Wednesday, May 11, 2011

Big Capacitors!

Get a load of these babies!
Giant 1 Farad capacitors.  Used originally on super loud car audio systems.

I found them at a car boot sale, going for a tenner for the pair.  They'd cost about £30 each when new.

They have these so called protection circuits on the top.  I thought that it was an anti-surge device for when they have power applied.  They have extremely low internal resistance (maybe as low as 0.002 Ohms) so can draw a huge surge current when connected to a source of current.   But no, the circuit turned out to be just a buzzer that sounds when they are reverse connected by accident, or when subjected to over-Voltage.  They have a quite small maximum working Voltage of 16V.  Given the near explosion that occurred when I over-charged the baby capacitor on the bike generator, these would pose quite a risk, if mal-treated.
The display reads Volts and the two other LEDs are really just for show (they're for car audio "enthusiasts" after all)...  Although, they do serve a useful purpose as discharge loads, to make the capacitors safe in a short time after the power is removed.

Luckily, the tops just unbolt, and you can remove the circuit board.  It's just held on the capacitor terminals with press contacts.  Then the bolts can screw back into the terminals directly.
The Voltmeter circuits are possibly useful later for something.  On their own, they're worth £5 each, so the capacitors were actually "free".
I thought that as I had a pair, I could wire them in series to allow them to be charged from one of the left-over small amorphous solar panels.  Wiring them in series increases the Voltage they can handle, but reduces the capacitance, so you get a 0.5F capacitor that is safe up to 32V.  If you wire them in parallel, you get a 16V 2F capacitor, and mind bending amounts of current (the internal resistance is halved again to 0.001 Ohms, giving a short circuit current of up to 16,000 Amps!).

The panel puts out about 23V, open circuit, in the sun, but you can't use that Voltage, as the current is so low.  But you can charge a 0.5F capacitor from that current, over a minute or so.  The capacitor, unlike a battery, does not seek to limit the Voltage it charges to.  So, eventually, it will store a unit of energy at the highest Voltage attached to it, and can later deliver that energy with massive current.

I disconnected the charge source, and started measuring the Voltage across the capacitors, thinking the charge would leak away in a few minutes.  To my surprise, only about 10% of the energy had leaked away after 24 hours!  The energy level of a capacitor is exactly proportional to the Voltage it reads, unlike batteries that can read 12V, but be completely flat.

But what use are giant capacitors..? I'll let you know when I think of something :D  They're just cool toys right now.  A bit dangerous though.  I shorted the wires of the pair when charging it from the small solar panel, and the entire lug at the end of one wire completely disappeared in a green flash of copper vapour discharge light.


  1. What use are they - could use them to make a mini welder?

  2. You certainly can.

    I'm building one of these CD welders for making battery packs:

  3. Hey, I am impress with this setup.Please answer the following questions based on your post.

    You said:

    "The panel puts out about 23V, open circuit, in the sun, but you can't use that Voltage, as the current is so low. But you can charge a 0.5F capacitor from that current, over a minute or so."

    When you say charge a 0.5F capacitor in a minute or so do you mean from the solar panels you are using? The one with the low current?

    Also could you say if it makes a difference whether the overvoltage reverse connection warning circuit makes a diiference when removed from the capacitor?

    Keep up th good work!


    1. Yep, even with such a low current at 23V, the panel can charge up the capacitor. It does not leak much energy by itself (10% in 24 hours) so the capacitor can charge from a very weak source. At 18V the solar panel puts out about 650mA, so the capacitor charges up quite quickly to that level and then more slowly to 23V. The over voltage / display readout thingy uses a fair bit of power (especially the LED display) and it's actually designed to leak the capacitor charge away in under a minute so that you don't have the energy stored for a long time and risk unsuspecting folk vaporising their spanner when tinkering with the capacitor some time after turning the power off.

      Without reverse protection or over voltage protection, the capacitor can do what all electrolytic capacitors do really impressively - explode.