FAQ
TL;DR: Yes—open DMM leads “float,” so you may see noise, often around 10–20 mV, and even up to 1–2 V on cheap meters. “All of my meters indicate voltage on the AC range.” [Elektroda, Steve Lawson, post #21666312]
Why it matters: Knowing this prevents you from misdiagnosing a good meter as faulty and shows how to reduce false readings.
Quick-Facts
- Open probes = floating input; small, wandering readings are normal on sensitive DMMs. [Elektroda, Steve Spence, post #21666296]
- Typical with shorted leads on AC: ~10–20 mV on quality meters; cheap meters can show 1–2 V. [Elektroda, Steve Lawson, post #21666312]
- Leads act like antennas for mains and RF fields; shorting cancels pickup. [Elektroda, Frank Bushnell, post #21666298]
- Reduce noise by twisting leads, keeping them parallel, and minimizing loop area. [Elektroda, Steve Lawson, post #21666311]
- Nearby Wi‑Fi routers/computers can inject measurable noise; turning them off helps. [Elektroda, Chuck Sydlo, post #21666310]
Quick Facts
- Open probes = floating input; small, wandering readings are normal on sensitive DMMs. [Elektroda, Steve Spence, post #21666296]
- Typical with shorted leads on AC: ~10–20 mV on quality meters; cheap meters can show 1–2 V. [Elektroda, Steve Lawson, post #21666312]
- Leads act like antennas for mains and RF fields; shorting cancels pickup. [Elektroda, Frank Bushnell, post #21666298]
- Reduce noise by twisting leads, keeping them parallel, and minimizing loop area. [Elektroda, Steve Lawson, post #21666311]
- Nearby Wi‑Fi routers/computers can inject measurable noise; turning them off helps. [Elektroda, Chuck Sydlo, post #21666310]
Is it normal for a DMM to show voltage with open probes on DC or AC?
Yes. With open probes the input floats and the leads pick up ambient fields, so the reading drifts. This behavior is expected on sensitive digital multimeters. Shorting the probes should bring the reading near zero if the meter is healthy. [Elektroda, Steve Spence, post #21666296]
Why do readings jump more on the AC range?
On AC, the meter is designed to detect alternating signals. Your leads behave like antennas and pick up mains hum and RF. Shorting the probes forces both inputs to the same potential, collapsing the pickup and stabilizing the reading. [Elektroda, Frank Bushnell, post #21666298]
What quick test confirms my meter isn’t faulty?
Short the probes together. A good meter will settle at or near 0 on DC, and to a very small value on AC. If it does not, suspect a setup or hardware issue. “Try shorting the probes together.” [Elektroda, Steve Lawson, post #21666297]
What numbers should I expect with shorted leads on AC?
Quality meters often show about 10–20 mV with shorted leads, while cheaper units can indicate 1–2 V. Both go to zero with ideal shielding, but small residuals are common in normal environments. [Elektroda, Steve Lawson, post #21666312]
My Agilent U1253B shows ±mV to ±0.1 V open-circuit. Is that within reason?
Yes. Users have observed ±mV up to ±0.1 V when the probes float. The value depends on ambient interference and lead geometry. Shorting typically stabilizes near zero, which indicates normal operation. [Elektroda, Joseph Sam, post #21666307]
How do I reduce the wandering reading without special gear?
Use this 3-step fix: 1. Short the probes tip-to-tip. 2. Keep the leads parallel and twist them loosely. 3. Separate the setup from EM sources like transformers. These steps cut loop area and RF pickup. [Elektroda, Steve Lawson, post #21666311]
Can Wi‑Fi routers or PCs in the room cause false readings?
Yes. A nearby wireless modem or computer can couple RF into the leads. Powering them down often reduces or eliminates the apparent voltage. This is a quick diagnostic to try first. [Elektroda, Chuck Sydlo, post #21666310]
Why does the meter show a few millivolts even when probes are shorted on AC?
Small residuals occur from internal noise and residual coupling. Readings like 6–7 mV AC with shorted leads have been observed and are not unusual in typical rooms. [Elektroda, Joseph Sam, post #21666307]
My DC mA range shows ~0.1 mA with open leads. Is that a defect?
Not necessarily. RF can induce currents in the lead loop, especially with quarter‑wave effects. Twist the leads, keep them close, and test again; remove the leads to compare. Persistent DC mA offset may indicate shielding issues. [Elektroda, Steve Lawson, post #21666311]
When should I suspect a real meter fault?
If shorted DC voltage does not settle near 0, or AC readings remain high after twisting and isolating from EM sources, investigate further. That behavior suggests a setup error or a defective meter. [Elektroda, Steve Lawson, post #21666297]
What does “floating input” actually mean on a DMM?
It means the meter’s high-impedance input is not referenced to a defined potential. With open probes, tiny fields move the input, so readings drift until you reference it by shorting or connecting. [Elektroda, Steve Spence, post #21666296]
Could input impedance design affect these symptoms on the U1253B?
Yes. Discussions note input-impedance behavior can influence apparent readings. Reviewing known notes and tests on this model helps separate normal floating behavior from design-specific quirks. [Elektroda, Mark Harrington, post #21666301]
Do analog (moving‑coil) meters behave differently from digital meters here?
Analog meters can rectify RF through protection diodes and show deflection due to nearby RF sources. Digital meters manifest drifting digits instead. Both are reacting to energy picked up by the leads. [Elektroda, Rodney Green, post #21666305]
Can a nearby magnet make the reading jump?
Yes. A strong magnet moved near the leads can induce signals and make readings jump, especially on sensitive ranges. This demonstrates how easily fields couple into open or looped leads. [Elektroda, Chuck Sydlo, post #21666300]
What is a DVM/DMM, and what’s special about the U1253B display?
DVM stands for Digital Volt Meter; DMM measures several quantities. The Agilent U1253B features an OLED display, which is bright and high-contrast compared with LCDs. [Elektroda, Mark Harrington, post #21666314]
