Mosfet switch used to protect a load

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shortbus

Joined Sep 30, 2009
10,045
May be I'm wrong, but isn't there a flaw in this whole idea of the O/P? A "circuit breaker" should do just that, break the circuit. A mosfet does not fill that function.
 

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newbieateverything

Joined Feb 25, 2013
95
May be I'm wrong, but isn't there a flaw in this whole idea of the O/P? A "circuit breaker" should do just that, break the circuit. A mosfet does not fill that function.
LM393 is actually a comparator (almost the same as an opamp)
the config makes it a differential amplifier.
for the circuit breaker to break the circuit, there has to be a reason.
the comparator compares the reference voltage to the overvoltage, if there is no difference between the 2 no signal is sent. if there is a signal is sent.
It gives the breaker some sort of intelligence so to speak, as the overvoltage is being compared to the reference voltage and breaks the circuit based on that.
versus just breaking the circuit at random.

the mosfet will then stay on or off based on the signal being sent to the 555 timer, the timer determines the on off times.
the mosfet gives the breaker the ability of fast switching the microsecond region
 
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newbieateverything

Joined Feb 25, 2013
95
One slight issue with your circuit, the references are all created by voltage dividers.

You'll need to pick something like a 22 Volt Zener diode to have a solid voltage reference.

Without a Zener/bandgap ref or similar stable voltage reference (independent of source voltage), measuring supply voltage is rather difficult.
one slight problem i have, my results dont show that the whole circuit responds to a fault current in the LED+2.2Kohm resistor, it protects the circuit from overvoltage by comparing, reference to supply, will fix by adding the 22V zener, to stabilize ref voltage that much is true but what can i do to actually ensure that once the current in the LED+2.2kohm resistor goes past 4.5mA*root 2(peak current) the current gets reduced to 4.5mA again as quickly as possible to ensure that the LED doesn't burn out.

i think so far what my circuit does is to compare the voltage reference, which is the voltage at the source of the Mosfet to the overvoltage being introduced into the system.
The comparator will then act based on:
• If reference voltage= or >Overvoltage no pulse signal will be sent to the 555 timer, hence leaving the Mosfet in its off state
• If reference voltage<Overvoltage a pulse signal will be sent to the 555 timer, the timer will then send a square wave to the Mosfet turning the Mosfet on or off

however i am not sure how my circuit reacts to a spike in the current at the LED based on this configuration
 

shortbus

Joined Sep 30, 2009
10,045
So what you want (are trying to do) is a current limiter not a circuit breaker. A circuit breaker physically "breaks" a circuit, not just shuts it off.

Wouldn't a current mirror be a better type circuit to do what you want?
 

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newbieateverything

Joined Feb 25, 2013
95
So what you want (are trying to do) is a current limiter not a circuit breaker. A circuit breaker physically "breaks" a circuit, not just shuts it off.

Wouldn't a current mirror be a better type circuit to do what you want?
It is a circuit breaker that responds to a fault current caused by an overvoltage.
The breaker is to break the circuit when the current in the load exceeds root*its rated value.

The switching device used is a mosfet, fast switching, voltage controlled etc. (best choice for the application)

A current mirror seems like a good idea, but i want my device to protect my load against higher than normal voltages such as (lighting strikes)

The idea here is to see if this breaker can be used onboard an aircraft to protect a load against a lightining strike

i realize the voltages i am using to simulate my lightning strike are much lower than an over voltage produced by actual lightning strike, it is what i have available
 

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newbieateverything

Joined Feb 25, 2013
95
@ thatoneguy

My simulation is showing a problem i didnt expect.
At the output of my 555 timer i get a decaying voltage signal as opposed to a pulse voltage determined by the RC combination
Am i doing something wrong or is it the choice of resistors i am using that is the problem?
 

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thatoneguy

Joined Feb 19, 2009
6,359
If you are trying to get a 1 shot out of a 555 at power on, the reset needs to be held low for the first little bit.



Change the left resistors/caps for pulse width, the ones to the right of them for startup delay.
 

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newbieateverything

Joined Feb 25, 2013
95
If you are trying to get a 1 shot out of a 555 at power on, the reset needs to be held low for the first little bit.



Change the left resistors/caps for pulse width, the ones to the right of them for startup delay.
i would like the 555 timer set up as an astable oscillator
your suggestion turns it into a monostable oscillator feeding only 1 pulse
i need the multiple shots out of the 555
 

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newbieateverything

Joined Feb 25, 2013
95
yes through a diode as well as the 10k in series with it
Its a push pull configuration
did i implement the push pull wrongly?

push pull to give the mosfet about 15mA to kick start it.
at least that was the idea.

edit: my push pull transistor is connected wrongly
i connected two NPN together its supposed to be 1 NPN and one PNP
 
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newbieateverything

Joined Feb 25, 2013
95
There's no other signal, though. No connection to pin 3?
from what i understand connecting pin 2 and pin 6 together in that config
allows the 555 timer to re trigger itself, so the same pulse will keep on being generated. the pulse duty cycle is dependent on RC

Is this wrong?? i only ask because my voltage signal at the source of my mosfet has alot of distortion

I want the voltage signal at the source of my mosfet to be without distorion because i am going to feedback that signal into the inverting side input of my opamp
 

Georacer

Joined Nov 25, 2009
5,182
It has been well established that AAC doesn't tolerate automotive threads. To expect us to accept aircraft threads is farfetched.

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