# Circuit Design Struggle - Diagram and Breadboard Setup

• USC_ME
In summary, the conversation is about the design of a circuit on a breadboard and its function. The circuit is called an over-voltage crowbar and is meant to protect powered circuits from excessively high voltages on the supply rail, should the power supply fail. It is designed to take an input of 12V and output a constant voltage, while also protecting the device on the output side from reverse polarity. The layout of the circuit has been discussed and some have suggested using a MOV instead of an SCR for the crowbarring action. The function of the circuit has been questioned and clarification has been requested on how it will provide a constant voltage at the output.

#### USC_ME

As an ME major I am less than comfortable with circuit design, so I have been banging my head against a circuit I need to layout on breadboard for a couple hours now. Below is a link to a diagram of the desired circuit, as well as a picture of the breadboard setup I am using.

USC_ME said:
As an ME major I am less than comfortable with circuit design, so I have been banging my head against a circuit I need to layout on breadboard for a couple hours now. Below is a link to a diagram of the desired circuit, as well as a picture of the breadboard setup I am using.

Welcome to the PF.

What problems are you having? How much of the circuit have you wired up so far? The breadboard picture is too far away for me to tell how much is wired up...

The layout isn't critical. What sorts of currents are you handling? What is the input voltage? What are you doing exactly? You could lay it out so it looked nearly like the schematic, or rotate components to take less space. Shorter wires are generally better. Hard to say for sure without more info.

When you go above 18V+scr trigger, you fire the scr and blow the fuse. Why have you chosen that action?

Thanks for the replies, I'll try and get back to you all with some more details. I just got done with an exhausting day working on a home renovation so am turning in for the night, but wanted to let you know I'm still interested. Thanks again!

I think there is some thing wrong with this schematic,

As you can see when the thyristor is conducting, the + and - terminals are shorted!

What is the purpose of the circuit that you are designing?

meBigGuy said:
When you go above 18V+scr trigger, you fire the scr and blow the fuse. Why have you chosen that action?

As I said

meBigGuy said:
When you go above 18V+scr trigger, you fire the scr and blow the fuse. Why have you chosen that action?
The circuit is called an over-voltage crowbar. It is designed to protect powered circuits from excessively high voltages on the supply rail, should the power supply fail.

Obviously it will crowbar. Is that really what he intended? I was wondering why he chose that. Maybe he has good reasons. Just asking.

Sorry its taken a while to get back to you all. With some help from others I have put together what I think is a serviceable, if ugly, layout. Here is a picture of it here: http://i.imgur.com/DDol18I.jpg

As to the function of the circuit, it is meant to take an input of 12V that is constantly turning on and off and output a constant voltage, as well as protect the device on the output side from reverse polarity, etc.

Can I get a check from you all on the way I've laid out the circuit?

edit: Here is useful pic for my nomenclature: http://i.imgur.com/HUWIhU3.jpg

meBigGuy said:
Obviously it will crowbar. Is that really what he intended? I was wondering why he chose that. Maybe he has good reasons. Just asking.

It is to protect equipment from over voltage. There really is no other reason to do it. It is not at all uncommon to do this. A good 12 volt power supply will have this on the output in case of regulator failure. If the over current protection still works then the supply will go into current limit. If the over current protection has been damaged as well then usually the fuse on the primary side of the transformer will blow provided it has the correct fuse installed.
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Think about what can happen if this is in a power supply being used to float a battery in parallel with equipment. Not uncommon to do this for battery backup in communications systems.

Is there a reason he isn't just using a MOV to do the crowbarring?

Typically aren't MOVs designed for higher voltages? They are pretty fast and clamp the voltage off but do not short circuit like an SCR will. Once the SCR is fired it will stay on until the current is removed. Very little power will be dissipated in the SCR. To allow an MOV or zener type device to just clamp off at the preset voltage and pass max current will get cause it to dissipate lots and lots of heat compared to the SCR.

A MOV has a wide voltage range and cannot be accurately adjusted. An SCR with a zenner diode is much more predictable.

Interesting, thanks for the info.

USC_ME said:
Sorry its taken a while to get back to you all. With some help from others I have put together what I think is a serviceable, if ugly, layout. Here is a picture of it here: http://i.imgur.com/DDol18I.jpg

As to the function of the circuit, it is meant to take an input of 12V that is constantly turning on and off and output a constant voltage, as well as protect the device on the output side from reverse polarity, etc.

Can I get a check from you all on the way I've laid out the circuit?

edit: Here is useful pic for my nomenclature: http://i.imgur.com/HUWIhU3.jpg

Any opinions?

The connection is correct as per single line diagram, however you have to verify the connection to thyristor terminals with the data sheet of the thyristor (what is the thyristor number that you are using?).

it is meant to take an input of 12V that is constantly turning on and off and output a constant voltage

Regarding the function of the circuit, i have doubt about its functionality as you described. If you can explain more how it will provide constant voltage at the output.
As per your single line diagram, this connection with zener diode, resistor and thyristor will make over voltage protection, so when the voltage increase above certain level, the SCR will conduct thus shorting the +ve and -ve terminals and the fuse will burn and opens the circuit, thus clearing the fault.

When the input voltage goes away the 1000uF will supply current and droop accordingly until the input comes back. Have you calculated and are you OK with the ripple voltage that will naturally be created?

I would lay it out such that the ground for R1 C2 went to SCR1 rather than the long way around like you have it. Or move SCR1 closer to R1-C2

1 person
I was not the one who actually designed this circuit, but was simply trying to lay it out. I plan on testing it with the end device today, so fingers crossed, hopefully everything will work according to plan.
I took your advice on the layout meBigGuy; It looks a bit better now to me. http://imgur.com/aMwCtdL Thanks!

I got the circuit up and running on Wed, and it is giving exactly what we want: a constant 12V at the output. Thanks for all the help. We'll see later today whether the end device functions properly.

The next improvement in layout would be for the fuse and SCR to be next to the diode and capacitor with the zener and stuff on the far side. Make the input to output ground and power as short and fat as possible. But what you have will work.

The SCR symbol used is an alternate symbol. Not an MOV symbol The circuit diagram is correct for 18 volt crowbar/overvoltage protection. The +DC should be from a single point and the grounds should be from a single point.. The bread boards usually have separations in their power strips and require jumpers to make them contiguous. I can't see what kind of SCR is plugged into the circuit. Under power it could be tripped due to a disconnect in the multiple ground and +dc points.

USC_ME how much is your input voltage?

And why you didn't use LM7812 voltage regulator instead of all this?

His input voltage is 12 Volts and provides a current the an LM7812 can not provide...and I can guess that the supply can be any kind of 12Volt source and a 12Volt 3 terminal regulator does not act as an Overvoltage Crowbar that can short and pop a breaker or blow a fuse when the supply fails. Only the circuit he showed it the issue.

## 1. What is circuit design?

Circuit design is the process of creating a schematic representation of an electronic circuit, which includes all the components and their connections. It is a crucial step in the development of any electronic device or system.

## 2. What are the basic components needed for circuit design?

The basic components needed for circuit design include resistors, capacitors, inductors, diodes, transistors, and integrated circuits. These components are used to control the flow of electricity and create specific functions in a circuit.

## 3. What is the difference between a diagram and a breadboard setup?

A diagram is a visual representation of a circuit, created using symbols and lines to represent components and their connections. A breadboard setup, on the other hand, is a physical layout of a circuit on a breadboard, which allows for easy prototyping and testing before soldering the components onto a PCB.

## 4. How can I troubleshoot common circuit design issues?

Some common circuit design issues include incorrect component values, faulty connections, and short circuits. To troubleshoot these issues, you can use a multimeter to check for continuity and resistance, double-check the component values, and ensure all connections are secure and correct.

## 5. What are some tips for creating a successful circuit design?

Some tips for creating a successful circuit design include starting with a clear understanding of the circuit's purpose, using standard symbols and conventions, organizing components logically, and testing the circuit thoroughly before finalizing the design. It is also important to consider factors such as power consumption, heat dissipation, and signal integrity.