chinese psu kit - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: April 26, 2019, 10:25:07 am
content: hi all ive one of thesehttps://www.ebay.co.uk/itm/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-Short-with-Protection/143085522518its a 0-30v psu at max 3 amp,ive changed the op amps for mc34071pg op amps,if i run 2 or 3 2n3055 pass transistors with ballast resistors in place of the original plus use a 30amp bridge in place of the original rectifier diodes along with beefing up the tracks and changing the current sense resistor from a .47 ohm 5w to a .22ohm 25 watt should it be pos to get say 6-10 amps from it also using bigger filter caps?,cheers m3vuv.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 26, 2019, 11:17:44 am
content: With a 24 V AC source the DC voltage be at around 28-33 V with load. At that voltage a 2N3055 is good for maybe 2-2.5 A. So 2 of the 2N3055 may allow up to 5 A with a really good heat sink. Even at that current level the simple circuit without tap switching is not really useful.The circuit is supposed to be up to 30 V - however real world maximum is more like 25 V due to ripple and other losses, maybe 30 V with no load and a relatively small transformer and thus high no load voltage. However this puts the OPs at even greater danger.The easy fix is to use a 18-20 V transformer and change 2 resistors (lower current and voltage setting) to make it a 15 V 2 A supply. It may take a larger filter cap at the negative supplyThe filter cap for the kit is one the low side - though more capacitance adds to the ripple in CC mode - the layout is still the same crap for the last 20 years or so. It can be fixed with a bodge wire (green) and cutting the trance (blue) - see attached picture.

author: m3vuv
author_rank: Super Contributor
timestamp: April 27, 2019, 12:29:45 am
content: what is that going to achieve the track does the same as the wire,seems pointless

author: m3vuv
author_rank: Super Contributor
timestamp: April 27, 2019, 05:58:18 am
content: im supprised by the lack of useful answers!

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2019, 06:40:29 am
content: The change is to fix a layout problem: The connections are the same, but the effect of trace resistance is different.There is quite some ripple current to charge the filter cap. This can easily reach the 5 A range, possible 10 A at full load. How much current depends on the transformer and fuse used.Though the traces are a little wider this gives a drop, especially in the short path from the diodes and the filter cap. The voltage at the shunt is relatively low, so that 1 mV ripply would leads to some 2 mA of current ripple. I have not measured it myself, but the problem came up in a German forum: the original board showed quite some ripple (just like the current) and the changes essentially removed it.Adding an extra copper wire to thicken the current path would be a first fix. It is not the worst point, but one that is relatively easy to fix - and for a kit that is sold for some 20 years now, it's a shame not to fix the layout, if it is so easy.The worst point is having to much voltage at the OPs. Even if it may initially work, chances are the TL081 may not like 40 V very lond and fail. I don't know the failure mode, but failing with to high an output voltage is possible.This circuit has been discussed over an over gain

author: m3vuv
author_rank: Super Contributor
timestamp: April 29, 2019, 10:13:10 am
content: still no answers to my original question!,ive changed the opamps already to higer voltage items!.,all the answers so far are not very helpfull!,always seem to go off track!.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 29, 2019, 10:44:51 am
content: they actually did ... the PCB is poorly designed and is most likely 1mil thick.That is barely enough for the 2A specification that they list given how thin those tracks are.Good luck pushing it to 5A at 30VNot to mention the power dissipation of the output pass element, so on and so forth.If you want the right tool for the job, you buy the tool for the job (or make one if you know how to). follow dave's tutorial on building a simple CC-CV power supply which is capable of 5A.

author: m3vuv
author_rank: Super Contributor
timestamp: April 29, 2019, 02:20:38 pm
content: the idea was to use it to control a few fan cooled heatsinked pass transistors,ive a 50a bridge 6x 2n3055 on heatsinks and a 50v 100,000ufd cap,the way i see it the original pcb will only carry the current to drive the pass transistors,was going to run wires from the pcb to them.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 29, 2019, 02:57:03 pm
content: just don't50A bridge? you must be joking ... that's about 70 watts continuous ... I have not seen any bridge that is suitable for that ratingthat's besides the point, if there is a legit application for a 50A bridge, then it must be an industrial application, so don't cheap out on the bloody power source for your fans! I'd be more curious as to what fans are you using that draw about 150 watts (5A 30V that you mentioned)! even the most insane bloweymatron delta fans don't draw that much, not even close! you have to either be trolling with that numbers or you need to seriously reconsider how you design circuits ...

author: Simon
author_rank: Unknown
timestamp: April 29, 2019, 03:26:25 pm
content: we regularly use 190mm fans at work that pull 125W. But yes this project looks a bit insane and if all you are powering is fans then a full blown regulator is not required. Wy buy a low power regulator and try to beef it up. Just buy something suitable or make one from scratch.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 29, 2019, 03:43:05 pm
content: he mentioned a bridge rectifier, so I assumed something small cooled with a PC fan. the most insane ones are these delta fans used for server racks which require a lot of static pressure to force the air through:this one is a mid range one and as you can see it's rated for less than 24 watts!their top of the line ones can draw about 60 watts but will literally fly off due to their own force if not secured properly and sound like a jet engine too!

author: Simon
author_rank: Unknown
timestamp: April 29, 2019, 03:50:10 pm
content: Yes and as I just said I have used fans just like that from ebmpapst that pull 125W! yes they sound like a jet and have enough force to move themselves around and do a lot of damage. We once put a cooler for a server rack for a submarine in a door with about 12 of those fans. The ones I use come in a 32.5W, 65W and 125W variety but all look identical, they just go faster and faster topping out at 11'000rpm. doubling power into a fan will only get you a 20% increase in speed but a 40% increase in pressure so going up to 125W is not that insane when you think that it is only pushing 20% more air than a 65W fan.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 29, 2019, 04:30:50 pm
content: There is nothing bad with using a 50 A rated bridge rectifier at a lower current. However there is a problem with using 100 mF of filtering cap - this causes quite some current peaks, that can cause quite some ripple if not careful with the layout. For beginner It's better to start small - so more like the original size, reduced to some 1.5-2A.As already noted the proposed the 2N3055 is good for something like 2 A max at some 35 V input voltage. So it could work to higher current, but it is not such a good idea to use the simple circuit to much higher power. It's likely better to use a slightly more complicated circuit and get away with about half the heat sink and fan power. Another point is the drop on the shunt: with higher current it gets attractive to reduce the drop at the shunt and use a better OP instead of a higher power resistor. Already the original configuration produces a lot of heat and thus not really stable current regulation. The .22 Ohms 25 W shunt would be more suitable for 2-3 A.When replacing the rectifier, filter caps, shunt, power transistors, OPs there is not that much left of the kit that can be reduced. Expect very low quality pots to come with the kit - so better replace them too.

author: m3vuv
author_rank: Super Contributor
timestamp: April 29, 2019, 06:53:27 pm
content: dont you guys read?,firstly the bridge is what i have to hand secondly the fan is a small one to cool the heatsinks i have for 6x 3055 pass transistors,if i need that many,the filter cap is 100 thousand uf at 50v rating not 100 uf,i want to try and controll the 3055s with this psu for more current outputthe pass transistors will be off the pcb on heatsinks,i could use a 723 but i like the cc idea of this psu,having the value of the sense resistor should let it produce 2x the current as i see it,as long as the other components ie output transistors bridge etc are man enough?,thats basicaly what i was asking ie if it wouls work,not a sermon on heatsinks etc which i already know!.,ppl not reading the post properly or reading too much into it make this forum hard going!!

author: Simon
author_rank: Unknown
timestamp: April 29, 2019, 06:59:35 pm
content: Listen, if you don't like what people have to say then sod off! No one here is under any contract to tell you what you want to hear and you have been told: It's a bad idea, just make a suitable power supply for whatever it is you want to power. Why are you taking this shitty thing and trying to turn it into something it is clearly not? you either buy something for what it is or just build something from scratch.What are you trying to power with it?

author: m3vuv
author_rank: Super Contributor
timestamp: April 29, 2019, 11:55:56 pm
content: i think everyone is missing the point,ive no interest in buying anything,the kit is built and works fine,i just wanted to know if my ideas to get a few more amps from it would work,if i wanted a basic psu id use a 723 etc,ive built a homebrew cnc mill and turning it into a laser engraver,if i can get 5 amps out of the psu it would be handy for powering the lased diode,approx 4 amp.,i dont need lectures on heatsinks etc,ive been into electronics for 40+ yrs keep the advice for the newbies!,dont mean to sound rude but i ask a simple question and it doesent get answered just waste my time reading posts that are not realy helpfull in regards to my original question.

author: m3vuv
author_rank: Super Contributor
timestamp: April 30, 2019, 12:06:44 am
content: maybe even swiching taps on a transformer to reduce the power dissipated in the pass elements,its more the cc mode im after,i could use a 723 but the current foldback doesent start until .6v is across the current sense resistor,with this kit half of the current control is already there and better than the 723 can do overall,look upon it as more of a hack.

author: Simon
author_rank: Unknown
timestamp: April 30, 2019, 06:21:08 am
content: Well it sounds like you don't want advice so why are you asking?

author: Simon
author_rank: Unknown
timestamp: April 30, 2019, 06:36:16 am
content: The transformer is 24V for a 30V output, erm yea, this sounds like the regulator as it stands can hardly cope as they are relying on the peak voltage to be over 30V but if you pull enough current it will loose regulation. Theoretically increasing the transistors will work and yes sorry to give the advice you don't want but heatsinking will need to be looked at again. yes if you drop the sense resistor value it will work to 5A thinking it's 3A. Without seing the schematic I have no idea.Wy don't you just buy a cheap switchmode unit that is already on a frame and ready to use?

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: April 30, 2019, 06:56:30 am
content: Quote from: Simon on April 30, 2019, 06:36:16 amBecause he is 40+ years into electronics, so he knows everything that he needs to redesign this shitty PSU to power his laser. Well, obviously nearly everything. How can you not understand that?Some basics in kindness and using the space bar seem to be lacking, though.

author: soldar
author_rank: Super Contributor
timestamp: April 30, 2019, 09:18:39 am
content: Quote from: m3vuv on April 27, 2019, 12:29:45 amQuote from: m3vuv on April 29, 2019, 10:13:10 amQuote from: m3vuv on April 29, 2019, 06:53:27 pmYou might try adjusting your attitude if you want to ask for people's help.With an attitude like that I am not surprised even your electronics refuse to cooperate.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 30, 2019, 10:43:53 am
content: all the shitty attitude aside, he's been into electronics for 40+ years and still doesn't know lasers need to be modulated and a basic CC PSU (a cheapy chinese one at that) isn't gonna cut the mustardand it goes from powering fans for a 50A bridgeto powering a laser in a few postsfirst you should know what your main goal isalso as pretty much everyone pointed out, this kit is running at it's limits which means upgrading one part doesn't do anything and you must upgrade EVERYTHING for it to be any better ... you get to keep the PCB which still seems to have issues and others pointed outalso everyone pointed out buying / building a better PSU which you said you have no interest inthe answer is pretty obvious here

author: m3vuv
author_rank: Super Contributor
timestamp: May 01, 2019, 10:31:33 pm
content: jeez i give up,its like trying to tech your granny to suck eggs!