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0-30V Stabilized Power Supply


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can i use MC33171 instead of 34071? I compared datasheets and i think it is possible, but i rather ask.
For me is much easier to get MC33171 in local store...

The MC33171 is low power like an old LM324 but is improved.
Therefore its output current is too low, its bandwidth is too low and its slew rate is also too low.
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The opamps will not affect each other in a quad package but it might overheat.

But looking at the schematic U3 if feed by +10V -1.3V supply if I'm looking at correct and U1,U3 are fed form the high plus side of power supply(41V), I was not sure if this would affect the operation of it if it all was in one chip which would maje a smaller neater design well I think so any way
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My name is Fernando and I'm from Argentina.

First I would like to thank you all for the quality of the discussions and specially to Audioguru and RedWire  for the excellent job regarding the new PS design.

I have built the new version and I'm very pleased with it (btw, I built the old version too, but I use it nowadays as spare one for low power circuits only).

Some thing that I'm concerned about is power dissipation. I use one heat sink of 12 x 5 x 3 cm (about 5 x 2 x 2 1/2 inches) for each Tr  plus a salvaged fan from a PC PSU. So far all is well (despite the death of a power transistor and consequently the death of a power led during testing) but I'm worried about delivering 3 amps at say, 5 volt.

So here is my question.

I've read in this forum (I think it was Hero999) about tap changing as a way to reduce heat.

In fact, the transformer that I got has two secondaries: one at 30V and the other at 9V (it used to power a huge dot matrix printer).

I tried to google a tap changer circuit but to no avail.

Does anyone know how to do it? I thought about using a SCR but sadly, at least for now, I lack the knowledge to face such a task.

Thank you all


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I am currently constructing the updated version of this power supply,
I have a fwe questions about this power supply.
Q1 ; What happens when current limiting kicks in, does the output voltage drop or does it stay steady. If current limiting is then does this mean that the power supply is a constant current source rather than power supply.

Q2 ; What is the output regualation of my power supply. Input transformer is 31.6 V AC.

Also what is the rated load of this power supply.

Thanks for your time especially redwire and audioguru.


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Q1 ; What happens when current limiting kicks in, does the output voltage drop or does it stay steady? If current limiting is then does this mean that the power supply is a constant current source rather than power supply?

It doesn't have current limiting. Instead it has current regulation (constant current source) which is more exact.
Ohm's Law says that the voltage must be reduced for the current to be regulated when the load resistance is reduced. For example if the voltage is set to 30V and the current is set to 1A and the load is 15 ohms then the output voltage becomes 15.0V because the current is still regulated at 1.0A.
The LED lights to warn that current regulation is reducing the output voltage.

Q2 ; What is the output regulation of my power supply? Input transformer is 31.6 V AC.

Its error amplifier consists of U2, Q2 and Q4. Its open loop gain at DC is about 200,000 so the output voltage drop from no load to 3A is a few milli-volts depending on the resistance of the output wiring.
At high frequencies then the circuit gain drops but the smoothing effect of C7 produces good voltage regulation.

What is the rated load of this power supply?

3A or less at 30V or less. The heatsink must be pretty big or have a fan because if the output voltage is low or shorted and the current is set to 3A then the output transistors will have about 38V across them x 3A= 114W of heat that must be dissipated.
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Opamp U3 is the current regulator.
The 0.47 ohm resistor R7 develops a voltage across it due to the load current in it and this voltage feeds the (-) input of U3. The (+) input of U3 is fed a variable voltage set by the current regulation pot. When the voltage from R7 is higher than the voltage set by the pot then the output voltage at the output of U3 drops which causes D9 to reduce the voltage of the project until the load current is reduced to the setting. When the output voltage at the output of U3 drops then it turns on Q3 that lights the LED to warn that the current regulator is reducing the output voltage.

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I built the old version with TL081 long time ago and I update it without changing the PCB following these instructions, or is it better to redo the PCB with the new version?

The OPA445 high voltage opamps were used about 5 years ago. Today they are available only sometimes (Digikey has some today) and are very expensive (nearly $12.00US each).

The new version has a choice of two inexpensive opamps that are available at many places in North America and maybe in Europe.
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Hi everyone,

If I used the latest circuit with 5 power transistors can I achieve a 10A constant supply with a verry large heatsink and fan, it works out to about 65W per transistor? I have a 26.3VAC 10A+ transformer with massive 2.5" wide core.

Would anything else need changing other than larger supply caps - guessing about 50000uf or more?

Can I draw 10A at 1V up to 30V 10A theoretically with a massive heatsink?


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Thanks for the reply guru, would the circuit still work if I replace R7 with a 0.1R power resistor for minimum power dissipation or will this mean further modifications are required?

Regarding the BD139 burning, could I just parallel 2 of them with another 1k resistor?

I wasn't so sure about the best way to resolve the lack of drive?

I have a large transformer and 100000uf cap.

Thanks alot.

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If matt09 uses several MJ11016G Darlington transistors instead of the 3055's    then the BD139 should be able to drive the output transistors.   Now he may loose one volt of max output.  

And yes R7 would need to be a very low value and large (something like a 0.1 Ohms  25W aluminum housed resistor mounted to the frame)  I would suspect that the current limiting adjustment would be very coarse with low value of R7.

Are you using this for a battery charger or a room heater?

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Thanks redwire, I'd prefer to use the 3055's as I have a few already but can the problem not simply be solved by paralleling two 139's or using a more powerful substitute such as the D44H11?

I'm guessing I could change the current limiting adjustment by playing around with the value of R18?

I am using this as a powerful test bench supply.

EDIT: Just seen the price of those darlington transistors they're like

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You don't want to parallel the BD139's  it will become a mess.  YOu would need to add resistors to ensure one doesn't hog all of the current just like what is done for the 3055's.  You are correct in that the easiest solution is to change  the 139.  Audioguru would need to comment on the D44H11 subsitute. 

As for the current limiting issue.  I have a single turn potentimeter so if the original design was based on 3 amps one turn covered 3 amps.  If you move to 10 amps then the single turn would cover 10 amps.    As was suggested you may want a big knob to provide better control. 

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Would this be much better than thhe 3055's, I have loads lying around? http://www.datasheetcatalog.org/datasheet/mospec/2N5683.pdf -- 2N5686
They're massive things with a 50A collector current and 300w handling, could I sub them and get away with just 2 handling 175w each for a 10A output?

I don't suppose the bd139 could drive two of those together if the 3055's weren't used and it would solve the problem?

If not I've also looked around for a BD139 replacement and have a TIP transistor (I'd have to go out and buy the D44 one), would this be a suitable replacement?

http://pdfdata.datasheetsite.com/web/70588/TIP41C.pdf -- TIP41A

http://www.fairchildsemi.com/ds/BD%2FBD135.pdf -- BD139

If guru could check over this I would be eternally grateful, pretty much all the specs match or are better, ie 6A collector current.  :)

Would the emitter resistor R16 value need changing from 1k or need to be a higher power value also (or both) with the new transistor ?

Regarding the current control I think I'll use two pots in series of different orders in resistance for course and fine control.

I assume this still doesnt solve the problem of U2 not driving this new transistor hard enough?

Sorry there's a hell of alot there!

Thanks so much guys  :)

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The very powerful transistors will work well at 5A each but you will not be able to cool them unless your heatsink is submeged in liquid nitrogen.
They are spec'd to dissipate 300W when their internal temperature is max and their case is held at 25 degrees C somehow. Of course an ordinary huge heatsink will get hot so the power must be reduced.

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Thanks guru, the derating graph shows a temperature of 100'C at 175W per transistor so providing they are cooled below this temerature they would work?

You say about cooling them in liquid nitrogen but isnt the total dissipation to the heatsink the same except with the 3055's it's spread across more lower power transistors?

I have a huge heatsink able dissipate 350W with a rise in temperature below 80'C


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A heatsink works best when the heat is spread out on it. A single point of heat on a heatsink makes that spot hot but the remainder of the heatsink is much cooler.
There is a loss of cooling whenre the transistor is mounted on the heatsink. Even thermal grease blocks some of the cooling effect.

I never operate transistors at their max allowed temperature.

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