Regulated Power Supply Circuit design

[techguru]

Dear all,
            I want to design regulated power supply for my lab which should be programmable for both voltage and current. I bought a transformer of 24V@ 5Ampere centre tap. it should provide output DC voltage from 0-30V @ 3 ampere of both polarity.I request you all to provide me the circuit which would accomplish my aim.Thank you all gentle men.

[tooki]

Assuming you want to make a linear power supply, you can't do what you want with that transformer.

When you say 0-30V/3A of both polarities, you mean you want two 0-30V outputs, each 3A*? The transformer you have is, in essence, 2x12V. Nowhere near enough.


*Though there have been power supplies that provide symmetrical "tracked" outputs (where if you set the controls to, say, 14V, then you get –14V on a – output, +14V on the + output, and one common terminal in the middle), what's more common now is to see two independent supplies, with separate + and – terminals for each supply. You can then put them in series (connect the – of one to the + of the other) as needed, or use them separately if you want.

[techguru]

Hi to all !!!!!!!!!!!!!!!!!!
 Transformer Specification

primary:230V ac
Secondary:24V-0-24V @ 5 ampere.

[eblc1388]

Transformer Specification
primary:230V ac
Secondary:24V-0-24V @ 5 ampere.

Good only for a 0~20V 3A regulated supply. If you wanted 3A at 30V, use a 35V-0-35V transformer.

[techguru]

Sir,
       How to design 0-20V power supply.as per your specification.

[eblc1388]

You should do a search on this subject in the forum. There are numerous discussions about regulated power supplies from simple to sophisticated design to suit everyone. 

[Kleinstein]

A 24 V transformer is too low to get a regulated 30 V DC output. It is more like a maximum of something around 20-25 V (maybe 28 under light load), depending on the circuit.

The usual way is to build 2 largely separate supplies, that can be connected in series and this way used as a +- supply. It is usually easier to design and more flexible. However the transformer needs fully separate windings in this case. At this power level it would also be a good idea to have a kind of pre-regulation or transformer tap switching otherwise the power dissipation can get rather high.

It is a good idea to use the search function, but be aware, that there are lot's of poor power supply circuits around. A good lab supply is a project that is way more difficult than it seems at first.
It might be a good idea to look at circuits of usually older commercial supplies, where circuit plans a available.

To release less magic smoke, it is a good idea to start with a much smaller version (e.g. like 0-20 V and 1 A). The tricky control loop is not that much different.

[Kalvin]

If the transformer had two separate 24VAC windings, you could build a decent two channel PSU using these nice modules: https://www.eevblog.com/forum/blog/eevblog-1030-$20-diy-bench-power-supply!/ With a 24-0-24 your options are now somewhat more limited.

[Lautrec]

Dear all,
            I want to design regulated power supply for my lab which should be programmable for both voltage and current. I bought a transformer of 24V@ 5Ampere centre tap. it should provide output DC voltage from 0-30V @ 3 ampere of both polarity.I request you all to provide me the circuit which would accomplish my aim.Thank you all gentle men.

I think that you can. You need 20000µF (or more) capacity + 0,1µF/50V for each diod from the rectifier. I made one one with transformer 21V to 0-30V regulated voltage. After the rectifier I add 6x 6800µF/50V and I got max 30V in output. But remember in your case you must use components as for 5A load and provide a stable voltage control to base of transistor or use IC like XL4015. My transformer is 21V/30A and PSU is with 9x TIP3055 the output is 0-30V/30A+. I love it!

[Damianos]

Dear all,
            I want to design regulated power supply for my lab which should be programmable for both voltage and current. I bought a transformer of 24V@ 5Ampere centre tap. it should provide output DC voltage from 0-30V @ 3 ampere of both polarity.I request you all to provide me the circuit which would accomplish my aim.Thank you all gentle men.

I think that you can. You need 20000µF (or more) capacity + 0,1µF/50V for each diod from the rectifier. I made one one with transformer 21V to 0-30V regulated voltage. After the rectifier I add 6x 6800µF/50V and I got max 30V in output. But remember in your case you must use components as for 5A load and provide a stable voltage control to base of transistor or use IC like XL4015. My transformer is 21V/30A and PSU is with 9x TIP3055 the output is 0-30V/30A+. I love it!

21V * 30A = 630VA,
30V * 30A = 900W
How you managed that? What physics/electrical/electronics rules apply on this?

[Kleinstein]

Transformer ratings are long term average. No problem getting more power short time if you check the transformer temperature.
On the long term, there is no way around the power limit of cause. Just a normal rectifier and filter cap even has a poor power factor. So even getting 3 A DC from an 5 A transformer might be on the edge with a stiff (e.g. toroid or large) transformer. A very large filter cap will bring the power factor down.

I have seen a power-supply circuit that uses voltage doubling rectifier (and thus a low of filter caps) as the higher transformer tap and normal rectifier for lower voltages. So it is possible to get away with the lower transformer voltage.

[Damianos]

Of course, if we short-circuit the capacitors we can have a few hundreds or even thousands of Amperes for short time ... ...
Chemistry problem?

[techguru]

Dear All,

          Could anybody give an idea to start my design with the schematic of nice design which could be simple and be a good regulator in controlling voltage and current.  help me in this regard.thank you all.

[IanB]

Could anybody give an idea to start my design with the schematic of nice design which could be simple and be a good regulator in controlling voltage and current.  help me in this regard.thank you all.

"Simple" and "good regulator" don't go together when you ask for extreme specifications like 30 V and 3 A.

If you want to keep it simple you need to aim lower. Keep the current below 500 mA or so and keep the voltage down around 20 - 25 V.

[not1xor1]

Transformer Specification
primary:230V ac
Secondary:24V-0-24V @ 5 ampere.

Good only for a 0~20V 3A regulated supply. If you wanted 3A at 30V, use a 35V-0-35V transformer.

I think it is better to give a way to get to the result rather than bare numbers

supposing the transformer is able to provide 24V at full load (resistive 5A), one has to take into account:

-1) line voltage variations (usually specified as +/-10%), so in this case the voltage might be as low as 21.6VAC (24-10%)

-2) peak voltage i.e. 21.6 * sqrt(2) that is about 30.5V

-2) rectifier drop at peak current... with a 10A bridge that should be around 2-2.5V (according to the GBJ10*, maybe optimistic, datasheet it should be 2.2V at 100A and 25°C) so worst case now is 28-28.5V

-3) ripple voltage: some books provide the formula Vpp = I / C * f, but usually the real value is a bit better (i.e. less)
in case you have a 10mF capacitor and a 50Hz frequency (100Hz thanks to the bridge) you get: 3 /( 10e-3 * 100) that is 3Vpp or more likely something around 2-2.5V, so the minimum voltage now is 25.5-26.5V

-4) regulator drop-out voltage, which might be as low as 1V using a low drop out design or more likely around 2-3V, so now you get the worst case maximum output voltage which might vary between 22.5V and 25.5V regulated DC

please correct this if there is anything wrong

[techguru]

Clear Perspective.

but my secondary Ac Voltage is 24v.Hence i could take a peak voltage of 24*sqt(2)=nearly 34Volts provided i should use good filter capacitor.Hence i think i could take about 24V dc at full load condition.

Thanks a lot sir...

[eblc1388]

The "peak" voltage information is only useful for specifying the rating of the smoothing capacitors only.

First we start from the DC load side. If the output is 24V 3A, then its rms voltage is also 24V, because it is DC. If there is no loss in rectification and regulation, then a 24V rms AC supply input is perfect. Note here we couldn't care less of the peak voltage of AC input, that's irrelevance.

But then one has to cater for ac mains fluctuation, rectification losses, regulation headroom...., that would require additional voltage headroom on the AC input voltage. So any circuits in the internet which says 30V 5A using a 25V secondary transformer is a joke. Perhaps if you look closely, what it really mean is 0-30V, 0-5A instead. It can never output 5A at 30V. Someone might tell you he can by using a very large value smoothing capacitor. No, he can't.

I challenge him to post a photo showing his regulated supply supply 30V 30A using a 21V secondary transformer. Don't forget to stick an AC voltmeter to show that it is really 21V at the secondary.

[techguru]

Hi to all,

         Can you suggest some technique to get 0V output from any voltage regulator with out going for additional transformer..

[Jeroen3]

The recent mailbag showed a SmartSupply by ThomasVDD on github.
It has a method of getting down to true 0V. I think it was based on the uSupply Dave never finished.

[Kleinstein]

It is rather simple to get a low power auxiliary supply with a kind of charge pump from the transformer. So going all the way down to 0 is not a difficult part in building the regulator.

The difficult part is getting both the voltage and current regulation stable (that is not oscillating) for any realistic load.

With circuits that start with a voltage regulator and add a kind of current limit later, there is the tendency that current regulation wil tend to oscillate quite easy. This problem is what caused Dave's µ-supply to kind of fail  .  So I would be kind of suspicious with any design that uses a voltage regulator that is not directly made for an adjustable current limit.

Transformer Specification
primary:230V ac
Secondary:24V-0-24V @ 5 ampere.

Good only for a 0~20V 3A regulated supply. If you wanted 3A at 30V, use a 35V-0-35V transformer.

I think it is better to give a way to get to the result rather than bare numbers

supposing the transformer is able to provide 24V at full load (resistive 5A), one has to take into account:

-1) line voltage variations (usually specified as +/-10%), so in this case the voltage might be as low as 21.6VAC (24-10%)

-2) peak voltage i.e. 21.6 * sqrt(2) that is about 30.5V

-2) rectifier drop at peak current... with a 10A bridge that should be around 2-2.5V (according to the GBJ10*, maybe optimistic, datasheet it should be 2.2V at 100A and 25°C) so worst case now is 28-28.5V

-3) ripple voltage: some books provide the formula Vpp = I / C * f, but usually the real value is a bit better (i.e. less)
in case you have a 10mF capacitor and a 50Hz frequency (100Hz thanks to the bridge) you get: 3 /( 10e-3 * 100) that is 3Vpp or more likely something around 2-2.5V, so the minimum voltage now is 25.5-26.5V

-4) regulator drop-out voltage, which might be as low as 1V using a low drop out design or more likely around 2-3V, so now you get the worst case maximum output voltage which might vary between 22.5V and 25.5V regulated DC

please correct this if there is anything wrong

With a 24-0-24 V transformer one can use a 2 diode rectifier and thus reduce the rectifier drop to about half.

[techguru]

Hi sir,,,

      What should i do know. where to start and how to proceed. how to start my design with out conventional voltage regulators.yes i came to know the difficulty in building 3 ampere from your kind previous reply sir.  help me in this perspective.

[techguru]

Hi to all,

               
                     Looking for beginner circruits, that are instructive and useful to  understand to construct my regulated power supply. thank you all.

[mariush]

Here's a schematic for a commercial power supply designed for up to 18v and 2 A of current

It uses a transformer with two big windings each capable of probably around 12v AC and depending on the desired output voltage, a relay switches between using only one 12v winding or both windings for higher AC voltage.

The transformer also has a winding which gives around 15v AC which is rectified using a separate bridge rectifier to provide 12v for the relay and maybe fan (so low current) , and two separate windings of around 8v each to create  +5v and -5v for opamps and the digital multimeter chips ICL7106 that would show the voltage and current on LCD displays (with old style segments).

Basically, you can buy all the components for this power supply separately, and even the lcd displays you can buy maybe not exactly the same but something with a few seven segment digits and dots is easy to find.  ( or you can buy the icl7106 chips designed for seven segment led displays instead, since these multimeter chips are available in both lcd and led display versions)

I'm also linking to  a PDF with the project for making a bench power supply which also has a good amount of information in it :  http://savedonthe.net/download/2010/Variable-Adjustable-Linear-Power-Supply.html

[techguru]

Thanks a lot sir...

[rstofer]

There is a falacy in using gigantic capacitors to hold up the average voltage and it is related to conduction angle of the rectifiers.

http://www-personal.engin.umd.umich.edu/~fmeral/CIRCUITS/05*Unit%205/Diode%20Rectifiers.pdf

Bottom line:  If the conduction angle is very short, the rectifiers have to deliver the total energy in a short period of time.  The current goes way up!  Of course, the transformer also sees the high peak current and drops its output voltage.

I wonder why anybody wants to go through the anguish of trying to build a PSU when they are dirt cheap:

https://www.banggood.com/DPS3003-32V-3A-Buck-Adjustable-DC-Constant-Voltage-Power-Supply-Module-Integrated-Voltmeter-Ammeter-p-1062475.html

Here's another thread re: DIY power supplies (among other things):
https://www.eevblog.com/forum/beginners/cheap-oscilloscopes-and-psus/

It's real easy to build PSUs based on the 7805,7905,7812 etc.  But these are fixed voltage with built-in current limiting.  Unfortunately, the current limiting can't be set down to a really low number like 100 mA.

The LM317 is a better approach for adjustable supplies but current limiting may take a 2d 317.

Overall, you will wind up with a bunch of op amps around a current loop and voltage loop all working to drive a pass transistor.

Search around this site and the rest of the Internet.  There are a lot of projects and none of them are simple.  Adjustable current limiting is a bit hard to do.

Here's one approach to creating a PSU with current limit.  It is a lightweight PS though, it only delivers 1.5A 0..26V.  But it does serve to show the complexity of a PSU with adjustable voltage and current limiting.

http://dangerousprototypes.com/blog/2014/04/27/app-note-simple-current-limiting-power-supply/

Where does the 3A requirement come from?  Is it just copied from a catalog?  I can't imagine building up a breadboard that needed even 1A, much less 3A.  If I needed more than 3A, I would probably use a purpose bought fixed PS or a set of batteries.  In my corner of the sandbox, 100 mA is a lot!

I do like the idea of adjustable current limiting when I bring up a new project.  It kind of keeps the smoke inside the chips.

For fixed voltages, a PC PS works well.  It won't have adjustable current limiting so it is more useful for things like motors.  It also provides very high current so the smoke will leak out in a great big hurry!

Add a connector:

https://www.sparkfun.com/products/12867

Here's a teardown video of the Rigol DP832.  Ignore the UI components and the CPU/networking and you still have a bunch of parts involved in a 3 output supply.

[rstofer]

Dave did some videos re: power supply design:

https://www.youtube.com/results?search_query=eevblog+power+supply+design

[Kleinstein]

Daves Lab supply design videos have some good points, but also some pretty stupid ideas. Using an LM317, LT3080 or similar is not a good starting point to build a lab supply. These are nice chips if you need an adjustable voltage for a well behaved circuit, but they have no adjustable current limit and adding one is like more complicated than the full thing.  If at all use a regulator chip that has provisions for an adjustable current limit, like the L200, lm723.

Dave's LT3080 based lab supply did not really work and the project thus never finished. I doe's not hurt watching the videos, but I would not at all recommend that circuit.

The linked circuit that mariush gave is a very simple standard circuit, maybe a little on the simple side, but reasonably easy to understand.  At low power one can leave out the transformer tap switching.

[orolo]

Kleinstein, you are right, but you are also speaking from a professional point of view. For a hobbyist, building equipment is a learning experience, also an imperative. You build a limited LM317/LM337 linear supply, learning a lot (morale boost), and move on to use it for other experiments. You become aware of its limitations, and learn more. Then you decide to build a better supply, and you're ready for it. My first homebrew power supply was a bipolar 317/337 inspired by the Elliot sound page, mains kit ripped from a IBM computer, huge capacitance multiplier included, with a voltage meter taken from a ultracheap chinese analog multimeter. I still use it a lot (I'm not a pro, obviously), though I've upgraded to a 723 design. Using this first supply, I soon learned to detect a short because the transformer started humming and the voltage dropped almost to zero (clack!, the cheap analog meter dropped and bounced). As long as you don't bypass the 317 with transistors, the crude protection is good enough for a beginner.

[rstofer]

The linked circuit that mariush gave is a very simple standard circuit, maybe a little on the simple side, but reasonably easy to understand.  At low power one can leave out the transformer tap switching.

Yes, the linked supply circuit is the kind of thing I was talking about when I mentioned op amp control loops and such.  It probably works very well but, in my opinion only, it is over the top for a first hobby PS project.  It's also complex enough that it probably needs a PCB - another stumbling block for newcomers.

I wasn't really recommending Dave's circuit either but Dave's circuit has the advantage that every single aspect is discussed including the bit about referencing the datasheets.  It might be worth building just to follow along - an educational project, not the final solution to power supplies.  A breadboard solution is workable.

One point I did make was something Dave hit on in the first video:  Be realistic with the specs!  It seems that 3A is some kind of a universal constant, something like 'e' or 'pi'.  Everybody wants 3A!  Why?  Same for 30V!

[hermit]

Is this the series of videos I learned that Microchip seems to have the lowest cost chips?  That there is a wide range between "typical" and the "max" rating?  That Dave seemed to be unlucky enough that all of his chips seemed to fall at the end of the range than near "typical"?  In other words, is this the series I learned that Microchip probably isn't very honest with their "typical" listing?

[Cliff Matthews]

Maybe if you're stuck with that transformer...
You could add a 10% mains boost switch with an small auto transformer? A 50VA 12v lighting transfo's more than enough for those times when you just have to run up the output. https://www.eevblog.com/forum/beginners/rolling-my-own-boost-auto-transformer-(sort-of-)/msg1284154/
This one's a bit high, but you get the idea..

[techguru]

Thanks a lot, sir...

                 I am a student of Electrical Engineering. can anybody suggest the book for current control in regulated power supply. no book gives the circuit for current control in linear regulated power supply design.

[rstofer]

Study the linked schematic carefully,  you will see where Q2 is the output pass transistor and Q1 is part of a Darlington arrangement to increase the gain.  Power transistor typically have fairly low gain.

The signal to pass more or less current comes over the connector CN4.  The rest of the upper circuitry is just the transformer winding and the rectifiers.  Below are two fixed voltage supplies.  The center 5V supply controls the relay that selects the transformer voltage and the lower PS provides + and - 5V.

Now for the control loops:  Note that CN4 Pin 2 has is held high (turning the pass transistor on) by R45.  Two things can pull this voltage down.  The upper op amp, IC3A, measures current (see how IS is generated across the 0.1 Ohm resistor)  If it goes higher than the setting on the pot, the op amp output starts to pull the control voltage down through D2.  There's a bit of integration going on in the feedback for the op amp to slow things down.

Voltage control comes from the lower op amp, IC3B, through diode D3.  That long resistor chain comes from both VS- and REF.  It is compared to the voltage setpoint and is the primary control of the signal going to CN4 Pin 2.  IC3B also has a little integration going on to slow things down.  At least I think that's what the capacitor is doing in the feedback path.  I suppose it could also be considered a filter.

So, that's it!  Two control loops, one for current and one for voltage are combined to control the pass transistor.  The rest of the circuitry has to do with warning LEDs and I have no idea what the lower left op amp is doing.  It's too late at night to figure it out.

Regardless of where you come up with a schematic, it will have to accomplish the same thing.  Clearly there needs to be a voltage comparator the drives the pass transistor.  That is basic voltage regulation.  Next, there has to be a way to diminish the drive when the output goes overcurrent.  The rest is just details.

[Jwillis]

I'm confused about that transformer 24-0-24 @ 5amp.Is that 5 amps series 48 volt or parallel 24 volt .Also I've also been looking into current control for my Frankenstein PSU and came up with this as an add on.For the CCR I figure a NSI50350AD http://www.onsemi.com/pub/Collateral/NSI45030-D.PDFshould work.   

[IanB]

I'm confused about that transformer 24-0-24 @ 5amp.Is that 5 amps series 48 volt or parallel 24 volt.

A 24-0-24 volt transformer is a 48 volt transformer with a center tap (in this case 48 V 5 A). It is not possible to wire a center-tapped transformer like this in parallel. You could only do that if there were two completely separate secondary windings with no connection between them.

[Jwillis]

Sorry I'm a beginner.By parallel this is what I mean. 

[techguru]

Sir,
     There is no link attached to it...

[rstofer]

I was talking about the schematic embedded in Reply 22

[Kleinstein]

I'm confused about that transformer 24-0-24 @ 5amp.Is that 5 amps series 48 volt or parallel 24 volt .Also I've also been looking into current control for my Frankenstein PSU and came up with this as an add on.For the CCR I figure a NSI50350AD http://www.onsemi.com/pub/Collateral/NSI45030-D.PDFshould work.   

That circuit looks like horribly slow  . The extra PV optocoupler is a really bad idea for a control loop, that should be relatively fast for a voltage regulator.

For learning about the design of power supplies it is a good idea to look at the Application notes from the big IC companies. A good example is AN1148 from National Semi (now Ti). There are other good ones from Ti and Motorola too. For an EE student they can be understandable, though not always easy.
There is also a rather long explanation on lab supplies from HP. 

The failure of Daves µSupply idea has nothing to do with parts being low quality of not reaching typical performance.  It is more about start with a voltage regulator and trying to add current limiting as an afterthought.

[Neomys Sapiens]

One point I did make was something Dave hit on in the first video:  Be realistic with the specs!  It seems that 3A is some kind of a universal constant, something like 'e' or 'pi'.  Everybody wants 3A!  Why?  Same for 30V!

Well, the 30V seem understandable to me, as getting into electronics involves not only circuit experiments, but also playing around with anything interesting and some repair attempts as well. And a lot of industrial things (control and sensor stuff, relays, PLCs and other modules) use 24V DC nominal.

[techguru]

Hi to all,

         Thanks to Kleinstein, i went through National istrument document(application Note-28). Thank you sir. today i went through the shop for buying component to test the circuit. but i found difficult in finding Zener diode(Lm113,1N821), Lm395(power transistor), and OPamps. But the application note gives promissing thoughts in getting result. these component were obselete in market. please any body could suggest  alternative component present in today's component to build the circuit.
 Thank you all...
           

[Kleinstein]

The zeners / refs are rather easy to replay something like TL431 and LM329 would be a possible choice.
For the protected power transistor, a replacement is more difficult. Without the protection just an NPN could do it, but this will not work with 3 in parallel, but just one (Darlington-) transistor. Something like am TIP 140 could be suitable. Parallel connection would need extra resistors. The lower LM395 would need extra current limiting added. In some circuit the LM317 can be used to replace the LM395.

The LM308 could be replaced by a more modern OP - for the LM101, the LM301 (reduced temperature range) is usually possible. With this op the compensation pins are used and thus a replacement difficult. 

Anyway the circuit has some interesting aspects, but is a little unusual.

[paulca]

For fixed voltages, a PC PS works well.  It won't have adjustable current limiting so it is more useful for things like motors.  It also provides very high current so the smoke will leak out in a great big hurry!

Sorry to hijack but is there a defacto PC PSU thread on the forum? 

I can find lots of example breakout boards for the ATX supply, but it seems an awful waste to just leave the high amp PCI 12V rails disconnected.  I am thinking of using it as a DC power supply for DC-DC buck/boost Bench power supply type things.

[techguru]

The zeners / refs are rather easy to replay something like TL431 and LM329 would be a possible choice.
For the protected power transistor, a replacement is more difficult. Without the protection just an NPN could do it, but this will not work with 3 in parallel, but just one (Darlington-) transistor. Something like am TIP 140 could be suitable. Parallel connection would need extra resistors. The lower LM395 would need extra current limiting added. In some circuit the LM317 can be used to replace the LM395.

The LM308 could be replaced by a more modern OP - for the LM101, the LM301 (reduced temperature range) is usually possible. With this op the compensation pins are used and thus a replacement difficult. 

Anyway the circuit has some interesting aspects, but is a little unusual.

Sir please suggest some other circuit involving opamp for current regulation. Thank you sir..

[not1xor1]

The LM308 could be replaced by a more modern OP - for the LM101, the LM301 (reduced temperature range) is usually possible. With this op the compensation pins are used and thus a replacement difficult. 

I always appreciate your posts, but this time you missed a relevant detail 

it is not just a matter of temperature range, the LM101 has a lower offset and withstands an higher supply voltage, +/-22V, while the LM301 is not specified for voltages above +/-18V, although real samples may survive nicely since are probably just 101s marked as 301

the supply voltage is already a bit above the limit in the original circuit as 35+10 = 45V

another possible problem is the current limit LED which might get too much current with some other opamp (the LM108 short circuit current is less than 10mA)

[techguru]

Hi all,
        I have decided to go for building power supply with two transformer for 2 separate channels which could be used to power opamp as well.
now how to place two transformer in same cabinet. Are there any constraint in placing them?.Could you please all help me in this regard...

[kingoft]

There is a falacy in using gigantic capacitors to hold up the average voltage and it is related to conduction angle of the rectifiers.

http://www-personal.engin.umd.umich.edu/~fmeral/CIRCUITS/05*Unit%205/Diode%20Rectifiers.pdf

Bottom line:  If the conduction angle is very short, the rectifiers have to deliver the total energy in a short period of time.  The current goes way up!  Of course, the transformer also sees the high peak current and drops its output voltage.

I wonder why anybody wants to go through the anguish of trying to build a PSU when they are dirt cheap:

https://www.banggood.com/DPS3003-32V-3A-Buck-Adjustable-DC-Constant-Voltage-Power-Supply-Module-Integrated-Voltmeter-Ammeter-p-1062475.html

Here's another thread re: DIY power supplies (among other things):
https://www.eevblog.com/forum/beginners/cheap-oscilloscopes-and-psus/

It's real easy to build PSUs based on the 7805,7905,7812 etc.  But these are fixed voltage with built-in current limiting.  Unfortunately, the current limiting can't be set down to a really low number like 100 mA.

The LM317 is a better approach for adjustable supplies but current limiting may take a 2d 317.

Overall, you will wind up with a bunch of op amps around a current loop and voltage loop all working to drive a pass transistor.

Search around this site and the rest of the Internet.  There are a lot of projects and none of them are simple.  Adjustable current limiting is a bit hard to do.

Here's one approach to creating a PSU with current limit.  It is a lightweight PS though, it only delivers 1.5A 0..26V.  But it does serve to show the complexity of a PSU with adjustable voltage and current limiting.

http://dangerousprototypes.com/blog/2014/04/27/app-note-simple-current-limiting-power-supply/

Where does the 3A requirement come from?  Is it just copied from a catalog?  I can't imagine building up a breadboard that needed even 1A, much less 3A.  If I needed more than 3A, I would probably use a purpose bought fixed PS or a set of batteries.  In my corner of the sandbox, 100 mA is a lot!

I do like the idea of adjustable current limiting when I bring up a new project.  It kind of keeps the smoke inside the chips.

For fixed voltages, a PC PS works well.  It won't have adjustable current limiting so it is more useful for things like motors.  It also provides very high current so the smoke will leak out in a great big hurry!

Add a connector:

https://www.sparkfun.com/products/12867

Here's a teardown video of the Rigol DP832.  Ignore the UI components and the CPU/networking and you still have a bunch of parts involved in a 3 output supply.

I was about to post the same thread, thanks to you 

[techguru]

Hi my question is what will happen ,if I place two iron core step down transformer adjacent to each other. Or what is the way to place them in same cabinet.

[techguru]

hi to all,

            Can anybody help me in constructing Circuit using L200 IC where i could adjust current as well.Give some prons and cons of L200 IC. Thanks to all of you....

[tooki]

Hi my question is what will happen ,if I place two iron core step down transformer adjacent to each other. Or what is the way to place them in same cabinet.

Others can correct me if I'm wrong, but I don't think anything will happen. You see this done all the time.

I know that my Keithley 2015 multimeter uses two transformers (one for the multimeter part, one for the DSP board), and my Tascam studio headphone amplifier uses two, I think one each for the positive and negative rails.

[schmitt trigger]

For a hobbyist, building equipment is a learning experience, also an imperative. You build a limited LM317/LM337 linear supply, learning a lot (morale boost), and move on to use it for other experiments. You become aware of its limitations, and learn more. Then you decide to build a better supply, and you're ready for it.

That is my personal experience also. Although in my case I started with the LM723.
The good thing about the 723 is that the essential regulated PSU building blocks are all exposed and there, one can learn about a basic architecture, which one can improve later.

And the good thing about building your own power supply is: you are no longer at the mercy of batteries!! One can really start experimenting with stuff, without a concern that the batteries are flat.

[techguru]

hi to all,

            Can anybody help me in constructing Circuit using LM723 IC where i could adjust current as well.I Request  to Reply to this if you could help me out in getting a solution. Thanks to all of you.... Good day...

[hermit]

This forum, and indeed the  internet at large, is littered with examples of these circuits.  At some point you need to show a little initiative and do some of the work.

[techguru]

HI hermit,

              I have tried the following circuit which i found while browsing. but non of them is usefull for adjustable current regulation. for example this circuit is using 500ohm variable resistor for current control but i found 3 of my regulator IC got breakbown. It may be the voltage between pin2 and pin3 may exceeded 5V or voltage across 500 potentiometer may be greater the breakdown voltage of base emitter junction. What may be the reason?

                  There is no guarantee for output when we copy circuit from google.

I know to get current limitation function for IC either by Foldback Regulation or normal current limitation to protect series pass transistor from LM723 Data sheets.

                        My question is how to control current at output by selecting appropriate potentiometer between pin2 and pin3?

[hugo]

Why don't you try this one:

http://www.ve2ums.ca/chasse/Serge/Atelier/Projets/Membres/VE2EMM/alimentation_ang.htm

It works fine for me.

[oldway]

You could use the schematics of the Philips PE1542 bench power supply.....it was a very good power supply, very reliable.

But it only need one 24V secondary.....
For two units, you need two separate 24V secundaries.

You can download the service manual with schematics here:
https://elektrotanya.com/philips_pe1542_service_manual.zip/download.html
Wait processing became Get manual....

[techguru]

Hi oldway,

    I couldn't understand the technique to analyse the circuit.At what point I should start and how to analyse the circuit. Which electronics book such schematic and analysis.

what will happen if i place two Iron core transformer close to each other? How to place them? Will coupling coefficient and Flux created on one coil will affect each other?

[IanB]

what will happen if i place two Iron core transformer close to each other?

Nothing.

How to place them?

According to wiring layout and mechanical security.

Will coupling coefficient and Flux created on one coil will affect each other?

No. Transformers are designed to keep the flux inside the core, not outside. If the flux were outside the transformer would be inefficient.

[techguru]

Hi to all,
Good morning today , I was working in my laboratory for circuits lab. I connect class A amplifier and switched on my power supply. The supply gave a hum when I switched my mains power. What does it say the class A amplifier overload the regulated power supply or there is short in breadboard. Else the RPS is poorly designed? What are the technique to be used reduce hum?

[techguru]

Good morning to all,

          From previous two replies, I have got the circuit for current control but I don't know to explain how it works. When some thing go wrong, then I may not know to troubleshoot the circuit.

So i want to work with opamp based current control , then i will add this current control circuit to voltage regulator. Hence I want good circuit book to understand theory behind the operation of circuit.. Thank you all .. good day..

[techguru]

Dear all,
            I want to design regulated power supply for my lab which should be programmable for both voltage and current. I bought a transformer of 24V@ 5Ampere centre tap.How much DC Current i can take from the Regulator. What decides the current.give me some reference for this .Thank you all gentle men...

[techguru]

Good evening all,
                 Now I am interested in constructing variable current limiting circuit. I constructed a simple circuit using LM723 IC. Please help me construct the same using operational amplifier , so that I could include with voltage regulator.

[tooki]

Do your own homework! There are tons of existing discussions here and elsewhere on such things, if you just do a little legwork yourself. 

[Cliff Matthews]

Do your own homework! There are tons of existing discussions here and elsewhere on such things, if you just do a little legwork yourself.

After a week of only his posts, I think Guru got the message...  Three cheers for self moderating forums!

[techguru]

Here is the circuit i am working,

                                           Now i want adjust the current limiting resistor to adjust my current but the problem i cannot have variable resistance of 0.3 ohm(potentiometer). how to implement the resistor to adjust the current ? help me in this regard..

[Cliff Matthews]

Seriously, you have beat the p#ss out of this thread. It's time to start another thread and be more specific, OK?

[techguru]

Cant Get you Cliff!!!!