I think it will be at or about 1800W by the time you are done with calculating all the losses.If you are serious about building this, you need to learn more, and will then decide to build a switch mode supply.

author: jahonen
author_rank: Super Contributor
timestamp: January 21, 2012, 08:19:26 am
content: Also, note that power factor of such simple rectifier with caps is abysmally low, something like 0.5, so transformer VA rating would need to be twice compared to DC output power (drawn after filter caps) to ensure that VA rating is obeyed. This is due to that when filtering capacitors are added, then the conduction angle of the diodes reduces. Since average current is constant, the peaks get proportionally higher, and will rise to insane levels.It would make sense to use some kind of PFC to reduce the requirements. Then we are in the switcher world, although it would be possible to combine PFC and conventional transformer (PFC after the transformer), but seldom done.Regards,Janne

author: tekfan
author_rank: Frequent Contributor
timestamp: January 21, 2012, 10:38:06 pm
content: If you really want to build a power supply like that I suggest that you look at other tested designs. For example the HP6260b power supply. It has 0-10V, 0-100A output. Weighs 100 pounds, uses 3 phase power input and the peak power consumption is about 3500VA.To keep the head dissipation of the series regulator transistors to a minimum it employs a pre-regulator circuit. This circuit is sort of a PWM controlled AC source that is being applied to the transformer via a big triac. Then it is rectified by several huge diodes, filtered by five 100000uF capacitorsand regulated by 8 high current NPN transistors (believe me these aren't the average 2N3055's). The pre-regulator circuit is enclosed in a shielded enclosure so as to keep the noise of the output to a minimum. The regulator circuit is also very good as it keeps the output noise to less than 500uVIt also has quite a sophisticated protection circuit which prevents instant vaporization of the series pass elements. It has crowbar protection which is necessary for a power supply with such high currents.I think that once you fully understand the circuit and safety requirements of this power supply then you can try and build your own.Link to service manual:http://cp.literature.agilent.com/litweb/pdf/5950-1766.pdf-circuit diagram on the last page (a pretty bad copy but will give you a taste of the complexity of such a power supply)-you may want to read the principles of operation section of the manual (on second thought you must read it if you want to build a power supply with such high power)A few pictures of the power supply:Notice the power switches are actually two circuit breakers and that there are no output terminals on the front.A look at the output terminals. They strongly resemble a welding power supply. The two small wires bolted onto the output terminals are the two sense wires which are required because of the big voltage drops across the wires. I really would not want to know what happens if the terminals short on the chassis.

author: IanB
author_rank: Super Contributor
timestamp: January 21, 2012, 10:41:08 pm
content: Quote from: tekfan on January 21, 2012, 10:38:06 pm(From the picture on the back it looks like 240 V split phase, as commonly found in the USA.)

author: tekfan
author_rank: Frequent Contributor
timestamp: January 21, 2012, 11:10:20 pm
content: Quote from: IanB on January 21, 2012, 10:41:08 pmYou're right.The European version of this power supply has a 3 phase 380V input.

author: Unknown
author_rank: Unknown
timestamp: January 22, 2012, 05:43:20 am
content: Would it be possible to persuade an old buzz box to do the job?Something like this?Or this?The lincoln one even has multiple taps, so maybe (?) jelly bean 20A rectifiers could be used on each tap instead of a single big rectifier?

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 22, 2012, 07:09:26 am
content: Quotedoes it hum?

author: Unknown
author_rank: Unknown
timestamp: January 22, 2012, 07:32:11 am
content: Sure they do. Why?

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 22, 2012, 07:47:17 am
content: QuoteQuoteQuotethen its possible

author: metalphreak
author_rank: Frequent Contributor
timestamp: January 22, 2012, 08:21:45 am
content: HobbyKing sell a 60A supply for $130. Considering their mass production volume I doubt you could build your own for less. Would be interesting to know what sort of design topology they use for it.http://www.hobbyking.com/hobbyking/store/__20612__Turnigy_1080W_220_240V_Power_Supply_13_8V_18V_60amp_.html

author: BravoV
author_rank: Super Contributor
timestamp: January 22, 2012, 09:21:37 am
content: Remember, the OP is still busy thinking & tinkering with the circuit diagram, while the big questions still unanswered are :a. 100 dollar budget and DIY style, henceb. Must find big enough transformer which the OP planning to salvagec. Must unwind and rewind the coil <this includes the material needed like fresh copper wire> if the transformer winding is not suitable for this job.d. Must find and salvage enough caps (maybe thousands of used pc psu and assuming they're still good enough)And all of this is backed by his self taught electronic which I think still in very early stage and also this is the "FIRST" electronic project.I guess its easy to predict where this ended up.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 22, 2012, 09:48:00 am
content: Quote from: BravoV on January 22, 2012, 09:21:37 amSmoke on the water and fire in the sky.

author: Unknown
author_rank: Unknown
timestamp: January 22, 2012, 10:13:07 am
content: Quote from: BravoV on January 22, 2012, 09:21:37 amHe will be over budget, clear as daylight. I think if he ends up 3 x the original budget that will be extremely good result.The lincoln on the last picture waspurchased for 30$ in the USand is capable of 225A welding current (no idea about the voltage). As far as I understand these models are very common there. Since they have multiple taps it may be possible to obtain usable result without rewinding.Also since it has a DC setting I presume it has rectifiers in it. By adding capacitors salvaged from digikey or ebay it is possible to obtain an unregulated power supply of very high power. Thinking of it, this much can be done without opening the lid of the welder at all.From that point on it is the realm of power electronics of course and the learning curve gets steeper. But isn't this how power electronics experts are made? And if he doesn't succeed at least he'll have a welderedit:the images hereclearly show that there is plenty of room inside the case for capacitors etc. also there is a cooling fan .the wiring diagram isavailable hereedit: apparently only some of the more expensive models have DC capability and rectifiers.

author: Unknown
author_rank: Unknown
timestamp: January 22, 2012, 03:37:34 pm
content: Also it seems that welding machine rectifiers are surprisingly cheap.This 180A one is 35$ + shippingeBay auction: #http://www.ebay.com/itm/Clarke-240AC-180DC-rectifier-welder-parts-WE22400061-/320719986104?pt=LH_DefaultDomain_0&hash=item4aac669db8

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 22, 2012, 04:00:44 pm
content: just read a page on welding transformer. it has poor leakage of some sort (intentional). it cannot stabilize voltage very well at high ampere, so ymmv and further study is needed if ones is to use welding transformer as PSU transformer.

author: ron
author_rank: Regular Contributor
timestamp: January 22, 2012, 07:13:06 pm
content: A welder might be a source for parts. Just keep in mind that welders aren't designed for continuous load and many have a duty cycle less than 25%

author: Unknown
author_rank: Unknown
timestamp: January 22, 2012, 07:18:44 pm
content: Quote from: Mechatrommer on January 22, 2012, 04:00:44 pmWelding transformers are wired as constant current sources. I dont know how exactly but it seems like the it has something to do with the reactor winding. It is a current limiting "feature" and may have some usefulness actually.@ron this one seems to have 60% duty cycle at 225A

author: IanB
author_rank: Super Contributor
timestamp: January 22, 2012, 07:34:51 pm
content: Quote from: HAL-42b on January 22, 2012, 07:18:44 pmNeon sign transformers are designed similarly. A gas discharge lamp has infinite resistance before it has struck, but a very low resistance once the arc is established. A current limited source is therefore ideal to prevent the lamp drawing an excessive current.The similarity with arc welding is easy to see.

author: cyberfish
author_rank: Regular Contributor
timestamp: January 22, 2012, 09:47:10 pm
content: I'm still not convinced you need a regulated supply for your application.The charger probably doesn't care about voltage all the much since it probably has an internal SMPS.The quadrotor also shouldn't depend on a fixed voltage because lithium batteries drop in voltage as they discharge, so you'll need closed loop speed control on them anyways. It should be able to accept a voltage range of say 10V - 15V.If you get rid of the regulation part, the supply essentially boils down to the transformer and rectifier, and a few caps. Something like 0.5V ripple. THAT you can build relatively cheaply (maybe even within the $100 budget if you salvage a lot of parts).By the way, motors take a lot more current to start up. You can save yourself some money on power supply by doing staggered startup (second motor starts 0.25 seconds after the first, third motor starts 0.25 seconds after the second, etc). That way it only needs to be rated for 3 motors continuous current + 1 motor startup current.