741 op amp for attenuation - Page 1
Forum section: electronics / beginners

author: ChiroZi
author_rank: Newbie
timestamp: May 01, 2023, 11:22:18 am
content: Hi all.I have a sketchy circuit where i want to use a potentiometer with a 741 op amp to attenuate the entire circuit down to 0V.The circuit currently works, (but i have a slight offset that i would like removed. but most of all i want to be able to explain what i am doing with calulations to show my choices here make sense. I am having some trouble finding the right information for this setup.i know the 555 timer circuit can probably be improved and it might be drawing too much current through the 555, but i just want to focus on everything after the 555 output.many thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: May 01, 2023, 11:34:19 am
content: Comes from:https://www.eevblog.com/forum/beginners/555-timer-setting-frequency-and-pulse-width/Readhttps://xyproblem.info/and see if that fits your situation.The reason why I'm calling this out as an XY Problem is you haven't given us the 'big picture' e.g. what is the desired final output or effect and what is it used for? You also are still showing us snippets of the circuit, in this case we need to know what the 741 output is connected to."Sketchy" is an excellent description. Leaving aside any criticism of the 555 section*, I wouldn't expect a 741 in inverting configuration, powered through a 42K resistor (single supply, no decoupling) , with gain setting resistors an order of magnitude lower and its +in grounded, to do *anything* useful or predictable.* 555 section:My opinions of it aren't fit for polite company to hear!You should know better - see your previous topicFor simulation its unnecessary to show us cr@p like that - it could all have been replaced with a pulsed voltage source

author: ChiroZi
author_rank: Newbie
timestamp: May 01, 2023, 03:53:39 pm
content: The circuit is simply just for learning by generating a pulse width of 500 micro seconds, with a frequency of 10Hz and the ability to attenuate the output with a potentiometer. The output going off screen is simply going into a multimeter and an oscilloscope so thats as far as i need it to go, i need a measurment after this point.Yes i could have done it differently and used a digital circuit But i didn't, i did it this way. I'm hoping to learn something about the mistakes i am making and i don't mind criticism but telling a noob it is "cr@p" and to look at a post saying basically what i am asking is stupid is quite rude? we all start somewhere and i can understand if you spend a lot of time answering question on here it can get frustrating hearing the same silly questions, but if it irritates you then please don't bother, wasting my time and your own.

author: Ian.M
author_rank: Super Contributor
timestamp: May 01, 2023, 05:26:45 pm
content: I thought you'd already got the pulse source figured out in your previous topic. I was trying to make the point that if you are working on a design for the attenuator circuit, there's no need to simulate the internals of the pulse source, let alone post one that you *know* isn't satisfactory. Part of the art of effective simulation is knowing what you can simplify or leave out.With that out of the way, we are getting somewhere, as we know the required result and the load it has to drive. Unless I'm misunderstanding you, you want a variable amplitude positive going 0.5 ms pulse, with frequency 10 Hz, and it doesn't have to drive a large load (a scope input is typically 1 Meg || 15 to 40 pF).You are aware that the OPAMP configuration you are trying to use is inverting? That means that when the 555 output goes high, the OPAMP is trying to drive its output below ground, but cant because it doesn't have a negative supply powering it. Add a negative supply and it will produce an output pulse, but not what you want.You should reconfigure the 555 to produce a 0.5 ms low time pulse (as it can do 99.5% duty cycle far far better than it can do 0.5% duty cycle), and invert the output with a small signal PNP transistor(as suggested by WattsThat and modified to PNP by myself in your previous topic), with the potentiometer track as its load, between its collector and ground. That also gives you more output swing than a NE555 (or other bipolar 555 clone) can provide on its own, as the PNP will pull up to within a couple of hundred mV of the supply, but the 555 has two Vbe drops in its high side output structure (look at a transistor level internal schematic for it) so can only drive high to a level well over a volt below Vcc, worse if there is significant output current (expect around 2.7V drop sourcing 100mA!)You can now take your variable amplitude pulse from the pot wiper, then buffer it so it can drive reasonable loads. For this you *could* use your 741 OPAMP, configured as a unit gain buffer (input to +in, -in connected to output), but its not going to be capable of rail to rail operation as the 741 isn't a RRIO OPAMP. That means the output wont go all the way down to ground unless you give the OPAMP a negative supply. It doesn't need *much* negative supply, -3V would be sufficient, but that's a PITA to provide. Of course you could substitute a single supply OPAMP with an output that can go right down to ground, but I suspect you don't have one handy, especially not one for >12V supply, that doesn't have annoying input common mode voltage range limitations, or at least is free from annoyances like phase inversion if you exceed the input common mode range.An cheap and easy fix for the failure to swing down to ground is to use a NPN emitter follower instead of an OPAMP, and lift the bottom of the potentiometer track by one diode drop to roughly compensate for the Vbe of the transistor. It will also need a load resistor from the emitter to ground to pull it low - lets make that 1K.The transistors are nothing special. Whatever TO-92 package small signal 'jellybean' parts you have should work OK, as long as you don't mix up the NPN and PNP devices! Any small silicon diode will do for D1. The potentiometer doesn't have to be 10K - if you've got a 12K one as shown in your schematic, that will be fine. I wouldn't recommend under 5K or over 20K there. If you want the frequency and pulse width to be reasonably stable, the timing capacitor needs to be reasonably good quality, fairly close tolerance (+/-10% would be suitable) and low leakage, which makes electrolytics and ceramics a poor choice. Any sort of metallised plastic film capacitor will do nicely, though if its too far from 1uF you'll need to change the fixed timing resistors to get enough adjustment range to bring the pulse back into spec. N.B the NE555 model in my simulations is near ideal, and the timing resistor values (in black) on my schematics, while close wont be exact for a real NE555 even if the timing capacitor is 'spot on' hence my suggestions (in blue) for the series combo of an E12 standard value fixed resistor + a trimpot, for each.ZippedLTspicesimulations attached.Edit: corrected a couple of typos + expanded explanation of the inverter and buffer sections of the circuit.

author: srb1954
author_rank: Super Contributor
timestamp: May 02, 2023, 07:39:40 am
content: Quote from: Ian.M on May 01, 2023, 11:34:19 amInterestingly, while the 741 has a high value current limiting resistor in its supply rail, the LED is connected directly across the battery with no current limiting resistor. The lack of any activity from the 741 will be made up for by the very bright, but short, burst of light from the LED.It is amazing the truly stupid things you can do with a circuit simulator if you just hook components up willy-nilly without having any basic understanding of how those components work. However, the saving grace of a circuit simulator is that components won't explode of catch fire like they would in a real circuit

author: Ian.M
author_rank: Super Contributor
timestamp: May 02, 2023, 09:24:43 am
content: Good point.Even a humble PP3 9V alkaline battery typically can deliver about 2A (initially) when short circuited, and can easily get hot enough to melt its battery clip! If the LED doesn't self-disassemble the circuit certainly wont work as the failing LED will clamp the voltage too low for the OPAMP or 555 to work. Meanwhile the LED's busy letting the magic smoke out, and melting a hole in the breadboard or burning the PCB!If the O.P. is using a 6x AA alkaline battery pack, the probability of fire is much higher, as their initial short circuit current can exceed 10A. It doesn't take long for thin hookup wires to ignite if you try to put 1A through them!In LTspice, its easy to check the instantaneous dissipation of any component: Alt click it to add a trace with its calculated power dissipation, or alt double-click it to replace all traces in the current plot pane with its dissipation. Its worth checking what tools *your* preferred simulator has for plotting dissipation, as real world parts are far less forgiving and cant be 'un-burnt' for another run by mouse click!Also, if the circuit smells like something is overheating, *DONT* try to locate the part that's getting hot with your finger tip as when you find it, it will hurt!