Supercapacitor + LED for a FRED?

Here is another choice for a supercapacitor.

0.22F 5.5V, 10mm diameter.

Would probably need a resistor between it and the LED.

 

I use these,or something like it on the regulated side of a 7805 circuit for passenger lighting.

 

7805 circuit Described here

 

www.jcstudiosinc.com/BlogShowT...=408&categoryId=

 

Mine are 1 farad each,not .22, I use 5 farads for 6 LEDs on a strip.

 

When voltage is removed the car lights stay on reasonably bright for about 2 minutes. They dim slowly but still glow,even an hour later.

 

If you charge the circuit to 5 volts with a 7805,then use a cl2-n3 chip to protect the LED instead of the resistor shown here

 

www.jcstudiosinc.com/BlogShowT...=409&categoryId=

 

A single red LED should flash 2 minutes or so I think with 1 Farad,just a guess might go longer.

 

If no resistor or CL2-n3 is used you could only charge the cap to about 2 volts,the usual forward voltage of a red LED.

 

Dale H

 

How about a little dangling pickup to sip a charge from the tracks on and off.

Because that would mess up the trackside Equipment Fault Detector that I'm working on.

I think a removeable, rechargeable, micro LED, FRED is a great idea and would last long enough. Just charge, and  drop the base into the coupler. Gravity or twist lock tab to hold it, touch with battery power to recharge once in a while. Reminds me of HotWheels Sizzlers and choppers of the 70s. They were motorized hotwheels driven by capacitors, recharged by battery packs, very cool, very fast. Like the little one minute recharge, micro remote controlled cars of today. Ever see the bottle caps with flashing micro LEDs for advertising in the bars? I had one last 2 years before its micro button cell died. Even three stacked would still be smaller than a normal capacitor. Micro buttons, or a super cap in some thing like this would be cool. I wonder what shapes and sizes could be found with focusing on the "special" productions of caps and batteries. I seen some pretty odd new stuff over time. 

One challenge with supercaps is their voltage decays as they discharge...as opposed to batteries which essentially hold their voltage for most of their discharge cycle.  The Evan Designs FRED claims to run 100 hours on a 3V CR2032 battery. 

 

Some back-of-envelope calculations:

 

CR2032 capacity is ~200 mA-Hrs.  This provides ~2000 Joules of energy (3V * 0.2A * 3600 sec).  A 2.5V 1 Farad supercap stores ~3 Joules of energy (1/2 * 1 Farad * 2.5V * 2.5V).   So if you were able to use all the energy in that supercap, it would last only a few minutes.  It's worse than that since the supercap is constantly losing voltage as it discharges. Say you can only use the supercap between 2.5V and 2.0V to power a flashing red LED.  That's about 1 Joule of usable energy.  So this would be only a few minutes of operation.

 

Some of those cheap RC fast-charge cars mentioned above use tiny Nimh batteries such as 1/3AAA (1/3rd the size of a AAA battery).  IF (big IF) you can fit two of those (to get 2.4V), their voltage would hold across the discharge cycle and you could get hours of operation.

That's why I thought the idea of having some little pickup that "sips" power now and then might work.

Stan

 

Never tried it but if you charge the button capacitor I mentioned (1 Farad,5.5 volts) to 5 volts and use a CL2-N3 chip the decay is less of an issue. I imagine the LED will flash even down to 1.5 volts or less. The original poster does not want to use a battery which is a whole different approach.

 

Dale H

Hi Dale,

 

There are a couple issues with the CL2-N3 approach which I'd be weary of.  First, per the datasheet, the operating voltage spec V(a-b) is 5V minimum which is the difference between the input and output.  So if the flashing LED takes 1.5V, this requires a supercap voltage of 6.5V which would damage it.  I realize this V(a-b) spec is undoubtedly conservative and the actual drop-out voltage is surely less but the spec is the spec.

 

Second, the CL2-N3 is a constant current chip and my guess is a flashing red LED device is NOT a constant current load.  I suspect a flashing LED takes minimal current 99% of the time and then consumes a burst of current only when it flashes...and that neither of these currents or even the average current is 20 mA as a CL2-N3 is hardwired to deliver.  So if space allows additional electronics, I'd go with a 1.5V constant-voltage regulator rather and a 20mA constant-current regulator.  A CL2-N3 will try to apply ever increasing voltage to "force" 20 mA into the device which may or may not damage it.

 

My post was to highlight the difference in energy storage density between supercaps and batteries with what's available to the hobbyist today.  When I read the OP's post, I inferred that the problem was the size of batteries vs. batteries per se.  That said, I think it's worth taking another look at hearing-aid or watch button cells - about 10 cents each on that auction site.

I'm open to the idea of button cell batteries. I just can't find a suitable holder for them.

Hi Stan

 

Your points are well taken. Only way to really know is to try the circuit. As for the LEDs I think over current,not over voltage is what damages them. As long as the current is 20 ma it is protected I think.The voltage gap could be a problem. A fixed resistor would work valued for the 5 volts. I think a small battery is a better way.

 

I do use the supercaps on my lighting LED strips and am very happy with them. As a conventional operator it is nice to still have passenger car light when power is removed when the train stops.They slowly fade when the train is parked.

 

Dale H

BTW, on instructables.com there are several references to supercapacitor powered LED flashlights. Most of them seem to be using a 1F 5.5v supercap + small resistor + white 3.2v LED. Some claim to last 10min. Some claim 10hrs.

I'd take the 10 hour estimates with a grain of salt.

Yup.

I'm just gonna have to jump in, try it, and find out for myself.