Coil Resistance
The DC resistance of a solenoid winding in ohms. It determines which voltage class the coil belongs to and how much current it will draw; on mobile proportional coils 12 V types typically measure a few ohms and 24 V types about four times that.
Resistance measurement is the fastest way to evaluate a coil in the field. The coil connector is removed and a measurement is taken across the terminals; since the measurement is affected by oil temperature, it is normal for cold and hot values to come out different.
- Infinite resistance: the winding is broken, the coil must be replaced.
- Noticeably below expected: there is a short between turns; the coil heats up and strains the drive.
- Low resistance to the body: the insulation has broken down, there is leakage. This also puts the drive board at risk.
Resistance is also the basis of the current calculation: nominal voltage divided by resistance roughly gives the steady state current. On current controlled drives, since the drive already forces the target current, resistance does not directly determine flow rate, it only affects how much voltage the drive must apply. Keeping a known good second coil on hand for comparison is the most practical method when measuring.
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