External Moderators tmoulder Posted April 25, 2011 External Moderators Report Share Posted April 25, 2011 No, I'm not doing yard work I'm using a potentiometer to pipe 0-10 VDC to an analog input on an E1B snap IO board. The problem is resolution - the active portion of the pot is very small, and since the resolution on the analog input is also tiny, the active area encompasses all of 24 measurement units out of 1024 for full swing. To overcome this, I applied full 24 VDC to the pot, and set up the mechanism so the active portion ranges from 9-10 VDC. This brought my resolution to 92 units, sufficient to my purposes. The problem - at 15VDC or more, my input will go *poof*. Not a big deal right now, but in the future, if somebody changes the pot or spins it somehow without consulting me first, they may well let the smoke out. So I've been exploring ways to limit the potentiometer output by restricting it to 12 VDC or less. A zener does not seem suitable because I need the full voltage range below 10 VDC. I don't want to divide the voltage and lose resolution again. A diode clamping circuit I built was unsuccessful in tests. And lastly, all the documentation I can find on the web is all about clipping and clamping AC signals, not DC outputs. This seems like it would be a common problem in these analog circuits, and I was hoping somebody might have an idea about a solution. Thanks! TM Quote Link to comment Share on other sites More sharing options...
External Moderators tmoulder Posted April 25, 2011 Author External Moderators Report Share Posted April 25, 2011 Digging a bit deeper, it seems the zener might be the way to go. I'm going to pick some up and test them. TM Quote Link to comment Share on other sites More sharing options...
External Moderators tmoulder Posted April 25, 2011 Author External Moderators Report Share Posted April 25, 2011 Success, I'll through a drawing up at my blog tomorrow. TM Quote Link to comment Share on other sites More sharing options...
Cara Bereck Levy Posted April 26, 2011 Report Share Posted April 26, 2011 Looking forward to reading it We're all back at work now, after the holidays...<sigh> I like holidays! Quote Link to comment Share on other sites More sharing options...
Emil Posted April 26, 2011 Report Share Posted April 26, 2011 Hi, Attached is a scheme to connect potentiometer to AI. It "kosher" - it means it's generated bu our R&D and is checked here. FYI. POT_V100.PDF Quote Link to comment Share on other sites More sharing options...
External Moderators tmoulder Posted April 26, 2011 Author External Moderators Report Share Posted April 26, 2011 Hi Emil! That's a great circuit, and will work perfectly if you want to divide down to 10 V for the entire travel of the pot (0-10 VDC full range of motion). The problem I had was a bit different. I needed 0-10 VDC over a *potion* of the potentiometer, to get better resolution over a specific range of physical movement. This is tricky - you can't divide a potentiometer up, obviously. By applying 24 VDC to the pot directly (no R1), and adjusting the physical mechanism, I could get 0-10 VDC over the lower range of the pot while not allowing greater voltages to reach the AI and blow it. The problem is, it's always possible to get a situation where you do end up with >10 VDC at the pot. I had this happen while testing this - loose wire in the connector. Or when the pot is replaced or being adjusted. To prevent this, I used a zener diode and resistor to clip the pot output to 12.7 VDC maximum - overscale, but under smoke point. It provides a safety to the AI in this configuration. Thanks! TM Quote Link to comment Share on other sites More sharing options...
External Moderators tmoulder Posted April 26, 2011 Author External Moderators Report Share Posted April 26, 2011 Posted to blog. By the way, I want to take a second to thank Emil for posting that circuit. In mine, the 10K resistor is in series with both the Zener Diode and the Analog input. It got me to thinking about the voltage drop on the resistor and how it would affect the analog reading. But looking at his circuit, I see the input impedence on the input is over 1 Megohm, and so shouldn't pose any issues, a 0.01 attenuation. If you need more accuracy than that, you probably shouldn't be using a potentiometer TM Quote Link to comment Share on other sites More sharing options...
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