adc voltage limit

#1
I am using an 18x running off 5 volts. The adc pin 1 is reading amplified voltages from a thermocouple. If the thermocouple leads get disconnected the full voltage (9 V) will be feed to pin 1. I assume that this will kill the chip. I have placed a n114 diode between pin 1 and vcc (cathode of diode to vcc). This limits the overvltage to 5.6V. Is this ok, or would any voltage above vcc do damage.

Thanks,
Robert
 

hippy

Technical Support
Staff member
#2
This is a question to which you are not going to get a straight yes or no answer, because it's not possible to.

According to the 16F88 (18X) datasheet I have, under Absolute Maximum Ratings it says, "Voltage on any pin with respect to VSS (except VDD and MCLR) ... -0.3V to (VDD + 0.3V)", and, "Stresses above those listed under "Absolute Maximum Ratings" may cause permanent damage to the device".
 

premelec

Senior Member
#3
As well as Hippy's good remarks - if you do try it be sure to have a series limiting resistor between the amplifier's output and AD input - say 5K.... that at least keeps the current down some in case of failure.
 
#5
i strongly suggest you download the pic16f88 datasheet from microchip and get familiar with the rated limits of the chip before anything else certain things can be pushed a fair way
(eg the eeprom endurance testing thread)
and some limits are quite literally what it says on the data sheet

 
 
#6
RObert, as said above, READ the DATASHEET from which hippy has kindly cut'n'paste for you.

Can you 'feed' the ADC input via a suitable pot divder circuit? Maybe post your circuit so that people can suggest a mod.

It is vitally important taht you get Data Sheets and READ the relevant parts - you can't always rely on people to answer on this forum within a couple of hours. Remember, it is spare-time-voluntary and not a technical helpline.
 
#8
Thanks for all the replies. I looked at all the datasheets for the picaxes but did not find references to maximum voltages. I foolishly did not think of looking at the sheets for the pic chip being used. I feel very silly. Thanks again for the replies.
Robert.
 
#9
Note that a 5 kilohm resistor in series might not be enough to protect an IC pin from over-voltage; how ICs react depends on their internal design, and usually diodes are effectively formed between inputs and surrounding areas of the chip - that might be fine for limiting damage from very low current static electricity, but anything like a milliamp might be a problem
 
#10
...but (I should have continued) the 16F88 seems to be able to withstand 20mA input clamp current, so 5k is probably okay. In general I would also use external PNP transistors to be extra safe - emitter to pin that needs protection, collector to earth, base to preset pot giving a voltage roughly 0.6V below Vdd. A very cheap way to protect IO pins from over-voltage.
 
#11
How about a 5.1V Zener diode from the Pin to the ground?
I saw this trick while I was trying to figure out how to protect the pin from voltage above 5v
 
#12
Transistors, zeners, diode clamps and any other device will simply vapourise if you stick a 12v car battery on the input.
You MUST limit the current.
All that is needed is a simple series resistor. Choose a value that will limit the current to around 2-5 mA as a general rule.
Most (but not all) inputs have internal clamp diodes already fitted.
 

womai

Senior Member
#13
By the way, the trick with a current limiting resistor + using the internal protection diodes is also used on the standard Picaxe download circuit - standard RS-232 drives voltages far above 5V and below 0V, but the Picaxe-internal diodes clamp that to Vcc and Gnd; current is limited with the 22kOhm resistor. (that said, many serial ports on today's computers, especially laptops, violate the RS-232 spec and simply drive 0 to 5 V).

Wolfgang
 
Top