Coil RFQ checklist: turns, wire, bobbin, termination and test

Why coil quotes diverge

Two winding shops can quote the same drawing and land far apart. The gap is rarely the hourly rate. A coil price depends on the winding, the wire, the bobbin, the termination, the test and the volume, and each of those can be read two ways when the request leaves it open. A supplier that assumes a simple layer winding will price less than one that plans for an encapsulated winding with a thermal cut-out. Both are answering the same email.

The fix is a pack, not a longer email. The pack follows the order in which a winding shop makes decisions: what the coil must do, how many turns of which wire, what holds it, how it connects and how it is proven. Once those five answers are visible, the differences between quotes become explainable, and a buyer can compare them without guessing.

Start with the electrical duty

State the supply voltage, the frequency and the current. An AC coil at 50 or 60 Hz, a DC coil with a rated voltage and a must-operate window, and a coil fed from a rectified supply are three different parts. Give the resistance target at 20 C with its tolerance, because resistance is usually the parameter that sets the ampere turns and the heat.

Add the duty cycle. A coil that is energised continuously needs a different wire class and impregnation from one that pulls in for two seconds and then holds at reduced current. Say whether the coil is a relay coil, a contactor coil, a solenoid or a valve coil, since those labels carry different expectations for force, travel and endurance.

Close with the environment: ambient temperature, humidity, vibration, pollution degree and whether the coil sits in a sealed enclosure. If the part will be potted, encapsulated or conformally coated, say so. Each of those changes the thermal path and the wire grade, and a quote that ignores them will be revised later.

Specify the winding, not just the turns

Give the turns, the wire diameter and the winding direction, then add the details that decide how the coil is built. The wire grade matters: an enamelled wire to a named IEC 60317 specification, a self-bonding grade, a solderable grade or a high temperature grade are not interchangeable at the same price. State the insulation class, for example class 155 or class 180, and the temperature rise the design allows.

Say which parameter is controlled. A coil can be wound to a turn count and a resistance, but holding both at tight tolerance costs money. If inductance is the function, give the target and the test frequency. If resistance is the function, give the target and the measuring temperature. The supplier needs to know which one wins when the two disagree.

Describe the winding method if the design depends on it: layer wound for a predictable build, wild wound for a low cost bobbin fill, bank wound for a short axial length, or progressive winding for a controlled traverse. Give the maximum outside diameter and the maximum build height, because the winding has to fit the space and the cover.

Put the bobbin drawing in the pack

The bobbin is part of the coil, not an accessory. Send the bobbin drawing with the material, the wall thickness, the cheek diameter, the winding width, the core hole and the pin layout. If the bobbin carries pins, give the pin diameter, the plating, the pitch and the pin numbering. If it has a locating key or a mounting foot, give the position relative to the pins.

Creepage and clearance belong on the drawing. State the distance from the winding to the pins, from the pins to the core, and from the winding to any metal part of the housing. A coil that meets a resistance target and fails a creepage check is scrap, and the winding shop cannot fix a bobbin that was designed without those distances.

Say who supplies the bobbin and who owns the tool. If the customer supplies it, state the incoming inspection and the acceptable variation in the winding width. If the supplier moulds it, state the flame class, the comparative tracking index and the colour, and ask where the gate sits, because a gate mark on the winding surface can damage the wire.

Termination and leads

Name the termination: pins for a PCB, tags for a quick connect, flying leads, a connector, insulation displacement or a welded joint. For flying leads, give the wire gauge, the strand count, the insulation type and temperature, the length, the strip length, the colour and the sleeve. Say whether the ends are tinned, crimped or fitted with a ferrule.

For a polarised coil, state which pin is the start of the winding and which is the finish. A DC coil wired the wrong way may still pull in, but a coil with a suppression diode or a permanent magnet will not behave as intended. Mark the polarity on the drawing and on the label.

Add any strain relief, lacing, tape, varnish or encapsulation around the lead exit. A lead that is potted into a housing needs a defined jacket length and a defined bend radius. Those details are cheap to write and expensive to discover at first article.

Define the test before the price

List the routine tests: DC resistance, inductance where relevant, turns ratio, insulation resistance and a dielectric test. Give the test voltage, the duration, the ramp and the points where the voltage is applied. State whether the test is applied between winding and core, between winding and pins, or both.

Then set the sample plan. A 100 per cent resistance and hi-pot check is normal for a coil. A turns ratio or inductance check may be sampled. Say whether the supplier must keep the test records and send them with the shipment, and whether a first article needs a full dimensional report in addition to the electrical results.

Separate routine tests from type tests. A humidity test, a thermal cycle, a salt spray on the terminals or a vibration test is a qualification activity, not a line check. If the programme needs one, give the standard, the duration and the acceptance, and expect it to be quoted as a separate item.

The pack that gets a usable quote

The table below is the structure of the pack. Each line closes a variable that would otherwise be filled by an assumption. It is short enough to send with a drawing and complete enough that two suppliers are pricing the same part.

What to do when the design is not final

A coil is often specified before the housing is frozen. In that case, send the current concept and mark the parameters that are fixed and the ones that can move. Ask for a price at two winding builds or two wire grades, and ask the supplier to flag the points where the design is likely to change the cost.

That is more useful than a single number built on an assumption. It also gives the programme a way to compare a low cost option against a version with more thermal margin, without reopening the whole quotation.