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I'm trying to measure a fork's lift battery by an ADC. The problem that the output voltage isn't predictable and not constant, so that I can make a constant voltag divider. What I have measure is it goes from 7 to 30VDC, and the max input voltage of my adc is 1.2V. How would I design the voltage dividers and the op-amps in that case?

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It seems you want to measure a voltage that can be from 7 to 30 volts with a A/D converter with a full scale range of 0 to 1.2 V. What you need is a "voltage divider". This is just two resistors in the simplest case:

In this case, 30 V needs to be scaled down to 1.2 V, which is a factor of 25. That means R1 needs to be 24 times R2. For example, R2 could be 10 kΩ and R1 240 kΩ. That is exactly on mathematically, which doesn't leave room for tolerance of the parts. It would be best to make R1 a little higher, like the next higher up standard value 270 kΩ. In that case the divider will divide by (R1 + R2)/R2 = 28, so the input voltage range the A/D can measure will be 0 to 33.6 V.

That's the basic answer to the question you asked, but a few things seem strange. Are you really sure the A/D range is only 1.2 V? That is possible, but surprisingly low. Also, 7-30 V sounds like a very large range for a battery. That implies the battery can be drained down to less than 1/4 of its maximum charging voltage, which is very low and can damage most types of batteries. What kind of battery do you have? Where did you get the 7 and 30 volt figures from?

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  • \$\begingroup\$ The battery is the acid battery that's in the fork lift. The ADC is the one's that in SAMSUNG Exynos4412. What about the buffering Op-AMP ? is there any design issue ? \$\endgroup\$ – Mahmoud Jan 21 '13 at 18:38
  • \$\begingroup\$ @Mahmoud: A factor of 4 in voltage is way too large for a operating lead-acid battery. The unit should be stopped long before the voltage gets that low else the battery will be damaged. Whether you need a buffer depends on the maximum signal impedance required by the A/D. You can make the resistors lower, but that also drains more power. That should be no problem though if it's only on when the forklift is operating. \$\endgroup\$ – Olin Lathrop Jan 21 '13 at 19:42
  • \$\begingroup\$ Should I count and recalculate the power rating of the resistors based on the total current that will be drawn from all the circuits or the current will be branched to the resistors on its on, like kirchof's law ? \$\endgroup\$ – Mahmoud Jan 21 '13 at 23:26
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You need to make a signal conditioning circuit (I would include plenty of protection & a good bit of extra headroom for over-voltage conditions) that divides the voltage so that its maximum output is 1.2v. The core of this can be a simple potential divider.

You should read up on transient voltages, back-emf, etc. as any vehicle (especially electric) has lots of ways to generate nasty voltage spikes and harsh conditions for sensitive electronics.

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