Author Topic: DIY 0-30V 0-3A ...Again  (Read 15812 times)

0 Members and 1 Guest are viewing this topic.

Offline JwillisTopic starter

  • Super Contributor
  • ***
  • Posts: 1725
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #25 on: January 06, 2019, 11:40:15 pm »
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. 
 

Offline Cliff Matthews

  • Supporter
  • ****
  • Posts: 1911
  • Country: ca
    • General Repair and Support
Re: DIY 0-30V 0-3A ...Again
« Reply #26 on: January 07, 2019, 12:44:13 am »
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..
 

Offline JwillisTopic starter

  • Super Contributor
  • ***
  • Posts: 1725
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #27 on: January 07, 2019, 01:15:16 am »
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..

I 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?
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15157
  • Country: de
Re: DIY 0-30V 0-3A ...Again
« Reply #28 on: January 07, 2019, 06:07:44 pm »
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.
 
The following users thanked this post: Cliff Matthews

Offline Audioguru

  • Super Contributor
  • ***
  • Posts: 1507
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #29 on: January 07, 2019, 08:20:05 pm »
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.
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7681
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #30 on: January 07, 2019, 10:21:25 pm »
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/#msg2097889
But D10 is incorrect :palm:
« Last Edit: January 07, 2019, 10:23:50 pm by floobydust »
 

Offline JwillisTopic starter

  • Super Contributor
  • ***
  • Posts: 1725
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #31 on: January 07, 2019, 10:37:44 pm »
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.

Thanks 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.


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.

I 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.
 

Offline JwillisTopic starter

  • Super Contributor
  • ***
  • Posts: 1725
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #32 on: January 07, 2019, 10:53:58 pm »
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/#msg2097889
But D10 is incorrect :palm:


Really? 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.
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7681
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #33 on: January 07, 2019, 11:12:04 pm »
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.
 
The following users thanked this post: Cliff Matthews

Offline JwillisTopic starter

  • Super Contributor
  • ***
  • Posts: 1725
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #34 on: January 07, 2019, 11:47:30 pm »
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.

Thanks 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. 
 

Offline Cliff Matthews

  • Supporter
  • ****
  • Posts: 1911
  • Country: ca
    • General Repair and Support
Re: DIY 0-30V 0-3A ...Again
« Reply #35 on: January 07, 2019, 11:51:36 pm »
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.
I 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?
« Last Edit: January 08, 2019, 12:51:21 am by Cliff Matthews »
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7681
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #36 on: January 08, 2019, 01:16:35 am »
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.
MC34071 around 2V lost and TL081 over 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.
« Last Edit: January 08, 2019, 01:23:29 am by floobydust »
 
The following users thanked this post: Cliff Matthews

Offline RCM

  • Newbie
  • !
  • Posts: 1
  • Country: cn
    • RCM Labels, RCM Stickers and RCM logo
Re: DIY 0-30V 0-3A ...Again
« Reply #37 on: January 08, 2019, 01:54:58 am »
Using 3 separate zener's could produce temperature drift.
RCM Labels, RCM Stickers and RCM logo.
Custom made with any size and color.
Shopping: www.rcm-labels.com
 

Offline JwillisTopic starter

  • Super Contributor
  • ***
  • Posts: 1725
  • Country: ca
Re: DIY 0-30V 0-3A ...Again
« Reply #38 on: January 08, 2019, 02:06:14 am »
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 5.08V
Checked U2 and U3 and got responses from pin2 , and pin6
switched 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.

Conclusion
This 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.   

 

Offline Cliff Matthews

  • Supporter
  • ****
  • Posts: 1911
  • Country: ca
    • General Repair and Support
Re: DIY 0-30V 0-3A ...Again
« Reply #39 on: January 08, 2019, 02:16:12 am »
Using 3 separate zener's could produce temperature drift.
Kleinstein 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..
« Last Edit: January 08, 2019, 02:36:35 am by Cliff Matthews »
 

Offline Cliff Matthews

  • Supporter
  • ****
  • Posts: 1911
  • Country: ca
    • General Repair and Support
Re: DIY 0-30V 0-3A ...Again
« Reply #40 on: January 08, 2019, 02:23:00 am »
I give up!
This circuit is a complete piece of crap and isn't worth any further investigation.
I 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 @ 2a
http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html
« Last Edit: January 08, 2019, 03:21:36 am by Cliff Matthews »
 
The following users thanked this post: Jwillis

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15157
  • Country: de
Re: DIY 0-30V 0-3A ...Again
« Reply #41 on: January 08, 2019, 12:32:20 pm »
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. :horse:

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.
 

Offline Tek Tech

  • Contributor
  • Posts: 21
  • Country: pl
Re: DIY 0-30V 0-3A ...Again
« Reply #42 on: April 24, 2019, 12:20:36 am »
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.

You 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.

Given 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.
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15157
  • Country: de
Re: DIY 0-30V 0-3A ...Again
« Reply #43 on: April 24, 2019, 06:51:09 am »
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.
 

Offline m3vuv

  • Super Contributor
  • ***
  • !
  • Posts: 1738
  • Country: gb
Re: DIY 0-30V 0-3A ...Again
« Reply #44 on: November 27, 2019, 11:51:23 am »
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?
 

Offline xavier60

  • Super Contributor
  • ***
  • Posts: 3034
  • Country: au
Re: DIY 0-30V 0-3A ...Again
« Reply #45 on: November 27, 2019, 12:00:42 pm »
Is the -5V ok?
HP 54645A dso, Fluke 87V dmm,  Agilent U8002A psu,  FY6600 function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.
 

Offline m3vuv

  • Super Contributor
  • ***
  • !
  • Posts: 1738
  • Country: gb
Re: DIY 0-30V 0-3A ...Again
« Reply #46 on: November 27, 2019, 12:05:52 pm »
in a word yes,well i actualy measure 5.1v,forgot to say i have used MC34071 ops
 

Offline xavier60

  • Super Contributor
  • ***
  • Posts: 3034
  • Country: au
Re: DIY 0-30V 0-3A ...Again
« Reply #47 on: November 27, 2019, 12:13:52 pm »
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.
HP 54645A dso, Fluke 87V dmm,  Agilent U8002A psu,  FY6600 function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.
 

Offline m3vuv

  • Super Contributor
  • ***
  • !
  • Posts: 1738
  • Country: gb
Re: DIY 0-30V 0-3A ...Again
« Reply #48 on: November 27, 2019, 12:22:30 pm »
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.
 

Offline xavier60

  • Super Contributor
  • ***
  • Posts: 3034
  • Country: au
Re: DIY 0-30V 0-3A ...Again
« Reply #49 on: November 27, 2019, 12:39:30 pm »
Check the voltage range on pin 3 of the CC op-amp. The CC Pot should smoothly vary it from near zero to about 1.5V
The 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
HP 54645A dso, Fluke 87V dmm,  Agilent U8002A psu,  FY6600 function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf