LF411 Doubt - Page 1
Forum section: electronics / projects

author: Sarvesaa
author_rank: Contributor
timestamp: April 25, 2023, 07:36:29 am
content: Hey,I am a beginner and I am designing a linear power supply. I wanted it to based on HP3610A. They used LF411 OP amps which are obsolete nowadays.I want to use TL081 OP amps instead of LF411. The doubt is what does the OP amp do in the circuit. The Gain of it and configuration of it.Can anyone help me with this.

author: moffy
author_rank: Super Contributor
timestamp: April 25, 2023, 08:00:19 am
content: Your snippet has left out a lot of essential detail:https://neurophysics.ucsd.edu/Manuals/Hewlett%20Packard/Hewlett%20Packard%20Operating%20and%20Service%20Manuals%20for%20HP361XA%2030W%20Bench%20Series%20DC%20Power%20Supplies.pdfsee Figure A-8.It in association with the series pass transistors appears to control the output voltage, as it states on the schematic "Voltage Error Amplifier".

author: Swainster
author_rank: Frequent Contributor
timestamp: April 25, 2023, 08:13:02 am
content: Yes, the snippet confused me too. I scribbled on it so that it makes a bit more sense:Looks like LF411 are still technically in production?However, I'd certainly give TL081 a try - the specs seem close enough, maybe a little noisier and less precise, but not far off. Can probably tweak the feedback a bit if the TL081 is too fast (oscillates).

author: Sarvesaa
author_rank: Contributor
timestamp: April 25, 2023, 08:15:39 am
content: I want to know what is the configuration and the gain of the OP amp.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 25, 2023, 11:04:17 am
content: This is my non analog guy understanding of the circuitR15 (14.3K) is basically a constant current source as the voltage across it is fixed to 5V. You can ignore the diodes as they are just for clamping transients - they should have close to 0V across them in normal operation. By the usual op-amp magic, no (very little) current flows into the op-amp inputs, therefore in order for the positive and negative inputs to be equal, the exact opposite current must flow through the 50K pot. The voltage across the pot is basically the output voltage, so the result is that the output voltage goes from 0V when the pot is at 0 ohms, to 5V*50K/14.3K = 17.5V when the pot is at the 50K end.It may help to think of it as the negative side of the output being fed back into the non-inverting input of the op-amp i.e. becomes negative feedback.NB, I can't work out how the voltage range is limited in high current mode, so this explanation could be total BS

author: tooki
author_rank: Super Contributor
timestamp: April 25, 2023, 11:32:56 am
content: Quote from: Sarvesaa on April 25, 2023, 07:36:29 amThe capacitor in series in the feedback loop means it’s an integrator.P.S. You copied the schematic wrong: the resistor you have going from the diode anode to ground (which should be Vsense) is supposed to be between the inverting input and Vsense, with the diode anode also at Vsense.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 25, 2023, 11:53:24 am
content: I was able to buy new old stock National Semi LF411’s from a US eBay seller for several 36xx supplies I was repairing. The LF411 and the supply pico fuses, F1 & 2 were the only problems found. Others had mechanical issues like broken power switches which are still available.IIRC I did substitute a TL081 to test with as it had the same pin out. It appeared to function correctly but I did not test thoroughly since I was able to source original parts.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: April 25, 2023, 02:13:36 pm
content: This what I suggest:1.Redraw the HP circuit and sort the functions into 3 groups:Essential, protection, compensation2. Enter the essentials into your fav simulation tool, check for stability and phase margin, using TL081 or your OPA choosing3. Reapply some of the compensation techniques to see what they do to stability4. Reapply the protections, I’m sure this thread will be happy to comment which is relevant or not with the new OPA5. Do measurements on the real circuit - stability over full load range, impulse response, ripple rejection, constant U/I transition, stability in constant current mode6. Do stress tests - short circuit, input power loss with large capacitive load, inductive load. Don’t bother if you loose a few components.

author: switchabl
author_rank: Frequent Contributor
timestamp: April 25, 2023, 03:49:06 pm
content: Maybe I am missing something but TI lists the LF411 as "active" and seems to have plenty in stock (I think it's just the A version has been discontinued for a while).

author: macboy
author_rank: Super Contributor
timestamp: April 25, 2023, 06:53:06 pm
content: You drew the circuit wrong.You have the 50K pot and 1K resistor going to ground. They don't connect to ground. The +S node (connected to +OUTPUT) is used as a virtual ground. The negative output -OUTPUT isnot ground. It is always going to be some voltage below ground. Now consider that the +5V node in your circuit is actually the voltage reference. The circuit is a little confusing because everything is referenced to the positive output (the +S virtual ground) instead of the negative output. This arrangement gives great performance and very simple high-side current sensing. Simple once you wrap your head around it I guess.

author: exe
author_rank: Unknown
timestamp: April 25, 2023, 08:15:02 pm
content: https://octopart.com/search?q=LF411&currency=USD&specs=0

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 26, 2023, 12:36:00 am
content: LF411 was one of my favorite opamps for undemanding applications. Cheap, stable, JFET and clean. Great jellybean opamp.

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2023, 08:37:04 pm
content: Quote from: Conrad Hoffman on April 26, 2023, 12:36:00 amThe LF411 is the economy version of the others in that series. It is also available as a dual in the LF353. The TL071/TL081 is practically identical

author: Zero999
author_rank: Super Contributor
timestamp: April 26, 2023, 08:46:33 pm
content: Quote from: Sarvesaa on April 25, 2023, 07:36:29 amWho told you the LF411 is obsolete?The TI website says they're still in production and there are plenty available on Digi-Key.https://www.ti.com/product/LF411https://www.digikey.co.uk/en/products/detail/texas-instruments/LF411CP/1509946The TL071 has slightly lower noise, but the LF411 has a lower offset voltage, which is probably more important here.