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Old 14th Sep 2019, 9:24 am   #1
crackle
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Default Adjustable voltage 15 amp power supply

There are many circuits on the internet which claim to be a 15 amp power supply.
Some examples are below.
from the national semiconductor lm338 data sheet
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and
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others found on the internet.
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Has anybody experience of making any of these types of circuits?
Are there traps in the layout and types of components?
Which circuits give the best voltage regulation under varying loads?
Which ones are best to avoid?

Has anyone got a better circuit they think is easy to construct and is better than the above.

Thanks
Mike
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Old 14th Sep 2019, 12:55 pm   #2
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Default Re: Adjustable voltage 15 amp power supply

Hi,
I'm only a beginner at this, but do you want the regulated voltage to go down to 0 volts ( like in a bench power supply ? ).
if so some of the circuits on the web using the lm723 and series pass transistors won't do this. whatever you decide upon you will probably need a large heatsink. I hope other more experienced people here will have more complete suggestions.
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Old 14th Sep 2019, 1:36 pm   #3
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Default Re: Adjustable voltage 15 amp power supply

Mike, I think you need to consider what you want (in terms of both the end result and your effort) in a bit more detail. For example if you want say 25v to 5v, adjustable at 15A max you will need to manage well over 300W of heat if you use pure linear regulators like those above. This is serious heatsink/fan territory. In this case I guess a thyristor pre regulator operating at 50Hz, is the way to go to limit dissipation in the pass devices. The modern approach would be switchers operating at say 100kHz but you are getting into major non trivial design.

Ken
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Old 14th Sep 2019, 2:09 pm   #4
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Default Re: Adjustable voltage 15 amp power supply

The really old-style bench PSUs used a variac to supply a fixed step-down transformer, and rectified and smoothed the output of that.

A more sophisticated variation on that idea would be to have a similar arrangement supplying a linear regulator, with the variac turned down to limit the input voltage and hence power dissipation. If a multi-tapped transformer is available, and some suitably-rated relays, you could even implement automatic tap-changing logic, based on an op-amp and several comparators, to keep the voltage dropped across the regulator within a fixed range.

Or go hybrid, with a switched-mode PSU feeding a linear regulator with the actual adjustment on it; take the feedback to the SMPSU from the output of the linear regulator, with an op-amp to boost the feedback voltage by a fixed amount which the voltage drop across the linear regulator will then become equal to.
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Old 14th Sep 2019, 3:32 pm   #5
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Default Re: Adjustable voltage 15 amp power supply

Thanks for your replies.
The main use of the power supply would be to supply about 14v at 10 amps.
But being able to make it more versatile and adjustable voltage seems attractive to me.
No I wont need to take it down to 0V probably 6v would be the minimum required.

Mike
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Old 14th Sep 2019, 6:13 pm   #6
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Default Re: Adjustable voltage 15 amp power supply

I built this one Mike, see attached and have had no issues with it. I built a circuit board for mine following the schematic pretty much. You can leave out a lot of the diodes as they're there because the chap who designed it was doing a lot of HV work, as you could the metering part.

Andy.
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Old 14th Sep 2019, 6:48 pm   #7
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Default Re: Adjustable voltage 15 amp power supply

The above circuit is basically a regulator followed by an emitter follower. There is no feedback from the actual output so voltage out will vary slightly depending on load current and temperature.

This may or may not matter depending on the application.
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Old 14th Sep 2019, 6:55 pm   #8
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Default Re: Adjustable voltage 15 amp power supply

For a decent current 14v supply it is worth reading the PW 'Marchwood' article. The design is reasonably thorough and well explained in the text.

Just be cautious with 2N3055. Their spec is so wide that the type is a dumping ground for failures of much fruitier devices so you can often see dramatically higher gain or Ft in parts bought cheaply. There are also out and out fakes with tiny die inside which could never do the 2N3055 current and power specifications.

David
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Old 14th Sep 2019, 7:11 pm   #9
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Default Re: Adjustable voltage 15 amp power supply

Hi Andy
Thanks for the circuit, I was surprised to see it had no small value resistors on each transistor.

I found the link to the page https://www.pocketmagic.net/variable...-power-supply/
There is another design further down the page which may have better voltage stability, can any one confirm.
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and another which has a current limiting control.
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Mike
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Old 14th Sep 2019, 8:01 pm   #10
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Default Re: Adjustable voltage 15 amp power supply

I forgot to ask, can somebody please explain the advantage of running the controlling regulator from a separate rectified supply, as in the above 2 circuits.

thanks
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Old 14th Sep 2019, 8:52 pm   #11
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Default Re: Adjustable voltage 15 amp power supply

The three terminal reg needs some overhead voltage input to output on it in order to work. Then there is the base-emitter voltage of the power transistors.

Running a separate rectifier and smoother for the reg input will run a bit higher voltage than the main smoother because the lower current will not have the voltage drops the main path has due to the main path's current. Thus the overall reg can drive the output a bit higher than if the same rectifier fed the three terminal reg as well as the main current path through the power transistors.

David
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Old 14th Sep 2019, 8:54 pm   #12
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Default Re: Adjustable voltage 15 amp power supply

It helps to isolate the low-level control circuitry from the ripple in the pass transistor supply- the 723 requires a few volts (I forget exactly, but at least 3V seems wise and preferably a bit more) of headroom in its supply, and as soon as any output assistance transistors fed from the control circuit rail appear, you can add at least 1V per transistor and again preferably a bit more. This means that the control circuit can duck-out surprisingly early if you don't keep an eye on things and keep its supply hygenic and separated from the main grunt. The alternative is one rail with necessarily higher headroom against ripple getting into the control-circuit- meaning that the pass devices get very sweaty!

Crossed with David!,

Colin
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Old 14th Sep 2019, 10:01 pm   #13
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Default Re: Adjustable voltage 15 amp power supply

At least we agreed

David
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Old 15th Sep 2019, 8:20 am   #14
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Default Re: Adjustable voltage 15 amp power supply

You could just build one of the big Farnell design's Mike and bung in a few extra Q's if needed. Guess it depends how complex you want to go, whether you need CC mode etc.

Andy.
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Old 15th Sep 2019, 10:56 am   #15
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Default Re: Adjustable voltage 15 amp power supply

I have decided on which to attempt to make, mainly based on the fact that I found some LM723 round shaped IC's which I had always wondered what they were for.

I spent the morning having a look at the layout of the control board. It will be built on veroboard with important tracks reinforced with additional copper and solder.

I think I have included all the components except the 2N3055 transistors their DC supply and their emitter resistors.

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Name:	30 amp PSU 2b layout.jpg
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Mike
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Old 15th Sep 2019, 11:00 am   #16
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Default Re: Adjustable voltage 15 amp power supply

The round metal can (TO100 or whatever) LM723 is somewhat hard to find now and is also sought-after by people repairing devices (HP, DEC, etc) that used them and have PCBs laid out for them.

The 14 pin DIL 723 is quite common and a lot cheaper. I would recomend using that in your project unless there is a very good reason to want the metal can.
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Old 15th Sep 2019, 11:25 am   #17
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Default Re: Adjustable voltage 15 amp power supply

Hi Tony
I am using the round one because I have about a dozen, and no other use for them. If someone wants to swap a couple of the other DIL types I am open to offers. There are a couple of UA723 ones as well
Do you think it will need a clip on heat sink?
What is that little bump on the bottom of the IC, it looks like ceramic.


Mike
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Old 15th Sep 2019, 12:33 pm   #18
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Default Re: Adjustable voltage 15 amp power supply

Here is the circuit diagram with the pin numbers for the TO-100 LM723.
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and in pdf format
30 amp power supply.pdf
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Old 15th Sep 2019, 12:58 pm   #19
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Default Re: Adjustable voltage 15 amp power supply

That looks a good candidate Mike, I have some big caps and I've got some big heatsinks for TO3/2N3055's should you need any, they have room for four and have old 2N3055's in some holes as well as Welwyn emitter R's, ideal for big PSU's. Drop us a PM if any use to you.

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Old 15th Sep 2019, 1:57 pm   #20
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Default Re: Adjustable voltage 15 amp power supply

On the heatsink thing, I would use one.

The LM723 is rather fragile in fault conditions. Hereís one that blew up on me in an HP 1740A

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Thatís the internal series pass transistor bond wire that has melted
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