Hello, I have a relay based circuit as shown below and the full schematics is in the attached document.
The circuits based on PC375 relay, how does it work?
Why they need all of those diodes and resistors?
There is no spice model so i am having trouble to know if this acctually will work. https://pickercomponents.com/pdf/Relays/PC375.pdf
Hello FVM. Pin 1 and pin 16 could be switched in the role of who is getting the hight voltage?
The goal is to make pin 4 connect to pin 6, maybe switching the 12V location will not make the pin 4 connect with pin 6.
You need to read a book or something. If you don’t understand the basic operation of a relay and a flyback diode, you’re certainly not going to understand the rest of the circuit. But you seem to be willfully ignorant. You’ve been on this board long enough, yet your questions here seem to show a complete lack of any knowledge of electronics. Maybe you’re just messing with us.
Hello FVM, regarding the SCR.
I have a comparator U6A it has +12V -12V(althought its not painted in the schematics)
This comparator between -3.52V and -0.74V is open drain so it can only lower the output voltage.
-0.74V is higher so we get on the output -12V which is going to the gate of the SCR.
If we look at some schmatics of an opamp we have a connection of the output to negative voltage source.
Why it cant draw current in that way?
What do we need to add to U6B to make it function properly?
can my SCR be opened with -12V?
I tried to read the datasheet and i am not sure its possible.The photo below fro the data sheet says its not.
Is it correct? SCR datasheet is attached.
Thanks.
It is not correct. This design has no power sequence spec so your 3 diode logic shutoff design cannot work and the dual relay design has many problems with this choice.
The SCR turns ON with Vgt= 0.7V, Igt<= 200 uA with 10 ohm load, then it latches ON. You must turn OFF +12V to reset your SCR's. Bad choice of logic.
Also -12V must turn ON before +12V to SET, to activate the Open Collector to shunt the Vgt gate voltage.
You have previously stated a high-power FET design with a voltage sequence for RF ON and OFF. That would be a simple Mealy-Moore logic design. Here you are using dual relays, SCRs as memory instead of Flip Flops or dual gate registers.
Your analog choices use special parts as simple diodes. are poor for precision voltage references. Then you have a precision programmable Zener just being used as a forward diode.
e,g, U7 a Programmable Zener used only as a forward diode with a pot to tune it to 600 mV.
e.g. using D16 is wrong and inverted on the schematic. It's a unipolar TVS diode but only using it as a forward diode as a negative V reference - 0.74V.
Dry contact switches create a lot of noise both in contact surge currents ON and inductive coils switched from both ends but only applying one diode clamp to the opposite rail being switched. The SCR's are useful as power shutdown safety latches but not for timing logic.
I agree with Barry. This is a mess and you need to learn the Art of Electronics (Horowitz) and understand component characteristics before using them. A much simpler quicker process is to create functional design specs for each block diagram you make with timing and tolerances. Break the system down into smaller blocks until understood. But make sure you have good design specs to validate each result (DVT) after testing.
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It seems you think the trigger is to apply -15V to the gate. This is wrong. That is the Fault Region . It is like switching -15V to an NPN Base with emitter grounded.
All you need is 0.7V to drive an SCR or more with a current-limiting resistor because the input is a PN junction. (200 uA @ 10 ohm load)
If the load is less than 6 mA, it may not operate as an SCR and latch, but cutting off the current is how it is unlatched or with an AC on the load at the zero-crossing current below the Ih.
The scr logic makes sense if it's intended as latching overvoltage/overtemperature circuit, no matter that the feature can be implemented more effectively. There are however obvious schematic errors, so it may be hopeless to read sense into every detail. I have e.g. problems with U6A comparator path. D16 is drawn as if it would be a 0.74 V zener, but such a part doesn't exist and there's no SZ1SMA1 available, lowest breakdown voltage of this TVS series is 5 V. Designer might have decided to use a silicon diode in forward direction as rough 0.7V reference, but forgot to correct the schematic. As shown, comparator trips if -3.52 V falls below -0.74 V. If any of the SCRs latches, relay LS2 is actuated and shuts down upper circuit part through N.C. contact.
The scr logic makes sense if it's intended as latching overvoltage/overtemperature circuit, no matter that the feature can be implemented more effectively. There are however obvious schematic errors, so it may be hopeless to read sense into every detail. I have e.g. problems with U6A comparator path. D16 is drawn as if it would be a 0.74 V zener, but such a part doesn't exist and there's no SZ1SMA1 available, lowest breakdown voltage of this TVS series is 5 V. Designer might have decided to use a silicon diode in forward direction as rough 0.7V reference, but forgot to correct the schematic. As shown, comparator trips if -3.52 V falls below -0.74 V. If any of the SCRs latches, relay LS2 is actuated and shuts down upper circuit part through N.C. contact.
I agree, and realized the same and expressed the same issue as reversed. I got my initial experience from reading every design example in electronics journal in the Eng library before graduating my BSc then read every National Semiconductor design databook before attempting any serious design and still made newbie schematic errors checked by another Sr. Engineer in my 1st yr. More experience reading how to design can be learned from reading Test Equipment manuals with Theory of Operation and partial schematics. I also suggest you learn how commercial designs are done such as in Test instruments, TV's etc rather than searching the web full of bad examples until you can recognize the difference. You will forget 99% of the details you learnt in undergrad but hopefully will have learned how to learn easier and faster with exponential experience.
Hello FVM,U6a is a comparator in its data sheet shown below they say that we need a pull up resistor in order for the device to function.
In the diagram in the end the output of the comparator is open collector NPN.
Pull up resistor helps to pulls the voltage up because we connect the Vcc to the output.
In my case the of U6A minus voltage is larger then plus voltage ,so the output is -12V.
How pullup resistor will help me reducing the voltage to -12V?
Thanks.
As previously stated, there are many schematic errors. Every question reveals new circuit faults.
I still don't understand the scenario. A former project colleague has left the erroneous design and you are trying to figure out what he wanted to achieve? Or even attempt to fix it?
Specifically, yes the comparators can't trigger the SCRs without pull-up, U6A and U6C are presently functionless. Another fault is that C59, C67 and C77 have wrong polarity.
This is absurd. It started out with a question about relay logic and has now devolved into people posting pictures of comparator internal circuitry and such. I’ll leave the rest of you to deal with this. I’m done.
Maybe @yefj will heed my advice to toss this design and start using timing charts, and state diagrams and reduce to simple CMOS logic. For each memory cell or binary function, you need 1 register or FF.
Inputs must be exclusive otherwise if both SR= 1, then both Q, Qbar=0
Hello,This is important point and maybe there is an error in the logic of this comparator.
My comparator has 12V and -12V supplies.
Basicly if i have only a pullup resistor in the data sheet that meeans the this comparator only works 0V to 12V.
It cannot pull down the voltage so its a basic mistake to put -12V in the supply and think that output could be -12V?
it cannot bet -12V because we would need some pulling down.
the output for this comparator can be only 0 to12V.
Am i correct regarding the logical error i have?
Thanks.
It should be obvious from the schematic that the output always goes to Gnd which in your case is -12V instead. So if you pulled up to +12V your output will have a 24V swing going to an SCR that looks like a Vbe input trigger of only 0.7V. That is not good for many reasons.
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For using a single V+ supply and Vref can be used to compare a V- but you must ensure V+ is in spec. and Vin does not exceed specs.
Hello , the SCR point is important but you and FVM mentioned the multiple DIODE issues which is very intresting and my goal is to learn from these mistakes.
In the attached schematics there are no zenner diodes in the place you mentioned .
What is the problem? why do we need this bandaid correction?
also regarding D16 diode what is the problem in that?
As you can see in the simulation bellow i put a similar zenner diode in the same way and i got the breakdown voltage on the output.
Why its a problematic connection?
Thanks.
Hello , the SCR point is important but you and FVM mentioned the multiple DIODE issues which is very intresting and my goal is to learn from these mistakes.
In the attached schematics there are no zenner diodes in the place you mentioned .
What is the problem? why do we need this bandaid correction?
also regarding D16 diode what is the problem in that?
As you can see in the simulation bellow i put a similar zenner diode in the same way and i got the breakdown voltage on the output.
Why its a problematic connection?
Thanks.
1. Q6 is a low side switch with D14 for BEMF to 12V via J23 is OK but LS2-4 is a high-side dry contact switch to the high-side of LS1 coil which has no BEMF clamp diode to gnd. My Correction image may look like a Zener but was just moving early diode using an image paste.
The other diode issues have been mentioned already.
What's all this Stuff about D16?
D16 is a 400W unidirectional TVS, shown being used as a forward diode when any small signal diode is required. Although SZ1SMA1 is the start of a valid part number as the suffix controls the voltage not being used.
Future discussions are not needed on this schematic as it has no specs and the implementation logic is awkward with errors. It's a time waster.
TVS diodes absorb voltage surges to protect other semiconductor devices whereas Zener diodes provide other semiconductor devices with a constant voltage.
Hello Tony, generaly speaking about the opening the SCR properly.
I understand that with pullup resistor we can output positive voltage too, and its possible to do negative voltages because its open collector with straght path to the negative supply.
My goal is to open the nyc0102blt1g Thyristor ,its the ON state.(page 3 Voltage/Current Characteristics of SCR)
how do i know that my pullup resistor of the LM339APR will provide enough current to open ?
The data sheet of the SCR does say exactly how much current needed to be open and the datasheet of the Thyristor doesnt say what value of output current i will get from the pullup resistor .
what could be done?