Electronics > Beginners
DIY 0-30V 0-3A ...Again
Cliff Matthews:
--- 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 ---
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?
floobydust:
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.
RCM:
Using 3 separate zener's could produce temperature drift.
Jwillis:
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.
Cliff Matthews:
--- Quote from: RCM on January 08, 2019, 01:54:58 am ---Using 3 separate zener's could produce temperature drift.
--- End quote ---
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..
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