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0-30 Vdc Stabilized Power Supply


Sallala
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If you have a perfect heatsink for a 2N2219 so that its case is held at 25 degrees C, then its max dissipation is only 3W when its internal temperature is at its max of 200 degrees C. A perfect heatsink is impossible so its max dissipation is only about 2W.

A 2N3055 output transistor has a minimum current gain of 25 with a collector current of 3A. So its max base current is 3A/25= 120mA. Its driver transistor will have 32V across it (24VAC transformer) when the project's output voltage is low, and 120mA through it which results in a dissipation of 3.84W. The 2N2219 will melt. A TIP31A can dissipate a max of 40W and will be fine with a little finned heatsink.

You might be able to use a 2N2219 if it has a very good heatsink with a fan, and select a 2N3055 for very high current gain.

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i think i located the problem and that is the voltage on q1, collector is 0.8~0.9V so i'm asking if that could be the problem( i have no output )

If Q1 is connected wrong, shorted, doesn't have R13 and R14 the correct values or doesn't have a negative supply (-5.6V) for R14 then U2 will get hot.

Q1 is supposed to turn on and short the output of U2 to 0V when the negative supply disappears quickly when the AC power is turned off so that the output voltage doesn't rise.
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Hi MonSSter,
I don't think its negative supply is working.
One end of R14 connects to the base of Q1. The other end connects to the -5.6V supply. Check the parts that are supposed to make the negative voltage.
Without the negative voltage then Q1 conducts all the time and shorts the output of U2 to ground.

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Can i use 2n3055 in place of 2n2219 , it is also a power transistor

Q2 doesn't need a rating of 15A and 115W, but a 2N3055 transistor will work fine.
I can buy a TIP31A power transistor for $.68US and a 2N3055 power transistor for $1.68US.
There are many power transistors with ratings and prices like a TIP31A.
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hi AG,

1. How to decide the value of R9? Why don't we use 10K or 470R?
2. What's the purpose of C5? How to calculate the value?
3. Is C9 used to improve the stability of U2?
4. Can I connect output(pin6) of U2 to base of Q2 (bypass R15)? If not, why?
5. If the system should be grounded, which part of the circuit should connect to ground as well?

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1. How to decide the value of R9? Why don't we use 10K or 470R?

R9 feeds the adjusted reference voltage to U2 and limits the loading effect of C4 on the output of U3. If its value is higher then U2 might pickup interference. If its value is lower then U3 might oscillate due to C4 being at its output or R9 won't discharge C4 quick enough during current limiting.

2. What's the purpose of C5? How to calculate the value?

It is a supply bypass capacitor for high frequencies, because C1 is so big it has high inductance. Analog electronic circuits oscillate without it or if its value is too low.
C5 could be 0.1uF to about 1uF and a ceramic disc is best at high frequencies.

3. Is C9 used to improve the stability of U2?

Yes, because Q2 and Q4 are in its negative feedback loop and are much slower than U2 and cause delay, the combination could oscillate without C9.

4. Can I connect output(pin6) of U2 to base of Q2 (bypass R15)? If not, why?

U2 oscillates if its load is capacitive and is directly at its output. R15 isolates the capacitance of Q2 and I recommend reducing its value to 100 ohms.

5. If the system should be grounded, which part of the circuit should connect to ground as well?

Connect the 0V output to ground and the project supplies variable positive voltages above ground.
Connect the Positive output to ground and the 0V connection supplies variable negative voltages below ground.
Don't ground any part of the output and the project can be connected in series with another supply that might be grounded.
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