USB 6501 Digital Input Protection

[JHall92]

Hi,

Currently I am using a USB 6501 to read the relay contacts on electronic timers/counters at our manufacturing facility. These relay contacts are supposed to be isolated, allowing the end user to connect whatever they would like to them to switch signals at timeouts etc.

I connect DGND to all relay COMs and connect each individual relay N.O. contact to a digital input pin on the 6501. This way the input is pulled LOW whenever the relay turns on.

The issue I have ran into now is that the product we are making occasionally is soldered incorrectly and the input power (120/240VAC) may accidentally be soldered to one of these relay contacts. This ends up frying the 6501.

My solution in the past has been to wire a low current fast blowing fuse in series with each digital input and DGND. This causes a headache if I have many digital inputs and requires having spares lying around just in case this happens.

Is there a simpler way to protect the 6501? Perhaps only using 1 fuse in series with DGND? I'm not really concerned about where the blow occurs, but protecting the 6501 and maintaining its full functionality.

Best regards,

Jeremy

[crossrulz]

Continuity and Isolation checks should be done first.  Verify the 120V lines are not soldered to your relays before you apply any power to your board.  Other checks should be done at the same time (power not shorted to GND, all grounds are actually connected, signals not shorted to 120V or GND, etc.).

[JHall92]

Hi crossrulz,

Yes, that is another option that I considered, but this would acquire additional measurement devices, external relays, and software modifications. Though I do agree that this method is the proper way for testing product.

The issue with this, is that when I designed this tester, I was given $0 and literally pulled whatever I had lying around my office to implement it. It's worked great for over a year, but now we have finally seen this issue.

Best regards,

Jeremy

[crossrulz]

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@JHall92 wrote:

Hi crossrulz,

Yes, that is another option that I considered, but this would acquire additional measurement devices, external relays, and software modifications. Though I do agree that this method is the proper way for testing product.

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Just do a manual check before hooking up the board to the test fixture.  All you need is a simple handheld DMM.  Just make sure this test step is fully documented in whatever procedures to make sure it is not skipped.

[AnalogKid2DigitalMan]

Two fingers would work too and no cost either 🙂 Just Kidding!

Sidenote- the other day I was trying to use an inflatable bed pump. It was not working and the power adapter was 12VDC so of course I did a quick check with my tongue.

Well I jump backed and let out a wail. I think the rectifier bridge/filter failed I am guessing it was ~60VAC (current limited).

-AK2DM