voltage regulating?

Discussion in 'General Electronics Chat' started by redlong82, Jul 5, 2013.

  1. redlong82

    Thread Starter New Member

    Jun 21, 2013
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    Hello, everyone,

    I'm working on a scientific experiment. We need to charge magnetic coils up to 500A. Due to the limited budget, we are using car batteries. The good things are: first, car batteries is much cheaper; second, car batteries provide no ripples; and third, the internal resistance is low . The bad thing is after each discharge for about 5 seconds, the voltage of it drops a bit.

    Is there anyone knows whether there is any low-cost method to provide a more stable voltage for like 1 minute? I mean, some circuit using batteries power (~6.5 to 7.5V when fully charged) as input, and output a stable 6V or 5.5V dc voltage for 1 minute?

    I did think about the possibility of switch-mode power supply or linear power supply. But one issue concerns me. We want to pulse the coils and observe how the magnetic field in our device rises, which usually takes about 100ms. With the batteries, the coils take <1ms to reach the maximum. But I'm not sure any (low-cost!) switch-mode or linear power supply do that.

    Anyone has any idea?

    Thanks!
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
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    The low resistance value of the coil is what is causing the battey voltage to dip. If you were to and some resistance in series with the inductor the charge current would reduce and it would take a little longer to charge the inductor and you would gain some battery life. As I remember the time constant for inductors is T=L/R where R is the series resistance(including the actual resistance of the wire to make the inductor)
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    What on earth do you mean by

    "Charge magnetic coils up to 500A" ?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't see a problem with that. The analogue is charging a capacitor to a voltage.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What is this mechanism "which usually takes about 100ms"and is not a battery?

    Please do note that "high current" and "low-cost" should not be found in the same sentence together.
     
  6. vk6zgo

    Active Member

    Jul 21, 2012
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    500A is a huge current--car batteries have low internal resistance,but not that low!

    In their intended service,a drop in terminal voltage under large current draw during starting is accepted,& allowed for.

    Older cars had an 8 volt ignition coil,so that under start conditions,when the battery terminal voltage fell due to the high starter current,the spark output would still be adequate.

    During normal running,a resistor was switched in series with the coil.

    You can reduce the drop in terminal voltage by having a large number of batteries in parallel,but you will not eliminate it.

    Maybe you can graph the drop in voltage & use it as a correction factor.

    You could obtain a very large number of capacitors,connect them in parallel,charge them up,& discharge them through the coils.

    This would reduce the internal resistance problem to an extremely low level,but not completely eliminate it,& you may have problems due to LC resonance.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    And as usual everybody is forced to guess just so they can try to help the O.P...

    Because the O.P. did not bother to post what it is, what it does, why it does it, etc. :(
     
    PackratKing and wayneh like this.
  8. Shagas

    Active Member

    May 13, 2013
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    Individual car batteries can provide 150-250 amps depending on the size . And at those ratings their voltage tends to drop from 12 to 9 or 10 volts .
    You will probably need 4-6 identical car batteries in parallel to give you a solid 500 amps for 1 minute
     
  9. Austin Clark

    Member

    Dec 28, 2011
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    Have you considered using ultracapacitors? You'll likely need quite a few to get a usable voltage, and you'd have to charge them up periodically, but they will supply more than enough current without fear of damage.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Looks like this has turned into another "drive-by posting". :)
     
  11. redlong82

    Thread Starter New Member

    Jun 21, 2013
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    0
    Thanks a lot for replying.

    Sorry for replying later. I was very busy these days.

    Let me explain it in more details. Our experiment need an external magnetic field generated by 2 coils. The current in the coil needs to be up to 500A. There are high-current DC power supplies in the market. But these things are too expensive to our project. Car batteries are much cheaper. Since the duration of each discharge is about a few seconds. 3 or 4 batteries in parallel can do the job.

    The drawback is quite obvious. After each charge, the battery voltage will drop a little bit. So I'm wondering whether there is something similar to zener diodes to limit the output voltage. The idea is that such thing can bypass extra current and make the current flowing to the coils constant as long as the car battery voltage is higher than some critical value. But as far as I know, there is no zener diode can withstand such high current.

    So my question is whether there is any low-cost device which takes 6-7V from car battery as input, and output a stable voltage (say, 5.5V) so that a bit of change of the car battery voltage doesn't change the output of the device.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I am very concerned with the safety aspects of this project.

    Throwing hundreds of amps about, even at low voltage could cause injury to persons or property, by way of flash burns, flash eyesight damage or fires or explosions or acid burns, to name but a few.

    This is a dangerous activity.

    You were asked for proper details of you application at the outset and have responded that you were 'too busy', even though many here tried to help.

    And you actual response suggests to me that you are not properly aware of the electrical implications of what you are doing and not prepared to discuss your project fully.

    Are you making a rail gun?

    You say that you cannot afford a proper 500 amp supply, is there some El Cheapo device that will do this for you from a battery.

    There is another supply option, but I will not post it until satisfied that your use is legitimate.
     
  13. redlong82

    Thread Starter New Member

    Jun 21, 2013
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    0
    Well ... if you are interested in what's the project I'm working on. Here is the link:

    http://physics.nmt.edu/~dynamo/index.html

    It's a astrophysical project about how the magnetic field is generated in the universe. We've considered all safety issues. Things about batteries are only a small part of it. We control the batteries discharge remotely (100 feet away) so nobody is gonna be hurt.

    I don't know what make you think I'm designing a rail gun. 500amp is a large current, but I don't think it's enough for a rail gun (500A x 6V = 3kW). I've mentioned we need to discharge the coils for a few seconds. Is it possible to accelerate a bullet for a few seconds? How long the rail would be?

    I'm sorry for not answering the replies one by one. Since the physics about the experiment is so complicated, it's impossible to explain it in a few sentences. To understand it, one needs to know Navier–Stokes equations, Maxwell equations, Couette-Taylor flows, boundary layers, turbulence, Kelvin-Helmholtz instability, Ekman layers, dynamo theory, magnetohydrodynamics ...

    So I have to say we want to observe some phenomenon which may last for about 100-200mS. To observe it, we need to discharge the coils for a few seconds. But the most important data should be obtained during the first 100-200ms after the button is pressed. One criteria is the current in coils must reach the maximum in less than 1ms. We did consider switch-mode or linear DC-DC converters. My concern is the response time of the converters, plus, the ripples they may generate. Capacitor banks are very expensive and time-consuming. I'm seeking whether there exists some low-cost device similar to zener diodes which can bypass a portion of current so that the voltage on the coils can be stable at a fixed voltage for about 20 discharges. If there is none, we have to consider effects caused the slightly different coil currents. If there is, it will save the trouble.



     
  14. wayneh

    Expert

    Sep 9, 2010
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    You need a constant current regulator capable of 600A or more. (You don't want to design it - or anything - to operate at max specs.) I think it should be possible with MOSFETs in parallel or perhaps with IGBTs, which I know little about.

    The normal thing is to use the voltage across a shunt resistor as a measure of the current flow, and use that voltage - after buffering and amplification in an op-amp, to control the transistors that regulate the big current. I have no idea how you would find a 500A shunt resistor so I think you'll need another sensor strategy, perhaps a hall effect sensor. But the concept will be the same; use the sensed current and low-power electronics to control a large current via transistors.

    Another "solution" is simply more batteries. Every time you double the capacity you'll cut the problem in half. Just sayin'. They make some pretty darn big batteries.

    Oh, and yes, 500A is plenty to kill the unsuspecting railgun experimenter.
     
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