Very basic concept my brain does not believe - Page 1
Forum section: electronics / beginners

author: watchmaker
author_rank: Unknown
timestamp: December 20, 2023, 12:26:02 am
content: Thanks. I am serious.Attached is a simple network problem. When I do KVL/KCL I do not get 4mA thru R4. It is counterintuitive to me that the 60Vdc does not provide current to R4. I want to presume R4 is in parallel with Vs2 as well as Vs1. But the sim and BB do not agree with me.When I breadboard it and sim it, clearly only 4mA is going thru R4. The voltage drops are not an issue for me. I know this is simple stuff.I even understand calculating the voltages and currents in a single transistor ckt. But the current thru R4 has me baffled.Why does Vs2 not send an additive current through R4?Thanks!Dewey

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 20, 2023, 12:36:22 am
content: As your circuit is drawn the ONLY thing that matters to R4 is V2 the 60 volt battery. Unless the DMMs placed to measured current have voltage drops across them. Or the battery has internal resistance. This would surprise me, but the weakness of simulations is that there are hidden "features" like this.In your breadboard and sim, what is the voltage across R4? What is the voltage across the two current meters? Answering these questions will give clues as to what is going on.

author: The Soulman
author_rank: Super Contributor
timestamp: December 20, 2023, 12:42:27 am
content: Don't know, I think something is fishy with that (terrible drawn) simulation.Also when bread-boarding, (multi)meters aren't perfect and introduce some resistance of their own, as do the board contacts and jumper wires etc.

author: The Soulman
author_rank: Super Contributor
timestamp: December 20, 2023, 12:47:55 am
content: Looking at it the numbers on the left side and on the right side do add up as they are not connected to each other.Reverse polarity on xmm6?

author: Fryguy
author_rank: Regular Contributor
timestamp: December 20, 2023, 01:03:58 am
content: Something ist very wrong here - in this circuit you will get 7.5mA thru R4 (xmm4) and 24mA thru R1 (xmm5) .XMM6 will give you 31,5mA - XMM1 and XMM2 are correct . XMM3 will show 60V .These are the values this will produce in real lifeBtw - XMM5 and XMM6 have their polarity reversed

author: pqass
author_rank: Super Contributor
timestamp: December 20, 2023, 01:04:34 am
content: Something is definitely off. XMM6 seems to disconnect left from right side.See my simulationhere.

author: Zenith
author_rank: Frequent Contributor
timestamp: December 20, 2023, 01:19:34 am
content: Assume that the ammeters have negligible resistance. Then everything connected directly to the anode of V2 will be at 60V.The right side of R1 will be at 60V and the left side at 36V, so there will be a 24 V drop across and 24mA of current through R1. XMM5 should show -24mA. V1 would be driven backwards.I'm not sure what the consequences are of driving backwards a power supply, or battery, or the V1 voltage source in the simulation are.

author: Fryguy
author_rank: Regular Contributor
timestamp: December 20, 2023, 01:27:49 am
content: Simple - if they are psu's the 36v one will eventually show reverse current , up to 60v and maybe release the magic smoke .If they are 100Ah Batteries he will get a nice stand-by charging current of 24mA towards the 36V set - and if he shorts the 1k resistor - this will release a lot of magic smoke or blow up the 36V batteries in the very same second depending on the cables used

author: RFDx
author_rank: Regular Contributor
timestamp: December 20, 2023, 08:14:57 am
content: Works here as expected, no matter what meters or probes I use. The software also shows that the 1k and 8k resistors blow up if you leave their power rating at the default value of 0.25W.

author: The Soulman
author_rank: Super Contributor
timestamp: December 20, 2023, 08:46:00 am
content: Quote from: RFDx on December 20, 2023, 08:14:57 amI'm not familiar with this software, are those current meters really polarity dependent?If you reverse polarity on your xmm2 does it produce the same result as the op has?

author: watchmaker
author_rank: Unknown
timestamp: December 20, 2023, 01:05:18 pm
content: Quote from: Zenith on December 20, 2023, 01:19:34 amThat is how I analyzed it. It was when I checked my result with multism that it all went south. Guess I need more practice with multisim. No idea what went I did with the BB; could have been furstration and tired.THANKS!

author: watchmaker
author_rank: Unknown
timestamp: December 20, 2023, 01:21:31 pm
content: Just redid the drawing the simple way (all to ground) and I verified my results. Thank you.You guys are great, no one called me an idiot!The disadvantage to self study is that you do not have others around you to learn from. The Boylestad books are great for examples and I am getting good at redrawing networks. With Multisim, the lack of printed solutions "should" not be an issue. But I am learning MultiSim on the fly (did order some recent books) as well as matrix algebra.All of this increases my appreciation for what you all learned and use every day. Thanks for helping.Regards,Dewey

author: RFDx
author_rank: Regular Contributor
timestamp: December 20, 2023, 02:18:41 pm
content: Quote from: The Soulman on December 20, 2023, 08:46:00 amThe meters are not polarity dependent. Connected "the wrong way" they just show you a negative voltage/current value.

author: The Soulman
author_rank: Super Contributor
timestamp: December 20, 2023, 02:58:43 pm
content: Quote from: RFDx on December 20, 2023, 02:18:41 pmThanks, that is what I was expecting, but if that is the case, what was wrong with the original simulation?

author: RFDx
author_rank: Regular Contributor
timestamp: December 20, 2023, 05:37:37 pm
content: Quote from: The Soulman on December 20, 2023, 02:58:43 pmThe voltage displayed by XMM3 (32V) clearly shows that the burden resistance of the XMM4 current meter was changed somehow to 7 kOhm and therefore the current in question (4mA) was indeed correct but did not match the calculations the OP did by hand. XMM5 and XMM6 are also fishy but I can't tell in what way the settings of this meters were altered.

author: ledtester
author_rank: Super Contributor
timestamp: December 20, 2023, 09:43:58 pm
content: Quote from: The Soulman on December 20, 2023, 02:58:43 pmI think it's just a bug in Multisim.Note that the current through XMM6 is 355 pA and all of the other results are correct if you assume there is a break where XMM6 is.I bet initially there was a break at XMM6 and then you added XMM6 and then Multisim goofed. If you can replicate this I'm sure NI would be interested in knowing about it.

author: Zenith
author_rank: Frequent Contributor
timestamp: December 20, 2023, 11:02:03 pm
content: Quote from: watchmaker on December 20, 2023, 01:21:31 pmNot at all. I never had the impression you were an idiot, or I wouldn't have posted to the thread.Problems you are stuck on are interesting, at least looking back on them. It could be a maths problem. it could be a software problem. It could be something else. Preparing to explain them to someone else might make the solution obvious. In the act of explaining you might see the fallacy, or the person you were explaining it to might spot it straight away. A fresh pair of eyes.I was wondering what would happen if V1 and v2 cathodes were connected together and the anodes were connected through a 1K resistor, so V1 was driven backwards. Would the simulation cope with it or would it produce undefined results?

author: Fryguy
author_rank: Regular Contributor
timestamp: December 21, 2023, 12:03:33 am
content: Quote from: watchmaker on December 20, 2023, 01:21:31 pmGlad we could be of help ! Keep it up - the future needs people like you when we (the ripe stuff) are gone . . .

author: watchmaker
author_rank: Unknown
timestamp: December 21, 2023, 12:33:22 pm
content: One very good outcome is that I busted my ass(up till midnight) figuring out why I did not like Pspice for TI. I reloaded it twice before I realized and found the modelig application to set up independent voltage sources.I will add more in my post about Multisim vs TI for future readers.Regards,Dewey