Modulating Ic on a BJT?

Discussion in 'The Projects Forum' started by juansg, May 3, 2013.

  1. juansg

    Thread Starter New Member

    May 3, 2013
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    Hi,

    I am working in a project with a circuit that a college gave me, but he is not with me anymore, so I am trying to figure this out with no luck.

    The idea is to be able to modulate the current in Ic, using the voltage source (ultimately, the Vs will actually be the pwm output of a microcontroller). I am simulating this circuit, but it's not working as I was told it should work. Anybody has any insight on how to fix it?

    thanks
     
  2. #12

    Expert

    Nov 30, 2010
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    When the voltage on the left becomes positive compared to ground, the op amp and transistor allow 100 amps per volt (of input) to flow from the source on the right side to ground.

    This is not a practical circuit because there is no bipolar transistor that will pass 100 amps and survive.
    Increasing the resistance of R2 will bring the results into a reasonable range.
     
  3. juansg

    Thread Starter New Member

    May 3, 2013
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    Hi, thanks for the quick reply. When you say, the voltage on the left, do you mean in the Vin+ of the Op Amp?

    Also, the final goal of this circuit is to use the BJT as a varying load (from short-circuit to open circuit) for a solar panel, to be able to measure its IV curve. The panels I'll be measuring reach up to 10 amps in short-circuit, and 50 volts in open circuit. The current source on the right was just to try to simulate the solar module, but the values are random.
     
  4. #12

    Expert

    Nov 30, 2010
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    1) Yes.

    2) R7 becomes significant here. Is 100 ohms the intrinsic impedance of the solar panel or just a guess you stuck in to simulate the circuit?

    3) You are still in the range of more than 100 watts, and it all happens in the transistor. You are going to have to use multiple transistors, and using a fan on your heat sinks would help a lot.
     
  5. juansg

    Thread Starter New Member

    May 3, 2013
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    The current source + resistors on the right side was just a poor attempt to model the photovoltaic module. It's a 285W module, with the I-V curve that I've attached. The idea is to be able to use a PWM ramp at the Op Amp's Vin and being able to regulate a current flow from 0A to about 8A. The circuit was using just one BJT with a big heat sink.

    The original circuit was actually working, and I was given a hand written version of it. I just got access to the full circuit (see it attached), but I don't get how's that possible. Please, note that the wire connected to the emisor and going upwards in the diagram, just goes to a current sensor and then to ground. Also, somebody made a mistake drawing the 500uOhm resistor, it's actually a 0.01Ohm resistor in the circuit.

    The main problem I see is that using such a tiny load resistor, the voltage and current at the base of the BJT go crazy high. Any comments are more than welcomed.
     
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    Last edited: May 4, 2013
  6. #12

    Expert

    Nov 30, 2010
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    1) Yes. Fast modulating can keep the heat to a reasonable range.

    2) The voltage and current at the base of the transistor are under the control of a closed loop amplifier. The fact that this circuit was built and working should be some assurance to you.

    The difficulty that concerns me is that you have to use the right size wire and make perfect connections to keep the resistance in the current measuring circuit low...way lower than the sense resistor. .01 ohms is hard to do with anything less than soldering all your connections, and the diameter and length of the wire can get to .01 ohms pretty fast. When you build that part, no skimping is allowed.
     
  7. juansg

    Thread Starter New Member

    May 3, 2013
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    In case it helps, everything was implemented in a PCB. Could you elaborate a bit more your explanation about controlling the base? I am trying to draw a "concept electric diagram" of how to control the module's current with this system, but I'm always getting really high base currents...

    thanks a lot
     
  8. #12

    Expert

    Nov 30, 2010
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    You are supposed to have high base currents. The rule is that the base current should be 1/10th of the collector current. Of course, the base current gets measured and has to be subtracted (with a calculator) to get the solar panel current.

    The "other" way to do this is with a MOSFET. Their control current can be rather low, but it depends on how fast you switch them. Fast enough and you wind up at the same place with high control current. They have been improved dramatically in the last 10 years, but they come with their own set of limitations and complications.
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    If you go with an N-channel mosfet as #12 suggested you usually only need to drive it with 10 volts at the gate or even less with logic level mosfets. Driving the gate is like driving a small capacitor, but if PWM frequencies are not very high it's not an issue to drive it with a normal op-amp.

    Unless I'm missing something, you can't get around the watts in this application so you likely need multiple devices with heat sinks/fans. ;)
     
    Last edited: May 4, 2013
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