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Topic: Protecting Analog Inputs: How often and how bad will it fail? (Read 2 times) previous topic - next topic

tms8c8

I'm looking at developing a commercial product and I need to measure the voltage across an n-channel MOSFET when the MOSFET is conducting (i.e., the voltage between source and drain). This voltage is expected to be between 0.02V and 0.12V. With AREF at 1.1V, this is measurable and safe for the inputs. However, when the MOSFET is not conducting, the Arduino input sees 48 volts. When the MOSFET is "off", I do NOT need to measure any voltage, I just want to prevent my analog pins from being damaged.

My thought was to use a zener diode. When the voltage is less than the zener voltage (1.1V) then the inputs see the real voltage. When the voltage rises above the zener voltage, it conducts and the input sees the zener voltage. How often do zener diodes (chosen properly for current and voltage) fail? If it fails, what kind of damage can I expect to the board? A single pin fried or something more serious?  Would using a relay to completely isolate the input be a better choice? Unfortunately, I need to protect 12 analog pins so using relays represents a non-negligible increase in cost and PCB size.

Any thoughts? Is there a better way to do this?

CrossRoads

Why not use a P-channel instead, sourcing current into a 10K resistor to Gnd.
When the P-channel opens up, the resistor pulls the analog pin low.

Oherwise, post a schematic so we can see where this 48V is coming from.
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be80be

I would limit current to under the clamping diodes spec. At the 48 volts

tms8c8

#3
Jan 19, 2013, 04:29 am Last Edit: Jan 19, 2013, 04:33 am by tms8c8 Reason: 1

Why not use a P-channel instead, sourcing current into a 10K resistor to Gnd.
When the P-channel opens up, the resistor pulls the analog pin low.

Oherwise, post a schematic so we can see where this 48V is coming from.


I need to supply up to 10 amps (the load varies but can be as small as ~5 ohms). I'm controlling a variety of current hogging devices and I'm using a sort-of "multiplexing" system where I switch grounds on/off with N-channel MOSFETs and I switch V+ on/off with P-channel MOSFETs. I'm working under contract as a "consultant" and I can't provide too many specifics. I'm sorry - I know ambiguous questions get ambiguous answers but that's why I tried to ask a specific question regarding the reliability of zeners.

Basically, I am using the source-drain resistance of the MOSFET as a current-sense resistor. A crude measurement of the current is all that is necessary.

Jack Christensen

I certainly can't quote MTBF specs or anything, but regarding the reliability of Zener diodes, I would expect them to be more or less as reliable as any other semiconductor, which is to say very reliable. Like anything else, they will tend to be more reliable the more conservatively they are used, i.e. not close to their limits.

Not sure I'd lose too much sleep over it, unless every other part in the project is orders of magnitude more reliable than semiconductors. I don't recall seeing too many reliability specs in datasheets; if hard numbers are needed, contacting the manufacturer is probably in order.
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