Capacitor discharge

[Armagdn03]

Hello all

Thought I might put this one past the knowlegeable panel of electronic experts here.

I would like to discharge a capacitor once it gets to a certain voltage. I would like a near complete discharge (not supper picky, but the more the better) and I would like it to reset itself automatically (i.e. no 555 timers). I would like this requirement because the capacitor will be charged based on a variable speed rotor, and the timer would have to react to these conditions, which is way to dificult and impractical.

If anybody has any ideas, maybe a circuit that has an upper voltage limit that triggers it, and then a lower voltage limit that resets it....I would love to hear your advice

 

[dknguyen]

Where do you want to discharge it to? Just anywhere through a load resistor or something? How fast does it need to discharge? You might want to try looking at the voltage-trigger IC as well as the solar engine circuits that use them for BEAM robots.

I would like it to reset itself automatically (i.e. no 555 timers).

I'm fairly certain you can wire up a 555 timer to do the same thing. Just don't use the IC as a timer. A 555 timer really is just a upper and lower voltage trigger in one IC, as well as a separate circuit that can connect to ground or to Vcc. Nothing says you have to charge the capacitor with the same voltage supply as the timer itself. You could connect it the rotor instead and use some resistors to modify the trigger voltage of the 555 timer IC. Maybe a rectifier of some sort if the output voltage from the rotor is AC and not DC.

What exactly do you need the whole thing to do? More specifics would be needed to come up with a circuit using a 555 timer in it.

 

[Armagdn03]

Well, I need a rotating disk consisting of two metalic plates seperated by a dielectric, effectively creating a capacitor that is charged via its rotation (exact mechanism is not important) and what I really want now that I think about it is a fast cutoff for the discharge, ie a mosfet. Since the disk is spinning, it makes it dificult to make electrical connections to it, if I can house the electronics on the disk itself, all the better. So I want it to discharge maybe halfway through its total charge then cut off as fast as possible, then reset so that it may be recharged.

Actually, while im typin here, does anybody know of a good spray on dielectric with a high dielectric constat? I want this disk to have as much capacitance as possible.

 

[dknguyen]

Well, I need a rotating disk consisting of two metalic plates seperated by a dielectric, effectively creating a capacitor that is charged via its rotation (exact mechanism is not important) and what I really want now that I think about it is a fast cutoff for the discharge, ie a mosfet. Since the disk is spinning, it makes it dificult to make electrical connections to it, if I can house the electronics on the disk itself, all the better. So I want it to discharge maybe halfway through its total charge then cut off as fast as possible, then reset so that it may be recharged.

Actually, while im typin here, does anybody know of a good spray on dielectric with a high dielectric constat? I want this disk to have as much capacitance as possible.

How closely can you maintain the spacing on the plates as they spin? It'd have to be really really close together to get capacitance with just simple plates. I'm not sure about spray on stuff, but if the plates touched those would get scraped away anyways. Maybe submerge it in some kind of dieelctric liquid...like...water?
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/diel.html
Except that water itself might cause problems if you want to mount stuff fon the plates. Maybe not distilled water though or if you waterproof some stuff in a case or epoxy.

**broken link removed**
and similar voltage trigger products. Not sure if they are accurate enough for you though.

 

[Armagdn03]

The plates will be bonded together (by the dielectric if possible) and will spin together. It is essentially a spinning disk like capacitor. I figured with some sort of spray, that one could keep the distance minimal, and if the correct material was used, maybe I could get the capacitance up enough to be useful. (maybe a couple hundred nanofarads, maybe more, I dont know.)

 

[Nigel Goodwin]

You really need to tell us EXACTLY what you're trying to do, and not how you think it should be done - otherwise the thread will just get longer and longer as people try to guess what you're talking about.

Generally (not 100%, but certainly high 90%'s), if people here don't understand what you're asking, it's because you're trying to do something in completely the wrong way.