0-30 Vdc Stabilized Power Supply

[audioguru2]

Hi Thomas,
It is great that you can add a winding to the transformer to step-up its voltage. You aren't changing it much so don't worry about core saturation of additional temperature if the wire is the correct size. The project's max output current must be derated according to the transformer's power rating.

You said that you are going to gamble with operating the original TL081 opamps beyond their voltage rating. Somebody else in this thread said, "If your power supply isn't reliable, then it isn't worth having." Somebody else said, "You have spent months building a circuit. You connect it to this unreliable power supply which fails and suddenly gives max output voltage and ruins your circuit!"

How high a regulated output voltage do you need? With a 24V/4.2A transformer the modified project will give about 25VDC/3A. I recommend a 30V/4.2A transformer and those high-voltage opamps to get 30VDC/3A out.


Hi Tedy,
If you can't get the ICs then how are you going to build it?

 

[Thomas1]

Hi audioguru,

I think you have a point there. Using higher rated opamps would be a much better solution. Even if the tl081 won't go bang at the first time, they will fail at some time later because of getting too much voltage for a long time.
Okay, so I need to have high-voltage opamps. I'm going to read the other thread about this project in order to get more informations about the neccessary opamps and other needed changes. It's the one about getting 5A Output current from this project, isn't it?

Thanks for your great help

 

[audioguru2]

Hi Thomas,
There is very little difference beween the improved 3A project and the 5A one. The 5A supply requires a 30V/7.1A transformer, three 2N3055 transistors and its current sense resistor's value is reduced. Same OPA445AP opamps.

 

[Dosster166]

Hello everyone,

I'm a newbee, ;D

Well now that we got that out of the way my first of many questions.
I can't find the proper transformer for this projest, can someone point me in the right direction? Online source would be best

Thanks

 

[audioguru2]

Hi Dosster,
Welcome to our forum.
I shop at www.newarkinone.com and www.digikey.com .
I entered Hammond Power Transformer into Newark's search, selected Torroidal type and found a deluxe 30V/4A transformer with dual primaries and secondaries. They have less expensive ordinary ones too.
Shipping will be expensive since the thingy isn't made of feathers! ;D

 

[Thomas1]

Hi audioguru

So, when those (expensive ) opamps are used, the only thing to change in the project would be the layout, probably more 2N3055 with each one serial resistor and a big rectifier bridge, right? Getting 5A is impossible with my transformers but I'm sure I don't need that much current.

P.S.: I intend to go with TI's friendly sample policy. Did anyone manage to register by using netscape browser 7.02? I tried it several times but without success. Nothing happens when "register" is clicked after filling out the form. ???

 

[audioguru2]

Hi Thomas,
I've updated my recommended parts list for this 0-30V/3A project.
R14 should remain at 1.5K with your low voltage transformer as in the original article.

0_to_30V_Mod.txt

 

[Thomas1]

Hi audioguru,

Thank you for the new list.

I wondered whether the OPA604 could be used instead of OPA445. This is an audio-tuned opamp with +/-24V.
I did some measuring with my toroidal trafos and the unloaded voltage is 14,634% higher than the rated one. And after rectifying and filtering, this unloaded voltage gets multiplied by the root of 2.

26VAC*1,14634*1,414 equals 42,15VAC. Together with the -5,6V supply we are pretty close to the rated voltage of the opamps. The absolute max. voltage is +/-25V.

So can I tune my trafo up to 26VAC rated voltage and use this opamps, which are about 5 times cheaper?

Thanks.

 

[audioguru2]

Hi Thomas,
Gambling?
What are those 50V opamps going to do in the summertime when it gets cool at night and all the AC in your neighbourhood turns off? They will suddenly be powered with 53V or more.

Why not add a few turns to your transformers to raise their loaded voltage to 30V, and use $9.50US OPA445AP opamps from Digikey or free samples from TI?

 

[MP1]

Maybe his neighborhood is a little more regulated than yours ;D

MP

 

[Thomas1]

Hi audioguru,

Well, there a not that much AC in my neighbourhood and the line here is quite stable. But after all this power supply should be very reliable, so I'll take the 445APs. Little expensive, but so be it. I can get also more output voltage this way and probably even less ripple because the higher voltage should push more electrons in my filter caps.

Hi MP,

That leads me to an interesting question: Where are you people located? Quite strange time in this forum, ACs are a normal thing in every house... Probably Greece?

 

[audioguru2]

Hi Thomas,
This is an international forum.
I'm in Canada, eh, with Niagra Falls' generators (alternators?) producing umpteen gigawatts of power nearby. When I first measured my mains, I was surprised how much its voltage changes. Sometimes the utility reduces the voltage on purpose making a brownout but then it spikes when it returns to normal.
Electricity is fairly cheap here so many people waste it. Recently my city exceeded its record for electrical power used, not for Air Conditioning but for Christmas lights! The utility went around handing out free LED lights so hopefully the record won't be beat next year. ;D

 

[Tedy1]

Hi
I made this PSU, but voltage output is constantly 15.00V
Truning potentiometers doesent change anything. Voltage at C1 is 15.5V (I used 12V transformer).
When I connected car lamp output fallen down to ~9V.
Temperature of rectiifier diodes (1n5408) were around 100

 

[audioguru2]

Hi Tedy,
Of course it doesn't work, you didn't use the transformer that it specifies and you didn't read this topic about how to fix it.

Sure, those tiny rectifier diodes and R7 are going to smoke!
If you use a transformer with a higher voltage to allow it to work better, its tiny 2N2219 will also smoke.

 

[aergotic]

Doesn't somebody know to build PCB for modified parts in original shema. How to put two tranzistor 2N3055.

 

[audioguru2]

The two 2N3055 transistors do not go on the pcb. They are bolted to the big heatsink with insulating washers. They are simply in parallel but each has an emitter resistor to the output.

There isn't much difference to the pcb. A higher capacity main filter cap, but they are about the same size today as the original. A bigger and higher power R7, so stand it up away from the board. A rectifier bridge module that is bolted to the chassis or heatsink. The new TIP31A has a different pinout than the old 2N2219 so twist its legs to fit. What else?

 

[Blacque Jacque]

Hi all,

I've been considering building one of these supplies & reading this forum with great interest. I will be ordering a kit from my local supplier shortly & then upgrading it to the spec you guys sorted out.

I want to fit voltage & current meters & have obtained a couple of cheap ( 10UKP) DMM's with 3 3/4 digit displays (3999 count), Maplin order code N49AB. They are Mastech M320, and they're autoranging !! ;D

They're fine for the voltage display but the max current range is only 400mA. Is it possible to use some kind of low gain opamp circuit reading the voltage across R7 ( 1.41v @ 3A ) to provide a buffered & scaled signal to the DVM ?

 

[audioguru2]

Hi Blacque,
Welcome to our forum.
You have some good meters and a good idea to measure current. An external current meter will have a voltage drop and ruin the good voltage regulation of this project. R7 is outside the feedback loop so has no effect on output voltage. Sure, amplify the 1.41V up to 3.0V with an opamp and it will read output current directly.

Do you have a separate method to power the meters? Many meters cannot measure the voltage or current of their own supply.

 

[Blacque Jacque]

Hi Audioguru,

The meters are powered by 2 x SR44 watch batteries per meter so I figured I could use a separate isolated 3v supply ( another winding on the transformer + reg/rec circuit maybe, or take the guts out of a wall wart?). I'll try them battery powered initially then add a supply later.

For the ammeter drive, is opamp choice important ? A fancy opamp like the OPA445 isn't necessary for this circuit is it ? If supply voltage was a problem, couldn't I just add a small 78 type regulator to provide say +15 for the opamp & take the -5.6 from the existing rail ? The TL081 supplied in the kit seems an obvious choice.

Also, a cosmetic idea : Is it possible to add another LED which lights in CV mode ? I do like my test equipment to clearly show what it's doing

 

[audioguru2]

Hi Blacque,
Sure, use the existing spare TL081 to drive the current meter. You could drop-down the positive supply voltage to 5-10V (it doesn't need to be accurate) with a 2-resistors voltage divider, and use the 5.6V negative supply. A bypass cap at the positive terminal of the TL081 would be important.

You could make a CV indicator with a PNP darlington transistor driving its LED and connected similar to Q3. It would use very high value resistors at its base which would be driven from the collector of Q3. ;D