Chinese 0-30v 3a psu issues - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: September 10, 2019, 10:07:10 pm
content: just built this kit,schematic attached,it works fine but if i turn it off when its in cc mode i get a brief spike on the output,in cv mode its fine,is this normal?,if so it there a fix bar chucking it in the bin?,regards m3vuv

author: RIKRIK
author_rank: Contributor
timestamp: September 10, 2019, 10:42:35 pm
content: This reminds me of the old school power supplies , lm358http://www.electronics-lab.com/project/0-30v-laboratory-power-supply/, I'd say it's a keeper, have you tried turning the current and voltage fully down on power off, apart from that, have a tinker with it, for hobbyist electronics it should be fine. Fingers crossed .

author: ledtester
author_rank: Super Contributor
timestamp: September 10, 2019, 11:37:00 pm
content: This looks like the 2ma-3A, 30V power supply kit that has been much discussed on this forum.Have a look at this thread and, in particular, the comments by Audioguru:https://www.eevblog.com/forum/beginners/bangood-psu-enhancements/

author: m3vuv
author_rank: Super Contributor
timestamp: September 10, 2019, 11:44:11 pm
content: if the current and voltage are turned fully down it wont do it as its then not in cc mode,its only in cc mode that it does it.

author: floobydust
author_rank: Super Contributor
timestamp: September 11, 2019, 12:45:19 am
content: It's a design issue unless you assembled the kit wrong. The original design was copied from 1978 (Practical Electronics 1978 October) and used LM741's.https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/msg2239764/#msg2239764R13, R14 were changed for better control of switch on/off spikes, I think 160R and 12k in Rev. 7Normally, when power is switched off, C3 discharges first then Q1 turns on to shut off the output, until C1 is discharged. But under load, C1 can discharge first so the +ve rail sags and the op-amp outputs a nasty blip. The PSU needs an output off switch, or undervoltage lockout added.

author: Audioguru again
author_rank: Regular Contributor
timestamp: September 11, 2019, 01:59:56 am
content: You show the schematic of the old Greek kit that has many parts overloaded and wrongly uses TL081 opamps that have a problem where the output goes as high as it can when an input voltage becomes too close to the negative supply voltage. TR1 shorts the voltage driver opamp output to prevent it happening when the power is turned off.Recently Banggood and other Chinese companies sell their copy of the kit.

author: wraper
author_rank: Unknown
timestamp: September 11, 2019, 02:23:03 am
content: Try increasing C3 capacitance so negative rail holds voltage longer on turning off and see what happens. Most likely it happens because negative rail discharges faster than positive rail.

author: m3vuv
author_rank: Super Contributor
timestamp: September 11, 2019, 10:19:47 am
content: Hi i increased c8 to 100uf,all it does is delay the spike on switch off,still the same just happens a moment after switch off.

author: xavier60
author_rank: Super Contributor
timestamp: September 11, 2019, 11:06:34 am
content: Try replacing R14 with a zener diode with the same voltage as D7. Hopefully it will cause Q1 to mute early enough.

author: m3vuv
author_rank: Super Contributor
timestamp: September 11, 2019, 12:11:04 pm
content: i think ive sorted it,i replaced q2 with a bd139,only issue is now max volys out is only about 12v,thats running a 5watt bulb,voltage over c1 holds at 27v under those conditions,i suspect q1 is faulty.

author: m3vuv
author_rank: Super Contributor
timestamp: September 11, 2019, 02:06:05 pm
content: well i changed q4 for a 3055,still no voltage spikes but only just over 9 volts out into a 21 watt bulb,any ideas?

author: wraper
author_rank: Unknown
timestamp: September 11, 2019, 02:20:06 pm
content: BD139 has different pinout. So most likely that's the reason of 'fix' and becoming broken. You just assembled it incorrectly. Also BD139 is not a right part to go there.

author: xavier60
author_rank: Super Contributor
timestamp: September 11, 2019, 04:43:52 pm
content: Measuring the voltage drop across R15 will indicate how much current U2 is supplying to drive the output stage and how much current gain there is.

author: m3vuv
author_rank: Super Contributor
timestamp: September 12, 2019, 06:36:06 am
content: its not the wrong pinout!!

author: m3vuv
author_rank: Super Contributor
timestamp: September 13, 2019, 12:11:48 pm
content: For what its worth if i lift one leg of R11 and one leg of C9,it still behaves the same,voltage adjusts to a max of about 9v and the current limit still works.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 13, 2019, 12:48:13 pm
content: Is that still the same PSU discussed back in your thread from april?https://www.eevblog.com/forum/projects/chinky-psu-kit/msg2369598/#msg2369598

author: m3vuv
author_rank: Super Contributor
timestamp: September 13, 2019, 06:29:28 pm
content: same design but a new unit,seem fine just cant ger any output above 9v.

author: xavier60
author_rank: Super Contributor
timestamp: September 13, 2019, 08:29:41 pm
content: While an op-amp is in control of a loop, its input pins will measure the same voltage.

author: m3vuv
author_rank: Super Contributor
timestamp: September 13, 2019, 08:49:13 pm
content: well i removed q1 and tested it,it was sort of leaky in all directions,it now outputs to about 27v,the strange thing is now both the current and volt pot seem to adjust the output,the amp pot seems to vary the voltage less smoothly than the volt pot tho,the cc led is still working but back to a spike on turn off with the cc led lit,starting to tear my hair out with this now!.

author: m3vuv
author_rank: Super Contributor
timestamp: September 13, 2019, 08:59:00 pm
content: pin 2 of u sits at 7.63 volt in cv mode,pin 3 at 24.75,then in cc mode with the led lit pin 2 sits at 1.042v and pin 3 at 1.032.

author: m3vuv
author_rank: Super Contributor
timestamp: September 13, 2019, 08:59:40 pm
content: those last readings were from u3 pins btw.

author: m3vuv
author_rank: Super Contributor
timestamp: September 13, 2019, 09:11:09 pm
content: i changed d9 and its fixed it,cheers everyone,its been a pain but feel ive learnt something,it was the comment about both op amp inputs beeing equal if there in the loop led me too it,many thanks all.,also the spike hs gone in cc mode at turn off too.

author: m3vuv
author_rank: Super Contributor
timestamp: September 13, 2019, 10:20:17 pm
content: well maybe i spoke too soon,just noticed i cant get the output below 35mv,rv1 does nothing,earler i dis swap c3 for a 100uf from a 47uf,is that the cause?,cheers Paul m3vuv.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 13, 2019, 10:22:19 pm
content: You do notice that there is an edit function implemented into this forum software?

author: m3vuv
author_rank: Super Contributor
timestamp: September 13, 2019, 11:59:52 pm
content: in a word no i didnt,anyway swapped u2 and it goes down to 3mv,think thats good enough for me.

author: m3vuv
author_rank: Super Contributor
timestamp: September 14, 2019, 03:20:46 am
content: i have 3 of these working from an old gas analyser,would it be an idea to use them in place of the op amps?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2019, 06:43:19 am
content: The OP05 should work, but this OP is relatively slow. This can slow down the response. Especially the current limiting may than take quite some time to respond. In case of a short this can cause quite some current spike. On the positive side the drift is low. So the current limit would be more accurate - still the TC and self heating of the shunt limits the stability.An 741 type OP would likely give better performance.Normally the current and voltage settings should not directly interact. This may happen if the regulator is oscillating.

author: m3vuv
author_rank: Super Contributor
timestamp: September 16, 2019, 02:16:03 am
content: well ive had a mess about with this,im using a 50a bridge rectifier powered by a transfo from an old 20a psu,smoothed by a 100000uf 50v cap,the original q4 is a 2n3055 in a darlington configuration driving 4 2n3055 pass transistors with 0.1 ohm emitter resistors,ive 3x 25 watt current sense resistors for different current ranges in place of r7,can anyone think of a way of sensing the current on say on the feed to the bases of the pass transistors,im not sure how to connect a current sense resistor in the pass transistor base feed to u3 without blowing things up,could use it as is but as it is all the output current passes thru the current sense resistor so switching between resistors could be a issue as it will carry the full output current.,also in cc mode u3 output is low,in cv mode its high,could i drive a cv indicator led from it,ie is the op amp capable of outputting enough current ?,also all ic's are mc34071p ic's,regards paul m3vuv.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2019, 06:00:33 am
content: 0.1 Ohms emitter resistor are kind of at the low end for 2N3055. It may still work with well matched (e.g. selected or some luck) transistors and good thermal coupling. The 2N3055 are relatively slow, this might effect the compensation. One might consider to use at least for the 1st transistor a faster one (e.g. D44H...).Current sensing from the base current is not really accurate. It may still be used for an additional fast acting hard current limit. So this would not act on U3, but independently. Changing the current sens resistor to a different position would totally mess up the circuit - essentially start over from scratch.The OPs are powerful enough to drive a LED. Modern LEDs don't need 20 mA anymore - some 1-5 mA is usually still bright enough.For high power one usually prefers to have transformer tap switching on some other ways to reduce the power loss.