I'm trying to understand this schematic diagram and how it works - Page 1
Forum section: electronics / beginners

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 13, 2021, 09:11:45 pm
content: This is a schematic diagram of an old circular knitting machine.As a beginner I want to go through it, in order to understand how it works.First of all, what is the purpose of these two diodes across the relay coil (R5B) (witch is 24VAC)? And how does the circuit work?

author: Terry Bites
author_rank: Super Contributor
timestamp: December 13, 2021, 09:23:32 pm
content: The diode across the coil stop high voltages deveoping when it is de-energised.https://electronicsarea.com/why-is-there-a-diode-connected-in-parallel-to-a-relay-coil/2x6A might seem crazy but they are doing another job as rectifiers in the PSU via F4. I Imagine there are two of them for redundancy because are in the safety loop.

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 14, 2021, 06:55:22 am
content: Thank you..The relay coil is a 24VAC, does it need this diodes?And can it be energized with just a half wave without capacitor?

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2021, 09:41:51 am
content: Please attach it, rather than uploading it somewhere which isn't trusted by the firewall connected to the network I use.My guess is the diodes are transient voltage suppressor diodes. Like an ordinary diode, it will conduct when forward biased and block current, when reverse biased, like an ordinary rectifier, but when reverse bias voltage exceeds a certain threshold, it will conduct. The idea is the voltage across the coil will be limited to a safe level to protect the switch, be it a relay, or semiconductor such as a TRIAC.https://www.allaboutcircuits.com/technical-articles/transient-voltage-suppressors-tvs-an-introduction/

author: rstofer
author_rank: Super Contributor
timestamp: December 14, 2021, 04:26:18 pm
content: The diode may also be used to retard the opening of the relay contacts by slowing the collapse of the magnetic field. I don't know whether this is the reason but there are other relays that don't have diodes so it wouldn't seem that voltage spikes were the primary concern. It would only be a short time delay but it is noticeable.https://electronics.stackexchange.com/questions/100134/why-is-there-a-diode-connected-in-parallel-to-a-relay-coil#:~:text=The%20reason%20it%20is%20slow,that%20energy%20is%20%22spent%22.Is there something physically different about this particular relay?

author: Steffalompen
author_rank: Contributor
timestamp: December 15, 2021, 02:22:54 pm
content: There are many interlocking dependencies here, my guess is (if they really are what they seem) they are there to inhibit another event.Somehow the coil must hold with just half-wave, or T4K won't have time to kick in and release the brakes. If not for that function I would consider buzzing as a purpose. Breaking the start-latch which is another purpose would only require an impulse.It is very strange.

author: m k
author_rank: Super Contributor
timestamp: December 15, 2021, 06:08:10 pm
content: There's also that "photoc." switch.Is it some kind of a stop and go effect enabler?Being knitting and all.

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 16, 2021, 07:07:32 am
content: QuoteOk, done!QuoteNo, they are just normal rectifiers:https://www.tme.eu/en/details/by550_100/tht-universal-diodes/diotec-semiconductor/by550-100/

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 16, 2021, 07:12:31 am
content: Quote from: Steffalompen on December 15, 2021, 02:22:54 pmNo, this is the relay:https://www.ia.omron.com/product/item/6297/

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 16, 2021, 07:15:08 am
content: Quote from: Steffalompen on December 15, 2021, 02:22:54 pmI'm sorry, I didn't understand it well.

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 16, 2021, 07:18:50 am
content: Quote from: m k on December 15, 2021, 06:08:10 pmThe photoc. doesn't exict in my machine, so it's not important for the relay to do its job.QuoteThe relay breaks the latch, so the machine stops, and brakes.

author: m k
author_rank: Super Contributor
timestamp: December 16, 2021, 09:18:22 am
content: R4L is brakes, R5B is its enabler, like it is for Micro Gate.Your missing Photoc. doesn't change the reason why those diodes are there.

author: SteveyG
author_rank: Unknown
timestamp: December 16, 2021, 10:54:05 am
content: Quote from: Shaydzmi on December 14, 2021, 06:55:22 amThe diodes are forming a half wave rectifier. When they are conducting, it's supplying power to the elements at the bottom.

author: Steffalompen
author_rank: Contributor
timestamp: December 16, 2021, 02:44:53 pm
content: Why would you need to rectify it?

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 16, 2021, 03:38:15 pm
content: Quote from: Steffalompen on December 16, 2021, 02:44:53 pmI don't need to rectify it, it's already an existing circuit, I'm trying to understand its work.

author: Steffalompen
author_rank: Contributor
timestamp: December 16, 2021, 06:05:28 pm
content: Shaydzmi, Yes, I was asking SteveyG.And I can't really clarify my post yesterday any better. At least not without any specific feedback.

author: m k
author_rank: Super Contributor
timestamp: December 17, 2021, 09:41:26 am
content: Nasty thing, must be a port from steam world and maybe I've had it all wrong but anyway.Those bottom switches seems to be limit switches.So when Upper Stop is active the coil R5B is half on and lamp is other half on and Line side can go forward.If SW3 is closed then other functions can also use R5B.Then after TEKEL TC delay(?) L4 is turned off and Neutral side can go forward.After all Micro Gate switches have changed their positions the lamp L5 goes off and machine is ready to go.Then Inching and Start can enable the Drive Motor through C3S and R3M.I don't know how T4x are different but they are clearly the main Drive Motor switches.The whole thing seems to operate through a diode above L6.Positive half cycle lights the lamp L6 if something is on and negative uses the relay R5B.Rosen Unit can bypass other functions but its switch is still behind that diode.Same with Iro For Lycra R7F but it is from positive side and just lights the lamp.It's knitting.Is it a free roller?L4 is called Rev. Counter.Is TEKEL TC a pulser?

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 18, 2021, 06:28:23 am
content: Quote from: m k on December 17, 2021, 09:41:26 amYes, It is a revolution counter with a preset.You are getting closer to understand this machine.. wow..

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 20, 2021, 06:48:17 am
content: Does the coil R5B get positve wave from wire 228, or it's been bypassed by the two diodes?

author: Steffalompen
author_rank: Contributor
timestamp: December 20, 2021, 09:45:22 am
content: Exactly the black magic some of us have been trying to rationalize in different ways.Would you mind measuring their forward voltage?

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 21, 2021, 10:33:47 am
content: QuoteAfter removing them from the circuit, each one measures:0.5 vapproximately.

author: vu2nan
author_rank: Contributor
timestamp: December 21, 2021, 10:50:53 am
content: Hi Shaydzmi,When a fault feedback switch closes, half-cycles of current flow through the indicator lamp and opposite half-cycles through the coil of relay R5B as shown.The NC contact of R5B shuts down the drive motor and it's NO contact energises the brake through a timer T4K and relay R4L.The indicator lamp is on for the operator to take corrective action.Sharing of half-cycles is observed in the fault indicator / trip circuits of these circular knitting machines.Lamp life would certainly increase. Any other reason for adopting this method is not quite apparent.

author: Zero999
author_rank: Super Contributor
timestamp: December 21, 2021, 11:35:14 am
content: Quote from: Shaydzmi on December 21, 2021, 10:33:47 amThat's a little on the low side, but acceptable, if the test current is very low.More importantly, do they block in the reverse direction, as expected?

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 21, 2021, 01:21:26 pm
content: Quote from: vu2nan on December 21, 2021, 10:50:53 amHi vu2nan!Thank you for this explanation.. It realy helped me.QuoteDo you think that there is other methods to do the same job? Bearing in mind that the switches my take up to 10amp if all of them are closed.

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 21, 2021, 01:23:38 pm
content: Quote from: Zero999 on December 21, 2021, 11:35:14 amYes they do block in the reverse direction..

author: Steffalompen
author_rank: Contributor
timestamp: December 21, 2021, 09:54:47 pm
content: Quote from: vu2nan on December 21, 2021, 10:50:53 amRight, but therein lies the problem.An impulse is enough to break the start/run-latch, but T4K likely needs a relatively long time before it actuates (a second perhaps?).My main question is will R5B coil hold steady (until T4K is finished) with just half the waves?I thought the reason for the diodes was to stop voltage going down the chain of dependency mechanical triggers in the schematic lower Right, or the lamps associated with them, since a diode is pointing the other way from there.But a new thought occurs, -if they could only get a DC coiled relay with those switches, perhaps they needed to botch it by half wave rectifying the signal for it?It would be very interesting to know what happens with 24V AC on that coil. (It seems to be current limited through the lamps, so beware)

author: vu2nan
author_rank: Contributor
timestamp: December 22, 2021, 11:02:22 am
content: Quote from: Shaydzmi on December 21, 2021, 01:21:26 pmHi Shaydzmi,The standard method would be thus:Nandu.

author: m k
author_rank: Super Contributor
timestamp: December 22, 2021, 06:52:29 pm
content: Quote from: Steffalompen on December 21, 2021, 09:54:47 pmI'd say that the machine can't operate if not.The thing is like sort of DC controlled.Lower left has Air-Blowers behind C4G and lower right has Quantum Sens. behind C4E.So R5B stops the machine when all of those low side switches are open and X MPF connector 8 is also not connecting.Normally a switcher right from TEKEL TC is stepping the process forward and the thing is a free roller, maybe slow but still.

author: Shaydzmi
author_rank: Regular Contributor
timestamp: December 29, 2021, 07:14:19 am
content: The machine is working perfectly.I tested the relay with a half-wave and it works fine.