JS220B - Software fuse tripping annoyance

Before “up dating” the software on my JS220B it would work on the test rig it is incorpoarted in. Since the update, the software fuses on the 220B are tripping during one of the product test sequences where there is a transient in the current flow. I try setting the fuse thresholds significantly above the maximum measured registered and the fuse still trips.
There does not appear to be any instructions in the manual as to what Fuse 1 or what Fuse 2 actually refer to nor any instructions as to why there are 2 thresholds to set on each fuse and what the time delta signifies. Is it possible to disable the fuses?
If anyone can expalin all the above I woudl be most grateful.

Plus - the front panel on the JS220B shows 3A as being the limit, so why can I set the sofware fuse to 10A?

Many thanks


Hi @NeilVP and welcome to the Joulescope forum!

Before the fuse implementation, the JS220 allowed you to exceed it’s current limit specifications. We had a few customers blow up the front-end shunt resistors and MOSFETs because of this. Ammeters should have protection circuitry to prevent damage, and the JS220 now includes that protection.

So, the likely explaination is that your circuit has been exceeding the current rating of your JS220 all along. Here are the specs:

From Joulescope JS220 User’s Guide, section 7.3 Recommend Operating Conditions.

The JS220 now includes four fuses: 2 user-configurable fuses and 2 fixed fuses, Range fuse and Max fuse. I suspect that you are seeing Max fuse trip. Hover over each fuse in the Device Control widget to see the tooltip explaination.

If you hover your mouse over each fuse setting, Threshold 1, Threshold 2, and Duration, you will see a tooltip that explains it. You can set the User fuse thresholds to whatever you want. However, if you set the User fuse higher than the Max fuse, the Max fuse will trip first making your User fuse irrelevant.

The fuse setting even includes a nice plot showing exactly when your user fuse will trip. Here is the setting for Max fuse which needs to make it into the Joulescope JS220 User’s Guide:

Does this make sense?

Does your system exceed the Max fuse condition? What is not shown is cooldown, which can also matter if you have a bunch of close events that exceed 4 A.

Hi Matt
Thank you for the prompt response.

The trace is showing a peak of 7.5A lasting for approx 130ms before it is shut off.
Looking at the table in Section 7.3 that would explain the problem.

Using the word search facility in the pdf I can find 3 references to “fuse” but no references to “Max Fuse” or “Range Fuse”.

Does the table in Section 7.3 refer to what the “Max Fuse” can cope with?
“Range Fuse”, does that automatically limit the current to that specified in the “Current Range” drop down dialogue?

Kind Regards


Hi @NeilVP - We have not yet updated the Joulescope JS220 User’s Guide with the soft-fuse feature. It’s on the list, but our recent focus has been the next UI release.

The plot above is exactly what the Max fuse implements. All four soft-fuses emulate tradition fuses that heat up and cool down. The plot above shows how fast the fuse will trip given a constant current. Estimating from the image, at 7.5 A, it will trip in about 100 ms.

The Range fuse is set for each current range to protect the shunt resistor and MOSFET. With current ranging set to auto, this fuse will never trip. You will only see this trip if you select fixed current range and provide significantly more current than specified for the range.

In your test setup, do you have any way to limit the magnitude or duration of existing 7.5 A, 130 ms pulse?

Yes, I will modify the product to reduce slug the current rise.

Thank you for your help and explanations.



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