# Solid state DC circuit breaker to protect a load

1. Feb 27, 2013

### knowledgeseeki

My choice of solid state Dc circuit breaker is the Mosfet, easily available( i got loads!!!)
i want the mosfet to interrupt the circuit to protect the load from overvoltages
This is what ive done so far, please help its not working the way i want it to.

note to mods: i also put this question up in the general eng section, please remove from there if this is an infraction.

The resistor connected to the inductor is regarded as my load that i want to protect.

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• ###### circuit1.pdf
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2. Feb 27, 2013

### skeptic2

Can you explain why you connected the op amp U1A in series with V9?
What supplies the pulses of V7?

3. Feb 27, 2013

### knowledgeseeki

The pulses of V7 are the overvoltages i am trying to protect my load from by interrupting the circuit via the mosfet
are you saying the Vs+ of the opamp shouldn't be connected in series with V9?
it does need a power source, should it be in parallel

4. Feb 27, 2013

### skeptic2

In order to make meaningful suggestions, I need to understand the circuit. What supplies the overvoltage pulses?

Without fully understanding the circuit I suggest disconnecting the V+ from V9- and connecting V9- to ground. Then I would connect the op amp V+ to V9+ and disconnect V9+ from everything else.

Why can't you connect the op amp V+ to V3+ and eliminate V9 altogether?

5. Feb 27, 2013

### knowledgeseeki

The circuit purpose is to protect the load. The mosfet is to turn off to interrupt current flow and on when the current has dropped down to a safe operating region.
The source of the overvoltage pulse is a lighting strike( but for purpose of testing i will simply a pulsed voltage supply)

Actually that circuit i posted previously is alot wrong.

Does this circuit make more sense?

(the load i want to protect is the inductor resistor combination)

the differential amp, is used to control the on off time of the mosfet so that the mosfet can turn off interrputing the circuit thereby decreasing the current in the load, once the safe limit has been reached the mosfet is to turn back on.

#### Attached Files:

• ###### circuit2.png
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6. Feb 27, 2013

### skeptic2

Yes, I like that circuit a lot more.

7. Feb 27, 2013

### knowledgeseeki

Actually skeptic2 i think the circuit i showed u is quite wrong at least for what i am trying to achieve.
i know the max current across my load is 2.3A ( source being 24 and impedance of load being 10.48)

however what i want to show is that the Overvoltage causes the current in the load to increase beyond 2.3A thereafter showing that the mosfet by switching off interrupts the circuit and allows the current in the load to dissipate and drop back down to 2.3A.

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• ###### graph.jpg
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8. Feb 28, 2013

### knowledgeseeki

The pulse voltage supply i was talking about is just connecting a power source to a 555 timer and using the pulse signal from the timer to determine the comprator signal hence when the Mosfet turns on or off