1. Home
  2. General Electronics
  3. Balanced grounding between devices & fault

Balanced grounding between devices & fault

I have designed a communication device that is being powered by 24V. It has two types connectors. The first provides the power and control signals, while the second provides communication to the outside world. The device is contained within a metallic box that is completely isolated.

It was found that some of the devices failed at initial power on, or due to a later cycling of the power. Those devices were found to pull the 24V DC rail down to < 5V DC and would draw in excess of 200mA. There is a possibility that the device fails to handle the system inrush current.

In my analysis we found there to be a short between the +3.3V supply and GND. When the +3.3V LDO and the Altera gate array that was driven by the +3.3V was replaced, the device functioned correctly.

On the input to the control signals, we are using a National Semiconductor LM2676 3A Step-Down Voltage Regulator to step down the voltage to +5V. We are then using a National Semiconductor LM3940 LDO Regulator to provide us with the required +3.3V for the Altera FPGA.

All the communication signals from the outside world are floating and are passed through transformers and Holt transceivers that are powered by the above +3.3V supply.

For those devices that the fault was found, the shield from the cables that provide communication to the outside world were not connected to the GND of the device. I have two devices where this connection was made and the fault did not occur.

Any suggestions of might be the source of the fault ? Suggestions have been made that a GND imbalance between my device and other devices it is connected to, may have caused damage to the components of my device (possibly large currents into my device). However I don't understand how this can happen when there is no connection between the other devices and the GND of my device via the cable shield ? Can spiking occur across a SM0805 footprint that could have caused damage to the gate array if the voltage difference gets high enough between the two grounds ? Also another point: In a normal connection where the cable shield does balance the grounds between devices ? why doesn't this balancing cause large currents to flow between the devices ?

Does anyone have any other suggestions ?

Reply to
Shmuel Davis
Loading thread data ...

While you are looking for the cause of the problem, send the failed devices to Altera for a failure analysis. They might be able to point you to a specific pin.

Good luck.

Reply to
The other John Smith

This sounds like there is a destructive EMF developing between the signal lines and ground at power-on. Mismatched transmission line effects? Can you add a resistor in the lines and a TVS? Also, if it works and does not fail with the shield connected, why not leave it connected?

It could cause large currents to flow if the source impedance of the impalance is low enough. However, the source impedance of the imbalance could be low enough to destroy a semiconductor without causing "large" currents to flow. What is the current due to ground imbalance?

Reply to
The other John Smith

In article , Shmuel Davis wrote: [....]

Imagine a walk across a carpet and touch the box. A spark jumps from my hand to the metal box. Where does it go from there on its way back to the earth?

--
--
kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

How would such an EMF cause this type of damage and how would your suggested solution solve the problem ? What is a TVS ? What would be the connection of this resistor/TVS - from shield to GND or some other form of connection (The signals on the bus are a differential pair).

it connected?

This was going to be one of the solutions. However, I wanted to try and come up with some sort of theory of what was causing the fault. I was also worried that someone had suggested that if we do short the shield to GND then we may cause large currents to flow, doing more damage than good (even though the devices with the jumper in it did not show this fault !)

Is there any way to measure these phenomena you mentioned above ? We do not have access to the system that the devices were placed in so we can't replicate the precise circumstances under which the fault occurred (we are not able to measure the "current due to ground imbalance" and I was only suggestion that this might be a cause of the fault). Is there any way within an "artificial" labratory environment for us to narrow down what the source of the fault is (and thus the remedy ?)

Reply to
Shmuel Davis

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.