Need Help with Current Controller

Discussion in 'The Projects Forum' started by synapticCircuit, Jul 29, 2011.

  1. synapticCircuit

    Thread Starter New Member

    Jul 29, 2011
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    Hi. I'm new here and haven't built a circuit since the 7th grade when I covered a large book with aluminum foil (anode one side, cathode the other) in which was hidden a 9V battery, a home-made transformer, and a mercury switch. I then handed it to my english teacher.

    I'm trying to build an inexpensive DC current controller. Here are the specs:

    Input:
    An AC2DC power source: 6V/400mA (motorola cell phone charger).

    Output:
    ~10mA to ~400mA

    Other:
    on-off switch
    LED to indicate that the thing is on
    fuse(?) in case I do something dumb
    potentiometer to manually control the flow

    Load:
    Electrolyte bath of unknown/varying resistance
    It would be nice if this thing maintains a constant current
    irrespective of the resistance of the bath

    I saw the page about the current mirror circuit, which looks neat, but I know nothing about transistors or how to select them for the power range of this circuit.
    Also, the power supply is not the heavy windings type, but rather includes a very light transistor circuit of some kind. Are these things built to be current regulated as well? In other words, irrespective of load do they try to maintain a constant 400mA output? And what does that mean for this circuit?

    BTW. I entitled the book "1001 Shocking Stories"
     
  2. JingleJoe

    Member

    Jul 23, 2011
    185
    10
    I don't think it's possible to maintain the same current regardles of the load resistance unless you increase the voltage. I = V/R (ohms law)

    What exactly do you want to do with this circuit? From electrolyte bath, I gather you may be etching metal? If so I find that larger currents are better, however I've never drawn more than 5 amps when etching metal electrolytically, I'd use a power supply that can deliver atleast 3 amps.

    I'm almost certain thats a switch-mode power supply you have there, I don't know if this rule applies to switch mode but it applies to ye olde power supplies; the rated current can be overshot, however this risks pushing components beyond thier limits and usually results in fluctuating output voltage. Like the pirates code it's more of a guideline, so one might want to connect an ammeter to monitor current and/or resistor to limit the current to 400mA.
    Required resistor would be 15 ohms ( 6 / 400 = 0.015 ) the answer there is in Kilo-ohms because I used mA instead of A.

    P.S. attached is a circuit diagram of what I'd do.
     
    Last edited: Jul 30, 2011
  3. SgtWookie

    Expert

    Jul 17, 2007
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    JingleJoe,
    The simple 15 Ohm resistor in series with the supply would indeed limit it to 400mA if the supply did not exceed 6v. However, our OP wants the supply to be adjustable, and your circuit does not provide that feature.

    Arguably, the easiest way to make a reasonably accurate constant current circuit would be to use an opamp controlling a power transistor to source current to the load, and have the opamp compare the voltage across a known load resistor to a reference voltage set by a pot.

    There are very simple current regulators one can make using an LM317, but these won't reliably regulate below 10mA (5mA for an LM317L), and require power resistors instead of pots for the current that would be flowing through them. Additionally, when such regulators are used as current regulators, there is a minimum "dropout" of ~3V; 1.7v from the IN to the OUT terminal, and another ~1.25v from OUT to ADJ, so 1.7+~1.25 ~= 3v.

    As JingleJoe mentioned, the resistance of the load vs the supply voltage will limit the maximum current that can be supplied. I=E/R, or Current = Voltage / Resistance. If R goes up, E must also go up to maintain I. Also, the current control circuit will need at least a couple of volts "overhead" to control the current source.
     
    Last edited: Jul 30, 2011
  4. JingleJoe

    Member

    Jul 23, 2011
    185
    10
    Ahoy Sgt Wookie, I remember you from BG :)
    Re-reading the initial post, I didn't perceive a need for it to be adjustable, however, if one wanted to do that would not a simple potentiometer of correct power rating be suitable?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    BG? :confused:

    One of the stated requirements was:
    which I interpreted to mean adjustable. I may be mistaken; they could indeed want a fixed-current supply.

    A passive resistor won't respond to changes in the circuit. For example, if the resistance of the bath changes, or perhaps the resistance of the load or the supply voltage varies, they will cause a change in current.

    There are still unknowns; don't know what the approximate resistance of the bath will be, or the load, etc. I'm suspecting that 6v may be too low for all but very low currents.
     
  6. synapticCircuit

    Thread Starter New Member

    Jul 29, 2011
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    Thanks for the help and sorry not to be clear: yes I want it to be a hand-adjustable current regulator with an output that varies from 10 to 400 mA. We're not sure what current we need to do the job and want to play with the current via a simple potentiometer adjustment.

    Also, the electrolyte bath is mixed by hand and won't always have the same resistance, so another reason for the adjustability. I'll probably attach a cheap ammeter to monitor the output.

    I like the LM317 idea. I've been reading up on that as a current regulator and it has some nice features like a wide range of input voltages. I see what you mean SgtWookie about the voltage supplied perhaps being too low. That is an easy fix, I can just scavenge a 12V cell charger instead of a 6V.

    If I understand the LM317 set up correctly, I can just replace the fixed resistor with a fixed 3 ohm plus a variable 50-100 Ohm. That would give me 1.25/3 = 417mA when the potentiometer is wide open and as little as 12mA with the potentiometer turned the other way. Theoretically that is. I suppose the potentiometer might have at least a few ohms of "background" resistance.

    I also like that the LM317 is under a dollar.
     
  7. iONic

    AAC Fanatic!

    Nov 16, 2007
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    A 3 ohm resistor with 400mA passing through it will require .522W, or a 1W variable resistor.
     
  8. synapticCircuit

    Thread Starter New Member

    Jul 29, 2011
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    Attached (I hope) is an example of what I might build. Anybody want to comment?
     
  9. JingleJoe

    Member

    Jul 23, 2011
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    Brass Goggles!
    Also I see what you mean now, I thought that was just the possible range of currents drawn which would be dictated by the load, however if one wants to circumvent that, you need some more magic blue smoke machines.

    Quick question; can you even get 1 watt variable resistors?
     
  10. synapticCircuit

    Thread Starter New Member

    Jul 29, 2011
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  11. JingleJoe

    Member

    Jul 23, 2011
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  12. synapticCircuit

    Thread Starter New Member

    Jul 29, 2011
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    I see that you can get them for about $4 each on ebay.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Here's a constant current circuit using a common, inexpensive single-supply LM324 opamp, a Darlington transistor to control the current, a few resistors and a common low-wattage pot.

    The opamp senses the voltage across the resistor Rsense, and compares it to the reference voltage that you set on the pot, VR1, and adjusts the output of the opamp to keep the voltage across Rsense the same.

    Rload is the items in your bath.

    You can set the desired current by shorting across Rload. If the ammeter shows less current than set after removing the short, then the supply voltage is not high enough for the resistance of the load.

    The LM324 has four opamps in the one IC package. Unused opamp channels should have their inputs connected to ground to prevent oscillations. Leave the outputs of the unused opamps disconnected.

    The Darlington transistor should have a heat sink on it; it will get quite hot if you are supplying a good bit of current to a load that has low resistance.
     
  14. synapticCircuit

    Thread Starter New Member

    Jul 29, 2011
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    Thanks SgtWookie! I'll have a look at that and see if I can fully understand it. What is the benefit of this over using the LM317?
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    For one thing, you can get all of the parts at your local Radio Shack if you're in a hurry.

    For another, you don't have to worry about getting one of those big, expensive pots that will probably get overheated anyway.

    As shown, the circuit needs to have a well-regulated 12v power source. If you are going to use other than 12v, then something will need to be done for the reference side; right now it's depending on the 12v supply for an accurate reference level. A Zener diode and another voltage divider could be put in there, but this is just a proposal at the moment.

    A single LM317 would certainly have a lower parts count.
     
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