Two Different Things
Dielectric constant tells you how much charge a capacitor stores. Insulation resistance tells you how well it holds that charge over time. They are not the same thing.
What Insulation Resistance Measures
Insulation resistance (IR) is the DC resistance across the dielectric. It is measured by applying DC voltage and measuring the tiny leakage current that flows after the initial charging surge settles. Typical specs are in megohms or as a time constant (megohm x uF). A typical aluminum electrolytic has IR of about 100 megohm-uF. Film capacitors can exceed 10,000 megohm-uF.
The Relationship
Higher dielectric constant materials generally have lower insulation resistance. The same atomic properties that enable high polarization also make it easier for charge carriers to move through the material:
- Polypropylene film (constant ~2.2): Excellent IR, near-zero leakage. Used in sample-and-hold circuits
- Aluminum oxide (constant ~8-10): Moderate IR. Leakage in microamps is normal
- X7R ceramic (constant ~2,000-4,000): Lower IR. Not suitable where DC leakage matters
- Y5V ceramic (constant >10,000): Poor IR. Self-discharges noticeably within seconds
Where This Matters
- Timing circuits: A 555 timer with a leaky capacitor will drift. Use film or NP0 ceramic
- Sample-and-hold: Holding capacitor must have extremely high IR. Polypropylene only
- Power supply filtering: Leakage of a few microamps is irrelevant when filtering amps. Aluminum electrolytic is fine
- Battery devices: Leakage drains the battery constantly. Choose low-leakage types
How to Check
Insulation resistance decreases with temperature (roughly halving for every 10 C rise) and humidity. If your circuit misbehaves only when hot or humid, suspect capacitor leakage. Measure by applying rated DC voltage, waiting 1-2 minutes, then measuring the steady-state current. IR = voltage / leakage current.
