Sunday, September 25, 2011

charging supercapacitors

Supercapacitors are super because they can have much higher energy densities than capacitors that aren't so super.  A capacitor becomes super in the tens to thousands of Farads range, compared to the measly pico to microfarad range of not-super capacitors.  The eventual development of cheap supercapacitors seems to be what has led to the 'standlight revolution' in commercial dynamo lighting systems.  Back in the old days, when you stopped your lights would go out, although I've seen references to a few obscure vintage lights with battery powered standlights.  Now, when you stop, a charge stored in your light's supercapacitor can keep the LED on for a few minutes.  Supercapacitors are a good choice for standlight power as they can be charged up very quickly and can go through many many more charge/discharge cycles than any kind of battery.
3V 20F supercapacitor. Yours for only $8.38 CAD!

While these commercial standlights work well, my main complaint about them is that the lights dim significantly when they switch to capacitor power at a stop.  Commercial standlights typically use a 5.5V 1F capacitor. This doesn't store enough energy to keep the LEDs on at full brightness, so the discharge of the capacitor is slowed by dimming the LEDs (this applies mostly to the standlight for headlights).  Personally, I'd rather have a brighter standlight, which should be possible with a larger supercapacitor.  Higher value supercaps are, of course, bigger and more expensive than the little Gold Caps that are used most frequently.
b&m taillight guts

The biggest issue with supercapacitors is that they can only handle small voltages, typically between 2.5 and 3.0V for supercaps over 10F.  If you charge them beyond that voltage they can be damaged, or worse.  So, strict voltage regulation is required.  Another big issue is that you need to limit the current going into a supercapacitor. They'll take what current they can get, as they appear to your dynamo as a much lower resistance load than your LEDs.  This means your lights won't come on until your capacitor is charged up.  Finally, the low voltage of of the capacitor creates a problem for lighting LEDs. LEDs typically have forward voltages (Vf) in the 2 to 3V range, but don't operate at any voltage below their Vf.  So, a 2.7V supercapacitor can only drive a 2V LED until it reaches 2V.  With 2V across it the capacitor still has lots of juice left in it, but the LED can't get it out on its own.

This is a functioning DFN-DIP adapter, but it took a while...
My first solution was to double them up in series. Two 2.7V 20F caps can become one 5.4V 10F cap, giving plenty of voltage overhead for powering LEDs.  Turns out, voltage balancing is crucial when charging a supercap stack, as any unbalanced voltage can cause one cap in the stack to go overvoltage. Linear Technologies makes some nice looking chips that take care of that balancing and limit the current too.  I started with the LTC3225, which, on paper, looked like a perfect solution.  I went to the trouble of soldering the tiny 8 pin leadless DFN package onto a homemade DIP board before I wizened up and found an adapter board from Ancaster, Ontario's very own Proto-Advantage.  The LTC3225 worked as advertised except for one crucial aspect: despite programming it to charge the caps at 200 mA, I could only get it to charge at 30 mA.  Either I was doing something wrong or the one they sent me was way out of spec.  Either way, I opted to try another Linear supercap charger, the LTC4425.

LTC4425 application schematic

The LTC4425 worked and I was able to get it charging up a supercap stack at 200 mA (or whatever current was programmed with the Iset resistor).  In practice, however, the LTC4425 couldn't limit the inrush current of a discharged stack, which meant that for the first few seconds, all of the available current went into the supercap stack and none went to the LEDs!  As soon as the stack had some minimum voltage across it, the current regulation would start working properly.
LTC4425 MSOP-12 package. Pain in the butt!

While this was kind of a bummer, it pushed me towards a cheaper and probably better solution. While working with the LTC4425 I was introduced to a wonderful little chip called the ZXSC310 by the wise folks at candlepowerforums.  This is a LED boost driver than can drive LEDs with a Vin as low as 0.8V.  With a supercap charged up to 2.7 or 3.0V, there's plenty of voltage overhead to drive power LEDs at reasonable currents.  In practice, the 310 worked great and eliminated the need for a stack of supercapacitors, which meant I only needed to accomplish the relatively simple task of charging a single supercapacitor.  This is easily achieved with a voltage regulator and a current limiter.

In my case, I chose a 3.0V linear regulator (potentially a bad idea as it has to drop a lot of voltage from the dynamo under certain circumstances), the MIC5209, and a current limiting load switch, the FPF2125.  This circuit very effectively limits the current going into the supercapacitor under all circumstances, so the LEDs get their share, eventually getting all the current from the dynamo once the supercap is charged.
Supercapacitor charging circuit

1 comment:

Kevin McDonnell said...

This seems like the last step in perfecting a Sturmey Archer lighting system upgrade to LED (aside from the reflector issue and the heat dimming issue).

From my naive perspective I'm curious to know what the setup you ended up with here looks like (chip, supercapacitor, voltage regulator, current limiter and connections). Can it fit inside a headlamp?