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@ instructables.com build a nice power supply for his lab. He writes:
From my point of view one of the best ways to get started in electronics is to build your own laboratory power supply. In this instructable I have tried to collect all the necessary steps so that anyone can construct his own.
All the parts of the assembly are directly orderable in digikey, ebay, amazon or aliexpress except the meter circuit. I made a custom meter circuit shield for Arduino able to measure up to 36V – 4A, with a resolution of 10mV – 1mA that can be used for other projects also.
Linear Lab Power Supply with digital meter – [Link]
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The ebay kit is a copy of the original Greek kit which is a project here at Ekectronics-Lab. It does not work and is not reliable. Ebay and other Chinese copies are not available anymore because the defective kit is replaced with one that might work better and has a voltage and current display.
Hello Audioguru, I am the author of the instructable, I think I should have made my own circuit instead in trusting in the KIT. By the moment it works but it has the problem of the voltage rising when powering down because of the negative biasing and it has absolutly no thermal protection..
Thank you for your comment.
Daniel.
Hi Daniel,
The kits use TL081 opamps that have a “Phase Inversion” problem where the output suddenly goes as high as it can when an input voltage comes within a few volts from the negative supply that collapses first when the power is turned off in this circuit.
Q1 was added to drive the output low when the power is turned off and it is set with R13 and R14 for a 24V transformer. But your transformer is less at 21V so R13 should also be less, maybe add 68k parallel with the 10k that is there now.
Hi Audioguru,
Thank you for your comments. I bought the power supply kit and thought it was going to work perfectly and I have focused in the rest of the parts of the power supply: the meter, the box, etc. I am definitely going to make a new power supply circuit using two LT3081 (3A), and a MAX44246 op amp (I think), that can be used for a lot of projects also even with SMPS instead of transformer, I will keep you updated.
Daniel.