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Analog input resolution, scaling and protection

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tmoulder

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Caution: This approach can be risky. Analog voltages greater and 15 VDC can fry an analog input on an E1B module. Use with caution, and at your own risk.

I had a project where I needed to use a potentiometer with an E1B snap IO to measure an angular position. The problem I ran into was resolution. With 10 VDC applied to the pot, and the E1B resolution of 1024 units for full sweep, the portion of the pot I needed was only seeing 22 units.

I had an idea - I put full 24 VDC to the pot, then positioned the pot physicially so I was at the lower end of it's physical scale. This gave me 10VDC at slightly less than half of the pot's travel, which took my measured sweep from 22 units to 92 units. Huge improvement.

The drawback was - there's 24 VDC on the pot, and E1B inputs fry at 15 VDC. If someone replaces it incorrectly, gets to playing around with it or even a wire comes loose, poof.

So to protect the input, I hunted high and low and came up with this circuit.

blogentry-19-060710300 1303825180_thumb.

It uses a 10k resistor in series with a 12V zener diode, all available at your local Radio Shack. It shunts the current in such a manner that the input can never see more than 12 VDC even with a full 24 VDC coming from the pot - overscale, but below smoke point. The zener begins regulating at 11.4 VDC, and below that, behaves like a regular reverse-biased diode.

Hope it's helpful!

TM

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