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Constant current source advice

  1. Aug 16, 2014 #1
    Some of my advanced chemistry students would like to try this pharmacokinetic model circuit outlined in an AAPT publication (Cock & Janssen, The Physics Teacher, Volume 51, Issue 8, pp. 482-484 (2013)). It is a simple RC circuit, but I've never built a constant current source and am unsure of what things to watch out for.

    http://scitation.aip.org/docserver/fulltext/aapt/journal/tpt/51/8/1.4824943.figures.online.f2_thmb.gif

    The article suggests using a 72 uF cap, 27k/297k/420k/890k resistors, and 172/344/520/700 uA current. Then the procedure is to leave S1 open, and charge the cap for 0.15 s (how can I get timing like that!?). The switches' states are then exchanged and of course the charge discharges with the time constant (over 10 seconds).

    I've looked at some schematics and I'm confident it can be done with just a few components (like this), but am less sure about how I can vary it over the 4 values stated above. I also don't know how to only allow it to run for 0.15 s...any thoughts?
     
  2. jcsd
  3. Aug 16, 2014 #2

    nsaspook

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    Why do you need to build it? Couldn't you just model it using a electrical circuit simulation program like SPICE.
     
  4. Aug 16, 2014 #3

    NascentOxygen

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    So that's the complete procedure of interest in its entirety--just a one-off 0.15 s charge of that capacitor followed by a 10 s discharge? You then toast to its success and pack it away in the cupboard?
     
  5. Aug 16, 2014 #4
    Great idea, we should definitely do that to get an understanding of whats happening. But, they really are amazed by soldering and want to try it out.

    Sorry, should have given more detail. The idea is that the charge on the capacitor represents the dose of medicine in the body. Charge decays over a day (10 s) before another "dose" is given, this represents the medicine being metabolized. The idea is to adjust the values so that after a few days, you don't have an huge/dangerous amount of medicine (by adjusting the time constant, the exponential function varies).

    http://scitation.aip.org/docserver/fulltext/aapt/journal/tpt/51/8/1.4824943.figures.online.f6_thmb.gif

    I'm sorry I don't have online access to the article, but in my notes I have that a tau value of around 30 is considered better by pharmacists than 64 for example. In the image above, the lighter blue line is what a pharmacist would try to achieve. The equation in that field is

    dx/dt = -αx

    where x is the amount of medicine in the body, and α is a constant. The jagged lines in the image above approximate the solution. See also wiki
     
  6. Aug 16, 2014 #5

    nsaspook

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    That's great but judging from what you just explained the physical creation of this circuit is not the best use of their time IMHO.
     
  7. Aug 16, 2014 #6
    Oh? I think science projects are great. Especially ones like this where they end up creating something, its a huge confidence boost for young girls especially. The 2 interested in this are really top of their class, and are considering pharmacology as a career. I think it would be neat for them to have a little hand held device that simulated the body which they could give medicine to in just the right way.
     
  8. Aug 16, 2014 #7

    nsaspook

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    I know you mean well and science projects are great but a understanding of exponential growth/decay and time constants (by easily changing results in a program) in general will help them much more than building a circuit unless they have a desire to explore electricity directly instead of indirectly in pharmacology.

    I really don't think you appreciate the complexity of building a little hand held device with results that can be displayed instead of a static display of electronic parts. I can see building a static display of resistors and capacitors to teach soldering and to provide a prop for a computer simulation.
     
    Last edited: Aug 16, 2014
  9. Aug 16, 2014 #8
    Well, I mean its an RC circuit with 2 switches and a multimeter hanging on...not that bad really (and I'm kind of a dummy when it comes to electronics). I was thinking they could use some block terminals, like the ones that resemble breadboard holes, to swap out the resistance values. Should fit in an Altoids tin or similar.

    I don't think they would have chosen this project if they weren't a little interested in learning electronics. They've also already been exposed to exponential functions in science. Its really more to give them a platform on which they can use some error analysis techniques in a context they are interested in (error analysis doesn't exactly blow high school girls away). They have soldered once before but it was not a perf board kind of thing (they made some electrodes for a piezoelectric crystal they grew).

    The article has some pictures of the commercial current source and I had to shamefully admit that I didn't own one/couldn't afford one. I'm just trying to find a way to make this happen, maybe there isn't.
     
  10. Aug 16, 2014 #9

    nsaspook

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    It's a simple RC circuit with a sub-second timing/switching and micro-amp constant current control requirement to provide a linear time to voltage slope for the capacitor charging. :smile:

    That makes it complicated in a circuit for beginners but trivial to program in a electrical simulator for them to manipulate.
     
  11. Aug 17, 2014 #10

    NascentOxygen

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    You have in mind a single-IC current source, you'll also need a timer IC so you can set the variable charge and discharge times and to oversee the switching. If you use a breadboard or two there needn't be a single soldered joint in the whole project. :cry:

    You'll need a DC voltage supply of some description to power the current source.
     
  12. Aug 17, 2014 #11

    jim hardy

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    Does the body stop metabolizing during injection? Why do they open S1 during charge interval ?


    It's almost an integrator or lowpass filter with a pulse input
    1/([itex]\tau[/itex]s+1)
    tau being rate of metabolism which could be a knob on 1 meg pot.
    But 72 uf? maybe 10uf polypropylene and 10 meg pot.


    But what sort of readout ?

    Computing does seem natural - but i appreciate the value of actually building a piece of hardware.

    Arduino anyone ?
     
  13. Aug 17, 2014 #12

    NascentOxygen

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    You might wish to reconsider the 0.15s and 10s durations if you'll be monitoring with a meter. The predominant feature will be the exponential, that 0.15s part will show as just some unsteadiness in the reading. They'll just have to picture in their mind what is happening with your proposed time scale.
     
  14. Aug 17, 2014 #13
    The article suggested the 0.15 s charge time, I have no real need for it. If it can be adjusted upwards then all the better. I simply don't know. Perhaps the values chosen by the author were convenient for the apparatus he had access to.

    Let me ask this, lets change things. Lets say the charge time is 5 s. Assuming things scale linearly (which I dont know) then the day could then be about 5 minutes. They charge it by using a handheld stopwatch and incorporate their reaction time (around 0.30s) as an uncertainty. Does that effect the design of the constant current source at all? Isn't it kind of arbitrary?

    I mean they are just taking measurements of voltage and calculating charge from capacitance, and getting 4 graphs of q vs t like in my post 4 above, for 4 different resistance values and 4 different currents. Wouldn't the current values just affect the time scale?
     
  15. Aug 17, 2014 #14

    NascentOxygen

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    So the two switches will be manually operated push-button switches?
     
  16. Aug 17, 2014 #15
    Any reason that wouldn't work given they recorded uncertainty? Or how about a double throw?
     
  17. Aug 17, 2014 #16

    NascentOxygen

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    That should be okay.
     
  18. Aug 17, 2014 #17
    So really, the design of the current source isn't critical at all? As long as I slap something together that can output 4 different values and not electrocute my students I should be fine? (thanks for bearing with me) What are the drawbacks of using say a voltage regulator vs a general op-amp vs a specialized IC?
     
  19. Aug 17, 2014 #18

    NascentOxygen

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    I see no great dramas in getting this to work. There's room for plenty of maths.

    Motorola boasts that chip as being almost blow-out proof. I guess that means blow-up proof.

    I asked earlier how you intend to power it. What DC supply do you have?
     
  20. Aug 17, 2014 #19
    Batteries would be convenient, but I've read current sources prefer a regulated voltage. I might see if I can get away with just a 5V wall wart without a regulator.
     
  21. Aug 17, 2014 #20

    jim hardy

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    A decent if not precision current source can be made easily from a transistor , zener diode and a couple of resistors.

    9 volt transistor radio battery might work fine.

    Simplest current source is a transistor, zener diode and resistor.
    http://www.ecircuitcenter.com/Circuits_Audio_Amp/BJT Current_Source/BJT_Current_Source.htm image002.gif

    This one uses two garden variety diodes instead of a zener. Maybe 1N4148 - abundant and cheap.
    Replacing one of them with a 1N5231 5.1 volt zener should improve temperature performance at expense of output voltage.

    For the zener version RE would be R = V/I = 5.1/desired currentand you could use a pot or a switch to select current.
    RB just needs to conduct about two milliamps to get past the knee of the zener.
    Its value would be R = V/I = (Vsupply - 5.1 )/ .002 ohms .
    max output voltage would be about Vsupply - Vzener - 1 volt, only about 3 volts for a 9 volt battery supply.

    For the two garden variety diode version RE would be 0.6/desired current
    and RB ~( Vsupply - 1.2)/.002 ohms
    and max output voltage about Vsupply - 1.2 - Vcb volts, about 7 volts.


    A 12 or 15 volt wall wart would work well with the zener source.

    2N3906 is a general use pnp transistor that should be easy to find.. i bought a hundred of them, lifetime supply for just a couple bucks.
     
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