What are the small 18 V to 33 V varistors used for?

Low-voltage DC rails and signal protection. Walsin lists 18 Vdc to 33 Vdc varistor voltage parts in a 5 mm disc at 100 A peak. They suit 12 V and 24 V DC inputs where the threat is inductive kickback or a load dump rather than a mains transient.

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How to Choose an MOV Varistor: Voltage, Energy and Disc Size

Q: Can I put two varistors in parallel for more surge current?

Not reliably. MOVs have a steep non-linear characteristic, so the one with the slightly lower varistor voltage takes most of the current and degrades first. Use a larger disc, or a higher-energy series in the same disc size, rather than paralleling parts.

Choose an MOV in three steps: set the maximum continuous working voltage above your
highest steady-state line voltage, size the peak surge current for the surge you expect,
then pick the smallest disc diameter that delivers that current. Working voltage stops
the varistor conducting in normal operation. Disc diameter, not voltage rating, sets how
much energy it survives.

What does an MOV varistor actually do?

A metal oxide varistor is a voltage-dependent resistor. Below its varistor voltage it looks
like an open circuit with a small leakage current. Above it, resistance collapses by orders
of magnitude and the device clamps, diverting surge current away from the circuit it
protects. It is a shunt element, always placed across the line it defends.

The mechanism is grain boundaries in a sintered zinc oxide ceramic, each behaving like a
back-to-back diode pair breaking down around 3 V. Disc thickness sets the varistor voltage.
Disc area sets the number of parallel paths, and therefore the current. The two are
independent, which is why the same clamping voltage is available in seven diameters.

Walsin Technology (Walsin) lists these under the SR family. Our catalogue extract holds 379
varistors across four series.

How do I set the maximum working voltage?

Take the highest continuous voltage the varistor will ever see, including supply tolerance,
and pick a part whose maximum working voltage exceeds it. For a nominal 230 Vac mains at
+10%, that is 253 Vac continuous — so a part rated 230 Vac maximum working voltage is not
adequate for a 230 V nominal supply. This is the single most common MOV selection error.

An MOV that sits slightly above its working voltage does not fail immediately. It leaks. It
warms. Its varistor voltage degrades. Then it leaks more, which is thermal runaway, and it
ends as a short. An MOV specified too tightly to the line voltage is a reliability problem
that takes months to appear.

The other side of the trade: choose the working voltage too high and the clamping voltage
rises with it, so the downstream circuit sees a larger let-through. Set it as low as the
worst-case continuous line voltage allows, and no lower.

Flagged for review: Walsin’s data gives a Max Working Volt. of 230 Vac against a
Varistor Voltage of 360 Vdc on the SR361K parts. That is the part’s own specification, not
a recommendation for a 230 V nominal mains. Confirm your supply’s maximum continuous
voltage including tolerance, and select above it, before treating any 230 Vac-rated part
as a mains part.

How much peak surge current do I need?

Size it from the surge standard you have to meet, not from the fault you imagine. Mains
equipment usually works to an 8/20 µs current waveform. At a fixed varistor voltage, peak
current across Walsin’s SR size range varies by more than seventy to one — from 400 A in a
5 mm disc to 30,000 A in a 32 mm disc.

Here are eight real Walsin parts, all at 230 Vac maximum working voltage and 360 Vdc
varistor voltage. The only variables are disc diameter and series. The right-hand column
states field by field what matches and what does not.

Walsin part number	Series	Disc size	Max working voltage	Varistor voltage	Tolerance	Peak current	Match / mismatch vs SR361K14D
SR361K14D	Standard (D Series)	14 mm	230 Vac	360 Vdc	±10%	4,500 A	reference part
SR361K05D	Standard (D Series)	5 mm	230 Vac	360 Vdc	±10%	400 A	match: series, working voltage, varistor voltage, tolerance · mismatch: 5 mm disc not 14 mm, 400 A peak against 4,500 A — 11× less surge capability
SR361K07D	Standard (D Series)	7 mm	230 Vac	360 Vdc	±10%	1,200 A	match: series, working voltage, varistor voltage, tolerance · mismatch: 7 mm disc, 1,200 A peak — under a third of the reference
SR361K10D	Standard (D Series)	10 mm	230 Vac	360 Vdc	±10%	2,500 A	match: series, working voltage, varistor voltage, tolerance · mismatch: 10 mm disc, 2,500 A peak — just over half the reference
SR361K20D	Standard (D Series)	20 mm	230 Vac	360 Vdc	±10%	6,500 A	match: series, working voltage, varistor voltage, tolerance · mismatch: 20 mm disc, 6,500 A peak, larger board area and lead pitch
SR361K25D	Standard (D Series)	25 mm	230 Vac	360 Vdc	±10%	18,000 A	match: series, working voltage, varistor voltage, tolerance · mismatch: 25 mm disc, 18,000 A peak — 4× the reference, substantially larger body
SR361K05E	High Energy (E Series)	5 mm	230 Vac	360 Vdc	±10%	800 A	match: working voltage, varistor voltage, tolerance · mismatch: High Energy series not Standard, 5 mm disc, 800 A peak — twice the 5 mm Standard part, still far below the reference
SR361K07E	High Energy (E Series)	7 mm	230 Vac	360 Vdc	±10%	1,750 A	match: working voltage, varistor voltage, tolerance · mismatch: High Energy series, 7 mm disc, 1,750 A peak — 46% more than the 7 mm Standard part

Two results fall out of that table. Diameter is the lever — every part above clamps at
the same voltage, but the 5 mm Standard part takes 400 A and the 25 mm Standard part takes
18,000 A. Series matters at a fixed size — SR361K05E takes 800 A where SR361K05D takes
400 A in the same 5 mm body. If board area is fixed, changing series comes before changing
diameter.

Walsin’s product index is at https://www.passivecomponent.com/products/ — the product
index page, not a direct link to any datasheet PDF. Peak current figures here come from our
own parametric extract of the SR family.

Which disc diameter should I use?

The smallest one that meets your peak current with margin. Walsin’s SR range spans eleven
sizes in our extract: 10 mm and 14 mm are deepest at 53 parts each, then 20 mm at 49,
34 mm Square at 42, and 5 mm and 7 mm at 41 each. Everything above 20 mm is a 19 to 21 part
line.

Margin matters more here than in most component choices, because an MOV degrades
cumulatively. Every surge it absorbs shifts its varistor voltage slightly. A part sized
exactly to the single-pulse rating will pass the type test and then drift over years of
smaller real-world events. Sizing one disc diameter up is the cheapest reliability decision
in a surge protection circuit.

What is the difference between Walsin’s D, E, V and Industrial series?

Four lines with different energy densities. Standard (D Series) is the volume line at 170
parts in our extract. Industrial follows at 101 parts, reaching 30,000 A in a 32 mm disc.
Super High Energy (V Series) has 60 parts and hits 4,000 A in a 10 mm body. High Energy
(E Series) is the smallest line at 48 parts.

The step from Standard to High Energy at a fixed diameter roughly doubles peak current.
SR201K05E at 800 A and SR201K10E at 4,000 A differ in both series and diameter, so read
those two variables separately.

Flagged for review: Walsin’s series naming does not map onto the part number suffix.
SR201K10E is listed as Super High Energy (V Series) but carries an E suffix, while
SR201K32D is listed as Industrial and carries a D suffix — the same suffix as the
Standard (D Series) parts. Identify the series from the series field, not from the last
character of the part number.

The catalogue is one part smaller than Walsin says

Walsin’s own product page reports 380 varistors. The underlying file holds 379. Their
counter includes the header row.

It is a trivial discrepancy and worth knowing anyway, because the same off-by-one appears on
the antenna and RF pages. If you are reconciling a catalogue count against a downloaded list
and you are short by exactly one, that is why.

Venatech Limited, an authorised Walsin distributor in Hong Kong, holds all 379 rows with
series, size, working voltage, varistor voltage, tolerance and peak current on every one.

How do I choose a varistor, step by step?

Establish the worst-case continuous voltage. Nominal supply plus tolerance plus any
regulation overshoot. For a 230 V nominal mains at +10%, that is 253 Vac.
Pick a maximum working voltage above it. Not equal to it. An MOV operating near its
working voltage leaks, heats and degrades.
Identify the surge you must survive. Waveform, peak current, repetition rate and
source impedance.
Choose peak current with margin. At least one step above the calculated requirement.
Pick the smallest disc that delivers it. Compare series at a fixed diameter first.
Check the clamping voltage against what you are protecting. A downstream device with
a lower absolute maximum needs a second stage.
Plan for end of life. A degraded MOV fails short. Fit a thermal disconnect or a
correctly coordinated series fuse.

Frequently asked questions

What is the difference between varistor voltage and maximum working voltage?
Varistor voltage is the voltage at a defined small measurement current, typically 1 mA — the
point where the device starts conducting. Maximum working voltage is the highest continuous
voltage it may sit at without degrading. Walsin’s SR361K parts are 360 Vdc varistor voltage
and 230 Vac maximum working voltage.

Can I put two varistors in parallel for more surge current?
Not reliably. MOVs have a steep, non-linear characteristic, so the one with the slightly
lower varistor voltage takes most of the current and degrades first. Use a larger disc, or a
higher-energy series in the same size, rather than paralleling parts.

How does a varistor fail?
Short, in almost every case. Degraded grain boundaries leak, the leakage heats the disc,
and the heat degrades more boundaries. The result is a low-resistance path across the line,
which is why an MOV on a mains input needs a thermal disconnect or a correctly coordinated
series fuse.

Which Walsin varistor series has the highest peak current?
In Venatech’s extract the highest peak currents are 80,000 A and 70,000 A, at 21 parts each,
and 40,000 A at 40 parts. At the size level, the Industrial series reaches 30,000 A in a
32 mm disc. Filter on peak current rather than on series name when energy is the driving
requirement.

Why does Walsin’s site say 380 varistors when there are 379?
Their product counter includes the header row of the underlying data file. The real part
count is 379: 170 Standard (D Series), 101 Industrial, 60 Super High Energy (V Series) and
48 High Energy (E Series). The same off-by-one appears on their antenna and RF pages.

What are the small 18 V to 33 V varistors for?
Low-voltage DC rails and signal protection. Walsin’s SR180K05D, SR220K05D, SR270K05D and
SR330K05D sit at 18 Vdc to 33 Vdc varistor voltage in a 5 mm disc at 100 A peak. They suit
12 V and 24 V DC inputs where the threat is inductive kickback or a load dump rather than a
mains transient.

Next step

Send us your worst-case continuous line voltage, the surge waveform and peak current you
have to meet, and the board area you can afford. We will return the Walsin SR candidates
with series, disc size, working voltage, varistor voltage, tolerance and peak current stated
field by field. We will not tell you a part is equivalent. We will show you exactly where it
differs.

Send us your BOM → ·
Browse Walsin MOV varistors →