# 50F Cap as a 1.5V Power Supply?....

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#### 2XS

##### New Member
I am currently working on my project for my Mid-Term assessment for my EE bachelor degree. The project I chose is basically the Forever Flashlite: using electromagnetic induction to generate a charge stored on a cap which, in turn, powers an LED for about 10 min.

I've found little information online (including this site) to aid in the design. So, I purchased the Forever Flashlite and ripped it apart. After some reverse engneering, the cicuit ending up being quite simple. The output from the coil went into 2 Shottky barrier diodes BAT54A & BAT54C; the tapped output from the BAT's went across a zener diode in parallel with a .22 F cap.

:?: THE PROBLEM

Applying a load > 60ohms (drawing 25mA) across my 3mF cap charged to 1.7V will (obviously) immediatley discharge the cap. Does anyone have any ideas on how to slow down the discharge of the 50F (thats right, 50F) cap I am getting???

I have everything working, but I sure would like to kick tao's (RC) a; damn the Conservation of Energy.

Any input on this topic will be greatly appreciated.[/i]

#### Klaus

##### New Member
Well, if I remember this right, the time constant of a cap is RC. You obviously are getting a very big cap , so, to extend the time of discharge you also have to increase the R part. Perhaps pulsing your LED very fast to decrease the mean current?
But you knew that already, right? :wink:
Oh yeah, 50F can hold a fair amount of charge, dont short it out accidentally
Klaus

#### 2XS

##### New Member
Klaus said:
...to extend the time of discharge you also have to increase the R part...
^This would be great advice if my load wasn't fixed at approximatley 60-200 ohms.

I spoke to one of my professors and he said to look into using a FET to regulate/dissipate the voltage off the cap (i.e. 5V on the cap, 1.5V on the output) as a method of slowing the drain off the cap.

Essentially, I am looking for a way to change the exponential drain off the cap to a linear drain...like a battery :wink:

Any comments/suggestions/critisism would be greatly appretiated.

#### Phasor

##### Member
Essentially, I am looking for a way to change the exponential drain off the cap to a linear drain...like a battery
Well, a battery isn't really linear, but we'll ignore that for the moment. To obtain a linear decrease in capacitor voltage, you must draw a constant currrent.

At first, a linear constant current regulator springs to mind, but this could be quite inefficient. I envisage a small switching PSU, whose input can be anything from, say, 2-5V (from the capacitor), and output is constant 1.5V (or whatever you need).

You can get all-in-one switching ICs, eg TOPswitch (http://www.powerint.com/topswitchproduct.htm). You may find one suitable for your needs.

#### bonanz

##### New Member
2xs,

Can you post some more info about the circuit you got from the forever flashlight? I am working on a model maglev train project, and want to use electromagnetic induction to charge a large capacitor on the car of the train to power a small emergency baking system that will only be utilized for very short intervals. i'll be able to induce a pretty good sized voltage to charge the cap (about 1-2v per coil for 4 coils). so source voltage won't be a problem (like shaking may be). and i can easily clean it up and get the voltages and time constants that i would need, i'm just curious to see a layout of how those forever lights work without having to tear one apart (although i am tempted).

any help/suggestions from you and/or other members would be appreciated.

bonanz

ps. don't mean to hijack thread, sorry if it seems like that, but i felt it was enough along the same lines and i'm asking specifically about 2xs's specific circuit.

#### 2XS

##### New Member

This is the basic schematic where the diode bridge is made with Shottky barrier diodes and 'R' is a zener diode.

#### 2XS

##### New Member
Phasors suggestion about a switching regulator led me to what I believe to be a solution to the load issue. National Semiconductor's LM2665 and LP2980 seem to be the missing link.

The LM2665 (switched capacitor voltage converter) doubles input votage and generates a 40mA current the LP2980 (micropower 50 mA ultra low-dropout regulator) regulates; both have very low current drops. Page 8 of the LM2665 data sheet depicts the schematic with both models working to generate a regulated 5V 25mA from a 3V 650uA source

In theory, the combination of the full wave rectifier input and the LM & LP devices regulating the output should allow for a gradual discharge of the 50F cap. Key Word: Theory.

Any follow-ups to confirm or deney this allogation would be greatly appretiated as I am waiting for the parts in the mail.

#### Roff

##### Well-Known Member
National, Maxim, and Linear Technology all have high-efficiency switchmode current sources designed for the purpose of driving LEDs. I think they work off the voltage range you are expecting from your rectifier (eliminate the zener). Go to each site and search for "LED driver".

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