# 150V voltage follower

I am working on a datalogger to measure the voltage of a series of solar modules. I have 4 solar modules in series, let say 35V each. I am using an ADS1115 for the measurements. First I wanted to do differential measurements, but this is not possible as the voltage between the some of the pins of the ADS will exceed the max.

Now I will something simple, reference the series solar modules to the the measurement circuit and measure the voltage from the divider as in the drawing:

I want to add a voltage follower before the the voltage divider in order to not load the sources in a different way. The problem is I cannot find a (cheap) op amp to do this, that is rated above 150V. Is there a reason for this? I there another way to achieve a nice way to measure the voltage in series? how is it done on high voltage battery packs for example?

• You could increase the resistors by a factor of 10 and then buffer the divider outputs with a cheap quad amp. The loading would be pretty minimal. – Peter Smith Sep 12 at 10:42
• High voltage starts from 1000$V_{DC}$. I hope you don't mind I removed "high voltage" from the title, as this title would give incorrect search results. – Huisman Sep 12 at 19:35
• @Huisman, that definition varies with the context. In power engineering, for example, 10 to 20 kV would be considered medium voltage whereas in, say, a valve amplifier the "high tension" supply could be considered high voltage - and certainly is in relation to the audio signal - especially if you touch it. The edit to the title is fine though. – Transistor Sep 12 at 20:10

## 1 Answer

A simple solution which also has approximately matched loading at the cells is:

simulate this circuit – Schematic created using CircuitLab

The resistance seen by each cell is just over 1M and the output span of all the amplifiers is 0 to 4V where 4V is the nominal maximum for each cell at 35V.

Given that this is a low speed application, a very cheap quad amp could be used (beware of rail to rail input devices - the 150k input resistance for output 1 amplifier can cause issues unless it is internally steered and not just overlapping input stages).

• "the 150k input resistance for output 1 amplifier can cause issues unless it is internally steered and not just overlapping input stages" - do you have a link/app note on this topic? I've not heard of this before. – Aaron Sep 12 at 19:46
• These are connected as ground referenced buffers. The Vo1 would measure BAT4, Vo2 would measure (BAT4+BAT3)/2 (I did not check the divider values) so the resolution of Vo4 would be 1/4 of Vo1 and you would have to subtract 3/4 x Vo3 from Vo4 and multiply by 4 to get the same scale as Vo1 when measuring BAT1 – KalleMP Sep 12 at 21:35