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overcurrent and reverse polarity protection

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hemnath

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Can someone explain the attached circuit. It is overcurrent and reverse polarity protection. How to choose the values and selection of mosfet.
 

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The left MOSFET is a low voltage drop polarity protection. It will be off if the input voltage is reversed.
When the current gets too high, the bipolar transistor will turn on and remove the gate voltage from the right MOSFET.
The MOSFETs are P-channel.
 
Hi,

Give each part in your schematic a name (R1 .., Q1...), so we can talk about it.

It would be good if you have some other specifications, too. Voltages (min, max...), current (expected max normal current, max allowed overcurrent...)

Klaus
 
This is a great opportunity to learn a simulation tool.

Download LTspice.
 
Designations are noted.

Input could be 9V battery. Nominal output load will be 5mA.
 

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Hi,

Sadly you didn't give more informatiin about current limit...

--> Just say you want it to be limit to 10mA nominal.
R1 .... is the "shunt" for current measurement.
Q1 .... should become conductive during current limit.

--> Q1 needs about 0.6V V_BE to become conductive. Now with 0.6V and 10mA you are able to calculate R1.

--> R2 is no needfed, as long as V_GS_max of Q3 is higher than the battery voltage.

--> R3 and R4 are not critical. They just need to drive the tiny gate current of Q3/Q2 (and the leakage current of Q1). Choose 100k down to 10k

--> Q1, Q2, Q3 need to widthstand battery voltage and max limiting current.

Klaus
 

For my application, I want to limit max allowable current to 50mA. Upon that, the circuit should be protected.

From this, I have calculated R1 as 0.6V x 50mA => 30mOhms
R2 is no needed means. Then should I short the connections of Q1 collector and emitter? Kindly clarify.
R3 = 100K; R4 = 10K;
Q2 and Q3 => BSS84-7-F, MOSFET P-CH 50V 130MA SOT23-3.

Are these values are ok.
 

hemnath said:
For my application, I want to limit max allowable current to 50mA. Upon that, the circuit should be protected.

From this, I have calculated R1 as 0.6V x 50mA => 30mOhms
R2 is no needed means. Then should I short the connections of Q1 collector and emitter? Kindly clarify.
R3 = 100K; R4 = 10K;
Q2 and Q3 => BSS84-7-F, MOSFET P-CH 50V 130MA SOT23-3.

Are these values are ok.
Click to expand...

Ohms law is U = R * I
Can be rewritten as R = U / I
In your case:
R = 0.6V / 0.05A = 12 Ohms
 

hemnath said:
...........
R2 is no needed means. Then should I short the connections of Q1 collector and emitter? Kindly clarify.
R3 = 100K; R4 = 10K;
Q2 and Q3 => BSS84-7-F, MOSFET P-CH 50V 130MA SOT23-3.

Are these values are ok.
Click to expand...
R2 is only needed if the input voltage is higher than the Vgs(max) rating of the MOSFET.

You leave the collector-emitter connection open.
If you short that connection how could the transistor work?

You could make both R3 and R4 100kΩ.
 

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