Variable capacitor (trimmer)

01

Set capacitance range and temperature characteristic

Murata's ceramic trimmer capacitor catalogue codes each part by maximum capacitance and temperature characteristic — NP0, N750, N1000 or N1200 ppm/°C — and lists series from 1 mm to 4 mm size for surface mounting. Its TZC3 series (3.2 × 4.5 × 1.6 mm, adjustable with 0.5 mm-thick tools, for automatic placement and reflow) runs from 1.4–3.0 pF (NP0 ±300 ppm/°C) and 2.0–6.0 pF, through 3.0–10.0 pF (N750 ±300 ppm/°C) to 5.0–20.0 pF and 6.5–30.0 pF (N1200 ±500 ppm/°C), with maximum-capacitance tolerance of +50/−0%. [1]

Editorial calculation: in an LC circuit with fixed inductance, frequency varies as 1/√C, so a 6.5–30 pF trimmer alone gives a tuning ratio of √(30 ÷ 6.5) ≈ 2.1; stray and fixed capacitance in parallel reduce that ratio. [1]

Editorial note: the temperature characteristic of a trimmer adds to the drift of the rest of the tuned circuit; state the characteristic code (NP0 to N1200) you need rather than a capacitance alone. [1]

Give the supplier the minimum and maximum capacitance and tolerance, temperature characteristic, Q at your frequency, self-resonant frequency, rated and withstanding voltage, insulation resistance, operating temperature, size and terminal type, adjustment direction and tool, torque, soldering process (reflow or flow), packaging and quantity.

02

Check Q, voltage rating, torque and environment

For the TZC3 series the catalogue lists Q of 300 or 500 minimum at 1 MHz and maximum capacitance, 100 Vdc rated voltage, 220 Vdc withstanding voltage, 10,000 MΩ insulation resistance, adjustment torque of 1.5 to 9.8 mNm and −25 to +85 °C operation. [1]

Its TZW4 series, for high-power use such as impedance matching in cellular base stations, lists 0.4–1.0 pF and 0.4–1.5 pF parts with NP0 ±150 ppm/°C, Q of 200 minimum at 500 MHz, 250 Vdc rated and 550 Vdc withstanding voltage, self-resonant frequency above 3 GHz at maximum capacitance, 1.5–10.0 mNm torque and −55 to +125 °C operation. [1]

The catalogue states that it lists only nominal specifications and asks buyers to approve the product specification or approval sheet before ordering. [1] A linked supplier lists trimmer capacitors among its passive components. [2]

Beyond ceramic chip trimmers, Knowles Precision Devices (with its Voltronics and Johanson Manufacturing brands) describes precision trimmers with air, glass, sapphire and PTFE dielectrics for use from 1 MHz to over 2 GHz and at voltages up to 20,000 V DC, in multi-turn and half- or single-turn formats. It also offers non-magnetic trimmers for MRI and NMR coils, whose multi-turn types use only materials with no measurable magnetism. [3]

Editorial note: the jump from 100–250 V DC ceramic trimmers to kilovolt-rated air or sapphire types is a change of product class, not a grade option; state the RF voltage across the trimmer, the frequency, and whether the part sits in a magnetic field before choosing a dielectric. [3] [1]

03

Compare what suppliers offer

Suppliers linked to this page list trimmer capacitors; each card links to the page reviewed.

The listing reviewed offers trimmer capacitors alongside film, ceramic, tantalum and electrolytic capacitors. Ask for the series datasheet with Q, voltage, torque and temperature characteristic. [2]

04

Check vacuum trimmer test voltage, RF current and trim span

Comet Plasma Control Technologies describes its Mini-Tune trimmers as vacuum capacitors between fixed and variable, used where small adjustments are needed to meet an individual circuit tolerance or to compensate for differences per tool and tool drift over time. They are modified Mini-Caps in the C-, D- and E-geometries, so the same body sizes interchange with the standard Mini-Caps. [4]

Comet lists nominal capacitance of 12 to 440 pF, Upt of 15 to 40 kV, maximum RMS current at 13.56 MHz of 14 to 175 A, diameters of 48 and 64 mm, heights of 65 to 95 mm and convection cooling. Each part is tuned with a no. 3 hex key through a self-locking screw, adjustable by ±1.7 turns for ±5 pF, and can be trimmed after mounting in the match network. [4] [4]

Editorial calculation: ±5 pF over ±1.7 turns is about 2.9 pF per turn; on a 12 pF part ±5 pF is about ±42%, on a 440 pF part about ±1.1%. Editorial note: give the supplier the RF current and voltage across the part at your frequency, not only the capacitance, since these vacuum parts are rated in kilovolts and amperes rather than the 100–250 V DC of the ceramic trimmers above. [4] [1]

05

Trace what each part controls

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your circuit

A variable capacitor quotation needs values for your circuit. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the range, frequency and process data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Range: [Cmin–Cmax pF]
TC: [NP0; N750; other]
Frequency: [MHz; Q min]
Voltage: [rated; withstanding]
Size/terminal: [mm; SMD; THT]
Process: [reflow; flow]
Torque/tool: [mNm; tool]
Quantity: [pieces; reel]

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