losses of mosfet transistor

Discussion in 'The Projects Forum' started by mah, Dec 29, 2013.

  1. mah

    Thread Starter Active Member

    Mar 15, 2010
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    i made an electronic load for 150 watt photovoltaic panel . i can control the circuit but the measured voltage by the voltmeter isn't the real volt relevant to the current , for example i got 3 amp it should give 30 volt on the drain ( where the +ve terminal of the panel is connected ) but the voltmeter reads 27 volt , where did this loss go ? , i used four IRF530 but they are from the same manufacturer ( they have different numbers on it) but i don't think that this is the problem . this is the circuit without using 10 KΩ in feedbackhttp://forum.allaboutcircuits.com/attachment.php?attachmentid=62147&d=1385954290
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,236
    Sounds like you have some resistance between the panel and the transistor drains.
    What is the voltage directly at the panel output?
    How are you measuring the current?
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
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    At 3 amps current you will have about 2.48 volts across the 3.3 ohm resistors plus some drop in the Mosfets.
     
  4. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    no, there is no resistance between panel and drain.
    the voltage of the panel is ranging from 0 to 40 volt.
    i measure the current using clamp meter.
     
  5. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    i measured voltage on the resistors connected to source , they aren't equal . the time i measured voltages on them were ranging from 0.5 to 0.8 v. two of them give .8 and the others .5 volt
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    How did you construct the circuit ?
     
  7. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    constructing on PCB
    do you think the heat will affect that?
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    How much heat ?
    Are Mosfets and/or source resistors warm, or too hot to touch?

    0.5 to 0.8 volts across 3.3 ohms doesn't make sense if you are drawing 3 amps split between the 4 devices.

    3 amps/4 = 0.75 amps per source resistor.
    0.75 amps * 3.3 ohms = 2.48 volts drop.
     
  9. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    they are warm.
    I am sorry i used 1Ω instead of 3.3
     
  10. inwo

    Well-Known Member

    Nov 7, 2013
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    Trust the voltmeter!;)

    The current will be the same as voltmeter with the same precision as resistance and voltmeter. Meter load should be negligible.

    Unless you have some high precision clamp on ammeter, much better than mine, I wouldn't bet on it.
     
  11. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    i can't trust the voltmeter because i put the panel on 50 watt potentiometer and it gave 30 volt and 3 amp with the same voltmeter so that mean the circuit has issue .
     
  12. tubeguy

    Well-Known Member

    Nov 3, 2012
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    It's always best to post your actual circuit values. ;)

    The 1 ohm resistors equate to 1 amp current per volt drop.
    Makes it simple to measure the current using your voltmeter across these.
    I agree with inwo. Use the voltmeter to measure the current!

    Add the currents through each leg (calculated using the voltage drops). The accuracy depends on the voltmeter and the accuracy of the 1 ohm resistors.
     
    Last edited: Dec 29, 2013
    inwo likes this.
  13. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    what is the relation between this talk and the drop in voltage problem, i measured the voltage directly on the drain
     
  14. tubeguy

    Well-Known Member

    Nov 3, 2012
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    With exact 1 ohm resistors the total current will then be:
    1 amp + 1.6 amps = 2.6 amps

    EDIT: When you used the 50 watt pot, did you set the current using the clamp meter or by setting the resistance to 10 ohms?
     
    Last edited: Dec 29, 2013
  15. inwo

    Well-Known Member

    Nov 7, 2013
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    If you are just asking about voltage drop from panel to circuit board (drain), then see post #2.

    Voltage drop= resistance.
     
  16. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    I am fed up with mah. This is the fifth thread he has started about this same topic. The threads are so fragmented and disjoint, that no one can follow them.

    I have posted several iterations of schematics for his electronic load for him to follow. Each time, he comes back, starts a new thread, complains that the circuit doesn't work, but each time he has made modifications which he doesn't tell us about which are likely the root cause of his problems...

    This time, by reading through this thread, I find that instead of the 3.3Ω source resistors (which are there to equalize the currents between the FETs), he has subsituted 1Ω resistors instead. One of his early problems was mismatched threshold voltages Fet to Fet so that the FET with the lowest threshold voltage turned on first and hogged all the current, causing a FET to burn up. Most likely caused by the substitution of the 1Ω for the 3.3Ω which he never mentioned till now...

    While we where trying to get to the bottom of his early FET failures, it was suggested that the circuit might be unstable, and is oscillating. To prevent that, I added frequency compensation to the feedback network to make the circuit unconditionally stable. Now by reading this thread, I find that he left out the 10K resistor specifically added to slow the step response of the circuit to make it overdamped.

    I'm finished. I have better things to do with my time...
     
    Last edited: Dec 29, 2013
    RamaD, tubeguy and THE_RB like this.
  17. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    i changed al the resistors with 1 Ω so they are all equal
     
  18. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    by setting the resistance to 10 Ω . i noticed that there is a difference between the potentiometer and the circuit by .4 v in creased to 2 volt when i increased current to 2.5 A before this value it was small i can ignore it.
     
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