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Old December 19th, 2011, 09:12 PM   #1
diat150
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reading 12 volt signal using 0-10 volt analog input

IM trying to find a cheap way to read system voltage using a 0-10 vdc input. my working voltage range is from 10-20 volts. Is there a voltage dividing circuit or something that I can put together that would keep my 10-20vdc working voltage scaled to my 0-10vdc analog input?
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Old December 19th, 2011, 10:44 PM   #2
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See picture and links.

http://www.google.com/search?rlz=1C1...viding+circuit
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File Type: png voltageDivider.png (5.1 KB, 135 views)
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Old December 19th, 2011, 10:57 PM   #3
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It's an iffy situation. It's cheap, but it might blow your analog input, because your analog input circuit is likely to 'see' 20 volts, not 10 volts:

You want to see from a 10 volts range which is 10-20 volts with respect to the source's low side. Whether your analog input will do that depends on how much common mode it can tolerate and how high it can float above ground.

A couple of 100k ohm resistors in series will provide a reasonable impedance to the source, and will load your analog input slightly, but scaling can correct for the error.



The problem is that the point labeled a (negative terminal at source) is probably grounded somewhere, so the your analog input will likely see the voltage with respect to ground, not with respect to its own (-) terminal.

Your low side is elevated above the source's negative.

Some AI's can't take much common mode voltage. Some will saturate so the signal looks full scale up or down and not be damaged, but others can be damaged by excessive common mode.

If your analog input is single ended, do not try this.
If your analog input is differential, it might handle it.
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Old December 19th, 2011, 11:05 PM   #4
diat150
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all grounds will be common. I should have mentioned that. The analog input would be on a ge versamax nano
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Old December 19th, 2011, 11:40 PM   #5
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I was going to recommend the voltage divider as well.

Make sure you test this first with your DMM and make sure to check the common as danw said.
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Old December 20th, 2011, 12:38 AM   #6
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Or use a transmitter. See link below

http://www.api-usa.com/signalconditioners_dc_input.php
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Old December 20th, 2011, 04:17 AM   #7
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Add 1N4740A (10V, 1W zener) in series. Add .1 uF Monolythic across input for noise. Cathode to +10-20V input. Put Zener in temp stable area to minimize drift. Add another 1N4740A across analog input if you want.
Even the cheapest opamps can take over 30 volts without damage. AD's analog module for the 205 series is rated at absolute max +/- 75 volts. No danger at 20 volts.
Note that this is the cheap way. I would never do it. Too much thermal drift. Use a signal conditioner.

Last edited by keithkyll; December 20th, 2011 at 04:25 AM.
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Old December 20th, 2011, 07:45 PM   #8
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Mickey's voltage divider circuit in post #2 is totally adequate. The voltage "seen" by the analog input will be one-half of the input voltage (both R's same value), providing the impedance of the analog input is much higher compared to the value of the lower resistor.

I have exactly this arrangement to provide 10V from a 24V supply to my training rig analog input potentiometers, although instead of two individual resistors I used a 20-turn cermet potentiometer, so that I could "trim" the output voltage to exactly 10V.

One advantage of using a potentiometer is that any temperature drift affects both the "upper" and "lower" resistances equally, so my 10V doesn't drift at all.
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Old December 20th, 2011, 10:39 PM   #9
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I will also be just turning the analog input on occasionally. so it wont be on 24-7. maybe an hour or two at most a day.
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Old December 21st, 2011, 05:30 AM   #10
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for daba better use a lm319 for a constant voltage, and diat use max 10 k as the input resistance of the analog port is not very high.
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