Capacitor Reforming

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This page is still under significant construction and may not read well as a whole yet

There are some instances where replacing electrolytic capacitors may not be necessary. There is a difference between a "bad" capacitor and one that has drifted out of spec. Reforming is a complicated subject, with sometimes strong opinions both for and against. Some opinions are due to misunderstanding or miscommunication as to when it is appropriate to reform a capacitor. If you don't see any signs (visible or measured) that a capacitor has failed, then you may be able to bring it back to with-in spec and continue using it. As long as there aren't any leaks, capacitor lifespans are usually not reduced by sitting unused; they may just need to be reformed.


You need to have a good understanding of how electrolytic capacitors work in order to know what the reforming process does. Here are some points about them that you should know:

  • Anode is a metallic sheet
  • Cathode is an electrolytic fluid
    • The cathode is also consumed to form a dielectric barrier
      • This consumption is part of the rated operating life of the capacitor
      • If the capacitor’s electrolytic fluid has been depleted it is not feasible to replenish it and the reforming process will only temporarily help
  • The dielectric between the two is an oxide
    • The oxide has a breakdown voltage that will allow DC to pass
      • The oxide degrades over time reducing the breakdown voltage
    • The oxide is formed when a voltage is applied to the capacitor
      • This is measured as DC
      • If the oxide is degraded it can be rebuilt by applying a low voltage and slowly increasing it to the capacitors rated voltage
        • This is the reforming process
        • It needs to be current limited to prevent the process from happening too quickly and heating/damaging the capacitor

Variacs or Externally Controlled Reforming

I do want to briefly mention the idea of using Variacs to reform all the capacitors in a device at once. This is relevant for purely analog electronics like CRT Oscilloscopes, early CRT TVs, and Tube Radios because the circuits are more interconnected. More modern devices with regulated power supplies do not derive voltages in the same way as analog devices do. Additionally, power and signals are usually controlled more in devices with integrated circuits and you are not guaranteed to be able to change the voltage going into all of the capacitors in a device. You cannot use a variac on more modern devices as a result and trying to may even damage them. So my general advice is to just focus on individual capacitors when troubleshooting rather than try something like a variac.

Reform or Replace

Reforming capacitors is only applicable if a device has not been used in a long time (as in years), and the capacitors are presumed good (or, put another way, "it ran when parked"). If you know that the device sat for a long time, then was powered on normally and and found not to work, the damage is likely permanent and any affected capacitors will need to be replaced. Shorted/failed capacitors can also cause other parts to fail, leading to further repair needs. If there are any visible signs of failure of a capacitor (leaks, etc) you should replace it; reforming will not fix those problems.

Reforming is a preventative measure to potentially reverse natural deterioration in the capacitor. Reforming does not “fix” capacitors, it just prevents potentially healthy capacitors from failing

  • 1980s and older Electrolytic capacitors do not fail as often as modern capacitors
    • Paper capacitors are more likely to fail
  • Typically, power supply capacitors and other large analog circuit capacitors are the only ones that are reformed
    • These capacitors can be costly, $30/ea, and potentially reusing them can save a significant amount on a restoration
      • Exact mounting types and values may also be difficult to match with modern replacements
    • Signal smoothing capacitors do not have a significant amount of current flow through them in circuit and will likely reform in place quickly without incident

How to Reform a Capacitor

  • You need to apply a DC voltage to the capacitor at a low current for an extended period of time to rebuild the oxide layer
    • Use an adjustable voltage PSU to regulate the voltage
    • Use a current limiting resistor to regulate the current
      • A resistor value of 100-1000x the capacitor’s rated voltage is recommends as it will produce a current between 1-10mA that will scale with the adjusted voltage
    • Recommended current limit varies between sources
      • Less than 1mA is easily possible
      • DoD recommends an average of 5mA but that is on the high side
  • You need to know what the voltage and current is at the capacitor which will require two meters.
    • A true dial analog meter for the voltage is recommended because it can double as a capacitance tester after the process is complete
    • The voltage at the capacitor will slowly rise as current consumption falls
      • There will be a point of diminishing returns that will be your indicator to increase the voltage to the capacitor
    • I recommend deciding on a max current limit, very slowly increasing the voltage until you hit that limit.
      • Leave the voltage alone until the current reduces down to a level where it takes significantly longer to see a noticeable decrease
      • Once that point has been reached, increase the voltage again until you reach your target current limit
      • Once you have reached the voltage rating of the capacitor you are at the end of the reforming process

Determining a successful reform

  • A capacitor has been successfully reformed when it is capable of handling its rated voltage again.
  • As long as the capacitor did not short during the reforming process, the reforming can be considered successful
    • There may be a significant increase in DC resistance once the oxide is reformed
      • This is only anecdotal evidence at this point though
  • That is not the only measure of a healthy capacitor
    • The capacitance itself still needs to be verified
      • This can be done by charging the capacitor to its rated voltage applying a small load to discharge it
        • Measuring the discharge rate is an indication of capacitance
        • Too fast is a fail
    • ESR is another measure of health
      • ESR is typically measured now with dedicated meters
        • They are unsuitable for capacitors that would typically need to be reformed