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DIY 0-30V 0-3A ...Again

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floobydust:
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:

Jwillis:

--- Quote from: Kleinstein 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.

--- End quote ---

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.



--- Quote from: Audioguru 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.

--- End quote ---

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.

Jwillis:

--- Quote from: floobydust 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:


--- End quote ---

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.

floobydust:
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.

Jwillis:

--- Quote from: floobydust 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.

--- End quote ---

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. 

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