Trace ESD current paths with the HP 547A

Article By : Kenneth Wyatt

The classic HP current tracer gets a new life as an ESD troubleshooting tool.

In the mid-1970s, HP developed a set of three handheld probes; a logic probe (HP 545A), a current pulser (HP 546A), and a current tracer (HP 547A). These were replaced later with the updated HP10525T logic probe and 10526T pulser sold together as the HP 5015T Logic Troubleshooting Kit. As far as I know, the current tracer was not upgraded. The combination of current pulser and current tracer were originally meant to locate shorts or latch-ups in CMOS and HCMOS circuits, circuit board shorts, and so forth.

I found a complete set of these several years ago on eBay and wondered how to utilize these, especially the current tracer, in my EMC consulting to track down damaging current flowing from ESD events.

The current tracer (Figure 1) can accept power 4.5 VDC to 18 VDC and comes with small finger-operated grabber clips for attaching to the circuit under test. I will probably, however, cut off the connectors and connect the two wires to proper banana plugs, as the grabber clips were kind of finicky to use. Some used probes don’t include the grabbers, but that’s probably not a great loss.

HP 547A current tracer probeFigure 1 The HP 547A current tracer probe comes with two clips that I’ll replace with banana plugs.

The original user manual says:

The HP 547A Current Tracer is a hand-held probe which enables the precise localization of low impedance faults in electrical systems. The probe senses the magnetic field generated by a pulsing current internal to the circuit or by current pulses supplied by an external stimulus such as the HP 546A or 10526T Logic Pulsers. Indication of the presence of current pulses is provided by lighting the indicator lamp near the Current Tracer tip. Adjustment of probe sensitivity over a 1 mA to 1A range is provided by the SENSITIVITY control near the indicator.

It occurred to me recently that the current tracer might help map out the path of ESD discharge currents, especially for complex systems with multiple circuit boards, power supplies, and cables.

I soon had an opportunity to try out the technique. One of my medical product clients called for help with an ESD compliance failure that had kept them busy for some weeks trying to troubleshoot. Without revealing details, the system was a desk-standing electro-mechanical system with a main circuit board and power supply, a keyboard and display board, several solenoid actuators, and numerous interconnecting cables.

Injecting ESD at anywhere from 2 kV to 15 kV caused an actuator to fire at the wrong time. The system was particularly sensitive at the two side-enclosure panels near the front.

After doing a fair amount of troubleshooting and applying potential fixes, we were getting close, but still not sufficient to meet the new 15 kV requirement per the IEC 60601-2 standard.

Part of the issue was the keyboard and display, which were poorly bonded to the enclosure. Furthermore, numerous system cables could have been related to the problem.

After some head-scratching (and some lunch), I pulled out the current tracer, set it to 1 A sensitivity and connected it up to an external 12 V power supply. We then turned the system on its side and injected a small 500 V ESD pulse into one of the sensitive areas. As my client applied repeated pulses every few seconds, I started probing around on the keyboard/display board and cables (Figure 2).

HP 574A troubleshoot ESDFigure 2 The HP 547A current tracer in use. We were able to isolate the dominant path of ESD current quickly by probing different cables and areas on the keyboard/display board.

We were easily able to isolate the path of ESD to just a few of the cables as well as the display board. The lamp brightness indicated the relative amplitude (and dominant path) of injected ESD current pulse. Now we could try various bonding and shielding methods to see the result in near real time. Between properly bonding the keyboard/display, rerouting cables to the solenoids, and adding one ferrite choke, we were able to confirm that the ESD current to the sensitive portion of the keyboard/display had been diverted back to chassis and we were able to achieve 15 kV compliance.

There are other more modern current tracers available, but it seems their probe tips are rather large and thus may not work in this type of application. The HP 547A has a very small tip and is sensitive enough to detect low-level ESD. I found it perfect and easy to use for the type application.

Current-tracer probes were more prevalent 10 years ago, but occasionally I see them pop up on eBay or other surplus outfits for $50 to $100. This just might be the right tool for those really difficult ESD challenges and I plan to keep this in my troubleshooting kit.

Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.

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