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Power supply help?

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ped

New Member
Hi guys, great forum.

Ive bought an variable psu kit (see pics) and i have a couple of questions, it uses a single 2n3055 for its output , basically what i want to do is increase the output section to use 6 2n3055's as i will be pulling more current, i'm going to use 6x .1R 5w resistors as emitter load resistors, my question is more about driving the Base's of the 3055's , will i have to use another transistor in place of the 2n2219 ? if so could i use any NPN power transistor?

Regards



 

audioguru

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Most Helpful Member
The project has many problems and is corrected at www.electronics-lab.com in a thread with about 190 pages and in other shorter threads.
1) Its power transformer, rectifier diodes, 2N3055, 2N2219 and some resistors are all overloaded.
2) The TL081 opamps operate at a supply voltage that is higher than their max allowed supply voltage.
3) The 24V transformer's voltage is too low for a regulated 30V/3A output.
4) Its main filter capacitor is much too small.
5) A part number is not specified for D8 its voltage reference zener diode and R4 has it operating at a very low current.
6) The voltage offset adjustment pot has a series resistor with a value that is way too high and it is connected to the wrong voltage.

We use a 28VAC 119VA transformer, two 2N3055 output transistors, a BD139 little power transistor as a driver instead of the tiny 2N2219 and opamps with a higher voltage rating.
The better opamps do not have the problem called "phase inversion" that the TL081 opamps have so Q1 is not needed to short the output of opamp U2 when the power is turned off.

There is another thread about increasing the output current to 5a and it uses 3 2N3055 output transistors.

Here is the latest schematic of the 30V/3A project:
 

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ped

New Member
Many thanks for the detailed answers,

Some of the problems listed wont affect me as the transformer im using is around 24v after retification (no load) , and it has its own bridge/ smoothing cap.

Effectevly what im doing is substituting this regulation circuit for the following circuit of the psu i have

 

audioguru

Well-Known Member
Most Helpful Member
With a 24VDC supply, the max regulated output will be about 17VDC.
1) The opamp has an output voltage loss of 1.5V.
2) The pre-driver transistor has a loss of 0.8V.
3) The driver transistor has a loss of 1V.
4) The 2N3055 output transistors have a loss of 1.2V.
5) The emitter resistors at the output have a loss of 1V (2A each).
6) The current sensing resistor has a loss of 1.4V.
 

audioguru

Well-Known Member
Most Helpful Member
25a max whats what the origonal psu chassis was built for
Then each 2N3055 transistor must pass 5A and their max voltage loss is 1.6V instead of 1.2V. Also the voltage loss in the emitter resistors is 2.5V instead of 1V. So the max output voltage is close to 15V and it might have rectifier ripple.

It depends if the transistors have high gain, then the project will be fine.
If the transistors have low gain then there will be ripple at 15V/25A.
 

ped

New Member
Thanks , in the psu i have there is space for another 2n3055 on the heatsink so i might throw one in to spread the load , ripple shouldnt be a problem as it wont be getting used on sensitive circuits , more on resistive loads and lamps.

Thans for all your help.

ps you agree with the previous poster when he says drive the 6 3055's with the circuits orignal 3055 output tranny? , how will that affect the adjustable current limiting?
 

audioguru

Well-Known Member
Most Helpful Member
If all five 2N3055 output transistors (or six) have a minimum current gain of 15.6 at 5A each then the driver transistor must pass 1.6A. The BD139 high frequency transistor that is recommended (for good transient reponse) will melt if the output voltage is set low or is shorted and the current is set to 25A, even if it has a huge heatsink. The TIP31 driver transistor produces a horrible transient response but might survive the extremely high dissipation of heat.
If the original 2N3055 output transistor is used as a driver for the many output transistors then the max output voltage will be 1.2V less.
 
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