UA741 op-amp power supply question

[Dannyx]

Good day folks. I'm playing around with this op-amp, the UA741, and trying to build an adjustable power supply with it (let's not go into the details here, as I'm sure many of you will scold me for not being the best choice, etc). I grabbed a beefy center-tapped UPS transformer which puts out 15v on each winding, built a small split-supply using an LM7812 (for +) and LM7912 (for-), but now I hit a snag: I'm trying to figure out what voltage the op-amp actually needs. The datasheet (see it here) says "supply voltage" is -18V to 18V (forgot to mention it's the C version, not the M) and index "2" says "voltage values are with respect to the midpoint between VCC+ and VCC-. This op amp does not have a GND or 0 volts pin to which I'd connect my center tap, so if I just connect my 12v output from the 7812 to VCC+ and -12V from the 7912 to VCC-, that would mean 24v across the IC, which is way over the maximum rating as far as I can make out. Is this assumption correct ? If so, I assume I need to swap out my LM7812s for LM7806s, so the total output is 12v give or take (6+6).

[budm]

Re: UA741 op-amp power supply question

You are feeding +12V and -12V which is fine, you are exceed the power supply limit.
I think you need to understand how the op-amp functions and how it is used first.
BTW, are using heat sink on those two regulators?

[Dannyx]

Re: UA741 op-amp power supply question

You are feeding +12V and -12V which is fine, you are exceed the power supply limit.
I think you need to understand how the op-amp functions and how it is used first.
BTW, are using heat sink on those two regulators?

I think you meant to say "you are NOT exceeding the power supply limit". I believe I know what an op-amp does - took me a while, but I finally got it down to an understandable level (props to Dave from EEV). Also yes, I know I should add heatsinks to the radiators, right now they don't have any - the power draw isn't too great, as the op-amp drives a 2N3055 transistor which does all the work, that's my idea. I build this crude design onto a breadboard and it works, but I did so using ground for the op-amp and not negative voltage, so now I'll try doing it this way, since I heard it's better (just a thought - may be wrong).

[budm]

Re: UA741 op-amp power supply question

""you are NOT exceeding the power supply limit" You are correct sorry for typo, so you are saying the -VCC pin is connected to the circuit GND? You can run OPAMP in single power supply mode if you bias the input pin correctly.
May be you should show the diagrams of the circuit.

[Th3_uN1Qu3]

Re: UA741 op-amp power supply question

For old, low-end opamps like the 741, a split supply is a better idea since this kind of opamps cannot do "rail-to-rail" output - there will always be a minimum output voltage if the negative rail is at ground potential. In simple words, output cannot be 0v if -VCC is 0v. It will always be a few 10s or 100s of mV. There are special opamps available which can do "rail-to-rail" output - an output voltage equal to the supply voltage, so if your -VCC is 0 volts then the output will have a range of 0 volts to +VCC, if your supply is +/-12v the output will have a range of -12v to +12v and so on.

With +/- supplies you ensure a healthy output voltage range for your application.

[budm]

Re: UA741 op-amp power supply question

BTW: When OPAMP runs on bi-polar supplies, the Voltage at the output pin will be 0V (but real opamp will not be at 0, but it will have offset Voltage, in mV range) ref to the circuit GND, but when runs on single supply, then the output will no longer be at 0V and can be almost as high as the VCC or close to GND (but will not be at 0V) it will depend on what Voltage you have at the + and - input of the opamp, so what is connected to the output pin will have to take that into consideration.
It sounds like you are trying to build Linear Voltage regulator with opamp base.
There are lots of application notes circuits from OPAMP manufacturers.
https://cdn.badcaps-static.com/pdfs/...e6dc429e79.pdf

[eccerr0r]

Re: UA741 op-amp power supply question

A lot of those old IC op amps are plenty fine running at huge voltage differentials. Without looking it up I thought 741's should be good to ±18V max, so a whopping 36V difference between the V+ and V-. ±12V is no problem, and in the past ±15V was actually typical. Of course now getting voltages up that high is pretty insane. Again depending on your usage, ±6V is fine too with these old op amps but the available operating range is getting kind of tight due to the older design (both input and output acceptable voltages are constrained by the supply voltage.)

Not much current needs to flow through it, the IC won't source/sink much anyway. Depending on what you're doing, you may not even need heat sinks on your LM7812/LM7912 regulators.

Another characteristic of op amps is that they should have a PSRR rating and as long as your application doesn't require a certain voltage, you don't even need to regulate it as long as it stays above the minimum and below the maximum voltage rating.

Yes theoretically if you short the in+ and in- inputs of an op amp, the output should go to the middle of the V+ and V- supply pins, at least that's in theory (all bets are off if the connected in+ and in- pins float towards the rails as the 741 is not a rail to rail input amp - best if you use a voltage divider to get into the linear region of operation). It should get pretty close. The offset null pins on the older op amps like the 741 can be used to trim it so that it gets closer to your desired mid/zero point.

[Dannyx]

Re: UA741 op-amp power supply question

BTW: When OPAMP runs on bi-polar supplies, the Voltage at the output pin will be 0V (but real opamp will not be at 0, but it will have offset Voltage, in mV range) ref to the circuit GND, but when runs on single supply, then the output will no longer be at 0V and can be almost as high as the VCC or close to GND (but will not be at 0V) it will depend on what Voltage you have at the + and - input of the opamp, so what is connected to the output pin will have to take that into consideration.
It sounds like you are trying to build Linear Voltage regulator with opamp base.
There are lots of application notes circuits from OPAMP manufacturers.
https://cdn.badcaps-static.com/pdfs/...e6dc429e79.pdf

Yes, that's what I'm trying to build: an adjustable power supply. I know it can be done with the LM317 or LM338 off the shelf, but I wanted to give the op-amp method a shot based on the design Dave from EEV showed on youtube in his lab power supply design. I got the voltage part down, at least on paper and in my mind, but the constant current part is giving me headaches: I want to limit the output current to around 1.5A. 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) which seems pretty insane for a resistor - I'd need a huge one, which is not readily available and is also bulky and expensive. Can't seem to get around this issue. Ideally, 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.

[eccerr0r]

Re: UA741 op-amp power supply question

Ah now this is making more sense.

Your current sense resistor needs to be very low value, and your op amp's gain will compensate for the very low drop across it.

But I'm not sure you're "measuring" right here? Why would that current sense resistor be dropping 12V, or is it 12V with your actual load?

There are some fairly decent reference designs out there. I ended up building a CV/CC PSU based on a LM317T reference design with a TIP42 series pass transistor and TL082 op amp as the CC feedback. A 0.2 ohm resistor is the current sense to reduce the sense losses, and yes I needed a negative voltage to be able to shut off the LM317T to do proper current limiting. Attached is the schematic I used. I think one thing that could make this design better is that the voltage regulation zero reference needs to be also brought down to negative 1.2V or something (disconnect the voltage adjust variable resistor and its capacitor at GND and connect it to a -1.2V instead, probably regulated with two diodes in series perhaps, referenced off the CC negative supply).

[Dannyx]

Re: UA741 op-amp power supply question

Have a look at this schematic and see if you can straighten me out, as I must admit it's a bit over my head. To my mind (and this is the important bit) it goes like this: all the current has to go through that series resistor. For simplicity's sake, let's say it's 1ohm. If the output voltage is 12v, that means the maximum calculated current through that resistor is going to be 12a (Ohm's law: V divided by R equals I. 12v / 1ohm = 12a). At the same time, the regulator's ADJ pin is being pulled to ground through the load itself, which means that the higher the load (its resistance is lower), the more the regulator is going to try and "close down", bringing down the voltage and thus the power along with it (since U*I = P, so if U drops, so does P). Now say I drop that circuit before the CV side of that Op-amp s**t I'm trying to build...what would happen ? :|

[eccerr0r]

Re: UA741 op-amp power supply question

Yes that CC circuit indeed drops and burns a lot of power in the resistor as the current pass gets higher. The CC/CV PSU schematic I usedfor my bench PSU (attached above) drops the voltage in CC mode in the transistors instead. Unless you go switching, something needs to eat that excess in CC mode.

BTW the LED lights up when the adjustable current limit is reached.

[Dannyx]

Re: UA741 op-amp power supply question

Yes that CC circuit indeed drops and burns a lot of power in the resistor as the current pass gets higher. The CC/CV PSU schematic I usedfor my bench PSU (attached above) drops the voltage in CC mode in the transistors instead. Unless you go switching, something needs to eat that excess in CC mode.

BTW the LED lights up when the adjustable current limit is reached.

I was going to ask what schematic you're talking about, but then I noticed you were kind enough to provide it further down below ) Thanks. Also, I too thought of replacing the resistor with a transistor, but wasn't too sure how to do it. I'll have a look at your schematic and come back with any questions. Cheers.

[eccerr0r]

Re: UA741 op-amp power supply question

The TL082 is a much better part compared to a µA741, so I'm not sure you can substitute it directly (with pin changes), so don't get too excited. But the concept is still the same.
The input leakage and offset voltage of the µA741 may require some circuit changes for it to work properly, and even then the noise in the µA741 may cause problems.

[Dannyx]

Re: UA741 op-amp power supply question

The TL082 is a much better part compared to a µA741, so I'm not sure you can substitute it directly (with pin changes), so don't get too excited. But the concept is still the same.
The input leakage and offset voltage of the µA741 may require some circuit changes for it to work properly, and even then the noise in the µA741 may cause problems.

I haven't actually soldered anything together so far - only breadboarded something that seemed to work fairly well, so it's not an issue to swap chips around, reconfigure some jumpers and voila....new circuit ) What I'm mostly interested in is stability at high loads. To put it to the test, I used some car lightbulbs and I'm now planning on testing it with a peltier cooler (haven't done that yet though - still need to rebuild the thing first). The lighbulb test was most satisfactory, even without proper traces and filtering.

[budm]

Re: UA741 op-amp power supply question

Have a look at this schematic and see if you can straighten me out, as I must admit it's a bit over my head. To my mind (and this is the important bit) it goes like this: all the current has to go through that series resistor. For simplicity's sake, let's say it's 1ohm. If the output voltage is 12v, that means the maximum calculated current through that resistor is going to be 12a (Ohm's law: V divided by R equals I. 12v / 1ohm = 12a). At the same time, the regulator's ADJ pin is being pulled to ground through the load itself, which means that the higher the load (its resistance is lower), the more the regulator is going to try and "close down", bringing down the voltage and thus the power along with it (since U*I = P, so if U drops, so does P). Now say I drop that circuit before the CV side of that Op-amp s**t I'm trying to build...what would happen ? :|

It is 1A constant current source, the output Voltage will vary to maintain the constant 1A through the load, so the maximum current flowing through that output current setting resistor will be at 1A max, the max load current will also be at 1A.
So basically if you put 1 Ohm load, the circuit will force 1A through 1 Ohm load so the Vdrops on 1 Ohm load will be 1V, if you put 10 Ohms load, the circuit will force 1A through the 10 Ohms load so the Vdrops on the resistor will be 10V. That is what Constant current power supply will do.

[Dannyx]

Re: UA741 op-amp power supply question

It is 1A constant current source, the output Voltage will vary to maintain the constant 1A through the load, so the maximum current flowing through that output current setting resistor will be at 1A max, the max load current will also be at 1A.
So basically if you put 1 Ohm load, the circuit will force 1A through 1 Ohm load so the Vdrops on 1 Ohm load will be 1V, if you put 10 Ohms load, the circuit will force 1A through the 10 Ohms load so the Vdrops on the resistor will be 10V. That is what Constant current power supply will do.

I kinda thought of that too. Say I put a fairly "hungry" load on it, it would theoretically bring the output to 1.25v (since that's the minimum ref. of an LM regulator). Anything to keep the current to 1A.

[budm]

Re: UA741 op-amp power supply question

You can short the output of that and it will still putout 1A, but you have to make sure you have heatsink on the LM317 because the Vdrops on LM317 x 1A on the LM317.
LM317 needs the diff between input and output of about 5V, need to look at the spec sheet again.

[Dannyx]

Re: UA741 op-amp power supply question

Ah now this is making more sense.

Your current sense resistor needs to be very low value, and your op amp's gain will compensate for the very low drop across it.

But I'm not sure you're "measuring" right here? Why would that current sense resistor be dropping 12V, or is it 12V with your actual load?

There are some fairly decent reference designs out there. I ended up building a CV/CC PSU based on a LM317T reference design with a TIP42 series pass transistor and TL082 op amp as the CC feedback. A 0.2 ohm resistor is the current sense to reduce the sense losses, and yes I needed a negative voltage to be able to shut off the LM317T to do proper current limiting. Attached is the schematic I used. I think one thing that could make this design better is that the voltage regulation zero reference needs to be also brought down to negative 1.2V or something (disconnect the voltage adjust variable resistor and its capacitor at GND and connect it to a -1.2V instead, probably regulated with two diodes in series perhaps, referenced off the CC negative supply).

Had a look at your schematic - WAY more complex that my design, must admit. Still trying to work it out - it's a mess in there ! Upon first glance, you don't seem to be using the Op-amp like I originally had in mind in my design. Seems to me you're using it to control that LM317 which in turn does the voltage control. I however was planning on using the op-amp to drive a transistor to do the CV part and was still struggling to figure out how to do adjustable CC. Can't figure out how to do it without having voltage drops...

[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).

[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)/