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Offline steferfootballdude13Topic starter

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Capacitor Question
« on: August 07, 2008, 11:10:44 AM »
Quick Question:
I have a situation where I Can Have a Tethered Robot, however i will only be able to get 1 line to it, so i deduced that instead of using a battery i could use a capacitor to temporarily hold the otherwise grounded current (if my capacitor is large enough.) I was wondering if it would work before i design my schematic.

This is a Rhetorical Circuit that shows what i am trying to communicate:


EDIT: I realized that i was unclear as to why the switch is there. It represents when the robot would discharge the capacitor.
« Last Edit: August 07, 2008, 11:12:56 AM by steferfootballdude13 »

Offline robokid

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Re: Capacitor Question
« Reply #1 on: August 07, 2008, 01:53:16 PM »
can you exsplain the problem more

Offline izua

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Re: Capacitor Question
« Reply #2 on: August 07, 2008, 02:25:19 PM »
if it's tethered, why can't you use two wires?
if you have only one wire, you can't really do much with it (unless you adapt something like a one-wire protocol).

and btw, what the hell is a rhetorical circuit? ;D i never though rhetorics cad anything to do with robotics (unless you're trying to sell them of course :D)
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Offline steferfootballdude13Topic starter

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Re: Capacitor Question
« Reply #3 on: August 07, 2008, 02:51:22 PM »
if it's tethered, why can't you use two wires?
if you have only one wire, you can't really do much with it (unless you adapt something like a one-wire protocol).

and btw, what the hell is a rhetorical circuit? ;D i never though rhetorics cad anything to do with robotics (unless you're trying to sell them of course :D)

They are going to be 1:87 cars driving around on a conductive road. The current will flow from the Conductive road into a hanging wire which will drag along from the car. (I only called it tethered for the sake of brevity) The conductive road, however will not allow for grounding, ergo my problem. So instead of wasting space with a heavy battery, i will use a capacitor.

and when i use the word "rhetorical" i just mean hypothetical or sample circuit. I was just wondering if it was theoretically Possible.

Offline robokid

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Re: Capacitor Question
« Reply #4 on: August 07, 2008, 05:51:32 PM »
you mean like a capacitor as a battery. yea i think that should work if like this.
 

« Last Edit: August 07, 2008, 06:13:28 PM by robokid »

Offline steferfootballdude13Topic starter

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Re: Capacitor Question
« Reply #5 on: August 07, 2008, 06:01:23 PM »
you mean like a capacitor as a battery. yea i think that should work if like this.

[img][C:\Documents and Settings\All Users\Documents\My Pictures\untitled.sch/img]
The picture didn't show up, you need hosting from a site like imageshaq or photobucket.

Offline Gertlex

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Re: Capacitor Question
« Reply #6 on: August 07, 2008, 06:21:46 PM »
I admit to having personally no idea as to how those cars that drag a single wire work (I know what you're talking about), but perhaps an example of a capacitor powered car is those Hotwheels that you could charge up and they'd provide their own drive power?  (But those have two terminals, rather than one)
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Offline izua

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Re: Capacitor Question
« Reply #7 on: August 07, 2008, 09:22:16 PM »
Can't you simply split the road in two, and have two "hanging" connectors?
You will still need ground. Current forms from + to - (conventionally), but because ground is 0, it doesn't mean it shouldn't be there. Another option would be adding a pantograph (that might look silly though)
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Offline slo

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Re: Capacitor Question
« Reply #8 on: August 07, 2008, 09:31:18 PM »
Methods I know of sending power with 0 conductors: induction (coil on car, another under track), photovoltaic (solar, or targeted laser), PM generation(moving magnet under track, coil on car), sonic(pair of matched transducers in resonance or piezoelectrics), RF(transmitter on track receiver on car), electrostatic fields(you figure it out).  None of these is likely to produce sufficient power in a suitably small package.
If you have only 1 conducting path there is no possibility of continuous current flow.  The circuit you drew does not represent the situation you described.  Both battery and cap are connected in circuit at both ends.  Even so, your circuit as shown will only conduct until the cap is charged.
If I were you I would use the road as a switch contact and possibly a 1 wire interface simultaneously.  The car would be powered  either by a precharged cap (like micromachines ) or battery.  If you are thinking of Bumper Cars as an example, they use a conductive pole to an energized ceiling grid and another contact to an energized metal floor.  2 conductors.

Offline dsheller

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Re: Capacitor Question
« Reply #9 on: August 07, 2008, 09:42:49 PM »
Not really sure who taught you electrical symbols... but you have a batteries positive source going to ground? Furthermore, a capacitor is a DC blocking component.... so for example say you have a 1.5 V battery hooked up to a capacitor. You'll charge the capacitor up according to the equation Vc = Vbat* ( 1 - e^-t/RC ). So... according to KVL there all the voltage drops across a circuit have to add up to battery voltage... and your battery voltage is appearing across the capacitor, therefore no more current will flow.

So in short, to get this right you're going to have to disconnect the capacitor everytime it charges to battery voltage, which will happen a lot. Not to mention a capacitor holds very very little charge when compared to the energy a lithium-ion battery, or any battery really, can hold.
« Last Edit: August 07, 2008, 11:48:38 PM by dsheller »

Offline steferfootballdude13Topic starter

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Re: Capacitor Question
« Reply #10 on: August 08, 2008, 10:55:52 AM »
Can't you simply split the road in two, and have two "hanging" connectors?

No, unfortunately this is for a modeling club that asked for my help. It would work but they want "Intersections"

Both battery and cap are connected in circuit at both ends..

The Switch (as mentioned in my edit) represents when the robot would discharge the Cap.

  Even so, your circuit as shown will only conduct until the cap is charged.

That is precisely what i was going for, if i could find a cap with a high enough Capacitance Value that would last longer than say, half a second.

Not really sure who taught you electrical symbols... but you have a batteries positive source going to ground?

Sorry I was in a hurry.


 
Furthermore, a capacitor is a DC blocking component.... so for example say you have a 1.5 V battery hooked up to a capacitor. You'll charge the capacitor up according to the equation Vc = Vbat* ( 1 - e^-t/RC ). So... according to KVL there all the voltage drops across a circuit have to add up to battery voltage... and your battery voltage is appearing across the capacitor, therefore no more current will flow.

So in short, to get this right you're going to have to disconnect the capacitor everytime it charges to battery voltage, which will happen a lot. Not to mention a capacitor holds very very little charge when compared to the energy a lithium-ion battery, or any battery really, can hold.

Essentially correct, i was asking just to see if it was therorictically possible, i was just kinda brainstorming ideas. I knew that once the cap is charged however the curcit would then be open. I now just have to find a very small high capacitance value cap if i want this to work.

Offline Gertlex

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Re: Capacitor Question
« Reply #11 on: August 08, 2008, 12:03:33 PM »
Well intersections in the track could just be done with gaps. I'm sure the vehicles will have enough momentum to carry them over such a gap.
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Offline steferfootballdude13Topic starter

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Re: Capacitor Question
« Reply #12 on: August 09, 2008, 01:14:15 PM »
OK, I haven't looked much but instead of a capacitor, i think i'll just use this capacitor battery: http://www.quality-deal.com/osCommerce/product_info.php?products_id=28. It is tiny and has a rating of .6 Farad! It also goes well with this tiny DC motor: http://quality-deal.com/osCommerce/product_info.php?products_id=31&osCsid=9095b9c23d2ad1874f298e345ab31c74

Thanks for all the Help,
Stefan

Offline Admin

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Re: Capacitor Question
« Reply #13 on: August 22, 2008, 06:28:03 PM »
Quote
So instead of wasting space with a heavy battery, i will use a capacitor.

For the same volume, a typical capacitor holds ~1% the charge a battery can . . .

Quote
i think i'll just use this capacitor battery: http://www.quality-deal.com/osCommerce/product_info.php?products_id=28.

Its rated at 3V, meaning your motors should run at about 1.5V. I couldn't find the datasheet, but I suspect it also has a very high internal resistance - meaning that if it outputs at high currents, it blows up. High farad caps tend to have high internal resistances . . .

 


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