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Overvoltage protection circuit

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mrclolveira

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I'm creating a circuit to protect an analog input, which can vary from 0 to 2.5V.
However I need divide this voltage to 2V.
And I need to protect for not exceeding 3.3 V.

I created this circuit in Multisim, simulation worked perfectly, but in practice No.

1434853600_1448501273.png

5808910800_1448501274.png



in practice, with 2V the result is 1.6 V, and with the result is 10V 2.6 V
Has anyone worked with this type of circuit or can you tell me what can I do to fix?
 

The datasheet for the zener diode shows that it needs a current of 76mA for its voltage to be 3.3V. With less current its voltage is less.
A tutorial about zener diodes shows that a low voltage one is almost like a resistor, it draws current at a voltage lower than its rating.
The low voltage zener diode voltage drops when the temperature drops.

It worked in Multisim because Multisim has very poor accuracy.
 

The datasheet for the zener diode shows that it needs a current of 76mA for its voltage to be 3.3V. With less current its voltage is less.
A tutorial about zener diodes shows that a low voltage one is almost like a resistor, it draws current at a voltage lower than its rating.
The low voltage zener diode voltage drops when the temperature drops.

It worked in Multisim because Multisim has very poor accuracy.

ok, so if I use a smaller resistor it can works?
 
Last edited:

Zener diode OVP will not suitable for measuring purpose. It can be used for power supply. You can use diode clamp methode. You must use low drop schottky diode as shown in the circuit.




Udhay
 

Hi,

The voltage across the 2k should be limited to 3V3.
Then the voltage at the input is 4.1V.
For precision it is better to connect a 3V3 zener directely at the input, so the zener current does not influence voltage.

But a simulation with a low ohmic 10V source is not usefull often.
Do you really want to protect against 10V DC, a battery for example?
Or do you want an ESD or surge protection?

A capacitor in parallel to the 2k may improve ADC performance. The value depends on analog frequency of interest and sample frequency.

Hope this helps

Klaus
 

Hi,

The voltage across the 2k should be limited to 3V3.
Then the voltage at the input is 4.1V.
For precision it is better to connect a 3V3 zener directely at the input, so the zener current does not influence voltage.

But a simulation with a low ohmic 10V source is not usefull often.
Do you really want to protect against 10V DC, a battery for example?
Or do you want an ESD or surge protection?

A capacitor in parallel to the 2k may improve ADC performance. The value depends on analog frequency of interest and sample frequency.

Hope this helps

Klaus

Thanks for you reply Klaus,
i want protect a sensor input, so it is a high ohmic source, and i want to protect against any wrong connection.

like you say, i put the zener on the input, and work fine with a limited current, and with a low resistence source, the zener burn.

do you know any better way to protect this input?

- - - Updated - - -

I found this Varistor,
https://www.littelfuse.com/products/varistors/multilayer/mla/v3_5mla0805.aspx

anyone ever used anything like this?
 

Why did you select a zener diode that requires A VERY HIGH CURRENT (76mA)? When you provide it with the high current then it heats which slowly increases its clamping voltage.
There are many other zener diodes that use a current that is only 5mA (1N5988B is one) and there are a few that use only 50uA (1N4684).

The circuit that uses Schottky diodes allows the voltage to clamp at 3.5V to 4.0V which might be too high for your protection.
 

Hi,

Why did you select a zener diode that requires A VERY HIGH CURRENT (76mA)?
I´d rather say it can withstand high current than it requires high current.

So i think a zener with higher current rating is not worse.

Klaus
 

Hi,


I´d rather say it can withstand high current than it requires high current.

So i think a zener with higher current rating is not worse.

Klaus
Its datasheet says its voltage is 3.3V plus or minus 5% when its current is 76mA. It is a low voltage zener diode so it is a poor voltage regulator anyway, with less current its voltage is less, maybe only 2V (my graph does not show currents higher than 20mA) and with 76mA it heats (1/4W) which slowly increases its voltage.
A 1N4684 zener diode that requires only 5mA has an 80mA max allowed current.
 

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