The Secret Life of Capacitors

[nul-points]

last Saturday, July 10th i setup the capacitor datalogging again, this time with a 2200uF capacitor

as before, the capacitor was enclosed within an aluminium case and it also had Neo magnets aligned with the capacitor can

the magnets are held in place with a rubber band - only two turns have been used here, only just enough tension to hold the Neos in place - so it's extremely unlikely that the rubber band is deforming the cap's aluminium can, as has been suggested

(it's also been suggested that the application of magnets only makes a "momentary" influence on the spontaneous voltage, but it's clear from Fig. 15, the 'magnet-test' graph in the PDF, that the magnets create a visible increase in the level of the voltage profile for the entire length of time that the magnets are applied - eg, 2 hours in one test - 45 minutes in the second test - hardly 'momentary'!)

the purpose of this most recent test was to compare the behaviour of the 'spontaneous' voltage on the capacitor in a cold, temperature-controlled environment (a domestic refrigerator) with that in a sustained hot environment (outlet of a laptop PC fan)

the test was allowed to continue throughout Sunday 11th, also


there are two main behaviours to note:-

a) the 'spontaneous' voltage does not follow the static level of the temperature - it responds mostly to the change of temperature

as you can see from the red trace on the dual-trace graph below, the temperature 'profile' to which the cap was subjected, was like a pulse waveform:  it started by decreasing rapidly from approx 28*C to 5*C, it remained at 5*C for 10 hours then it rose rapidly to approx 43*C and remained at around that level for 29 hours

what happened to the capacitor voltage profile whilst subjected to this temperature profile?

well, the voltage increased or decreased with the temperature changes, but then it gradually reverted back towards 0V after each change - it showed two 'overshoots', one negative, one positive, at the two major temperature transitions

it has almost 'differentiated' the temperature profile

so the temperature response of the spontaneous voltage is not too dissimilar to the electrical response of a capacitor to the passage of charge-separation current, in response to an electrical 'step' voltage

from this we can predict that spontaneous capacitor voltage will follow faster temperature increase/decrease trends better than steady-state temperatures


b) although the spontaneous voltage remained almost constant whilst in the 5*C environment, in contrast, at the elevated temperature and following the positive 'overshoot', it began to respond to the external influence which causes the daily periodic variation (similar to that seen in the collected data shown in the appendix of the PDF), at a lower peak-to-peak swing

the single-trace graph below is expanded vertically to show the detail of the spontaneous voltage profile - the two graphs below cover exactly the same time period

as you can see, on the cold-to-hot transition of its environment the capacitor self-charged to nearly +5mV, whilst still shunted by the 0.86Mohm resistor, and having sustained a negative voltage for the previous 10 hours


so the outcome of the test is that the spontaneous voltage does respond to temperature, mostly dynamically, but it is not dictated by it
  eg. the voltage was equally able to go negative when the test rig was heated to 43*C (21:00 July 11th) as when it was cooled to 5*C (10:00 July 10th)