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[on:]

As part of the #Shrimp Arduino-compatible circuit design described at http://shrimping.it we suggest a 10uF electrolytic capacitor to smooth the power supply following a discussion here on the forum.

However, I'm concerned that working with these components in a classroom could be dangerous, given the possibility (guarantee) of someone reversing the capacitor even in a 5V circuit.

Is this as serious as I fear?

Are there mitigating approaches I can take, such as reducing the capacitance to 1uF, which was an option discussed during circuit design (I'm guessing this means less explosive material) or selecting capacitors which are designed to be safer. Obviously leaving it out is an option, but I gather it will be needed for certain projects to keep the power supply regular.

Are mitigations such as gloves and eye protection considered to be enough protection when powering up a test circuit at 5V with a potentially reversed electrolytic? Other thoughts?

Currently the component I'm distributing in kits (on a non-commercial basis) are these...
http://www.taydaelectronics.com/10uf-25v-105c-radial-electrolytic-capacitor-5x11mm.html

Glad of your contributions.

[Reply #1 on:]

Try it - put one across a supply backwards, put a box over it, power up the supply. Give it a couple minutes, see if anything happens.

[Reply #2 on:]

Reassuringly I found this investigation...

The electrolytic capacitor can be modeled at an ideal capacitor with two back-to-back zener diodes in parallel with it, with each zener representing one of the cutoff voltages. The leakage slowly increases for voltages below these cutoff voltages, and then quickly for levels above the cutoff voltages. The performance is very consistent within these bounds as well, with differences most likely accounted for by heating effects due to the increase in current flow at higher voltages. So it seems that an electrolytic can be used backwards with negligible change in circuit behaviour, as long as it is within these bounds. These bounds change on both the positive and negative sides with capacitor voltage rating. A 1uF/50V capacitor was tested, and it operated up to -16V in reverse bias, which suggests a linear relationship between forward and reverse breakdown voltages. So if you plan to use a capacitor backwards, give it a quick check first to find out where its reverse breakdown begins. A DC current of 0.1uA is a good threshold to use.

at http://wiki.openmusiclabs.com/wiki/Electrolytic

If I interpret correctly, this experimentation suggests that a reverse bias of 5V on a Capacitor rated for 25V may be well-behaved as I hoped. It also offers a way to test the voltage threshold for the runaway overheating behaviour for a given cap, which may be worth experimenting with. I may well drop down to a 1uF cap and will consider bringing up the rated voltage to 50V to minimise some potential issues, though.

[Reply #3 on:]

Obviously you expect the students to be well-behaved. As compared to my Son's class where the trick was to connect a small electrolytic to the end of an extension cord, and later plug it into 120VAC.

[Reply #4 on:]

We do this in class, great fun!
Take a 10uF or similar, large V rating, 63V is great, connect it backwards and flip the switch, doesn't take long and you get a very satisfying bang, try it! just make sure you have a heap of capacitors as it is addictive.

[Reply #5 on:]

While capacitor explosions are fun they are not too dangerous. Most caps have vents or pre weakened tops, so just don't bend over one when powering it up as when they blow they blow upwards.

[Reply #6 on:]

Another possibility which was suggested by @oomlout was to upgrade/downgrade to a 1uF Ceramic, which eliminates the problem, and also probably eliminates the need for the 100nF ceramic in parallel (provided as a more responsive smoothing source before the electrolytic kicks in). It's only 18 cents from Tayda, but each decision like that adds another few pence. It's great to be able to say you can make one for £1.40, and that puts it nearer £1.50. Might make @ShrimpingIt a bit easier and more svelte, though (fewer wires pushed in the same holes and lower profile).

[Reply #7 on:]

was to upgrade/downgrade to a 1uF Ceramic, which eliminates the problem,

And probably stops it from being electrically an effective solution. Many regulators need a minimum of 2 to 3uF and so putting 10uF is the simple solution.
What is to stop students putting diodes or other orientation sensitive components in wrong. A simple rule of letting the teacher check before powering on would work. As would running your parts from a low current capacity power supply. You don't get anything much of an explosion from a 500mA power supply.

My fear is that this bogus safety concern will be used as an argument by the microsoft / health and safety muppits to ban the whole subject.
Do some experiments yourself with 5V and aluminum capacitors and see how unspectacular the results are.