Please help me understand voltage in this circuit - Page 1
Forum section: electronics / beginners

author: rthorntn
author_rank: Frequent Contributor
timestamp: June 30, 2025, 01:25:05 am
content: ### perpetual electronics noob alert ###Hi,I'm trying to scope an issue with a clock pulse circuit (NLC Cellular Automata eurorack module) with my Rigol DS1102E (out of interest should I:- dust off, relocate and use my 13kg Tektronix 2465B instead?), I can't scope this pulse, I can see the square wave, so I want to try and recreate most of the circuit in Falstad so I can visualise my nemesis.I'm putting in a 4V square wave so that is +4V to -4V so 8Vpp, why is it showing 6.381V after the 100p cap and 10K resistor?Thanks.Richard

author: ledtester
author_rank: Super Contributor
timestamp: June 30, 2025, 02:16:18 am
content: Here's a simpler circuit which shows the same behavior:(link to Falstad)The max and min really should be +/-8V but Falstad shows 6.38V due to the simulation timestep.Try changing the simulation timestep from 5u to 1u (Options menu -> Other Options -> Timestep size) and you'll see the max and min increase to 7.615V.

author: ledtester
author_rank: Super Contributor
timestamp: June 30, 2025, 03:07:02 am
content: Quote from: ledtester on June 30, 2025, 02:16:18 amThe RC time constant is 10us (calculated from 100pF*100kR).Code:SQWAVE --- CAP --- RES --- GNDA BThe square wave stays at 4V for several milliseconds and this allows the cap to charge up to nearly 4V. Hence the voltage at A is 4V higher than the voltage at B, i.e. V(A) - V(B) = 4.When the square wave generator flips to outputting -4V then the voltage at B becomes -8V because V(A) - V(B) = 4 and V(A) is now -4.But due to the simulation timestep, Falstad only shows the voltage at B after 5us so it reports a voltage which is a little less than -8V. With smaller timesteps the extreme values at B get closer and closer to +/-8V.

author: rthorntn
author_rank: Frequent Contributor
timestamp: June 30, 2025, 06:08:26 am
content: Thanks, this is fantastic!So I found my 40usec pulse entering NON INV B (at 200usec/div) but it isn't being output by my TL072C, all I can see on OUT B is -12V.INV B is a constant 1V.So now I'm trying to get my head around what this means, could a batch of TL072's exist (I've tried two from the same strip) that won't switch to outputting a +12V pulse with a 6V 40usec pulse being received on NON INV B?

author: PGPG
author_rank: Frequent Contributor
timestamp: June 30, 2025, 09:26:11 am
content: Quote from: rthorntn on June 30, 2025, 06:08:26 amYou are trying to use OpAmp as comparator.OpAmp is designed to operate all the time with Vin- = Vin+.When you do not ensure correct feedback and allow the inputs to be not equal one to the other OpAmp output goes to one of its limits - this time -12V and probably some transistors inside OpAmp saturates. I don't know how long OpAmp needs to go out of saturation but it can be longer than you expect.Comparators are designed to operate with Vin- != Vin+.

author: rthorntn
author_rank: Frequent Contributor
timestamp: July 02, 2025, 05:26:24 am
content: Going to a 470p worked for a +5V clock.220p gave me the attached scope plot, trying to escape the -12V.The 100p specced didn't even break the surface of the -12V.I have clocks that are under +5V that I would like to use...Maybe go to a 1000p or start playing about with different pin compatible opamps...https://www.eevblog.com/forum/beginners/difference-between-tl072-tl082-in-a-comparator-circuit/

author: Terry Bites
author_rank: Super Contributor
timestamp: July 03, 2025, 07:50:39 pm
content: Opamp woes. See end of reply.A small cap will form an RC differentiator which will (if too small) take a lot of energy out of the pulse and distort it.An effective coupling circuit should transfer as much of the input energy as possible into the resistor. A small capacitor cant hold much energy.[A rectangular pulse can be thought of as a combination of sine (cosine) waveforms.A filter will kill off the higher frequency ones. The RC input is a basic high pass filter.]https://www.electronics-tutorials.ws/rc/rc-differentiator.htmlLook familair?To maintain the waveform integrity the time constant of this circuit needs to at least 15x the pulsewidth (t=0.69RC). So @300Hz with 100k, the C will need to be >24n, 100n would be practical value.Lots of opamps don't want work as comparators, text books lie, teachers lie! Lots of web tutorials still repeat the same old drivel.Some older types may do but most modern ones definitely do not. Some oldies that will work: TL0xx, LM324, LM358 and the low life of opamps the 741.I'd ditch the opamp and diode in favour of a made for purpose comparator in practical circuit. eg LM393.I might even go against my principles and suggest using a 555.

author: rthorntn
author_rank: Frequent Contributor
timestamp: July 03, 2025, 09:31:32 pm
content: Thanks all.It's working - I just replaced the Tayda TL072 with an ST Micro TL082 from Mouser that I had laying around, it's like night and day, it clocks as low as 1.3V now