How much is my average current?

Discussion in 'General Electronics Chat' started by fjord, Aug 16, 2013.

  1. fjord

    Thread Starter New Member

    May 10, 2012
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    For the following situation:

    http://i.imgur.com/f7DM34A.png

    I'm driving a solenoid and need to know what my power supply requirements are.

    I measured with clamp ammeter and saw ~2A.

    Can you tell me how to compute this value, and how to use it to determine my PSU requirements?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,233
    You just use Ohm's Law. The current equals the supply voltage divided by the solenoid resistance, V / R.
     
  3. fjord

    Thread Starter New Member

    May 10, 2012
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    But it's a modulated PWM signal. V/R in this case is 27A and it's obviously not 27A average.
     
  4. Abovethelaw

    New Member

    Aug 2, 2013
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    Any supply must be able to handle the peak loads of the system.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    Some clamp ammeter may not response to the real current of pwm.

    Normally the I = V/R = 26.5A.
    Calculating the 50%/50% PWM, I = (V/R)/(100%/50%)
    So calculating the I_Pwm = (V/R)/(100%/Hi duty%)

    exp: pwm Hi duty 100%
    I_Pwm = (V/R)/(100%/Hi duty%)
    I_Pwm = 26.5A/(100%/100%)
    I_Pwm = 26.5A/1
    I_Pwm = 26.5A

    exp: pwm Hi duty 80%
    I_Pwm = (V/R)/(100%/Hi duty%)
    I_Pwm = 26.5A/(100%/80%)
    I_Pwm = 26.5A/1.25
    I_Pwm = 21.2A

    exp: pwm Hi duty 50%
    I_Pwm = 26.5A/(100%/50%)
    I_Pwm = 26.5A/2
    I_Pwm = 13.25A

    exp: pwm Hi duty 20%
    I_Pwm = 26.5A/(100%/20%)
    I_Pwm = 26.5A/5
    I_Pwm = 5.3A

    exp: pwm Hi duty 5%
    I_Pwm = (V/R)/(100%/Hi duty%)
    I_Pwm = 26.5A/(100%/5%)
    I_Pwm = 26.5A/20
    I_Pwm = 1.325A
     
  6. BobTPH

    Active Member

    Jun 5, 2013
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    Sorry, but is it not that simple. Since the load is an inductor, it will depend on the period of the PWM.

    Bob
     
  7. crutschow

    Expert

    Mar 14, 2008
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    3,233
    ScottWang's calculations are basically correct if the PWM frequency is high enough that the inductor ripple current never goes to zero. Anything above 10kHz should be fine for an inductance of 155μH.

    Note that a free wheeling diode needs to be added across the solenoid (cathode to plus) for the circuit to work properly and avoid large voltage spikes across the transistor.
     
  8. fjord

    Thread Starter New Member

    May 10, 2012
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    There is a diode, I didn't include it because it's not a part of the current calculation, what is shown is stripped down
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    But it is part of the current calculation for a PWM signal. If the inductive current has nowhere to go when the transistor shuts off then the current must stop, which generates a large inductive spike at the transistor collector.
     
  10. fjord

    Thread Starter New Member

    May 10, 2012
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    Cool fact. I have a protection diode from drain to Vcc that is not shown in the simplified schematic.
     
  11. fjord

    Thread Starter New Member

    May 10, 2012
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    OH jeeze! I just realized that my drawing is very deceptive! During the period of time labeled T2, there is NO SIGNAL, it doesn't continue as the three dots would have you thinking.

    That is, it's not just a simple continuous 50% duty cycle signal

    It's a square wave at 33.33Hz, with a duty cycle of 2.67%, modulating another square wave at 31.25kHz with a duty cycle of about 50% (though this varies, so a solution in general terms is preferable).
     
  12. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    As a rough method ......

    \text{I_{\small{AV}}=0.0267[LF_{duty}]x\frac{0.5[HF_{duty}]x53[Volts]}{2[Ohms]}=0.354 Amp}
     
  13. fjord

    Thread Starter New Member

    May 10, 2012
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    That is what i calculated before too.Interesting. Why do you suppose clamp ammeter would read 1.6a
     
  14. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    What clamp ammeter are you using?
    Do you own a DVM? If so what DC voltage do you measure across the solenoid?
     
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