Grounding Issues
Are you using the mains for powering your data logger, think about grounding!
Lately, we discovered problems while connecting a data logger thru the USB port to a PC.
The data logger was powered by a non-grounded power adaptor and ML-PB-DC power board.
Most of our data loggers are battery powered and used in a standalone environment.
But sometimes (like in a stationary line-up or in a workshop environment) they are powered from the mains. And one of the ML-PB-LI /SLA /DC power boards is mounted in the data logger.
Connected to the adaptor, the DC output voltage is floating in respect to the (protective) ground. De data logger is running fine. There is no problem.
But, problems can occur when the above described data logger configuration is connected thru the USB cable to a Laptop or PC.
Why is that?
Many modern AC /DC adaptors are based on an SMPS (Switched Mode Power Supply).
The advantage using (high-frequency) SMPS is that the used transformer can be way smaller than a conventional power supply with transformer working on 50 or 60Hz.
But, disadvantage of SMPS is the high frequency radiation generated.
Simple adaptors have no grounding. Power supply’s with grounding who are not connected to the (protective) ground wire in wall-outlet, experience the same problems.
In order to suppress the generated high frequencies, the manufacturer uses a capacitor on the DC output. At this point, we found that things could go wrong…
The capacitor on power adaptors is placed between the DC output and on one of the AC plug pens.
The capacitor used on a (shielded and grounded) power supply’s is correctly connected to the ground (shield of the power supply), but if the ground is not connected to the wall outlet, it has also a connection to the mains thru the mains net filter. (Which is also connected to the shielded ground).
In both cases (ungrounded adaptor and shielded power supply which isn’t grounded on the wall outlet) the mains are used for disposal of the high frequencies. Result is that the DC output is referenced to the mains!
When the data logger is running stand-alone, there is no problem.
But if you connect your Laptop or PC, which is grounded thru the wall-outlet, you will encounter an AC voltage between the data logger and Laptop USB jack!
What is the solution?
Our recommendation on all DC powered configurations:
** Always use a grounded power supply on data loggers with ML-PB-LI /SLA /DC power boards **
