Squeezing the power out of a Supercap

[steveo_mcg]

The old circuit discharged in about two minutes to <10ma and started about 40ma dropping to 20ma after about 30s. The new circuit starts ~100ma and takes about 5 minutes to drop to <10ma. I've got a 10r resistor in there so its discharging too quickly, I'm going to try and find one to keep it around 30-50ma as long as possible.

When i've missed out the link between the resistor and the super cap the current running out the supercap to the emitter via the diodes is very low, the resistor must be the path of least resistance.

I ended up with two silicon and 1 schottky which charges to about 5.3v.

 

[Bobblehat]

The old circuit discharged in about two minutes to <10ma and started about 40ma dropping to 20ma after about 30s. The new circuit starts ~100ma and takes about 5 minutes to drop to <10ma. I've got a 10r resistor in there so its discharging too quickly, I'm going to try and find one to keep it around 30-50ma as long as possible.

That's good data for anyone wanting to build any of these circuits.

When i've missed out the link between the resistor and the super cap the current running out the supercap to the emitter via the diodes is very low, the resistor must be the path of least resistance.

Yep! I got that wrong by the looks of it (I said I wasn't an expert). I had a feeling the diode route might have had more influence ..... good to know that the major control of the current is the resistor at the sort of currents we are likely to want.

I ended up with two silicon and 1 schottky which charges to about 5.3v.

A useful rise over 3.2V (or 2.7V, can't figure out why!) ...... and more energy to power the standlight. When you try a higher ohm resistor (50ohm ... 100ohm?) also try the diode in the feed line to the supercap ....... it might just make a difference to extending the run time ...... but then I was wrong last time :shakehead

I can't wait to get all my bits and start building the more complicated 2 front + 1 rear circuit Diodes arrived today ...... still gathering bits.

 

[Savvas]

A useful rise over 3.2V (or 2.7V, can't figure out why!) ...... and more energy to power the standlight. When you try a higher ohm resistor (50ohm ... 100ohm?) also try the diode in the feed line to the supercap ....... it might just make a difference to extending the run time ...... but then I was wrong last time

So Steveo and Bobblehat, just to clarify things a bit, are we left with a circuit that has a string of diodes in series with the standlight led with a supercap and series resistor paralleling this same led?

No need for a separate diode (or diode series) to charge up the supercap directly from the +ve rail?

BTW, with the 'standard' 2 led/ 1 standlight set up (using XR-Es) a 100ohm resistor and 1F supercap gives me 5-10 mins quite noticeable light.

ta,

Savvas.

 

[znomit]

No need for a separate diode (or diode series) to charge up the supercap directly from the +ve rail?

The point of the diodes in the original circuit is to drop the +ve rail 2x Vf (~6 or 7V) to something the super cap can handle (below 5.5V).
Here the power rail is below the super cap rating so no worries.

From testing I noted that the super cap voltage quickly dropped to around 4V... so getting the full 5.5V into it doesn't increase runtime significantly, it is nice and bright for short stops though.

 

[Bandgap]

The sudden voltage steps - rather than gradual changes - could easily be due to the internal resistance of the super caps.

the smaller types - sometimes called uA types - have a lot of resistance inside.

mA types less so. Large amp types have very little internal resistance and brightness would hardly change whe the dynamo stops.

so from your initial voltage drops, you could calculate internal resistance - which also by the way slows charging.

internal resistance will also be on the capacitor's data sheet.

Steve

 

[steveo_mcg]

A useful rise over 3.2V (or 2.7V, can't figure out why!) ...... and more energy to power the standlight. When you try a higher ohm resistor (50ohm ... 100ohm?) also try the diode in the feed line to the supercap ....... it might just make a difference to extending the run time ...... but then I was wrong last time :shakehead

As above, the 2.7v is the voltage sag with the (relatively) high drain from the emitter, the actual unloaded V was about 3-3.3v.

@znomit, I think for turning right that short bright stand light won't hurt, at lights I don't suppose it really matters once you've stopped in a traffic queue.