Measurement of the In-rush current

[johnsold]

I agree with Henrik.

What are you trying to protect?  The test apparatus or the device under test (DUT)?  What failure mode are you trying to protect against?  What kind of device do you have to interrupt the mains?  What is the maximum current or energy you can allow to pass under fault conditions?  How fast do you need to interrupt?

Integration is one way of emulating the behavior of a fuse, but if you know the source and fault impedances and the energy it takes to damage things in your system, you can probably simplify the process.  Integration is inherently a low pass filtering process and may not be the fastest way to do what you want.

Lynn 

[Mr B]

Hi Henrik, Linn,

 

Here, I’m trying to protect the Device under test when it is switched on for the first time. Unfortunately, errors occur during the various assembling phases in the assembly lines and it has been noticed that sometime the protection circuit doesn’t trigger properly. This is when my test rig must switch everything off quickly enough when a fault is detected, hence minimizing consequences. The failure can be caused by: a short circuit, parts incorrectly assembled or faulty components.

 

The mains is interrupted using a SSR (Solid Relay State) controlled via software and the current is measured using a current transformer. The current limit hasn’t been established yet; however it will be determined later.

Also, I’m already using LT spice for the simulation side but I am not too sure whether I doing the right thing! L

 

Regards,    

[Coq_Rouge]

Have you tried to use a NTC Thermistors as a inrush current limiter. You will find more on this page. http://www.ametherm.com/
            

[johnsold]

Ok.  You are trying to protect a switch mode power supply from the initial current at the initial turn on point.

The power supply input may have just a rectifier and a capacitor (if it does not have a power factor correction circuit).  With the capacitor initially discharged, the current is limited by the series resistance of the capacitor, the rectifier, and any wiring or switches in the input circuit.  Depending on the  ratings the capacitor may charge in the first half cycle of the power line frequency or it may take several.  Even if the time constant is smaller than the power line period, you will have considerable difficulty discriminating between a short circuit and the normal inrush in the first 100 µs.

As Henrik pointed out you cannot interrupt the current with a standard (SCR or TRIAC based) SSR in less that half a power line period.  So the first 100 µs does not much matter anyway.  The current after 2-3 time constants of the input circuit is probably more meaningful.

I would probably put a hardware comparator on the output of the current transducer and use that to shutdown the SSR.  Monitor the current with the computer and record what happened.  You could AND the comparator output with an output from the computer so that either could turn the DUT off but only when both agreed that conditions were safe could the DUT be on.

Lynn