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0-30 Vdc Stabilized Power Supply


Sallala
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  • 2 weeks later...

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.

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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?

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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?

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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

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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

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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.

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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?

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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 ?

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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.

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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

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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

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Hi  Blacque,
Sure, make your own darlington with a couple of BC557's. Make certain the 2nd one turns-off with a 100K resistor from its base (and the emitter of the 1st one) to its emitter (the positive supply). Use a 1M resistor from the base of the 1st one to the positive supply, and a 470K resistor to the collector of Q3. Use another 3.9K current-limiting resistor connected to the collectors of your darlington for the new LED. Use 1W resistors for the current-limiting resistors (R22 and the new one).

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Hi Aergotic,
I always design circuits for "worst case" conditions because you don't know what the current gain of your transistors will be, just a range of current gain. If your transistors have the lowest minimum guaranteed gain, you still want the design to work, and reliably.

The project's max output current is 3A. The modified circuit uses two paralleled 2N3055 transistors so each one will pass a max of 1.5A. The minimum current gain of a 2N3055 is 20 at 4A, but the datasheet has a graph that shows its gain doubling at 1.5A, which would be 40. Therefore the base current for each 2N3055 is 37.5mA and a total of 75mA max.

The 30VAC transformer will produce a rectified and filtered 40VDC at full load. If the project is set for 3A output current and the output is shorted, Q2 will have about 37.6V across it. Therefore the max dissipation of Q2 is 75mA X 37.6V = 2.82W.

The TIP31A can dissipate 2.0W at room temperature without a heatsink but will be extremely hot. If you want its max internal temp to be only 100 degrees C, then it will need a 26 degrees C/W heatsink and good thermal grease. 

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Hi All,

I have an idea which may solve a difficulty raised earlier in this thread ie: you cannot see the current limit set point easily, particularly if you want to use a multi turn wirewound pot.

Expanding my theory for current metering, you could use another opamp buffer to read the voltage on U3 pin3 ( current limit set point ) & feed it through a momentary switch to the ammeter. This would need to be done with no current flowing to the load, otherwise the

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Hi Audioguru,

I hadn't thought of a range switch, how would you implement this ???

Do you think current & voltage setting would be easier with fine & coarse pots instead of multiturn ones ? If the current setting range is from 2mA to 3A that's a big adjustment range, it would be difficult to provide accurate control at each end of the range. On the other hand, how much use is adjustment down to single milliamps ?

I think calibrating the meter scaling is surely easier than calibrating front panel markings & higher resolution too.

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Hi Blacque,
To use a range switch, you use a fixed main resistor and in parallel with it a high-value pot and a bunch of series connected scaled resistors in series. The switch shorts the resistors one at a time to change the range. Use a make-before-break switch to keep the current from jumping. Since the pot and resistors are high-value, the opamp's input that they feed must have a very high impedance, like a FET-input opamp.

I don't like having fine and course pots because it is difficult to see what their setting is, and you keep hitting the stop on the fine one.
I also don't like to use multi-turn pots. I just use a big knob and hold my breath when adjusing it. Sometimes I wish I could hold my heart too! Avoid coffee before adjusting.  ;D

I have never used a current regulated supply, but maybe I could use a few mA to measure the current gain of small transistors or something.

Use digital meters then you don't have to mark or calibrate them. With their 4 digits, they have very fine resolution.  ;D

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Hi Blacque Jacque,

Annother way to preset the current is to shorten the output with a suitable switch and measure the voltage drop across R7. This voltage gets to a 100k/27k potential divider. Now we have 0,3V @ 3Amp across the 27k resistor which can be fed to a panelmeter. A 1k pot can be added in series to the 27k for fine tuning.

A second switch can be used to disconnect the output from the psu. That gives us the ability to preset the voltage.

I intend to do it that way. Maybe there could appear a problem when R7 dissipates 4.23 Watts and gets mighty hot which will change its value. Those expensive Resistors in a TO-247 case could be a solution because their temperature coeffizient is less than 50ppm, but I can't find any with 0.47 Ohm.

Hi Audioguru,

1) Can a 0.56R or a 0.39R Resistor be used for R7?

2) I've read both threads about this psu and am a little confused about the emitter leg of Q1. Is the original schematic correct or not? ???

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