3xAA reverse protection and over voltage protection circuit

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Hi,

your picture has a lot of useless white area ... but the text is so small that I can´t read. .. especially FET type.

Mind:
* usual FETs don´t have body diodes
* but MOSFETs have body diodes inside
So if it´s a MOSFET it can´t protect against overvoltage.

The Zener most probably becomes conductive, but the way it is installed it can´t limit output voltage.

Klaus
 

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    hemnath

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Hi,

So this is a MOSFET with built in body diode....
Did you read the datasheet and see the "Graphic symbol"?

Klaus
 

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    hemnath

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Yes . After your reply.

Any equivalent transistor with the same package? I have designed the PCB.
 

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Hi,

there are two problems:
* the MOSFET internal body diode (which you may overcome by usuing a FET)
* the zener circuit can not work as overvoltage protection.

Overvoltage:
What overvoltage do you expect?
* ESD or other pulses that may be just "short circuited" by an overvoltage protection device.
* Or for long time overvoltage by making the FET high impedance? --> Try an LDO with reverse voltage protection, like LT1121 (just as idea. There may be more suitable ones)

Klaus
 

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No. its not working. when i increase input voltage to 6V . output is also closer approximately 5.8V. Zener diode is not breaking
 

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The last link you posted does not take one to a web page or document....


Regards, Dana.
 

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Hi,

The link is working.
I see the circuit and the description.

But I also see the comments below.

My recommendation:
Don't rely on random hobbyists words and schematics. Good descriptions/tutorials proove their design with scope pictures or anything else.

All semiconductor manufacturers provide good, reliable and well explained circuits. Other reliable sources are universities and well known electronics designers with good reputation.

Klaus
 

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As you state the PMOS reverse protection circuit is fine -


What you need the zener to do is breakdown and supply current to blow a fuse,
called a crowbar circuit. Use with resettable fuse. Or use an integrated part -


FR014H5JZ: High-Side Reverse Bias / Reverse Polarity Protector With Integrated Over Voltage Transient Suppression

www.onsemi.com

Your circuit, mistake was 100K resistor should be 100 or so ohms -

Design Guide - PMOS MOSFET for Reverse Voltage Polarity Protection

Design Guide - PMOS MOSFET for Reverse Voltage Polarity Protection
components101.com

But that circuit is protecting the MOSFET gate from over voltage, not the load.


Regards, Dana.
 

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crutschow said:
As noted that circuit cannot work for overvoltage protection.
Obviously, whoever published that circuit never built (or even simulated) it.

What to you want to happen when an overvoltage is detected?
Click to expand...
When the overvoltage is detected, it should maintain constant 5V supply.
--- Updated ---

danadakk said:
Your circuit, mistake was 100K resistor should be 100 or so ohms -



Regards, Dana.
Click to expand...
It is 100 in the circuit. mentioned as 100E. seems the picture is not clear.
 

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hemnath said:
When the overvoltage is detected, it should maintain constant 5V supply.
Click to expand...
Okay.
Below is the LTspice simulation of a circuit that blocks reverse voltage and limits the positive voltage to 5V.
M2 provides the reverse protection.
M1 and the TL431 2.5V adjustable voltage reference, along with the two BJT transistors form a 5V, low-dropout regulator, to regulate (limit) the output to 5V.

M1 and M2 must be logic-level [Vgs(th) of <2V maximum].

The simulation shows the output (green trace) staying a 0V for Vin being negative (yellow trace) and staying at 5V when Vin goes above 5V.

 
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    d123

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The circuit in post #16 is fine. As already mentioned, you need two MOSFETs to achieve the low voltage drop reversal and overvoltage protection. Depending on the expected input voltage range, it should be supplemented with the R-ZD circuit of post #1, otherwise the Vgs rating might be exceeded.
--- Updated ---

The post #1 circuit is a perfect reversal protection, just published with misleading title and wrong description.
 

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Since you mention 3AA batteries in your title, below is the circuit with higher resistor values to reduce the circuit bias current and maximize battery life;

 

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