Does my design make sense ? - Page 1
Forum section: electronics / projects

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: June 28, 2018, 08:53:07 am
content: Hi There,I am trying to develop one isolating circuit where...1. MCU control signals will operate a relay.2. Make a bridge with analog signals.3. Safe switching for relaying4. RC time constant is considaratble5. Diode action6. 6, and 8 number pin goes to stepdown transformer.Take a look the attachment and criticize what I did wrong.

author: spudboy488
author_rank: Regular Contributor
timestamp: June 28, 2018, 11:31:09 am
content: If I understand, in simple terms, what you want to do; you want the signals on the right to close a relay.1) You do not have voltage going to pin 16 of the relay. It will never close.2) The transistor side of the optos are all wired wrong. As wired, they will turn on and then (probably) burn out as you are just routing 3.3V to ground.3) You can eliminate Q1 and R2 and use the collectors of the 3 optos (tied to pin 1 of K2) to close the relay.4) Not sure what 120R (K2, Pin 4) is doing.5) Can't really comment on the driver side of the circuit without knowing what kind of signals are doing the driving.6) This looks like an OrCAD schematic. There are all kinds of connection errors. Pointer Snap To Grid is your friend.

author: wraper
author_rank: Unknown
timestamp: June 28, 2018, 11:52:51 am
content: Complete nonsense. In addition to previously said, ZD1-ZD3, D2, D4, D9 do nothing. No point in C1, C3. The rest of capacitors, and circuit in a whole is very dubious.

author: CopperCone
author_rank: Super Contributor
timestamp: June 28, 2018, 04:02:06 pm
content: Can you explain why you designed the circuit the eay you did.Im confused but people just saying stuff is dubious and unnecessary is irritating.. I want to know what you were thinking because it does seem bizzare at first glance.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: June 29, 2018, 07:57:47 am
content: QuoteThank you to take part in this chat. In a simple word, my MCU signal will control the relay.QuoteThere will be a connection relay port 1 and 4 ( will modify) together with the voltage applied from step down transformer.Quote

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: June 29, 2018, 08:02:56 am
content: Quote from: wraper on June 28, 2018, 11:52:51 amI do appreciate your response! When you make some comment you need to explain it more politely! Your tone looks crazy!Zener diodes are use to make constant 3.3v always across internal photo diode in isolators, explain why those diodes fails.Dont you see any filturing purpose for capacitors ?

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: June 29, 2018, 08:08:31 am
content: Quote from: CopperCone on June 28, 2018, 04:02:06 pmIt depends on how people see on different eyes.Its a 3 phase rectifiers control portion, take a look let and right side signals and previous description please.

author: wraper
author_rank: Unknown
timestamp: June 29, 2018, 08:10:52 am
content: Quote from: Md Mubdiul Hasan on June 29, 2018, 08:02:56 amZener diodes do exactly nothing because voltage is always limited to lower value by forward voltage of LED emitters in optocouplers. If voltage would ever reach a 3.3V so zener would start conducting, it would mean that optocoupler is already failed open and there is nothing left to protect. You already have caps before resistors but you use additional caps in parallel to optocouplers. And why do you need "filtering" with such big capacitor values at all? It's not a PSU power rail. I wrote "nonsense" because there was basically not a single thing done right, more like a random assemblage of components.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: June 29, 2018, 08:29:09 am
content: QuoteAs you said limited, the ZDs are also playing the role.Removing components will save money, but need to think a perfect design.

author: wraper
author_rank: Unknown
timestamp: June 29, 2018, 08:35:12 am
content: Quote from: Md Mubdiul Hasan on June 29, 2018, 08:29:09 amThey are not playing any role. If they will never conduct current, they do exactly nothing. I already edited my comment just before your post to add a bit wider explanation why.

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2018, 09:53:07 am
content: Quote from: Md Mubdiul Hasan on June 29, 2018, 07:57:47 amIt's likely the opto-couplers are unnecessary as the relays already provide isolation, from the device being controlled.The inputs could be connected to the transistors, via suitable base resistors, although that would make it so a high input turns the relay on, rather than a low input, as per the current configuration. If the relay needs to turn on, when the logic level is low, then an additional transistor could be used, rather than the opto-coupler, which would be much cheaper and easier.

author: spudboy488
author_rank: Regular Contributor
timestamp: June 29, 2018, 11:56:48 am
content: Quote

author: wraper
author_rank: Unknown
timestamp: June 29, 2018, 12:00:45 pm
content: Quote from: spudboy488 on June 29, 2018, 11:56:48 amThe issue is that optocouplers are generally too weak to drive a relay directly. LTV-816 are rated for 50mA only.

author: CopperCone
author_rank: Super Contributor
timestamp: June 29, 2018, 04:20:30 pm
content: Using relays and optocouplers improves isolation so long you have seperate supplies and drivers. Why not use two relays though? Not sure. Cost maybe. Systems diversification too.If you do inspection on relays it might save some labor not having to take an extra relay apart while also getting double isolation.But there are probobly more reliable single components that you can buy for increased cost. Maybe.

author: spudboy488
author_rank: Regular Contributor
timestamp: June 29, 2018, 05:40:33 pm
content: Quote from: wraper on June 29, 2018, 12:00:45 pmTrue but I was only using them for the purpose of explaining how to connect them. I probably should have removed the part numbers. Also, the relay that is in the original design (RY-12W-K) is a 12V, high sensitivity relay that should only draw ~12mA so the LTV-816 will actually work.

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2018, 10:38:27 pm
content: Quote from: spudboy488 on June 29, 2018, 05:40:33 pmBut there's no point in having the opto-couplers. They could be replaced with three BJTs in parallel, which will do a better job, as well as cost less.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 02, 2018, 02:24:15 am
content: QuoteA very useful point you have described. As long as I understand, relay input here is mismatched. If you are talking about transistor base resistor calculation, resistor value is halved so we MAKE SURE the saturation current is reached. In this case base resistor should be 1kOhm.What do you think?

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 02, 2018, 02:27:34 am
content: Quote from: spudboy488 on June 29, 2018, 11:56:48 am

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 02, 2018, 02:30:27 am
content: Quote from: CopperCone on June 29, 2018, 04:20:30 pmThank you for your suggesion, which point you think the isolation should be modarated ?

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 02, 2018, 02:40:56 am
content: Quote from: Hero999 on June 29, 2018, 10:38:27 pmThank you for taking part here. Let me assume that BJT could give proper isolation than optocoupler do. Need more explanation before I use them. Can you kindly recommend any document for this issue?

author: spudboy488
author_rank: Regular Contributor
timestamp: July 02, 2018, 11:36:48 am
content: QuoteSure. Everything can be done a different way. Maybe the circuit needs to have isolated grounds? Maybe the OPs boss said, "We have a glut of optos....use them". There could be some other design requirement that was not conveyed in the extremely vague original post. Based on the original circuit, you can only guess as to what the functionality is. If a full circuit requirement was given, we might find the whole thing can be done with a 25 cent PIC, a transistor, resistor, diode, and relay.

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2018, 01:19:11 pm
content: Quote from: Md Mubdiul Hasan on July 02, 2018, 02:24:15 amIt depends on the current taken by the relay. A general rule of thumb is the base current should be approximately1/10of the collector current and the base-emitter voltage is 0.7V.Quote from: Md Mubdiul Hasan on July 02, 2018, 02:40:56 amAre VCC and JD-VCC joined together anyway, i.e. do they share the same 0V/earth/ground connector? If so, the opto-couplers provide no isolation at all.The relay already provides electrical isolation, so the opto-couplers are superfluous.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 03, 2018, 04:44:57 am
content: QuoteCould you post a nearly similar circuit ?

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 03, 2018, 04:56:59 am
content: QuoteDo you agree how this user has calculate ?ftp://https://www.eevblog.com/forum/beginners/beginner-transistor-base-resistor-calculations/QuoteHow about consider 30-40 AC voltage applied from right side of original circuit? Does it make sense ?

author: spudboy488
author_rank: Regular Contributor
timestamp: July 03, 2018, 11:29:47 am
content: Quote from: Md Mubdiul Hasan on July 03, 2018, 04:44:57 amI can if you provide a better circuit requirement document.1) You have 6 inputs (along the right). What signal (or combination) is required to activate the relay? Just 1 being active? A combination? All?2) Is there a requirement to isolate the 2 sides of the circuit (opto-isolators)?3) What are the input signal levels? 3.3V? 5V? 100V? AC? DC?4) Is the relay specified in your original schematic the one you plan on using or was it just something you had the schematic footprint of? What load are you trying to switch?5) What is the reason for 120R?

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2018, 02:42:30 pm
content: Quote from: Md Mubdiul Hasan on July 03, 2018, 04:56:59 amYes, that's the correct procedure for selecting the base resistor.QuoteSorry, that doesn't make any sense. There is no mention of 30V to 40VAC in any of the schematics.If the control circuits to the relays need to be isolated from one another i.e. must have separate 0V rails, then yes, opto-couplers are needed, otherwise they're not and ordinary transistors will do.

author: CopperCone
author_rank: Super Contributor
timestamp: July 04, 2018, 12:14:40 am
content: I don't know what you mean, but basically if you had multiple transformers or power supplies for each barrier.Basically the idea is that the isolator is good for 4000VAC or whatever its rated for for like 1 minute. If you put 2 of them on seperate floating supplies (in series basically), it would have to fry through 2 isolators, but only in the direction of the isolators, if the isolator supply go to 4000V thats supposed to be isolated, you basically breach the circuit, so if its built right, you would survive twice as long or maybe a bit more since there would be voltage drop on that first charred 'shorted' isolator.Double isolation is sometimes specified as an industrial requirement.You would typically not put two isolators in a row though, industrially what would 'make sense' is to put one isolation between the motor sense and the control system, then another isolation between the control system and the operator keyboard. Putting two isolators between the control system and the keyboard would probably be less reliable because its more likely a single area of the PCB gets shorted for some reason like a dropped screw driver or something, and for the sake of saftey, you might get a ground connection, so the control system will relay some of the energy to ground if your lucky, to lessen the burden on the isolator. If you are going double isolation its typically for user safety, but it might come up with a complicated system topology.If you need to be so safe that you need more then 2 isolators, I would say that using a length of fiber optic cable, free air communication (like a little antenna in the PCB or laser) would be alot more safe, since you can easily make a gap that's say a foot long, which would offer insane isolation so long the fiber or chassis does not cake up with garbage that becomes conductive, unless you require triple isolation electrically for some reason, if you have like some kind of control system within a control system that uses sensitive sensors or something

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 06, 2018, 07:10:09 am
content: Hi Sir Boy488!Thank you to valuable information.Sorry for my late feedback, I am too much busy on other circuit since last week.Here we goQuoteIf you ask for any document, it will take long time. I think better to use block diagram for this application. If you can see other reply from potential engineers, things are confusing! Need proper understanding why and how we use this circuit.QuoteMy co-worker said, those are AC inputs of 30-40v, not sure if he misunderstood or not. If you look deeply to this original circuit you would see some phase inputs are used. I think those are occurs instantly in combination, all input meets in one relay.QuoteSure, It needs.QuoteSEE 1. If its DC, think 3.3v comming from MCU.QuoteThe relay I have posted is RY-12W. If you see the left side signals, those are DC outputs meaning that, load for lets say 20V 20amp.QuoteFor transistor biasing.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 06, 2018, 07:13:54 am
content: QuoteGive a lesson in detail.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 06, 2018, 07:28:39 am
content: Many thanks returns to you Sir!You are one of the best knlowledgeable person here!QuoteMultiple transformer I think.QuoteWell said. But, if you can draw some correction on this circuit on the basis of the theory, I could understand. Good point is the direction of the isolator, will it control through left or right signals?DC signals are on left.QuoteIn my case DC load should be sense, I think 1 isolator should this application. Can you imagine the Phase signal ?

author: spudboy488
author_rank: Regular Contributor
timestamp: July 06, 2018, 11:29:56 am
content: QuoteQuoteWhich is it? 30-40VAC or 3.3VDC?QuoteThat relay is only rated for 2Amp contact switching. You'll need to find a different one if you want to drive it directly.QuoteOh. I see you added as 12V power connection which is incorrect as well.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: July 07, 2018, 12:47:17 pm
content: Hi boy488,If you look at the design, left side signal does not contain any ground. Most probably those are AC signals generated from different transformers. 3 phases are available.The diodes I was adding were 1N4001~7.Suggest me a way to proper biasing for the transistor.If the relay works for 2 amps meaning that it would not switch the proper load if I imagine 20v 20amps.