Electronics > Beginners
DIY 0-30V 0-3A ...Again
soldar:
--- Quote from: Jwillis on January 05, 2019, 11:25:43 pm --- I have a variac so Would it be ok to set a lower voltage at the primary with the Variac just for testing?
--- End quote ---
I 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?
Audioguru:
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!
Jwillis:
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 pm ---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!
--- End quote ---
I 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 .
IanB:
--- Quote from: Jwillis on January 06, 2019, 12:25:41 am ---Still have full range of voltage without load but it still drops to 200mV at load full available current (30mA).
--- End quote ---
I do not think you have not clarified whether this restriction in current applies at all selected voltages, or just at the maximum voltage?
Audioguru:
--- Quote from: soldar on January 05, 2019, 11:44:55 pm ---You were 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?
--- End quote ---
That 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.
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