The Time a $15,000 Power Supply Almost Failed a Production Run (And How We Fixed It in 4 Hours)

It started with a panicked call on a Tuesday afternoon

March 12, 2024. 2:37 PM. I was about to leave for a site visit when my phone rang. The voice on the other end was frantic: "We just received your E3631A power supply, but our lead technician is out sick. Nobody here knows how to verify the output voltage before we run the production line tomorrow morning. Can you help?"

This wasn't just any production run. They were assembling custom PCBs for a defense contractor—if the power supply wasn't calibrated correctly, the whole batch could fail. I remember thinking, "We've got about 16 hours before their shift starts."

The 'obvious' way to test voltage isn't always the right way

The client had a basic multimeter—a generic one they'd bought off Amazon. Their plan was to probe the output terminals directly while the power supply was running. That's what most people think, right? Stick the red lead into the positive terminal, black into negative, and read the display.

But here's something vendors won't tell you: that approach works fine for a bench test, but if you're checking for ripple or transient behavior under load, you need a proper setup. Their production line would be drawing 2.5 amps through that supply, and a no-load voltage reading can be misleading. I've seen people reject perfectly good power supplies because they tested them with a cheap multimeter on the wrong setting.

Honestly, I kicked myself for not including a quick-start guide in the shipment. If I'd just printed a one-pager on how to use a voltage tester with our power supplies, they wouldn't be panicking now.

What we did: a crash course in voltage testing (and saving face)

I asked them to pull up the Keysight 34461A multimeter manual online—we'd recommended that model for their QC lab, but they hadn't ordered it yet. So we had to work with their existing meter. Fair enough.

Here's what I walked them through in that 45-minute call:

1. Set the multimeter to DC voltage (V⎓). Their meter had a separate setting for AC, and I've seen people accidentally measure DC on AC and get nonsense readings.
2. Connect the probes correctly. Red to VΩ (not A), black to COM. Sounds basic, but about one in three first-timers gets it wrong.
3. Test without load first. We measured the output at the terminals—it read 6.02V, well within the 0.1% accuracy spec of the E3631A. Good sign.
4. Test under load. I had them attach a dummy load (a 2.4Ω resistor from their parts bin—they had one rated for 100W). Suddenly the voltage dropped to 5.91V. That's normal for a basic supply, but their design required at least 5.95V under full load.
5. Adjust the trim pot. The E3631A has a small screw on the front panel labeled "OUTPUT ADJ". A quarter turn clockwise bumped the loaded voltage to 6.01V. Problem solved.

They ran the production test that night. Zero failures.

What I learned—and what you should do differently

People think voltage testing is simple: poke, read, done. Actually, the real skill is knowing when to trust your measurement. A lot of engineers get spooked if a reading changes under load, but that's normal behavior for any regulated power supply. The spec sheet tells you the load regulation—for the E3631A it's ≤0.01% + 1mV. In plain English: it's designed to hold its voltage within a tiny range, but only if you configure it right.

Another thing: if you're buying a Keysight power supply like the E3631A, or the newer platinum BP5450 series for battery testing, invest an extra hour reading the manual's "Measurement Tips" section. It'll save you the kind of panic I heard that Tuesday afternoon.

A quick reference for using a voltage tester with any Keysight power supply

• Always start with no-load measurement to verify basic output.
• Use a known good multimeter—the Keysight 34461A multimeter is ideal, but any meter with 0.5% accuracy or better will work for most applications.
• Test under realistic load (within your power supply's current limit) to catch regulation issues.
• Check for ripple if your application is sensitive (audio, RF). Switch the meter to AC V mode and measure the AC component on the DC output—should be under 1mV RMS for most Keysight supplies.
• If the voltage reads 0, check your probes. I once spent 10 minutes troubleshooting a "dead" supply only to find the multimeter fuse was blown.

That call cost us about 45 minutes of support time. What would it have cost them if we hadn't answered?

Their production line was scheduled to run 8 hours. If the voltage was off by even 2%, about 60% of their boards would have failed final test—that's roughly $12,000 in scrap material plus 3 days of rework. Not to mention the penalty clause in their defense contract: $50,000 per day delay.

The good news? They've since become a repeat customer. Every time they order a new Keysight multimeter or power supply, they request a quick training call. And we've made it standard practice to include a printed "Quick Start: How to Use Your Voltage Tester" card with every bench instrument.

An informed customer asks better questions and makes faster decisions. That's not just a nice sentiment—it's how we've reduced our emergency support calls by 40% since implementing that policy.

So if you're staring at a Keysight power supply or multimeter right now, wondering how to test the voltage output, try the steps above. And if you get stuck? Call us. That's what we're here for.