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I have a module with an ADC with a maximum input voltage of 2V. I would like to monitor the battery voltage of 15V with this input. I have successfully used a trimpot as a voltage divider to effectively divide by 10 to get 1.5V at the input.

The trimpot is to calibrate the reading because fixed resistors will never be exact. Is there something I can add to this in order to protect the ADC input against overvoltage if the trimpot is turned too far?

As an aside, I am planning to also add an op-amp voltage follower at the input to isolate the voltage divider from the ADC input.

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  • \$\begingroup\$ It's good that you are considering the input range of the ADC. However, pots are notorious for the wiper losing contact within the device. and depending upon it's construction you can get discontinuous operation while trimming. As a one of low use device it should be OK but not generally a good approach. \$\endgroup\$ – placeholder Oct 11 '12 at 4:18
  • \$\begingroup\$ without using a trimpot , you could use a FET as a variable resistor in place. I have read a application note on this. But I don't have enough experience to guide you on that. Some kind of feedback mechanism will auto calibrate it. as you could see there's no trimpots in modern measuring devices. \$\endgroup\$ – Standard Sandun Oct 11 '12 at 4:20
  • \$\begingroup\$ there are stand alone devices that perform this function in a device. One input moves the "wiper" up for every toggle and another moves it down and the setting is stored in EE prom on board. -> in case you were thinking of a production solution. \$\endgroup\$ – placeholder Oct 11 '12 at 4:30
  • \$\begingroup\$ Good additional ideas. I will spend some more time researching them. \$\endgroup\$ – TDL Oct 11 '12 at 4:33
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If you simply add a fixed resistor between the top of the trimpot and the 15V source, you gain two advantages. First, you can limit the maximum voltage applied to the ADC as you desire, and second, you get better resolution in terms of calibrating the exact divider ratio.

For example, if you have a 2K trimpot and a 13K resistor, the maximum voltage that you can get at the wiper of the trimpot is 2V, and now half the range of the trimpot covers a 1-volt span, rather than just 1/15 of the range.

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  • \$\begingroup\$ Great Answer! - Same thing I was just going to post. :-) \$\endgroup\$ – Michael Karas Oct 11 '12 at 4:21
  • \$\begingroup\$ Thanks! I don't know why this did not occur to me in the first place. \$\endgroup\$ – TDL Oct 11 '12 at 4:31
  • \$\begingroup\$ A second protection option is to put a zener diode across the lower part of the voltage divider, so its breakdown voltage is the highest that appears to the ADC. But a microcontroller for instance is likely to already have diodes to VCC, which is why you see ratings like Vss-0.5V to Vcc+0.5V (with 0.6V voltage drop in protection diodes). The fixed upper resistor you describe is more important and appropriate. \$\endgroup\$ – Yann Vernier Oct 11 '12 at 6:38

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