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over voltage protection

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bejoy_ak

New Member
hi,
i am designing a project to take the output of the fuel guage and convert the analog reading to digital by using adc...i hav desgned it such a way that if input vol is 12v the ouput is 5v.(one 15k and 5.1k,output across 5.1k),nw if the input is more than 12v hw do i protect my uc frm this..can anyone suggest me a circuit...??
 

crutschow

Well-Known Member
Most Helpful Member
The µC likely already has built-in overvoltage protection on the input (typically a diode connected from the input to V+). If you check the specs, it will list a maximum input current. As long as the overvoltage current through the 15K resistor (Overvoltage - 5V) ÷ 15k) is less than that, you will be okay.

It that isn't sufficient, you could also just add an external diode from the input (anode) to the µC +5V power (cathode). The diode will then conduct when the input gets above about 5.6V.
 

Sceadwian

Banned
There's an AVR appnote for a zero crossing detector that uses a 1meg resistor to directly connect the mains HOT line to an I/O line that relys on the internal clamping resistors.
It's VERY important you check the PDF for your exact model micro controller though as not every pin is gaurunteed to have this clamp circutry.
 

Boncuk

New Member
Generally a Z-diode is a good idea.

However using Z-diodes for an A/D circuit you must consider losses of the input signal (even within range) resulting in false measurement, increasing with increasing input voltage.

Transil-diodes have almost no effect on the input voltage until they break through.

Boncuk
 

Mr RB

Well-Known Member
hi,
i am designing a project to take the output of the fuel guage and convert the analog reading to digital by using adc...i hav desgned it such a way that if input vol is 12v the ouput is 5v.(one 15k and 5.1k,output across 5.1k),nw if the input is more than 12v hw do i protect my uc frm this..can anyone suggest me a circuit...??
You DON'T want to clamp the voltage, that will give you an innacurate signal!

The vehicle fuel float sensor is a resistive pot, the voltage is typically 0v to 8v or so. Just use your 2 resistor voltage divider and make sure that the highest reading (fuel tank real full) is less than 5v on the output to your uc. With modern vehicles they use a regulated voltage for the fuel sensor, but most older vehicles just use the battery/alt voltage and a pot in the fuel tank.
 

crutschow

Well-Known Member
Most Helpful Member
Generally a Z-diode is a good idea.

However using Z-diodes for an A/D circuit you must consider losses of the input signal (even within range) resulting in false measurement, increasing with increasing input voltage.

Transil-diodes have almost no effect on the input voltage until they break through.
A low capacitance signal diode to +V for protection also has little effect on the input signal.
 
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