A voltage dependant resistor (VDR), universally known in the industry as a varistor or MOV (Metal Oxide Varistor), is a non-linear passive component designed to clamp transient voltage spikes. Unlike a standard fixed resistor that obeys Ohm’s Law linearly, a VDR exhibits high resistance at normal operating voltages and drops its resistance exponentially when the voltage exceeds its specific clamping threshold. This shunts destructive surge energy away from sensitive downstream semiconductors and power supplies.
Whether you are repairing a switched-mode power supply (SMPS), designing an AC mains input filter, or debugging a fried microcontroller board, understanding how to specify, decode, and test these components is critical. Below is a bench-level guide to selecting the right varistor, reading manufacturer codes, and diagnosing failures.
Decoding Varistor Markings and Specifications
Most through-hole radial MOVs use a standardized alphanumeric coding system to denote their physical size, voltage rating, and tolerance. While Bourns, Littelfuse, and TDK have slight variations, the core EIA-style format remains consistent.
Take the common 14D471K as an example:
- 14: The physical diameter of the zinc oxide disk in millimeters (14mm). Larger diameters equate to higher thermal mass and greater energy absorption (Joules).
- D: Shape indicator. 'D' stands for circular disk. (You may also see 'S' for square in some older or specialized series).
- 47: The significant digits of the voltage rating.
- 1: The multiplier, expressed as a power of 10. Here, $10^1 = 10$. So, $47 \times 10 = 470V$. This is the nominal varistor voltage ($V_N$) measured at a specific DC test current (usually 1mA).
- K: Tolerance code. 'K' means $\pm 10\%$. ('M' would mean $\pm 20\%$).
When selecting a VDR for an AC mains application, the nominal voltage must be high enough to ignore the peak AC waveform, but low enough to clamp before downstream components fail. The general rule for continuous AC RMS voltage ($V_{RMS}$) is that the varistor voltage $V_N$ should be at least $1.25$ to $1.5$ times the peak line voltage.
Standard Radial MOV Specification Table
The following table provides real-world bench data for standard zinc oxide MOVs. Note that the Clamping Voltage is the maximum voltage the component will allow across its terminals during a standard 8/20 $\mu s$ surge pulse at the specified peak current. This is the voltage your downstream circuit will actually experience during a spike.
| Part Number | Disk Dia. | Max $V_{RMS}$ (AC) | Max $V_{DC}$ | $V_N$ @ 1mA DC | Clamping Voltage (Max) | Surge Current (8/20 $\mu s$) |
|---|---|---|---|---|---|---|
| MOV-07D180K | 7mm | 11V | 14V | 18V $\pm 10\%$ | 36V @ 5A | 250A |
| MOV-10D271K | 10mm | 175V | 225V | 270V $\pm 10\%$ | 710V @ 25A | 2,500A |
| MOV-14D471K | 14mm | 300V | 385V | 470V $\pm 10\%$ | 1,240V @ 50A | 4,500A |
| MOV-20D681K | 20mm | 420V | 560V | 680V $\pm 10\%$ | 1,650V @ 100A | 6,500A |
Varistor Types and Application Selection
While the radial leaded Metal Oxide Varistor (MOV) is the workhorse of AC power supplies, it is not the only voltage dependant resistor technology available. Selecting the wrong type for a specific job is a common reason for premature field failures. Here is how the primary clamping technologies compare.
| Technology | Construction & Packaging | Response Time | Energy Handling | Typical Application |
|---|---|---|---|---|
| Radial MOV | Sintered zinc oxide grains in epoxy-coated through-hole disk. | < 25 ns (limited by lead inductance) | High (10J to 1000J+) | AC mains input protection, SMPS primary side, industrial relay snubbers. |
| MLV (Multilayer Varistor) | Surface-mount (SMD) ceramic multilayer structure, similar to MLCC capacitors. | < 1 ns (very low parasitic inductance) | Low to Medium (0.1J to 5J) | ESD protection on data lines, low-voltage DC rails, portable electronics. |
| TVS Diode (Silicon) | Silicon PN junction (not a true VDR, but competes in the same space). | < 1 ps (fastest) | Low (usually < 10J) | High-speed data lines (Ethernet, USB), precision signal clamping. |
| High-Energy Block MOV | Massive square or rectangular zinc oxide blocks, often potted or housed in plastic shells. | < 25 ns | Extreme (1,000J to 10,000J+) | Surge Protective Devices (SPDs), utility grid protection, heavy motor drives. |
Which Type for Which Job?
Choose Radial MOVs when you need to absorb massive energy transients from the AC grid (like lightning-induced surges or inductive load switching) and have the physical board space for through-hole leads. Choose MLVs when routing high-density surface-mount PCBs for 5V/12V DC logic where ESD and fast switching transients are the main threat. Choose TVS Diodes when clamping precision is paramount; TVS diodes have a much sharper "knee" in their V-I curve and lower leakage current than MOVs, making them ideal for sensitive RF or high-speed digital buses where a MOV's parasitic capacitance would distort the signal.
Visual Failure Modes and Bench Testing
Unlike a standard carbon film resistor that typically fails open when overloaded, a voltage dependant resistor usually fails short. Understanding the physics of this failure helps in diagnosing burnt power supplies.
MOVs degrade with every transient they absorb. Each surge melts microscopic regions of the zinc oxide grain boundaries, slightly lowering the component's nominal voltage and increasing its leakage current. Over years of service, this thermal runaway loop accelerates until the MOV conducts heavily during normal AC peaks.
Visual Symptoms of Failure
- The Charred Short (Most Common): The MOV's epoxy coating is cracked, bulging, or completely burnt away, exposing the black/grey ceramic disk underneath. The PCB pads are scorched, and the series fuse is blown black. The MOV failed short, drawing massive mains current until the fuse cleared.
- The Catastrophic Shatter: If a surge exceeds the MOV's absolute maximum energy rating (e.g., a direct lightning strike on a 14mm disk), the zinc oxide ceramic can vaporize or shatter explosively. You will find ceramic shrapnel and a crater on the PCB. In this case, the MOV may actually fail open due to physical disintegration, leaving the downstream circuit unprotected but functional.
- The Silent Degrader: No visual damage, but the power supply runs unusually hot, or the MOV is too hot to touch during normal operation. This indicates severe internal degradation and high leakage current.
How to Test a VDR on the Bench
You cannot test a varistor's clamping voltage with a standard multimeter, but you can easily check for catastrophic failure.
- Isolate the component. Desolder at least one leg of the VDR from the PCB to prevent parallel circuit paths from skewing your reading.
- Set your digital multimeter (DMM) to the highest resistance range (or diode test mode).
- Probe both leads. A healthy MOV will read OL (Open Loop) or well over $10 M\Omega$.
- If the DMM reads near $0 \Omega$ or shows a dead short, the varistor has suffered thermal runaway and must be replaced.
Safe Substitution Rules When the Exact Part is Missing
When repairing commercial equipment or legacy hardware, you rarely have the exact OEM varistor in your bench stock. Substituting a VDR requires strict adherence to electrical limits; guessing can result in a fire hazard or immediate destruction of the replacement part.
Step-by-Step Substitution Framework
1. Match or Exceed the Voltage Rating ($V_N$):
If the original part is a 14D471K (470V), and you only have 14D511K (510V) in stock, the 510V part is electrically safe to use. It will survive the AC peaks easily. However, be aware that its clamping voltage during a surge will also be higher. Ensure that the downstream components (like bridge rectifiers or MOSFETs) have a voltage rating high enough to survive this slightly elevated clamping threshold.
2. Never Downsize the Physical Diameter:
The physical volume of the zinc oxide disk dictates its Joule (energy) rating. If the schematic calls for a 20mm disk (e.g., 20D471K), do not substitute it with a 14mm disk (14D471K) just because the voltage rating matches. The 14mm part has roughly half the thermal mass and will vaporize during a severe grid surge that the 20mm part would have easily absorbed.
3. Check Lead Spacing and PCB Footprint:
Standard 14mm MOVs typically have a 7.5mm lead pitch, while 20mm MOVs use a 10.0mm pitch. If you are upsizing the disk diameter for better surge immunity, you may need to bend the leads or mount the component slightly elevated off the board to accommodate the wider pitch. Ensure adequate clearance to prevent arcing to adjacent traces.
4. Verify the Series Fuse:
A varistor is only as safe as the fuse protecting it. If you substitute a MOV with a significantly higher voltage rating, it will draw less leakage current, which is fine. But if the original design relied on the specific thermal characteristics of the OEM MOV to blow a thermal fuse (often zip-tied directly to the MOV's epoxy coating in premium power supplies), a generic substitution might leave the thermal fuse blind to a slow-burn thermal runaway event. Always verify that a fast-acting overcurrent fuse is present in series with the mains line.
