Author Topic: Automatic charge and discharge of a capacitor  (Read 2829 times)

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Offline OpticalTopic starter

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Automatic charge and discharge of a capacitor
« on: October 13, 2010, 09:22:27 AM »
Hello, I am going to enter a design competition where we have to build a solar powered "crane" that runs off a 1.5V 400mA solar panel. In order to get more power to the motor, I would like to build a circuit that repeatedly and automatically charges and discharges a capacitor into a motor. Basically the same principle as a camera's flash. (At this point there does not appear to be any rigid time limit, but I would like to keep the run time of the device to something reasonable).

My question is what do I need to look into in order to build something like I just described? My knowledge of electronics is pretty limited, although I do know the basic theory behind most of the common components. Do I need to program a micro-controller in order to do this? Could I get an integrated circuit that does exactly this? Or is there someway that I could arrange some diodes and transistors in order to get the circuit to do what I want?


Offline madsci1016

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Re: Automatic charge and discharge of a capacitor
« Reply #1 on: October 13, 2010, 09:52:17 AM »
I think your approach is flawed. When you say charges and discharges into a motor, do you mean repeatedly starting up the motor to spin for a sec and then letting it stop while you charge the cap again?

This is extremely wasteful of power. Most if not all of the power you stored in the cap will be wasted just getting the motor to start spinning. DC motors waste a lot of power during start-up to overcome momentum and draw close to their stall current for a a good portion of a second.

A better design would be to charge up a larger bank of capacitors to a high voltage using a boost regulator from the solar panel, and then running the motor through a buck regulator to step the voltage down to the motors operating voltage.

What motor are you using? Hom much power/energy will it need to lift the weight?

Offline OpticalTopic starter

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Re: Automatic charge and discharge of a capacitor
« Reply #2 on: October 13, 2010, 10:48:37 AM »
I think your approach is flawed. When you say charges and discharges into a motor, do you mean repeatedly starting up the motor to spin for a sec and then letting it stop while you charge the cap again?

This is extremely wasteful of power. Most if not all of the power you stored in the cap will be wasted just getting the motor to start spinning. DC motors waste a lot of power during start-up to overcome momentum and draw close to their stall current for a a good portion of a second.

A better design would be to charge up a larger bank of capacitors to a high voltage using a boost regulator from the solar panel, and then running the motor through a buck regulator to step the voltage down to the motors operating voltage.

What motor are you using? Hom much power/energy will it need to lift the weight?



Ah, that's exactly the sort of feedback I was looking for. I was thinking of the principles behind those things you just mentioned, but I didn't know what the names of them were so googling and looking in textbooks wasn't helping.

As far as a motor goes, we plan on using this:

http://www.mabuchi-motor.co.jp/en_US/cat_files/re_260ra.pdf

Our goal right now is to lift 3 lbs. of weight. I'm not sure how much torque we can get out of our gearbox right now, so I'll get back to you on exactly how much power we need.

Offline OpticalTopic starter

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Re: Automatic charge and discharge of a capacitor
« Reply #3 on: October 13, 2010, 11:21:45 AM »
I've been googling and I've run across buck-boost ICs, could I use something like that to increase the amount of power to the the motor?

Offline z.s.tar.gz

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Re: Automatic charge and discharge of a capacitor
« Reply #4 on: October 13, 2010, 11:45:30 AM »
buck-boost (or any dc/dc converter for that matter) doesn't increase or decrease power (measured in watts) but simply changes the voltage and inversely the amperage.
say you had a 12V battery you wanted to run that motor with. normally just hooking it up would destroy it as you're looking at 4x the rated voltage. With a buck converter (what you're looking at for this application) you could reduce the voltage to 1/4 the original in exchange for 4 times the amperage. Because the hypothetical battery can already produce the current needed for the motor before it is run though the converter you will actually end up using 1/4 the current.

There are other techniques besides dc/dc converters but they mostly rely on dissipating extra power as heat which translates to you having 1/4 (or whatever) the voltage but still use the same amount of current. Not very efficient to say the least.

Please somebody correct me if I'm wrong as this is all self-hypothesized.  ;D
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Offline madsci1016

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Re: Automatic charge and discharge of a capacitor
« Reply #5 on: October 13, 2010, 12:13:09 PM »
You are right. regulators don't "increase power", only change voltage. But the energy a capacitor can store is =1/2 CV^2, so increasing the voltage lets you store exponentially more energy inbetween charges. 

Buck-boost voltage regulators both use switching technology and either an inductor or capacitor to operate.

Buck regulator usually refers to a step down regulator, ie from a higher voltage to a lower.

Boost regulator usually refers to a step up regulator, from low voltage to a high voltage.

There are some regulators that can do both, since they both operate on the same principle. I would not use a linear regulator on this project.

I will have to take a harder look (I'm at work right now), but 3lbs is an awful lot for a 1.5V motor. Have you done any math for power required to lift 3lbs, and energy required to move the 3lbs the distance you want?

Offline garrettg84

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Re: Automatic charge and discharge of a capacitor
« Reply #6 on: October 13, 2010, 12:41:38 PM »
...we have to build a solar powered "crane" that runs off a 1.5V 400mA solar panel...


How long can the solar panel be exposed to light before your crane must do work? The idea of using a small bank of capacitors doesn't actually sound like a bad idea. Are you able to also use a battery/charging circuit?

Maybe I am making the wrong assumptions, but does it need to run purely off of this panel and must run during the time that the panel is exposed or would you have time while the panel is exposed and collecting?

Some ridiculousness for thought:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G17930
3000F Cap at 2.7v

(Edit)
More food for thought - What about counter weighting the crane as well? At 1.5lbs to lift 3lbs, you would only ever be lifting 1.5lbs in either direction (if 3lbs is your imposed limit).
« Last Edit: October 13, 2010, 12:46:34 PM by garrettg84 »
-garrett

Offline madsci1016

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Re: Automatic charge and discharge of a capacitor
« Reply #7 on: October 13, 2010, 12:52:45 PM »
Quote from: garrettg84 link=topic=12357.msg92931#msg92931
Some rediculousness for thought: http://www.goldmine-elec-products.com/prodinfo.asp?number=G17930
3000F Cap at 2.7v


That's actually the opposite of what you would want for this. See Zach's thread on super Caps. Quickest reason is that a cap discharges linearly. It's better to have a high voltage cap regulated down to motor voltage, so for most of the discharge it's a constant motor voltage, and more usable energy out of the cap.

Offline OpticalTopic starter

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Re: Automatic charge and discharge of a capacitor
« Reply #8 on: October 13, 2010, 01:22:34 PM »
Quote
buck-boost (or any dc/dc converter for that matter) doesn't increase or decrease power (measured in watts) but simply changes the voltage and inversely the amperage.
Thanks for explaining that! I just wanted to make sure that I wasn't about to waste a lot of time building a circuit if I could get an IC to do the same thing.

Quote
How long can the solar panel be exposed to light before your crane must do work? The idea of using a small bank of capacitors doesn't actually sound like a bad idea. Are you able to also use a battery/charging circuit?
Right now, I'm going to say about 10 minutes. The rules don't say anything about that and the people putting on the competiton haven't responded to my e-mail yet.

Quote
I will have to take a harder look (I'm at work right now), but 3lbs is an awful lot for a 1.5V motor. Have you done any math for power required to lift 3lbs, and energy required to move the 3lbs the distance you want?

I'll have some math done later today (around 9 o'clock tonight) about how much weight I think the crane can actually lift, but we are allowed to use 10 pulleys or gears in the construction of the crane to get some more torque in the system. Just off the top the head the last time I ran the calculations it came out to just over 1 lb (ideally) without using batteries or capacitors to get more power. Hence, I want to build a circuit to deliver more energy to the motor.

Basically what it looks like Iím going to do right now is have the power from the solar panel first run through a boost regulator to step the voltage up to 25V, then through a capcitor (or capacitors) until the capacitors are sufficently charged.  Now to deliver the power to the motor I either want to use a single pole change over switch (if the people running the competiton will allow it) or a 555 timer to automatically change switch the circuit over to another voltage regulator that will step the voltage down to 3V, that will run the current through the motor. Can I use a 555 timer for that? If I can, could I get a good website about 555 timers?.

The way we have our gearbox set up we donít have to worry about the load falling back down when the motor stops turning.
Quote
ore food for thought - What about counter weighting the crane as well? At 1.5lbs to lift 3lbs, you would only ever be lifting 1.5lbs in either direction (if 3lbs is your imposed limit).
The problem with using a counter-weight in this case is that our score is based on how heavy our crane is vs. how much it can lift. So any points we would gain by using a counter-weight would be lost because it would make the crane heavier.

Offline garrettg84

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Re: Automatic charge and discharge of a capacitor
« Reply #9 on: October 13, 2010, 03:20:06 PM »
The problem with using a counter-weight in this case is that our score is based on how heavy our crane is vs. how much it can lift. So any points we would gain by using a counter-weight would be lost because it would make the crane heavier.

Spring loaded?
-garrett

Offline z.s.tar.gz

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Re: Automatic charge and discharge of a capacitor
« Reply #10 on: October 14, 2010, 06:44:57 PM »
Remember that capacitors discharge power linearly (right?) but that voltage is not a linear function over time so you actually have a small window when you will have a usable voltage. (that part I know is true)

Here's my source http://en.wikipedia.org/wiki/RC_time_constant.
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Offline madsci1016

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Re: Automatic charge and discharge of a capacitor
« Reply #11 on: October 14, 2010, 07:34:59 PM »
Remember that capacitors discharge power linearly (right?) but that voltage is not a linear function over time

Other way around. Voltage is linear, power isn't. Imagine a capacitor draining through a resistor. As voltage drops, so will current since V=IR. Since power = VI, and both V and I are dropping, it's not a linear curve.

Quote
you actually have a small window when you will have a usable voltage. (that part I know is true)

Correct, which is why I have been suggesting a high voltage system where voltage is regulated down to motor voltage. This lets you run the motor at a constant voltage for much longer over the discharge of the capacitor.

You can find hundreds of circuits on 555 timers if you google. Only problem with that idea is it will solely be time based. Even if your cap isn't charged fully, it will start to discharge.
« Last Edit: October 14, 2010, 07:37:37 PM by madsci1016 »

Offline z.s.tar.gz

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Re: Automatic charge and discharge of a capacitor
« Reply #12 on: October 14, 2010, 07:43:29 PM »
You can take an idea from beam robotics and use a voltage monitor ic to tell when the capacitor is charged and have the ic open up a mosfet when the capacitor is fully charged (or however much you want, it's adjustable)
Save yourself the typing. Just call me Zach.

Offline madsci1016

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Re: Automatic charge and discharge of a capacitor
« Reply #13 on: October 14, 2010, 08:07:25 PM »
I was thinking a schmitt trigger with a 5V regulator and some voltage dividers to reference the Cap voltage to less then 5V max.

This would let you set it so the motor would turn on when the cap gets fully charged, and off when it drops below a critical level.

Offline Admin

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Re: Automatic charge and discharge of a capacitor
« Reply #14 on: October 14, 2010, 08:12:21 PM »
What you are looking for is called a solar engine.

You'd probably want to take the schematics to the 21st century and modify them a bit to use MOSFETs.

Offline z.s.tar.gz

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Re: Automatic charge and discharge of a capacitor
« Reply #15 on: October 14, 2010, 08:53:04 PM »
Precisely. There are different versions but voltage trigger ones are most easy to get working.
Save yourself the typing. Just call me Zach.

Offline squareceo

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Re: Automatic charge and discharge of a capacitor
« Reply #16 on: October 23, 2010, 10:20:01 AM »
With all other variables being constant, does putting multiple capacitors in parallel affect the charge and discharge rate?

Offline Soeren

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Re: Automatic charge and discharge of a capacitor
« Reply #17 on: October 23, 2010, 11:48:32 AM »
Hi,

With all other variables being constant, does putting multiple capacitors in parallel affect the charge and discharge rate?
Yes of course it does. The formula is: C = A x s / V
(C = capacity in Farads, A = current in Ampere, s = time in seconds and V = Voltage drop)
Turning it around a bit you get: s = C x V / A and keeping V and A constant you can see that the time is directly proportional with the capacity - double the capacity and you double the time and so on.
Regards,
SÝren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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