I have a 96volt DC line, and I am trying to get it down to 3.3volts to put into an opamp, and then to put into an ADC so that I can read the voltage on the line. The 96volt line will be prone to spikes when it is turning on and off.

I am thinking of using a voltage divider to get it down to between 0 and 5 volts. The line can go up to 110 volts for a long period of time, so I chose R1 to be 11k and R2 to be 415 ohms. Any thoughts of problems with this design?

I also need to figure out how to prevent voltage spikes (up to 220v) for a short period of time. I am thinking about using a capacitor connected to ground coming off of the voltage divider line. I am worried that this may decrease the accuracy of measuring the voltage. Would this be the correct way to go, or is there a beter solution?

Thank you!

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With your setup you get a current of 100/11k = 10 mA, a bit big...I would go for bigger values, like 1M and 41K, to give an example. And make sure that the wires are distant enough. –  clabacchio Mar 27 '12 at 18:52

What you have will work to attenutate 110 V to 4.0 V. However, there will be considerable power dissipation. R11 will dissipate about 1 W at 110 V in, so you definitely can't use a ordinary 0805 resistor. If you really need the low output impedance of 400 Ω, then get a resistor that can handle the power.

If you can do with a higher output impedance, then you could use larger resistors. For example, 100 kΩ at top and 3.77 kΩ at bottom will dissipate less than 120 mW total with 110 V in. The output impedance in that case will be 3.63 kΩ, which is still low enough to go directly into many microcontrollers.

The capacitor as you show will filter short term spikes. No, it won't hurt your accuracy at all. It may actually enhance it due to reducing high frequency noise. Make the low pass filter rolloff frequency as low as possible without cutting into the signal you actually want. For example, with 100 kΩ and 3.77 kΩ resistors, a 1 µF cap will make a LPF with a rolloff of about 44 Hz. If your valid signal is only up to 20 Hz or so, that would work fine.

Clabacchio makes a good point in a comment. At up to 220 V you not only have to think about the power rating of the resistor, but also its voltage rating. This is the value it can safely withstand and stay within spec, not arc, or vanish into a puff of greasy black smoke. However, the resistor itself isn't the only issue voltage-wise. You also have to consider the closest distance between pads on the PC board. There are various regulations and guidelines for spacing and creapage distance depending on the intended use, the nature of the high voltage, etc.

All this means that a single 0805 resistor is probably not good enough in your case. You can use a bigger package, or in some cases you can string multiple smaller packages together. Some regulations require minimum spacing that has to be met somewhere, and it's not valid to accumulate it in pieces like with multiple resistors. High voltage standoff is one place where thru hole parts still make sense sometimes, although that's for a lot more than 220 V.

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How did you calculate the power dissipation, and how can I get it lower it? I would rather not waste any energy. –  Reid Mar 27 '12 at 18:55
@Reid: Power dissipated by a resistor is V^2/R, where V is the voltage accross it and R the resistance. When V is in Volts and R in Ohms, then the result of the equation will be in Watts. Since you're stuck with V, you make power lower by increasing R, as I did in the example I described. –  Olin Lathrop Mar 27 '12 at 19:03
Okay, thank you for your help! –  Reid Mar 27 '12 at 19:21
I would put a note about the fact that using 0805 resistors, he will have two pads with about 100V of difference and distant less than 1 mm. I would use larger components in any case. –  clabacchio Mar 28 '12 at 12:18
@clabacchio: Good point, added. –  Olin Lathrop Mar 28 '12 at 12:41

A voltage divider sounds like a fine idea.

A capacitor will keep very brief voltage spikes from harming the opamp. To prevent longer voltage surges from harming the opamp, we typically use some sort of crowbar circuit to clamp the voltage to a safe level. For example, perhaps something like:

+96 V -- polyfuse --+-- R3 --+-- R2 --+--+-- op-amp input
|        |        |  |
zener2   zener1    R1  C1
|        |        |  |
return -------------+--------+--------+--+-- GND


The simplest crowbar-like circuit is a single zener diode, but there are many others that can handle much higher voltage and current levels.

In normal operation, crowbar circuits have practically no effect on the signal at the op-amp. (Normally no current flows through the above zeners; normally the voltage drop across the polyfuse is insignificant compared to the voltage drop across R2 and R3).

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