Electronics > Beginners
Diy bench power supply trouble
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Jwillis:
As I explain in my original post that the calculations I made were base on the Maximum output of the BC558 and that the OP needs to take the appropriate measurements his/her self.I was simply addressing a possible problem and a simple solution to fix it based on the schematic presented.By presenting the formulas and how to plug in the variables based on measurements may not have been apparent .So its really a good idea to show the way the calculations work. You can't be afraid of the math or induce fear of the math because it's really not that hard or mysterious.
As for the use of a base resistor as a means to control the output of a BJT ,that is a common  method used because there is less heat produced there, because the amperage is lower, then if you attempted a control at the collector where the amperage is higher.If there is a solution that doesn't require a massive change to the design then by all means use it.I was presenting  the simplest possible method to use on the current design. Changing transformers for multiple outputs would be a solution but a bit extreme without massive changes to the design. But if the OP is considering it then that's fine.
As for  datasheets, I never implied that they were not an integral part to designing anything.I simply stated the component may not have the Exact characteristics stated in the datasheet an that adjustments may be needed to compensate for those variables.And that proper measurements and correct calculations are key to a successful out come.
xavier60:

--- Quote from: andrejsstrumfs on May 09, 2018, 06:37:40 pm ---Hi all!

I'm trying to build the power supply based on this circuit.

It kind of works, but the problem is when the power supply switches between voltage limit mode and current limit mode, the opamp output can swing to negative rail and the power transistors get reverse biased and burn out. I actually do not see any protection against reverse biasing the transistors in this circuit. Hovewer my understanding could be quite limited :) Anyone got any ideas how to improve this?

--- End quote ---
"when the power supply switches between voltage limit mode and current limit mode,", could mean that a short circuit has been applied  across the output? This is stressful for a power supply that doesn't have fast current limiting.
 
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