Current Regulator Help

Discussion in 'The Projects Forum' started by 5lo3, Sep 11, 2007.

  1. 5lo3

    Thread Starter New Member

    Sep 11, 2007
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    Hi, I'm trying to build a circuit that will regulate 0 - 1A of current. The amount of current will depend on the 0-5V signal coming from a microcontroller. Anyway attached is what I've been using so far. The problem is, with a 5ohm load, i can only reach 0.76A. Can this be fixed by simply using a different transistor?
    VCCR.JPG

    Thanks, Liam
     
  2. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    The base to emitter voltage for the TIP120 is around 1.2V and that leaves 3.8V for the resistor. Could you power the opamp with 12V?

    You could change the Darlington to a FET also.
     
  3. 5lo3

    Thread Starter New Member

    Sep 11, 2007
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    Wow, thanks for the quick answer! I could run the op amp off 12V, however this circuit is going to be used in a vehicle. I was under the impression that vehicle power supplies were hard on most electrical components without some sort of regulator or transient voltage protection. I wasn't sure how sensitive op amps typically are, so i just ran it off of an lm7805 along with the microcontroller, etc. Do you think it can take the raw 12V? If not, could you recommend a particular FET to use (preferably in a TO220 package)?

    Thanks Again,
    Liam
     
  4. ashokcp

    Active Member

    Mar 8, 2007
    50
    2
    You require atleast 5V at the output to drive 1A into 5 Ohms load. With about 1.5V for the TIP120, 6.5V is required at the output of the opamp (Base resistor can also be reduced). For this you could try replacing 7805 with a 7808 (8V regulator) for the opamp supply, or insert 3 Nos. 1N4002 diodes in series between 7805 ground pin and ground. The second option would give about 7V output. I've assumed this opamp to be rail-rail, and, also can be powered with 8V, which I've not checked.
     
  5. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    This would make the 0-5V become 2V-7V from the microcontroller.

    If you put the load "above" the TIP120 and use a <5 Ohm resistor as the reference resistor you can ge the desired 1A. The thing to watch out for is the power handling of the set resistor. You may need to use three in series-- like 1.3R + 1.3R + 1.3R (2W each). This will make the current almost independent for a wide range of loads. You can also get a 2-Ohm current sense resistor which is pricey but if you hook up an A/D on it you could get feedback on the current.

    As far as using the 12V direct, you should be safe as long as you put some good bypassing caps-- 10uF (or more) electrolytic (16V+) + 1uF ceramic (25V+) on the supply rail. A 12V TVS diode would also be good.

    When selecting a MOSFET you have to get one with a low gate threshold voltage. IRF3706 is one that is in a TO-220 but it still can be as high as 2V.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Just a thought.

    Maybe you could put a second 5 ohm resistor in parallel with the 5 ohm already there. This will give you 2.5 ohms for your current sense resistor. To compensate for this you would use a Vin/2 voltage divider at the control input. You could even work a potentiometer into the voltage divider and provide a calibration adjustment.

    hgmjr
     
  7. ashokcp

    Active Member

    Mar 8, 2007
    50
    2
    Thanks for pointing out, I didnt realize that the uC was also being powered from this 5V!
     
  8. 5lo3

    Thread Starter New Member

    Sep 11, 2007
    5
    0
    Ok, the op amp is rail to rail, and it's capable of running at up to 16V. I'll run the op amp off the +12 and keep the darlington transistor and the 5 ohm resistor (I've got a bunch of these). The 5V regulator already has bypass caps on the 12V side. I'll also put the load before the darlington, so that I can use a wide range of loads. Does this sound reasonable? So here's the new circuit:
    VCCR2.JPG

    Thanks again for everybody's input, especially nanovate, you've helped me out a lot.

    -Liam
     
  9. ashokcp

    Active Member

    Mar 8, 2007
    50
    2
    It appears Ok, but did you notice hgmgr's and nanovate's remarks about R2? At 1 Ohm, R2 has to dissipate 5W, probably rated 10W.
    If you reduce R2 to 0.5 Ohms (may be 0.5Ohms 1W or 2 Nos. of 1 Ohm 1W in parallel), at 1A the power dissipation is reduced to 0.5W. However this reduction in dissipation (4.5Watts) is now added to the TIP120, which can be sufficiently heatsinked. You will have to use a voltage divider on the uC output to divide by 10, or a POT as hgmgr's for calibration. In this case, you can use +5V for opamps, which would also satisfy your concern about vehicle power supply (I had a 17V+ on my vehicle's 12V when the regulator inside the alternator housing failed, so I'm being a little overcautious!).
    The added benefit would be you would be able to handle even wider loads (maybe upto about 9 Ohms).
     
  10. ashokcp

    Active Member

    Mar 8, 2007
    50
    2
    "At 1 Ohm, R2 has to dissipate 5W" should read as "At 1A, R2 has to dissipate 5W"
     
  11. 5lo3

    Thread Starter New Member

    Sep 11, 2007
    5
    0
    I see what you guys are saying - it doesn't make much sense to have to use a huge 5ohm 10W resistor when i could use a much smaller 0.5ohm 1W resistor. So here is the revised circuit - is this what you guys are thinking?
    VCCR3.JPG
    I added the cap to filter out the pwm from the microcontroller.

    Thanks again
    -Liam
     
  12. 5lo3

    Thread Starter New Member

    Sep 11, 2007
    5
    0
    Oh except the op amp would be running off of 5V again

    -Liam
     
  13. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Since your target maximum current is 1A you will need to make R3 the 47K and R5 the 4.7K.

    You only need 0.5 volts at the positive input to the opamp to obtain 1A of output current.

    hgmjr
     
  14. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    With such a large resistor in series with the base of the TIP120 you may not
    have enough base current to get your desired collector current. Is there any reason
    you don't want to use a MOSFET instead of the BJT?

    (* jcl *)
     
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