DIY 0-30V 0-3A ...Again  - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: January 05, 2019, 08:34:44 pm
content: I know this darn project has been done to death and I apologize for bringing it up yet again.I've poured over all the posts here and across the Net concerning this project and have found nothing related to my problem.I understand that it's unlikely to achieve the advertised output of 3 amps without over heating and possibly destroying the circuit.That would be more impressive than what I getting right now.With no load it does indeed supply 0-31VDC. The current control seems to work as it effects the voltage and the LED does change off and on.My problem is despite all attempts,checks ,rechecks and more rechecks I can't get more than 30ma.On load test at full voltage and full amps all it does is 30mA and drops the voltage right down to 0.8V.Current can be adjusted between 0 and 30mA with delicate movement of the potentiometer.The transformer is a Hammond 120VAC to 24VAC 350VA .Now this should be plenty to power this PSU.I purchased new TL081 op amps from Digikey and tried 3 different sets. No change on loadI replaced the filter Cap from 3300uFwith 4700uF.No change on loadReplaced all diodes including zener .No change.All diodes have checked OKRemoved Q1 suspecting it may be faulty.No change on load.Changed Q2 from 2SD882 to a BD139.No change on load.Checked the Q4 2SD1047 and it checks OK.Replaced Q1 (9014) and Q3 (9015) .No changeAll resistors check within 1%.All diodes have checked OKAll potentiometers work as they should.I've checked all voltages across pin4 and pin7 supplying the op amps .All around 32 volts.I checked the zener diodes and get the expected 5.1Vdc across D7 and -5.1Vdc across D8.I load tested pin6 on U2 and get 600mA at 31V which should be enough to drive Q2 with the (R15)1K limiting resistor.Which should give 31mA at 31Vdc.Checked all traces and compared to working PCBs and found no difference aside from pattern.Checked and rechecked orientation of components.Checked continuity of traces and all solder joints.All functioning.The only thing that seems odd is that RV1(100K Trim pot) doesn't do any thing .Despite all this I can't get more than 30mA.Frustrated and out of ideas.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 05, 2019, 09:05:09 pm
content: Double check the value of R7.If it is correct, load the output to 30mA (or whatever the maximum attainable is) and report the voltage across R7, and at pins 2 and 3 of U3 (relative to 0V side of R7).

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2019, 09:15:39 pm
content: The circuit is known from Chinese kits with several known issues.The most important is that with a 24 V transformer the supply for the OPs U2 and U3 will be too high (e.g. nominally some 38 V, worst case maybe more than 42V) So expect these to blow.Even if the OP survive (lucky or using other OPs like 741 , TLE2081) one would not get 30 V out under a significant load. With a 24 V transformer its more like 20-24 V. The current rating is more like short time peaks and should be reduced to something like 2 A.To keep the original OPs in spec, the transformer should be no more than 18-20 V AC and thus some 16 V maximum output voltage under load.For the broken circuit, having 31 V at the supply to the OPs suggests the negativ (-5 V) supply is not working. This could be a wrong part (e.g. zener D7 the wrong way around) or a broken OP, drawing to much supply current.600 mA out of U2 suggest it's broken. Without the negative supply Q1 blocks any significant current - a little odd to still get 30 mA. 30 mA would be more like the normal current limit out of U2.

author: IanB
author_rank: Super Contributor
timestamp: January 05, 2019, 10:53:34 pm
content: This may seem obvious, but if we take the transformer at face value, then with a supply of 24 V AC, the maximum output current at 33 V DC using a perfect rectifier will be zero amps. If we subtract some diode drops in the rectifier and the necessary voltage drops across the regulation circuitry, then the limiting voltage for zero amps out will drop down to 30 V or so. If you are actually managing to get 80 mA at 31 V DC I would say you are doing remarkably well. It is way better than zero which is all you have any right to expect.

author: Jwillis
author_rank: Super Contributor
timestamp: January 05, 2019, 10:57:41 pm
content: Oh wow! Thanks for the reply sQuote from: Andy Watson on January 05, 2019, 09:05:09 pmThanks Andy.I checked R7 and pis 2 and 3 on U3 as you said. R7 is 47.3 Ohms and under max load (a whole 30mA) it reads 1.667 VdcPin 2 is 1.672Vdc and pin 3 is 1.723VdcQuote from: Kleinstein on January 05, 2019, 09:15:39 pmThanks Kleinstein. I don't expect much from this kit without modifications. But was hoping to get at least 1 Amp.So far I've retested those Zeners and they both read 5.1V and the orientation is correct.Double checked all the diodes for orientation.The Ops are all fine .I've switched them out several times with the same results.Quote from: xavier60 on January 05, 2019, 10:33:58 pmThanks xavier. I recheck D10 and its the correct way. R15 is 1K ohms But I found another schematic where R15 is 100 Ohms and this person claims a much higher current output.http://electronics-lab.com/community/uploads/monthly_08_2014/post-107142-14279144694286.jpggiven the extra power transistor I don,t doubt it.I was kind of leaning toward the idea that R15 may be to high but the circuit is identical to others that function fine.

author: IanB
author_rank: Super Contributor
timestamp: January 05, 2019, 11:00:15 pm
content: The key thing is can you get a reasonable current at lower voltages. For instance, what can you get at 24 V, or at 12 V ?

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2019, 11:05:38 pm
content: audioguru made some modifications to that project many years ago, but the webmaster on electronics lab (that site has plenty of other broken/bad projects/schematics) didn't bother updating it. I strongly recommend implementing them.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2019, 11:07:49 pm
content: Quote from: Jwillis on January 05, 2019, 10:57:41 pmAt 47 Ohms R7 would be way too large - it more like 0.47 Ohms to be right. So the 30 mA could be about right given the resistor is high by a factor of 100.

author: soldar
author_rank: Super Contributor
timestamp: January 05, 2019, 11:14:04 pm
content: Quote from: Jwillis on January 05, 2019, 10:57:41 pmR7 should be 0.47 Ohm or 0.5 Ohm. A value too high will limit the current output sooner.

author: Jwillis
author_rank: Super Contributor
timestamp: January 05, 2019, 11:25:43 pm
content: Quote from: Kleinstein on January 05, 2019, 11:07:49 pmQuote from: soldar on January 05, 2019, 11:14:04 pmYou know you guys are right I neglected to notice that .The schematic even requires a 0.47 Ohm not the 47 ohm that was supplied.Damn my poor vision.Now that explains whats going on.A couple 0.25 in series should be alright to test.Edit: I have a variac so Would it be ok to set a lower voltage at the primary with the Variac just for testing?

author: soldar
author_rank: Super Contributor
timestamp: January 05, 2019, 11:44:55 pm
content: Quote from: Jwillis on January 05, 2019, 11:25:43 pmI can't see how that helps any. You wer using the stated 24 Vac and it was working well except for the current being limited. Why do you want to change something that is working as it should?

author: Audioguru
author_rank: Super Contributor
timestamp: January 05, 2019, 11:52:19 pm
content: You bought a cheap Chinese kit that has all the errors of the original Greek kit plus the value of R7 is 100 times wrong!

author: Jwillis
author_rank: Super Contributor
timestamp: January 06, 2019, 12:25:41 am
content: I didn't bother with the Variac. Anyway I changed the 47 Ohm for (2) 0.25 in series (0.5 Ohm ). No change to current output of 30mA and voltage is now 3mV instead of zero to 31V.Darn!Still have full range of voltage without load but it still drops to 200mV at load full available current (30mA).Voltages across pin 4 and pin 7 for U2 remains 31 volts with or without load. U3 and U1 those same pins drop to 28 volts with load.Going to attempt a lower value at R15.Quote from: Audioguru on January 05, 2019, 11:52:19 pmI checked the board for any errors in the traces and they match all working products.Most of the components have already been changed to fully tested ones from reputable dealers.Aside from the error with R7 that is partly my fault for not paying attention.Besides, changing R7 did little .

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2019, 12:51:50 am
content: Quote from: Jwillis on January 06, 2019, 12:25:41 amI do not think you have not clarified whether this restriction in current applies at all selected voltages, or just at the maximum voltage?

author: Audioguru
author_rank: Super Contributor
timestamp: January 06, 2019, 01:05:00 am
content: Quote from: soldar on January 05, 2019, 11:44:55 pmThat circuit cannot survive since many parts have too much voltage and current. The TL081 opamps have a maximum allowed supply of 36V but the 24V transformer produces a peak of 34V plus the 5.1V negative supply plus maybe 2V more with low load current= 41.1V. Then POOF go the opamps.The circuit has no voltage and current calibration trimpots.The main filter capacitor is WAY too small.My improved circuit uses a 28V or 30V 4.2A transformer, 44V opamps that use a 1.3V negative supply instead of the higher negative voltage needed for the old TL081 opamps, two output transistors to share the heat, some larger resistors to survive the heat, and calibration trimpots. It produces a regulated 30VDC at 3A as it should.

author: Jwillis
author_rank: Super Contributor
timestamp: January 06, 2019, 01:17:08 am
content: Quote from: IanB on January 06, 2019, 12:51:50 amOh Sorry Ian .The restriction is is the same at all positions of the voltage pot.At maximum load with the current 30mA of the voltage can be adjusted between 0 and and around 200mVIt's acting like Q2 and Q4 aren't turning on at all and all the current is being drawn from U2 which just doesn't make sense .I have some 3055 s to replace the 2SD1047 if need be.I'll take a fresh look at it again tomorrow .It's probably something stupid .

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2019, 01:24:20 am
content: Quote from: Jwillis on January 06, 2019, 01:17:08 amOK, thanks for the clarification. But I'm still having a little trouble following what you are doing.The normal way to test such a power supply would be to set the voltage with no load, set the current limit to maximum, and then increase the load gradually and watch what happens to the voltage. When the voltage starts to dip or lose regulation you have reached the limit on how much current can be supplied.A common way to adjust the current limit would be to short the output terminals and then turn the current adjustment until it is at the current you want. When doing this it would be normal for the output voltage to be close to zero (since the output is shorted).

author: Audioguru
author_rank: Super Contributor
timestamp: January 06, 2019, 01:43:06 am
content: The driver and output transistors are simple emitter-followers. The output transistor has an hFE of at least 25 and the driver transistor's hFE is at least 80. Then the total current gain is 2000 and only 3A/2000= 1.5V will appear across R15if Q1 is properly turned off.U2 will have a max output current of only 1.5mA.Maybe the voltage at pin 3 of U3 is not about 1.5V when the current-setting pot P2 is set to 3Ato match the 3A x 0.5 ohms= 1.5V of your R7. Because any current more than 3A will cause the output of U3 to go low enough to reduce the output voltage and current.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 06, 2019, 04:10:10 am
content: Note: the garbage pots from Banggood could be a source of a lot trouble (at least they were for me).I cut-and-pasted a side-by-side schematic to help future modders.

author: Mate_Well
author_rank: Contributor
timestamp: January 06, 2019, 03:00:15 pm
content: •Actually, i was very interested in this project some time ago, after digging into it, i found there is a great document ( in French and it is not idealy translated but it is understandable into english), it has very nice measurements, facts, and additions to ithttps://drive.google.com/open?id=1qaKc0H9951NUQiRNrA4dNyz8iehMwrSn(it has the most stable part list/diagram from all that are floating arround, i hope this will help out (also i did not tryed to build it yet)•I will also try to make it much more powerfull with more powerfull transformer, which leads to more powerfull opamps, and other parts, air coling and such, i will do it long time from now on (i need to save up for the parts sadly)

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 06, 2019, 03:49:01 pm
content: Quote from: Mate_Well on January 06, 2019, 03:00:15 pmQuote

author: Microcheap
author_rank: Frequent Contributor
timestamp: January 06, 2019, 05:09:43 pm
content: I am well aware of the limitation of this circuit but I've assembled this project over a decade ago when I started learning electronics, and the psu still works, of course I never pushed it to its limits, I think the only time was when I left it overnight recharging an old lead acid battery at around 13V@2A, and it survived. The only thing I remember I had to replace 1 or 2 times were transistors Q2 and/or Q4 but I was using Chinese knock-off transistors with improper heat-sink.Back to the original question, have you checked Q1, R13 and R14 already? As Audioguru pointed, Q1 must be off, test the Vbe of Q1, it should be lower than 0.7V (I measured 0.2V in my board, with R13=10k and R14=1k5).

author: Mate_Well
author_rank: Contributor
timestamp: January 06, 2019, 05:42:48 pm
content: •Any chance the u3 is damaged, or has wrong pins conection (could be caused by putting the chip in 180°rotated also) ?link fixed

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2019, 07:33:29 pm
content: Quote from: Richard Crowley on January 06, 2019, 03:49:01 pmI am able to download it from Google docs, however it is 5 MB so it is too big to attach here.

author: Jwillis
author_rank: Super Contributor
timestamp: January 06, 2019, 09:18:17 pm
content: Hello folks.sorry for not getting back sooner.All the information you guys provided is a great help.Quote from: Mate_Well on January 06, 2019, 03:00:15 pmThank you Mate_Well ,I'll call a friend to see if she can translate that PDF file for me.I'm currently testing the ops to see which one failed .The voltage adjust is fine but lost current control.I suspect U3 as you mentioned .Quote from: Cliff Matthews on January 06, 2019, 04:10:10 amCliff Matthews I had the same problem at the start and changed those to 2W 10K for testing.So I pulled all the Ops to measure the voltages at pin 4 and pin 7 in relation to output ground.For U1 I can't identify a true zero point to compare the values of pin 4 which should be negative and pin 7 which should be positive for any of the Ops .So from what I understand the TL081 requires a supply of 7 V ~ 36 V, ±3.5 V ~ 18 V (single or dual) So I just took a reading pin 4 to pin 7 and got 33.8Vdc.Since pin 4 looks like it gets a variable voltage off of U3 pin 2.That's of course a guess on my part. Taking a reading at pin 4 on U1 from ground gives 0Vdc.Pin 7 I measure 33.5Vdc.For U2 pin 4 reads from output negative for pin 4 -5.1Vdc and pin 7 33.8Vdc . Across from pin 4 to pin 7 is 38.9Vdc.Which is to high.For U3 pin 4 reads from output negative for pin 4 -5.1Vdc and pin 7 33.8Vdc. Across from pin 4 to pin 7 is 38.9Vdc.Which is also to high.Since the additive values of pin 4 and pin 7 can't exceed 36 V,I wondering if theirs any reason why I can't incorporate zener regulators for U1 ,U2 and U3 to bring pin7 voltage down so the additive voltage of pin 4 and pin 7 won't exceed 36 V ? Could even bring D7 down as well since it really only needs to be >3.5V

author: Jwillis
author_rank: Super Contributor
timestamp: January 06, 2019, 11:40:15 pm
content: Something like this to control voltage for pin 7 .Since that pin gets its voltage directly for the rectifier ,changing the voltage to something like 28 or 30 volts should be plenty but or interfere with the rest of the circuit.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 07, 2019, 12:44:13 am
content: Using 3 separate zener's could produce temperature drift.U1 doesn't need any help, but pin 4 of U2 and U3 might work fine with a 2.7 or 3.1v zener at D7..

author: Jwillis
author_rank: Super Contributor
timestamp: January 07, 2019, 01:15:16 am
content: Quote from: Cliff Matthews on January 07, 2019, 12:44:13 amI was wondering because zeners of higher values >5.6V tend to have positive temperature coefficients as compared to lower value zeners <5.6 volt which tend to have a negative coefficient.So wouldn't that reduce the possible drift because It's in the upper voltage range?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 07, 2019, 06:07:44 pm
content: The only important zener is D8. The other zeners are only for the supply of the OPs. So there TC does not really matter.U1 and U3 don't need a high positive voltage, so they could run from the 12 V regulator. The zener diodes should go the other side of the shunt, not to include the zener currents in the current limit.How much negative supply is needed depends on the OPs used. U3 may need a little more, to safely get it's output to about -0.8 V and work with an input near or slightly below ground. With the TL081 from the kit the 3 V lower limit (though often better) for the input common mode range can be a problem - to be on the safe side an extra diode from the output of U3 to ground could help here, by preventing the output to go too negative on transients.

author: Audioguru
author_rank: Super Contributor
timestamp: January 07, 2019, 08:20:05 pm
content: Why are you adding Zener diodes instead of using the correct 44V opamps, a -1.3V supply for them and a 28V transformer like the improved version of this power supply?The output transistors are simply emitter followers that have some voltage loss and no voltage gain. If you use a 30V Zener diode for U2 then the maximum output from the project will be only about +20V at 2A, instead of 30V at 3A.If you want to use the low voltage TL081 opamps that need a negative supply of -5V then use an 18VAC transformer instead of adding Zener diodes to reduce the voltage. Then the single output transistor will not overheat as much.

author: floobydust
author_rank: Super Contributor
timestamp: January 07, 2019, 10:21:25 pm
content: These power supplies blow up anytime there is higher output voltage present (large O/P cap or charging batteries) and main power goes off. Or if you have a low setpoint say 3V and connect to 12V battery.You get backfeed from the output transistors E-B junction to the op-amp and it damages it. Adding a 1N4004 protection diode across C-E of output transistor would fix that.I see a mod with 100R resistor R16 (R15) which should limit op-amp current.https://www.eevblog.com/forum/beginners/diy-0-30v-0-3a-again/msg2097889/#msg2097889But D10 is incorrect

author: Jwillis
author_rank: Super Contributor
timestamp: January 07, 2019, 10:37:44 pm
content: Quote from: Kleinstein on January 07, 2019, 06:07:44 pmThanks Kleinstein.I'll have a look at those suggestions I plugged in 27 volt Zeners in the configuration I describe in a previous post.So now all the Ops are running at an appropriate voltage .U2 I'll pull the 27 and exchange for a 30 V because the voltage will only go to around 28.5 V but I'll worry about that later. Because of the change in input supply voltage the CC LED will only dim and not go completely off.But I believe that's just a matter of adjusting the resisters for Q3. Good news is is I have control of the voltage and a fair control of the current but still can'y get better than 200mA.Better than before but still disappointing.I can work the other bugs out later If I could just figure out why I'm getting such minimal current.It's really weird because I've gone over every value of every component and every trace and connection several times.Quote from: Audioguru on January 07, 2019, 08:20:05 pmI really appreciate your suggestions. Your modification looks really good and is better and deserves the respect to be done as a separate project .The reason I don't go with other Ops is because I simply don't have them available. I don't have a transformer of lower voltage that has an adequate current output unless I winded one myself .The shipping costs are prohibitive to where I live so I only make orders that can justify the cost of shipping.Your correct about the output voltage drop because of the zener.I get 0 to around 28V right now.But I'll work that out later.But this project isn't being done for a usable power supply at my bench for I have better ones .I trying to figure out why I can't seem to get higher amperage at the out put .If I got 2A I would be ecstatic but I only get 0-200mA at best .This has been the main problem from the start. The output transistors Q2 and Q4 are cold like they're not turning on at all. I'm going to attempt changes in that area to see if I can get some kind of response.With or without the zeners I get the same problem.Although it's better than the 20mA before I changed R7.

author: Jwillis
author_rank: Super Contributor
timestamp: January 07, 2019, 10:53:58 pm
content: Quote from: floobydust on January 07, 2019, 10:21:25 pmReally? D10? That wouldn't surprise me. I'll change that around and see what happens .At this point I'll try just about anything .Hopefully I'll learn something in the process. I'll check that to make sure the marking hasn't been painted incorrectly.But every schematic on the internet I've seen shows it in that orientation.That would be great find if your correct.

author: floobydust
author_rank: Super Contributor
timestamp: January 07, 2019, 11:12:04 pm
content: Let me clarify - D10 protects only the transistors' E-B junctions from backfeed and R15 is there to limit op-amp current to some extent. Some schematics don't have them, others move D10 in series with the op-amp output.

author: Jwillis
author_rank: Super Contributor
timestamp: January 07, 2019, 11:47:30 pm
content: Quote from: floobydust on January 07, 2019, 11:12:04 pmThanks for the suggestion.But.Darn! No change in output.Time to change hands and approach "deep thought" in a different direction.Although the sledge hammer is close by I'm not ready to give up just yet.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 07, 2019, 11:51:36 pm
content: Quote from: floobydust on January 07, 2019, 11:12:04 pmI have 2 of these boards to mod, and from comments, now quite uninspiring on my workbench.1) Any side-effects expected with a diode on U2's output?1a) with that in place, and all op amps (V+) on supplied 24v regulator, can a benefit be observed with R15 below 100 ohms?2) I bought 6A05's for a reliable 3A bridge upgrade, are these OK (fast enough) to also bypass junctions of 2 x MJE3055T?

author: floobydust
author_rank: Super Contributor
timestamp: January 08, 2019, 01:16:35 am
content: It's always a tradeoff between cheap and simple, verses something tough and well behaved in this PSU.The design is bound by needing an op-amp U3 inputs that can go below GND for current-sense (or -ve rail), and an op-amp U2 with very high output voltage.If you lower the op-amp U2's +V rail, you are also lowering the max. Vout capability of the power supply.So we can't add a Vreg or zener there unless the loss is OK, or we reconfigure the output (transistors) stage.Example: 27V zener, if U2 was outputting 25V, you lose (D12, R15, Q2 E-B, Q4 E-B) drops, total around 5V. so 20V max. Vout.MC34071around 2V lost andTL081over 3V lost.So the penalty for D12 is another loss of 1V max. output. and probably Q2 E-B resistor for speed. C9 has to connect across pin 2-6 on the op-amp.My small opinion is audioguru's circuit looks good but I would add protection diode (O/P trans C-E), and current limiting resistors for op-amp and Q2, and need an undervoltage lockout so there is no overshoot possible during power up/down.To troubleshoot one of these PSU's just measure the IC voltages and post them, as well as some at the transistors.

author: RCM
author_rank: Newbie
timestamp: January 08, 2019, 01:54:58 am
content: Using 3 separate zener's could produce temperature drift.

author: Jwillis
author_rank: Super Contributor
timestamp: January 08, 2019, 02:06:14 am
content: I give up!This circuit is a complete piece of crap and isn't worth any further investigation.I took readings across the board and found that U1 to be faulty.pin 2 ,pin 3 and pin 6 had no change on pot adjust .Pin 6 remained at 10.19V and pin2 and 3 remained at 5.08VChecked U2 and U3 and got responses from pin2 , and pin6switched out U1 and placed it in a simple Astable LED flash circuit to test and it didn't respond the same way as another TL081 .A good one will have a consistent slow flash.U1 had a fast flash like one input side was not working.Replaced U1 with a new OP.U2 failed.Same response on test circuit as U1 .Replaced U2.U3 failed .Same response on test circuit.Either one or the other inputs are being burned out on the TL081 op amps.ConclusionThis circuit is not worth purchasing .A complete waste of money and time .Better to build the fully modified version from scratch and leave this product on the shelf to collect the dust it deserves.Thank you everyone that participated in this discussion.The information was valuable and I learned something about op amps in the process.So it guess it wasn't a complete waste of time after all.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 08, 2019, 02:16:12 am
content: Quote from: RCM on January 08, 2019, 01:54:58 amKleinstein covered that one a few posts back.. it wasn't so apparent (to me) that D8 is Vref. I've seen enough LM317 cct's with Vadj slightly negative that I instantly thought that D7 was it.. Now Floodydust is covering-off how U2's output swing effects Vout. The whole thing is like a balancing act..

author: Cliff Matthews
author_rank: Unknown
timestamp: January 08, 2019, 02:23:00 am
content: Quote from: Jwillis on January 08, 2019, 02:06:14 amI think the retailer over-rated this.. so expectations were too high (what did we expect for a $6 bag of parts?)For me, the one thing that saved my op amps during testing was to put a Variac in front of my 88VA, 2 x 22v transformer.*edit - Here's something for you to read from another builder (see what you think before you pull out that hammer..)** - I'd like to add Floobydust's protections and shave-off 1.2v for U2 (as shown) to get 0-25v @ 2ahttp://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

author: Kleinstein
author_rank: Super Contributor
timestamp: January 08, 2019, 12:32:20 pm
content: With the given instructions the kit is really bad. With a reduced voltage (e.g. 18 V transformer and thus a maximum output of some 15 V), the circuit is not that bad and could work.The typical board layout also has a poor connection for the shunt sensing. This can be fixed with a cut trace and bodge wire if one does not like hum in CC mode.With the power transistor it's a good idea to do an initial test with a short to see if actually survives. There is a chance to get fake transistors from china.The MC34071 looks good on paper, but might be difficult to in a DIP form factor. For the voltage regulator a 741 or TLE2021 (up to 40 V) should also work.

author: Tek Tech
author_rank: Contributor
timestamp: April 24, 2019, 12:20:36 am
content: Hi Jwillis,I wonder if you have given up on your project? I built several of these just because they are dirt cheap (less than $4 shipped), have a darn nice PCB, and they work really well within reasonable voltage and current range with a proper heatsink. I never had a problem with any of these. I did read about wrong parts being sent in the kit; that's how you got the 47 Ohm instead of 0.47. I never received the washers and nuts that should come with the 10k pots, which is so frustrating. BTW, these nuts are metric M7 x 0.75 (fine thread). I ordered cheap taps from China and will be making my own nuts out of washers.Quote from: Jwillis on January 05, 2019, 11:25:43 pmGiven the above information, your project worked just fine in the first place, as others have pointed out. The 30 mA current limit corresponded with the 47 Ohm resistance: 100x greater resistance gave you 1/100 of the max. limit - only 30 mA instead of 3000 mA.Given your report of further problems, I'm wondering if you actually measured the two 0.25 Ohm resistors in series, which replaced the 47 Ohm? And, these should be each at least 2.5 Watt resistors - because when you short the output at the max. 3 Amp current they will dissipate 2.25 Watts each. Lower powered resistors will burn at full current.As I said, I put together several of these kits and they worked perfectly, as expected, without any additional modifications.I found unmodified schematic of the original kit that displays all parts and their values in a clear layout. Credit for the schematic goes to VoltLog and his youtube channel.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 24, 2019, 06:51:09 am
content: The main complaint about the kit is that the TL081 OP is only specified for 36 V supply, but with a 24 V transformer as suggested the voltage could go up to some 40 V, depending in the transformer (higher with a small transformer as the no load voltage is higher) and local mains voltage. It would still have a hard time to deliver 30 V - even 25 V could be only with reduced current.One could still be lucky and get a TL081 that can stand 40 V.With a 20 V transformer things are much better. Maximum output voltage is than more like 20 V at low load and maybe 16 V at full current.The filter cap is relatively small and the drop on the .47 Ohms shunt is rather high so it runs quite hot - so one could consider reducing the maximum current to some 2 A or consider the 2-3 A range only as a bonus with limited performance.

author: m3vuv
author_rank: Super Contributor
timestamp: November 27, 2019, 11:51:23 am
content: Hi all,ive built one of these,works ok except when the cc led comes on it cuts the output to 0v,it doesent regulate the current,its all or nothing,any ideas where to look?

author: xavier60
author_rank: Super Contributor
timestamp: November 27, 2019, 12:00:42 pm
content: Is the -5V ok?

author: m3vuv
author_rank: Super Contributor
timestamp: November 27, 2019, 12:05:52 pm
content: in a word yes,well i actualy measure 5.1v,forgot to say i have used MC34071 ops

author: xavier60
author_rank: Super Contributor
timestamp: November 27, 2019, 12:13:52 pm
content: What are the voltages on the inputs and output of the CC op-amp while in the fault condition.Also the voltage across the CS resistor.

author: m3vuv
author_rank: Super Contributor
timestamp: November 27, 2019, 12:22:30 pm
content: what do you mean by the fault condition?,it just doesent limit current,if i adjust the current pot ,the output drops to zero when the cc led lights,if i adjust it so its off i get output but it will supply max power regardles of the current pot setting.

author: xavier60
author_rank: Super Contributor
timestamp: November 27, 2019, 12:39:30 pm
content: Check the voltage range on pin 3 of the CC op-amp. The CC Pot should smoothly vary it from near zero to about 1.5VThe voltage on pin 3 should decease with increasing load current, by about 0.5V/amp. Pin 2 should always be zero volts.The power supply is supposed to go into CC mode when pin 3 drops to zero volts.Use the negative output terminal as the reference for all measurements.Im looking at the schematic in Reply #42

author: m3vuv
author_rank: Super Contributor
timestamp: November 27, 2019, 04:57:28 pm
content: u3 voltages are as follows,this is with the output set at 6v in cv mode pin1:-5.2v pin2:0.0002v pin3:10.05vpin4:-5.12v pin5:-5.22 pin6:22.61v pin7:23.69v pin8:0.2v,pin 3 doesent vary smoothly with the cc pot butsits at 10v cc led off,0v led on,its like on or off,no smoth varyation but a sudden jump,hope this helps.

author: floobydust
author_rank: Super Contributor
timestamp: November 27, 2019, 06:00:36 pm
content: CC setpoint at U3 pin 3 should smoothly go from 0.005V to 1.5V as you rotate the pot. Using values P2 10k, R17 33R, R18 56k. You are getting 10V which is way too high.Either the potentiometer P2 is defective or wired up wrong, or R18 or R17 are the wrong value.

author: m3vuv
author_rank: Super Contributor
timestamp: November 27, 2019, 07:06:53 pm
content: well sorted it,it was the pot u/s,replaced it and its working fine,cheers all m3vuv.

author: xavier60
author_rank: Super Contributor
timestamp: November 27, 2019, 07:59:47 pm
content: Quote from: m3vuv on November 27, 2019, 07:06:53 pmWhat sort of Pot is it? I have had 2 cheap 10T Pots from ebay do the same thing. Real scary when it's the CV Pot.

author: Jwillis
author_rank: Super Contributor
timestamp: November 28, 2019, 12:11:03 am
content: Wow .Never thought this thread would pop up again.Anyway the supplied pots in the kit are crap. One was faulty at start and the other failed soon after.Yes I finally got this Kit to work properly by reducing the transformer voltage to 20 Volts. Also replace the pots with WXD3-13-2W 10K 10 Turn Potentiometer .The 2X 0.25 5W resistors in series made little difference to the final operation. Added a small 40mm X 40mm heat sink and 40mm 12v fan supplied by the onboard 12v regulator. Regulator only required a small 10mm by 20mm heat sink.All in all it works fine.

author: floobydust
author_rank: Super Contributor
timestamp: November 28, 2019, 12:39:43 am
content: Did you mean these green potentiometers that come with the kit are crap? I found they work OK.

author: xavier60
author_rank: Super Contributor
timestamp: November 28, 2019, 12:42:52 am
content: My failed Pots are fake Bourns 3590S series,10K via ebay. The resistance wire breaks.There are a lot of Bochen WXD3-12 Pots about. I hope they are not fakes.Looks like they are 5 turns which I think I can live with.

author: Jwillis
author_rank: Super Contributor
timestamp: November 28, 2019, 05:53:37 am
content: Quote from: floobydust on November 28, 2019, 12:39:43 amMaybe I just have bad luck

author: m3vuv
author_rank: Super Contributor
timestamp: November 28, 2019, 01:36:04 pm
content: With mine i used a 2n3055 for q4 driven by a bd139,this in turn drives 6x tip35 trannys,the transfo is one from a 30amp ham radio psu with 5 more turns on it,r18 is a selection of 3 different values selectable on a 3way rotary switch,the current sense resistor is 0.8ohm 25 watt wirewound,goes from 0-29v and 100ma to 20a ok,also quadrupled the size of the filter caps and used a 50a bridge rectifier,its working well,the heat sink is 8" square with a 12v quadbike radiator fan bolted to it runing from a thermo switch and relay,it has a cv led as well and a digital volt/amp meter with a 90a shunt.

author: xavier60
author_rank: Super Contributor
timestamp: November 28, 2019, 11:02:46 pm
content: Has anyone tried any mods to enable T2 to also function as a fast current limiter?

author: Jwillis
author_rank: Super Contributor
timestamp: November 29, 2019, 08:27:31 am
content: I've only done in depth tests and measurements on a modified circuit using the OP445.Of course all the transistors are of higher ratings than the original circuit.As far as I can tell, when messing around with the circuit , Q1/T2 is a current dump for when the circuit is turned off. Without it, the output voltage shoots up to the charged voltage in the filter capacitors then slowly drains off . Even when voltage and current is set to 0V before shut down.In my case the out put would shoot up to 40 volts.On tests I noticed that when power is on, the base ofQ1/T2 is at a negative volltage equal to the negative rail.Around -25 volts. When power is switched off the base goes positive and dumps any voltage at the output to ground.But the filter capacitors remain charged for a period of time until they to drain off. I did put a similar circuit to also discharge the capacitors and it worked pretty much the same way, but later removed it to simplify things for further testing and design.The breadboard was getting crowded.

author: xavier60
author_rank: Super Contributor
timestamp: November 29, 2019, 09:12:34 am
content: Quote from: Jwillis on November 29, 2019, 08:27:31 amHave you measured the size of the current spike when the output is short circuited?

author: Jwillis
author_rank: Super Contributor
timestamp: November 29, 2019, 09:50:12 am
content: On the original or modified version? The original I have not . I could set it up later and test it. but I can't imagine it being much higher than the 3 amps I was getting on load test.On the OP445 version it has a maximum current output of well over 20 amps shorted .But I only tested that once because it set the boards on fire and could not get an accurate measurement ..Right now its limited to just 10 amps until I can modify the bread board setup with heavier rails for current.

author: xavier60
author_rank: Super Contributor
timestamp: November 29, 2019, 10:13:43 am
content: The concern is what the current spikes to in the first few hundred microseconds. Because output BJTs are usually used at a fraction of their maximum ratings, it might seem that the BJTs are safe even if the spike is very large. Maybe they will never fail.What bothers me is not being able to figure out how much SOA derating to allow for at high temperature.