Electronic Design

# Dielectric Absorption, Soakage, And Voltage Discharge From A Supposedly Passive Capacitor

We can never charge a capacitor completely, unless we wait for infinity. The reverse is also true when discharging a capacitor. In the case of a CRT starting at a high voltage, it will deliver a painful shock for a long time.

My first experience with soakage was a pleasant one, not like my first experience measuring a power transformer.

When I was a teenager, a local “ham” operator (a mid-twentieth century term for an amateur radio hobbyist—am I giving away too much of my age?) repaired TV sets in his garage. I learned a lot from him, some by practical demonstration. He had a disconnected power transformer with bare leads lying on the bench. He suggested that I could measure the resistance with an ohmmeter on the same bench. Naively, I grabbed the two probes and pinched each probe to a bare wire. Zing! Even though the meter was only powered by 3 V, the inductive kickback was enough to make me remember not to do that again.

When it came to soakage, he took pity on me. (He wanted me to remember, not to kill me.) This time he grounded out the CRT like Bob Pease did1 and showed me what charge remained in a few minutes. Then I did it too and was amazed at how long the charge remained—it seems to go on forever. (I think that, after a while, I just stopped trying out of boredom.) Keith Snook2 continues the dielectric absorption (DA) discussion. It is a great subject that deserves more attention.

The answer is inherent in what we all have learned: we can never charge a capacitor completely, unless we wait for infinity. Most circuits are considered practically charged after five time constants when the voltage is at 99.3% of the total applied voltage. The reverse is also true when discharging a capacitor. In the case of a CRT starting at a high voltage, it will deliver a painful shock for a long time.

References

1. Pease, Bob, “What’s All This Soakage Stuff, anyhow?” Electronic Design, May 13, 1998, http://electronicdesign.com/analog/whats-all-soakage-stuff-anyhow.

2. Snook, Keith, “What’s all this Trapped Charge and Dielectric Compression Stuff Anyhow?” http://www.keith-snook.info/capacitor-soakage.html.

Bill Laumeister is an engineer in strategic applications with the Precision Control Group at Maxim Integrated. He works with customers who use DACs, digital potentiometers, and voltage references. He has more than 30 years of experience and holds several patents.