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Topic: Safety Critical Circuit (Read 5573 times)previous topic - next topic

raschemmel

#15
Jun 26, 2015, 03:35 pm
Quote
I would love any feedback on my second draft circuit
Where is it ?

TQ18

#16
Jun 26, 2015, 06:41 pm
Attached to post #7, I'll see if I can post it again.

#17

raschemmel

#18
Jun 26, 2015, 07:56 pmLast Edit: Jun 26, 2015, 08:00 pm by raschemmel
I'm not sure if the led colors make sense but since you're the user they only have to make sense to you. Also, from an electronics documentation standpoint, it would make more sense if the gate switches were labeled "Gate Interlock"

TQ18

#19
Jun 26, 2015, 10:58 pm
Do I need a diode between the load and the MOSFET? I think the solenoid could be fired with a current flowing the wrong way do do I need to protect against that?

raschemmel

#20
Jun 26, 2015, 11:13 pmLast Edit: Jun 26, 2015, 11:14 pm by raschemmel
Quote
Do I need a diode between the load and the MOSFET?
No. The DIODE across the load (solenoid) is called a FLYBACK DIODE (AKA FREE-WHEELING DIODE) which is a component adapted from a flyback circuit to PROTECT the MOSFET. Obviously , the back EMF from the solenoid is no threat to the solenoid , but it can easily blow the MOSFET. The short answer to your question is you already have the diode you need to protect the mosfet.

JohnLincoln

#21
Jun 27, 2015, 12:02 am
Your Schematic is confusing, due to the odd orientation of transistor Q1 and the Safety LED (Green).

You also have some errors that will prevent 2 of the LEDs from working.

A green LED will have around 2.2V across it when it is conducting, but there are 2 base-emitter junctions in series connected across the Safety LED (Green).  These two junctions will limit the voltage to around 1.4V, so no current will flow through the LED.

The voltage on the anode of the Ready LED (Green) can't be any higher than the voltage on the base of Q2, around 0.7V, so that LED won't light.

weedpharma

#22
Jun 27, 2015, 04:31 am
Q1 and  Q2 has no resistance to Gnd from 12v so wil destroy something.

Weedpharma

raschemmel

#23
Jun 27, 2015, 08:31 amLast Edit: Jun 29, 2015, 04:26 pm by raschemmel
I couldn't stand to look at your schematic any longer so I redrew it. (SEE ATTACHED)

Be advised , FYI, when you post a schematic with components all cockeyed and upside down and basically messed up 9 ways from Sunday (visually, not electronically), it screams " no electronics experience !". Learning how to draw a schematic might prevent that.

Some pointers:
1- It is not necessary to connect the grounds with a line. Simply draw a ground symbol.
2- It is not necessary to connect the +12V with a line. Simply draw a power symbol or draw a
terminal and label it "+12V"

3- All Common Emitter transistors should normally be oriented with the collector UP and the emitter DOWN , with the base facing to the left. There are recognized exceptions, such as a Common Base Series Pass Transistor

COMMON BASE SERIES PASS TRANSISTOR

notice that Q2 base is facing toward the right, because it is driven through the 470 ohm resistor which is on the right, justifying the orientation shown. Thus, one may surmise that, while there are specific classic exceptions, (such as the common base series pass configuration, there are also application specific exceptions where the exception is dictated by the application. Where no such application specific or classic specific exception exists, the base of a common emitter transistor should be oriented to the left, not to the right.

5- The convention is that Vcc is at the top , GND at the bottom and therefore loads that are between those two are oriented vertically with the GND connection down and the Vcc connection UP, so LEDs and there resistors do NOT go off horizonally to the right with the GND at the right margin.

6- Also, Flyback diodes (AKA Freewheeling diodes) would be across the coil with the rounded portions of the coil toward the outside (meaning the diode would go on the right side of the word LOAD. Does it make any electrical difference ? No. As an example, look how the diode is shown here

7- Are you familiar with Ohm's Law ?
8-Have you done the math to calculate the resistor values ?
9-Do you know how to read a transistor datasheet ?
10-Have you read the comments about the leds that won't light because you have them connected to the
base of a transistor that isn't going to be much more than 0.7V ?
I didn't see any response to that and those were some very valid observations. You should thank John
Lincoln for pointing that out. ( I didn't think to check that)

11- Draw a Truth Table that shows the LED status for different conditions. I still don't think a SAFE led
should be GRN when the system is ready to fire. I think the Safety Led colors should be swapped so
the GRN led means the system is SAFE (DISARMED) and RED when ARMED (gate closed, ready to
fire). But that's just me.

Quote
Q1 and  Q2 has no resistance to Gnd from 12v so wil destroy something.
As soon as Q1 turns on, the base of Q2 will be shorted directly to +12V and blow that transistor.
Q1, on the other hand, has resistance (UNKNOWN) in both it's base and collector current paths.
I think we can surmise that the absence of resistor values (either in the schematic, or in the text content of the OP's post , ie: listed by component number, indicates that the OP either does not know how to calculate these values or has yet to do so. Why he would post a schematic BEFORE doing so leaves one to wonder just how this circuit came about.

There is a proper way to design a circuit

Create a Desisgn Criteria which includes electrical specifications
Calculate resistor values based on the specs

There is also a proper way to post on the forum.

When posting a schematic on the forum, it is customary to include the values for the components (resistors) or give a reason why they have been omitted, or ask for help determining their values and so on.  None of that seems to have occurred here so it would appear that would be the next order of business for this post.

TQ18

#24
Jun 29, 2015, 05:00 pm
First of all I would like to thank everyone who has commented so far as all the feedback has been a great help. Having spent the majority of today reading and re-reading your posts and doing a lot of research on some of the basic concepts again to really try and understand I have come up with the following:

From your feedback I have changed the following:
• Re-labelled the safety switch as "Gate Interlock"
• Labelled all components including values etc...
• Swapped the safety LEDs from green to red as per a number of people's suggestion
• Tidied up the layout of the schematic and hopefully made it easier to understand
.

The truth table for the LEDs is below with a 1 representing the gate interlock in the position it is shown on the schematic.

raschemmel

#25
Jun 29, 2015, 06:09 pmLast Edit: Jun 29, 2015, 07:19 pm by raschemmel
Much better ! The other suggestion I would make is a 12V Red Lamp/HORN in parallel with the solenoid so that when the FIRE button is pressed the RED lamp lights/HORN SOUNDS while the FIRE button is depressed. Admittedly, that would only be frosting on the cake.  I think you're "Good to Go" now. I'm not sure if it matters to you but technically the Truth Table is incomplete because it does not include the FIRE button,
which admittedly is effectively a "SOLENOID ENERGIZED" button. (FIRE button input,Solenoid Energized Output)

TQ18

#26
Jun 29, 2015, 07:58 pm
Much better ! The other suggestion I would make is a 12V Red Lamp/HORN in parallel with the solenoid so that when the FIRE button is pressed the RED lamp lights/HORN SOUNDS while the FIRE button is depressed. Admittedly, that would only be frosting on the cake.  I think you're "Good to Go" now. I'm not sure if it matters to you but technically the Truth Table is incomplete because it does not include the FIRE button,
which admittedly is effectively a "SOLENOID ENERGIZED" button. (FIRE button input,Solenoid Energized Output)
Thank you for your help! I am much happier with the schematic now both electrically and visually. I shall get to work prototyping tomorrow and then move onto the other side of this project; measuring the speed of the falling object using a couple of laser modules.

123Splat

#27
Jun 29, 2015, 08:09 pm
This is SOOOOO wrong, in so many ways. Atmega's are fantastic little toys, but they aren't magic.  Atmel has a hazardous service disclaimer. someone has already pointed out the terms of use disclaimer.  So, you put in your control circuit. mechanics fail, as they eventually will, electronics fail, as they eventually will, you get your code wrong, or the gods sneeze on your setup, whatever, and someone gets injured or killed.  Who do you think is gonna get sued or sent to jail?  Hint: it ain't gonna be Atmel or Arduino folks.

raschemmel

#28
Jun 29, 2015, 08:57 pmLast Edit: Jun 29, 2015, 10:13 pm by raschemmel
@123Splat,

Quote
This is SOOOOO wrong, in so many ways. Atmega's are fantastic little toys, but they aren't magic.  Atmel has a hazardous service disclaimer. someone has already pointed out the terms of use disclaimer.  So, you put in your control circuit. mechanics fail, as they eventually will, electronics fail, as they eventually will, you get your code wrong, or the gods sneeze on your setup, whatever, and someone gets injured or killed.  Who do you think is gonna get sued or sent to jail?  Hint: it ain't gonna be Atmel or Arduino folks.
This is nonsense. The solenoid can not energize without the gate being closed and the USER pressing the FIRE button. The arduino is nothing more than an "ARM" signal. The system is either ARMED or NOT ARMED. Either way, the load doesn't fall until the OP presses the FIRE button. Why don't you take some time to learn electronics.

larryd

#29
Jun 29, 2015, 09:03 pm
Quote
Why don't take some time to learn electronics.
No technical PMs.
If you need clarification, ask for help.

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