Simple and effective method to determine the polarity of an aluminum electrolytic capacitor.
Here is a method that should work.
I've never seen it described before BUT it is based on very well proven practice.
It is well recognised that an effectively unpolarised capacitor can be formed by placing two electrolytic capacitors in series with opposing polarity. When a DC voltage or a half cycle of AC voltage is applied the "correctly" polarised capacitor acts to acquire charge while the reverse polarised capacitor has only a very small voltage drop across it. This method is well enough recognised to be mentioned by some capacitor makers in their application notes and is used in many real world designs.
Even Cornell Dubilier say that it works:-) . They say:
If two, same-value, aluminum electrolytic capacitors
are connected in series, back-to-back with the positive
terminals or the negative terminals connected, the
resulting single capacitor is a non-polar capacitor with
half the capacitance.
The two capacitors rectify the
applied voltage and act as if they had been bypassed
by diodes. When voltage is applied, the correct-polarity capacitor gets the full voltage. In non-polar aluminum electrolytic capacitors and motor-start aluminum electrolytic capacitors a second anode foil substitutes for the cathode foil to achieve a non-polar
capacitor in a single case.
The method relies on the validity of the assumption that a reverse biased electrolytic capacitor will "safely " pass reverse current without damage. This assumption seems proven to be valid for wet aluminum capacitors but may or may not be true for eg Tantalum capacitors. Caveat Emptor :-) - although no great harm should come apart from , worst case, the destruction of a Tantalum capacitor (which may be considered to be of net social benefit in some circles :-) ).
Ensure that capacitor orientation can be determined, either from markings or other appearance or by adding a mark such as a small dot with a marker.
Connect two capacitors in series with opposing polarity.
Connect a voltage of "a few volts" somewhat to much less than rated voltage. Say 5V for a 10V to 563V cap but not critical.
Measure voltage across each capacitor.
The capacitor with the largest voltage across it is (probably) correctly polarised.
Example only. Your Voltages Will Vary.
If the voltage across each capacitor is about equal or is dominated by meter resistance then the capacitors probably aren't electrolytic capacitors.
In a very simple test this method was exceptionally successful.
Two 25V, 100 uF capacitors were connected in series with opposed polarities and about 6V applied to the pair. The large majority of the voltage dropped across the correctly polarised capacitor. Under 0.5V dropped across the reverse biased capacitor. Changing the applied polarity resulted in a swap of relative voltages (as expected) so that the correctly biased capacitor again deopped most of the voltage.
The test was repeated with a 1uF and 100 uF capacitor in series with opposed polarities. The results were as before with the forward biased capacitor being very easily identified.
This test MAY fail if very low leakage and very high leakage capacitors were tested together.
The same effect could be used to identify correct polarity using reverse biased leakage current. Applying a voltage with each of two polarities should produce a much higher leakage current when inverse polarity was applied.
Use of a meter's highest Ohms range may also allow relative leakage currents to be measured but some meters may not apply enough voltage to do this well. (I tried two cheap meters with 2 megohms max ohms range - not high enough. Meter O/C voltage was only about 0.3V in each case.
Just using a power supply, a single capacitor and a series resisor will utilise the same effect. Using say +5V and a 100k resistor the capacitor will have greater voltage across it when correctly biased than when reverse biased. However, using two nominally identical capacitors allows you to let them "sort out" the effective equivalent resistance value required.