UA741 op-amp power supply question

[eccerr0r]

Re: UA741 op-amp power supply question

The difference is that you're changing the reference voltage, and the electroschematics design is changing the feedback voltage. It's probably better to change the feedback voltage as changing the reference voltage may have some strange effects when the load resistance is changed (though the 78xx/79xx used as a reference are resistant to load changes.)

[Dannyx]

Re: UA741 op-amp power supply question

This is an adjustable CV PSU (no current limiting) with a 2N3055:

http://www.electroschematics.com/703...-power-supply/ (Note: LM1558 is similar to LM1458 which is similar to two LM741s)

Here's another power supply Adjustable CV/CC PSU project:

http://www.electronics-lab.com/proje...rol-0-002-3-a/

These might be good guidepoints.

Dang. My PSU is so old, it uses analog movement meters to measure voltage/current... every one else use LCD...

The first one looks great: right along the lines of my design The only thing it does differently is the Op-amp's inputs: while I used the non inverting input for adjusting it via the pot, the other schematic uses the inverting one for that....does that work ? Is there any difference ? I've just "mastered' op-amps and still get easily confused when something is different that what Dave from EEV "thought" me Also, I see a diode that's connected in reverse to the output of the op-amp - is that correct ? How would it operate as such ?

[eccerr0r]

Re: UA741 op-amp power supply question

This is an adjustable CV PSU (no current limiting) with a 2N3055:

http://www.electroschematics.com/703...-power-supply/ (Note: LM1558 is similar to LM1458 which is similar to two LM741s)

Here's another power supply Adjustable CV/CC PSU project:

http://www.electronics-lab.com/proje...rol-0-002-3-a/

These might be good guidepoints.

Dang. My PSU is so old, it uses analog movement meters to measure voltage/current... every one else use LCD...

[Dannyx]

Re: UA741 op-amp power supply question

Yeah, my mistake on that part: it makes perfect sense to connect the collectors to the diode bridge directly (see the new schematic). To back up, what I'm trying to build in an adjustable CV/CC supply which can deliver quite some amps....don't have an exact figure in mind, but the more the better Let's build this CV part (which should go from 0 to 12) first and then we'll work around it to incorporate CC - that's my idea at least. I reuploaded my schematic.

[eccerr0r]

Re: UA741 op-amp power supply question

Need to back up a step, what DO you really want to achieve? Adjustable CV/Adjustable CC or just a current limited fixed voltage PSU?

With the regulator and the transistor in series, the circuit you have will still limit current to 1.5A or whatever your LM7812 can handle. Also your current sense resistor is missing. Right now your circuit looks like it...well... head "SPICE" seems to make it look like two cascaded voltage regulators (second one adjustable) which I'm not quite sure is what you want...

If you simply intended this to be an adjustable CV supply, you should simply use the LM78S12 output as the reference for the op amp and connect the collector of the darlington to the unregulated input directly.

[budm]

Re: UA741 op-amp power supply question

I do not understand why you are using the regulated output of the 7812 to feed the Collector of 2N3055, think about what current will be flowing through the 7812, 2n3055, and the load, and the max Voltage you will get in this CV circuit, lots of power is being wasted on that 7812, and that is just one part of the problem in the circuit..
Please look at those link I provided about regulator circuits.

[Dannyx]

Re: UA741 op-amp power supply question

Here's the diagram I put together. Now before you start bashing me with "professional comments", please bear in mind that, aside from that awful excuse for a schematic I scribbled in MS Paint, there are probably many other things wrong with my circuit like missing components or incorrectly installed ones. I know that's not the proper way of drawing an op-amp, but I wanted to keep it idiot-proof and used the exact layout of the chip rather than the triangle symbol. I breadboarded this thing with a single rail supply and it sort-of worked....the voltage adjusted up and down and was fairly stable even under a relatively high load. Let me know what other improvements can be made to this CV part and then we can expand on it. Feel free to "pain over" my schematic and then upload it again. I don't know what transistor should come before the 2n3055 that's why I didn't label it - I heard I need to build a Darlington array to drive the 3055, that's why I put it there, but without actually knowing what it should be. Same with those caps. In my tests so far, I played around with 2.2pF tantalum ones.

[eccerr0r]

Re: UA741 op-amp power supply question

I think most of the op amp CC controls basically measure the voltage differential across a low value resistor. A high gain op amp circuit will turn off the supply transistor when the voltage goes above the setpoint. The LM317-TL082 design is using R6 and R7 as a voltage divider to scale down the "input" or high side of the measurement resistor (R2). The low side of the measurement resistor is connected to the noninverting input directly. The capacitor C5 is there to dampen oscillation.

One key point that is also a problem with the µA741 is that the supply needs to exceed the maximum input voltage range of the amps. The unregulated positive filtered voltage from the bridge is used to power the TL082 which works as the LM317 and TIP42 will drop some volts guaranteeing that the inputs of the op amp will be lower than the op amp supply. The negative rail unfortunately won't work with GND (plus the fact the LM317 needs a negative voltage to shutdown completely) hence a separate negative supply is needed. Note that the op amp has unbalanced power, which is OK as it's being used as a comparator and it only needs to swing down to negative rail to shut down the LM317T when over current is reached.

After staring at this a bit, I wonder if I really need the regulated negative supply for current control... might be able to do away with it - but I would still need it for getting the output voltage control to 0 but too lazy to make the mod.

Another thing I wanted to point out, not sure if I already posted it or not - the 100W limit of the 2N3055 will limit your CC control severely. I estimated a voltage drop swing of 6 volts max is what you can do at 15A current limit before the transistor fries, so if you're really looking into 15A, this may not actually be feasible with a single 2N3055 if you also want a full range CV control that exceeds around 6 volts... If you keep your max current less than 5A or so, you'll have much better current limiting control.

And though my PSU was not meant for 15A, believe me, I've replaced the TIP42 several times before when running only 2-3 amps - because I was exceeding the max dissipation of the transistor.

I think the CV/CC PSU is one of my most important tools though it tends to be used as a battery charger more than an actual power supply...

[Dannyx]

Re: UA741 op-amp power supply question

I think the more experienced members are rushing to offer information on how to build the ultimate bench supply, whilst the OP was really asking about split supplies for his 741. My first supply used a 741 and there is nothing wrong with it. The op-amp provides lots of gain as an error amplifier and beats the earlier one or two transistor error amp design for output stability.

There is a good helpful reason to use a split supply on your 741 error amp and that is to allow your power supply to be adjustable down to zero volts output. An op-amp needs the (-) supply to provide the zero volt (or below) drive that the pass transistor needs to go to zero volts output.

And another thing. The 741, or other op-amp does not need symmetrical supplies. Use a 12v positive rail and a -5 volt negative rail for example. Keep in mind that the inputs need to be properly dc biased, as with any op-amp. Good luck. Don't be discouraged if you smoke some parts at first!

So far I managed not to fry anything thankfully ) I believe I'll get the CV portion going, as it was fairly stable to begin with - just wanted to check up on the proper voltages. It's the CC part that confuses me slightly.

[budm]

Re: UA741 op-amp power supply question

I think the more experienced members are rushing to offer information on how to build the ultimate bench supply, whilst the OP was really asking about split supplies for his 741. My first supply used a 741 and there is nothing wrong with it. The op-amp provides lots of gain as an error amplifier and beats the earlier one or two transistor error amp design for output stability.

There is a good helpful reason to use a split supply on your 741 error amp and that is to allow your power supply to be adjustable down to zero volts output. An op-amp needs the (-) supply to provide the zero volt (or below) drive that the pass transistor needs to go to zero volts output.

And another thing. The 741, or other op-amp does not need symmetrical supplies. Use a 12v positive rail and a -5 volt negative rail for example. Keep in mind that the inputs need to be properly dc biased, as with any op-amp. Good luck. Don't be discouraged if you smoke some parts at first!

His post #8:
'I want to build an adjustable current regulator, which can go from say 1A, all the way up to 15A or how much a 2N3055 transistor is able to handle...again, power seems to be the no.1 problem here. I'm not too good with formulas and math and sh*t, I admit, so I might be wrong.
He wants to build adjustable CV/CC power supply with lots of current, not sure what the max Vout requirement is.

Also:
My first idea was to use an LM338 and put a resistor in series with its output to act as a current sense resistor to pull down/up the ADJ pin depending on how much current the load draws, but the problem I always face when trying to design this thing (be it with an off-the-shelf regulator like the LM or with an op-amp like Dave showed) is the power dissipation of that resistor: since it's in series with the load, at 12v that resistor would need to handle 18W (according to my calculations U*i=P - 12v*1.5a=18w)
The current sensor resistor will be really low Ohms resistor and it is NOT going to have 12Vdrops on it, it is not going to be 8 Ohms current sensing resistor, that will be really bad design.

[Longbow]

Re: UA741 op-amp power supply question

I think the more experienced members are rushing to offer information on how to build the ultimate bench supply, whilst the OP was really asking about split supplies for his 741. My first supply used a 741 and there is nothing wrong with it. The op-amp provides lots of gain as an error amplifier and beats the earlier one or two transistor error amp design for output stability.

There is a good helpful reason to use a split supply on your 741 error amp and that is to allow your power supply to be adjustable down to zero volts output. An op-amp needs the (-) supply to provide the zero volt (or below) drive that the pass transistor needs to go to zero volts output.

And another thing. The 741, or other op-amp does not need symmetrical supplies. Use a 12v positive rail and a -5 volt negative rail for example. Keep in mind that the inputs need to be properly dc biased, as with any op-amp. Good luck. Don't be discouraged if you smoke some parts at first!

[budm]

Re: UA741 op-amp power supply question

That won't be a problem. The problem is building the darlington array...what should I use for the "first" transistor ? (the one whose base is driven by the op-amp and in turn drives that 2n3055). I heard something about a TIPxxsomething ? :|

You need to know what the required Base current is for the 2N3055 for 15A load of Ic current then you can get the drive transistor that is rated at twice the Base current required by the 2N3055. Then you need the Beta of the Drive transistor to see if how much current from the 741 to supply to drive the first transistor.
Since it is Emitter follower output (Output of the regulator is at the Emitter of the 2N3055), the IC current + the Base current will be flowing through the load.

[Dannyx]

Re: UA741 op-amp power supply question

Correction on the Wattage, it is 45W.
Can we at least see the diagram?

Don't have one yet - it's all "up here" as they say I only scribbled something so I don't get lost. I'll post it as soon as I make it....get ready to be horrified though, as it's a pretty ghetto design

[budm]

Re: UA741 op-amp power supply question

Correction on the Wattage, it is xxW, but it will not be running at Vsat, so the power dissipation will have to be calculated based on the input Voltage and the Output Voltage requirement.
Can we at least see the diagram?

[Dannyx]

Re: UA741 op-amp power supply question

Beside trying to get enough Base drive current, he is going to need really good heatsinK for the 2n3055 too, it is going to dissipate about 16W at 15A.

That won't be a problem. The problem is building the darlington array...what should I use for the "first" transistor ? (the one whose base is driven by the op-amp and in turn drives that 2n3055). I heard something about a TIPxxsomething ? :|

[budm]

Re: UA741 op-amp power supply question

Beside trying to get enough Base drive current, he is going to need really good heatsinK for the 2n3055 too, it is going to dissipate about 45W (3V x15A, Spec shows Vsat @10A is 3V typ., but the Ic will not actually be 15A dues to the Base current will be flowing through the load too).












.

[eccerr0r]

Re: UA741 op-amp power supply question

While the typical solution is a darlington or compound pair, anyone seen some high powered monolithic op amps? The only one I've ever come across was the LM759, though there are probably more. Yet even the LM759 won't get the ampere or so needed to get the 2N3055 to get 15A collector current.

[budm]

Re: UA741 op-amp power supply question

You want to have the power supply to be able to supply 15A of current, then you are not going to get enough drive from the 741 to drive the 2N3055.
https://cdn.badcaps-static.com/pdfs/...b5fd21cca9.pdf
Look at the graph of hfe VS Collector current.

[Andyy]

Re: UA741 op-amp power supply question

Here's another power supply circuit using an op-amp and pass transistor, with current limiting.

http://www.eevblog.com/forum/project...-kit-(k-3206)/

[eccerr0r]

Re: UA741 op-amp power supply question

That circuit is basically the same reference design in NS's databook. There are tricks in there basically to save components, and use the minimum number of LM317 to have both adjustable CC and CV. The LM317 is controlling voltage for the most part, but the high gain amplifier that measures the voltage across the sense resistor that deals with the current. If the voltage across the sense gets too high (too much current is flowing) it will pull the adjust pin to the negative supply, shutting off the current flow.

If you're planning to use a pass transistor directly, you'll need to make sure that the base circuitry can supply the current needed to make sure can continue conducting (usually need to have the pass transistor in a sort of darlington configuration as most op amps don't source/sink enough current for a 2N3055).