capacitor/battery question
Archiver
Posts: 46,084
hey,
I'm using 'muscle wires' which require a rapid pulse of high
amperage current to contract. I'm trying to use the smallest battery
that will be able to contract the wire. I'm currently using a camera
flash battery, which can handle pulse currents high enough to
contact my muscle wire. The problem is that this flash battery is
too large and heavy for my robot. Unfortunately, most other small
batteries (coin type batteries) have a really low rating for maximum
pulse current that they can handle.
So, to get to the point, I was wondering if I could skirt this
problem by putting a capacitor in series with the muscle wire. This
way, a small battery which cannot handle a high current could take
its time charging the capacitor. Then, to contract the muscle wire,
I would just discharge the capacitor. The high current necessary to
contract the muscle wire would come from the capacitor, not the
battery (therefore a battery which cannot handle high currents would
not be harmed). Does this make any sense? Is my understanding of how
batteries work correct? Thanks for any help you could give!
I'm using 'muscle wires' which require a rapid pulse of high
amperage current to contract. I'm trying to use the smallest battery
that will be able to contract the wire. I'm currently using a camera
flash battery, which can handle pulse currents high enough to
contact my muscle wire. The problem is that this flash battery is
too large and heavy for my robot. Unfortunately, most other small
batteries (coin type batteries) have a really low rating for maximum
pulse current that they can handle.
So, to get to the point, I was wondering if I could skirt this
problem by putting a capacitor in series with the muscle wire. This
way, a small battery which cannot handle a high current could take
its time charging the capacitor. Then, to contract the muscle wire,
I would just discharge the capacitor. The high current necessary to
contract the muscle wire would come from the capacitor, not the
battery (therefore a battery which cannot handle high currents would
not be harmed). Does this make any sense? Is my understanding of how
batteries work correct? Thanks for any help you could give!
Comments
>
> I'm using 'muscle wires' which require a rapid pulse of high
> amperage current to contract. I'm trying to use the smallest
> battery that will be able to contract the wire. [noparse][[/noparse]...] I was
> wondering if I could skirt this problem by putting a capacitor
> in series with the muscle wire. [noparse][[/noparse]...] Does this make any sense?
> Is my understanding of how batteries work correct?
>
Yes. Not only batteries, but nearly any source of electrical power you'll
run into has a practical limit on the current you can draw for any
significant time. Without belaboring the equations, that's what "power"
means. You won't put the capacitor in series by the way. It has to be in
parallel with the battery. We have a significant problem in this same regard
when designing circuits for microprocessors. Any very fast logic has large
jumps in current requirements when switching, and we do indeed store that
energy in capacitors to dump into the IC on demand when the switching
occurs.
Someone else will have to offer you a sample circuit since I've never played
around with muscle wire, but you have two issues that must be addressed:
1. The circuit must protect the battery against the high inrush current when
you start up. The capacitor will be completely discharged (or at least it
will be in many cases), and when you close the switch it will look like a
short circuit to the battery until it is charged. (This is not a problem,
just a requirement for a resistor to restrain that inrush current.) If
you're trying to use a battery type that does not tolerate sudden high
current loads, it would be bothered by those high currents at start up, so
you constrain them to a level the battery is designed to handle. How long
the current must continue will depend on the next question, but if you have
a very large capacitor the current can easily continue long enough to damage
the battery if you do not put in a resistor.
2. The capacitor across each muscle wire must store enough energy to heat,
and thus contract, the muscle wire you're using. My faulty memory suggests
it's the heat that contracts the muscle wire... The energy in any case.
Choosing the right capacitance is one issue. Once that's done, you'll be
able to calculate how long it will take to charge that capacitor after each
contraction, and that will give you the cycle time on that "muscle."
Sorry I can't give you a specific circuit, but I would expect you can find
one on the web if no one answers here in a timely fashion.
Gary
some values that might get you started.
Where do you source the muscle wire?
>> Battery >----/\/\----o---/\/\
muscle wire switch
>> 100ohm | 1ohm
>> ===== 100,000 uF Muscle
wire here
>> |
>> >
o
>> Common
Leroy
Original Message
From: "Sam" <hard-on@t...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, November 11, 2003 8:23 PM
Subject: [noparse][[/noparse]basicstamps] capacitor/battery question
: hey,
:
: I'm using 'muscle wires' which require a rapid pulse of high
: amperage current to contract. I'm trying to use the smallest battery
: that will be able to contract the wire. I'm currently using a camera
: flash battery, which can handle pulse currents high enough to
: contact my muscle wire. The problem is that this flash battery is
: too large and heavy for my robot. Unfortunately, most other small
: batteries (coin type batteries) have a really low rating for maximum
: pulse current that they can handle.
: So, to get to the point, I was wondering if I could skirt this
: problem by putting a capacitor in series with the muscle wire. This
: way, a small battery which cannot handle a high current could take
: its time charging the capacitor. Then, to contract the muscle wire,
: I would just discharge the capacitor. The high current necessary to
: contract the muscle wire would come from the capacitor, not the
: battery (therefore a battery which cannot handle high currents would
: not be harmed). Does this make any sense? Is my understanding of how
: batteries work correct? Thanks for any help you could give!
:
:
: To UNSUBSCRIBE, just send mail to:
: basicstamps-unsubscribe@yahoogroups.com
: from the same email address that you subscribed. Text in the Subject and Body
of the message will be ignored.
:
:
: Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
:
:
:
from your battery is valid, however, you place the capacitor across the
battery and then discharge the capacitor through the muscle wire when you
wish to activate it not in series with the wire.
jim
http://www.geocities.com/jimforkin2003/
Original Message
From: Sam [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=03bHv6dwfRbMJGDdzaKKycdbAn_zpW_YezJJwS8F41STx5R-0h6zij-eNT1rRJQIHBoH58GHGAiB0Kp2]hard-on@t...[/url
Sent: Tuesday, November 11, 2003 8:24 PM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] capacitor/battery question
hey,
I'm using 'muscle wires' which require a rapid pulse of high
amperage current to contract. I'm trying to use the smallest battery
that will be able to contract the wire. I'm currently using a camera
flash battery, which can handle pulse currents high enough to
contact my muscle wire. The problem is that this flash battery is
too large and heavy for my robot. Unfortunately, most other small
batteries (coin type batteries) have a really low rating for maximum
pulse current that they can handle.
So, to get to the point, I was wondering if I could skirt this
problem by putting a capacitor in series with the muscle wire. This
way, a small battery which cannot handle a high current could take
its time charging the capacitor. Then, to contract the muscle wire,
I would just discharge the capacitor. The high current necessary to
contract the muscle wire would come from the capacitor, not the
battery (therefore a battery which cannot handle high currents would
not be harmed). Does this make any sense? Is my understanding of how
batteries work correct? Thanks for any help you could give!
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
group. It is a portion of a circuit that I have used to drive high-
voltage trigger transformers. Basically, the capacitor charges from
the +V supply through resistor R1 and the primary of the trigger
transformer. Note that once the capacitor is charged and not pulsed
(DC conditions), the primary of the trigger transformer looks like a
wire (zero ohms) to circuit common. The pulse trigger element is an
insulated-gate bipolar transistor (IGBT). The gate looks like a FET,
the high-current section looks like a bipolar transistor. This gives
high switch-current capability without the base current troubles of a
pure bipolar transistor. When the IGBT is turned on quickly, it
pulls the end of the capacitor that was charged to +V down to zero
volts. Since a capacitor cannot change voltage instantaneously, the
end of the cap connected to the primary of the pulse transformer goes
to -V, and discharges through the primary of the trigger
transformer. The resistor provides current-limiting during the
pulse, and also sets the recharge time.
This circuit configuration allow for the gate of the trigger device
to be referenced to circuit common. If the switch element
is "floating" with respect to circuit common (as shown in your
original circuit sketch), you cannot drive it directly from the Stamp
outputs. This circuit configuration also requires only one active
switch element.
For your application, the trigger transformer could be replaced with
the muscle-wire (I have never seen one of these, so I do not know the
particular characteristics of this device). You would need to select
an appropriate value of capacitance based on the resistance of the
muscle-wire and the amount of energy that you need to provide to it.
You would also need to select an appropriate IGBT. The one shown in
my circuit was selected for switching a capacitor charged to
approximately 300V.
Good luck.
--- In basicstamps@yahoogroups.com, "Sam" <hard-on@t...> wrote:
> hey,
>
> I'm using 'muscle wires' which require a rapid pulse of high
> amperage current to contract. I'm trying to use the smallest
battery
> that will be able to contract the wire. I'm currently using a
camera
> flash battery, which can handle pulse currents high enough to
> contact my muscle wire. The problem is that this flash battery is
> too large and heavy for my robot. Unfortunately, most other small
> batteries (coin type batteries) have a really low rating for
maximum
> pulse current that they can handle.
> So, to get to the point, I was wondering if I could skirt this
> problem by putting a capacitor in series with the muscle wire. This
> way, a small battery which cannot handle a high current could take
> its time charging the capacitor. Then, to contract the muscle wire,
> I would just discharge the capacitor. The high current necessary to
> contract the muscle wire would come from the capacitor, not the
> battery (therefore a battery which cannot handle high currents
would
> not be harmed). Does this make any sense? Is my understanding of
how
> batteries work correct? Thanks for any help you could give!