# Zener Diode Question



## Bwells (Mar 30, 2014)

I want to use two 5 volt 1F super capacitors in series fed by 9.2 volts. From what I understand, some kind of balancing maybe required. My thought is to use two zener diodes rated at 5 volts wired in parallel with each cap. The max rated voltage of the caps is close to what I will be powering them with and from what I understand, it is preferable to run them at 80% and not 92%. Is this design dangerous or is the headroom OK? Any thoughts on this? I have no idea what a super capacitor does when it lets loose but I imagine it is not pretty!


----------



## D&J Railroad (Oct 4, 2013)

This is sorta like the math question of, "If one train leaves New York and another train leaves Chicago, and the New York train is going 5mph slower, in what city will they be in when lunch is served?"

Answer is:
I don't know.


----------



## Bwells (Mar 30, 2014)

Ken, I think we would need a little more info than provided, such as which direction and speed each loco is traveling. If both are traveling west, lunch is served between the hours of 11:30AM and 1:00PM. The towns could then be determined with the starting time.


----------



## J.C. (Dec 24, 2016)

Bwells said:


> I want to use two 5 volt 1F super capacitors in series fed by 9.2 volts. From what I understand, some kind of balancing maybe required. My thought is to use two zener diodes rated at 5 volts wired in parallel with each cap. The max rated voltage of the caps is close to what I will be powering them with and from what I understand, it is preferable to run them at 80% and not 92%. Is this design dangerous or is the headroom OK? Any thoughts on this? I have no idea what a super capacitor does when it lets loose but I imagine it is not pretty!


not knowing much about super caps , but sounds like smoke city.


----------



## Bwells (Mar 30, 2014)

Because the caps are operating at 92% of max or something else?


----------



## time warp (Apr 28, 2016)

A representative schematic would be helpful, no?


----------



## Bwells (Mar 30, 2014)

Yes, you are right. Try this one but replace the resistors with zener diodes. Scanner is down right now and this is all I could find. Thanks. Edit: the 8 volts shown is actually 9.2.


----------



## gunrunnerjohn (Nov 10, 2010)

For 5V caps, consider 4.5 or 4.7 volt Zener diodes. You really don't want to charge the cap right up to their voltage rating, they are fairly sensitive to over voltage. Also, the Zener diodes have a tolerance, they won't all be at their exact rating. You also don't want resistors for balancing as they'll add to the load and discharge the cap.


----------



## wvgca (Jan 21, 2013)

If the intent is to drive 2.2-3V LED's rather than something that needs to have more than 5V, it may be simpler to use a 7805 or similar voltage regulator in front, and then run caps in parallel as needed to get your desired capacity ..
Also most double stack caps are 5.5V or so ..check the spec sheet to make sure ..
A cap charge current limiting resistor is also a good idea, 100 Ohm or so may be a reasonable starting value..
enjoy ..


----------



## Bwells (Mar 30, 2014)

John, thanks, the size of the diode is what I was looking for.
Warren, I am powering 12v led strips and decided on 9 volts as plenty bright for passenger cars. My diagram is a bit misleading as I showed a single LED and not a strip.


----------



## J.C. (Dec 24, 2016)

Bwells said:


> Because the caps are operating at 92% of max or something else?


john summed up my concern in one word tolerance.

and fyi this 
To calculate the total overall capacitance of two capacitors connected in series you can use the following formula:
C total =C1 X C2 divided by C1+C2
*


----------



## Bwells (Mar 30, 2014)

J.C. your formula is correct but provided that the capacitance values for the two caps are the same, easier to divide by two, if three in series, divide by 3. Hence, two 1F in series gives you .5F, three in series gives you .33F etc. Kind of reminds me of selling on Ebay but they only take 10% whereas Mr. Farad steals alot more, 75% for two in series!
Warren: I looked at the 5.5 volt button ones and the ESR was much higher than the radial cans. It appears that I will cross my fingers and hope for the best as the max voltage is real close to what I am putting into them.


----------



## gunrunnerjohn (Nov 10, 2010)

The button batteries are only good for very low currents, I've encountered that issue before. They look attractive until you check the ESR.


----------

