Isolated DC Voltage Sensing

Thread Starter


Joined Oct 29, 2010
Hi there ... :)
I need to sense voltage of a 4 series connected batteries battery bank which are isolated from my electronics ....

The voltage is converted to current for the opto using the transconductance amp .... the opto's o/p current converts the current back to voltage to be sensed.

The transconductance amp is designed to run at 10mA at 10V/battery the opto coupler is rated to work at 40 - 80% CTR at that current (designing for worst 40% ) overall voltage gain is 0.8

Please advise have I overlooked anything?



Joined Nov 20, 2008
The LM339 is a comparator with an open collector output, not an opamp. Use an opamp and redesign your circuit.

The 40 to 80% CTR is the part-to-part variation so you will have to calibrate each circuit you build to suit the actual CTR of the part used. There is also a small variation in CTR with ambient temperature and Vce so be sure to account for this in your circuit accuracy calculations.



Joined Mar 6, 2009
There are other devices (such as HCNR200/201) which are designed to overcome some of the issues of calibration related to CTR. Depends on whether accuracy is an issue for you. It may be of interest to look at the broader options available even if you don't adopt them. The data / application sheet for the aforementioned device is readily available as a download.


Joined Oct 2, 2009
This is a problem that keeps rearing its ugly head. I have the same problem so I will put some thoughts together to come up with a battery sense circuit.

The requirements are:
1. Monitor individual voltage levels of 12V batteries connected in series while being charged.
2. Monitor individual voltage levels while batteries are in service or idle, using minimum current drain.

First thing that comes to mind is PWM output opto-coupled to a microcontroller.
I will post my design when completed.


Joined Jul 9, 2011
1. Monitor individual voltage levels of 12V batteries connected in series while being charged.
For single voltages I'd go with the "HCNR200/201" option TNK suggested, I used them several times and they work just great.

However to measure 12 individual voltages that would require 12 of them. Not a good option IMO. Maybe it would be possible to use a multiplexed option...

Thread Starter


Joined Oct 29, 2010
Thanks everyone! The optos after procurement are tested individually so CTR variation is not an issue.

I will look into the HCNR200/201 option thanks!

IMHO ... VCF and FCV conversion is highly linear and accurate but if a simpler circuit can do the job - then you just did your job! :)