Solenoid amperage question

Discussion in 'General Electronics Chat' started by Tobias, Sep 26, 2009.

  1. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Does a solenoid draw the same amount of current no matter the mechanical load? For example a solenoid is opening a needle and seat style valve against 100 psi and then the same solenoid opens against 200psi with the same needle and seat. Will the solenoid draw the same amperage with both psi scenarios?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Solenoids draws 2 amounts of current.
    one is to get the solenoid energized to pull the metal pin.
    second is the current needed to hold the pin in side.

    The energizing current is always higher than the holding current.
    Holding current can be half or even one third of the initial current capacity.

    When it comes to different pulling force, of different capacity I do not think the current will vary but a solenoid is made to that specific application.
    I do not think 100 psi solenoid could pull twice it is rated for.
    You might need a stronger solenoid to pull the 200 psi ap.

    Rifaa
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Hi,
    there are several factors with solenoid valves:

    With AC solenoids, the current drawn varies with the mechanical position of the solenoid armature. The inductance increases as the magnetic gap reduces, so the current reduces as the solenoid operates.

    With a DC solenoid, the current is purely down to the circuit resistance and voltage, the armature position has no effect.

    There are two other things to consider:
    Solenoid valves are often 'pilot operated'; the actual solenoid operates a small valve which in turn controls a hydraulic or pneumatic actuator to operate the main valve.
    This setup can equally be on large valves or small, manifold mounted ones, just to minimise operating power.

    DC solenoids sometimes have a double-wound coil and a switch that operates when the solenoid is fully operated. A high current coil provides the initial closing force, and once the magnetic circuit is closed a low current coil is switched in to provide the (much lower) holding force, without the current consumption and heating.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Yes, but at 200 psi valve may not open if 100 was maximum capability of solenoid. A DC pull type solenoid is working at a disadvantage with max load at its minimum force position. Digressing from your question a bit, thumb nail shows a possible way to reverse operation. Using a push type solenoid to hold valve closed at minimum current, a compressed spring supplies power to open valve at desired pressure; each component operates at its optimum point.
     
  5. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Thanks for the input. Let me rephrase the question...would the same solenoid draw the same current opening against 200 psi and just opening against atmosphere?

    I made this box that activates nitrous stages on a race car using MOSFETS. The solenoid manufacturer says the four nitrous solenoids on one stage take 26 amps. When we test fire my box to activate the stages while the engine is running and the nitrous bottles closed, everything works fine. On the race track my box will open the first stage fine but then not open the second stage.

    I sent a box to the solenoid manufacturer and they test fired four stages at the same time with the nitrous bottle open and all four worked great. I am just trying to figure out why this isn't working on the car. I am thinking there has to be a lack of amperage to open the second stage.
     
  6. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Attached are a couple pix of the board with the busbars and mosfets. I have two MOSFETS, in parallel, per stage. The MOSFETS are on both sides of the board. The busbar is 0.040" thick and 0.500" wide
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Yes, coil resistance and voltage determine current. Current rise time will be slowed by solenoids inductance.
     
  8. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    It sounds like the wiring is not heavy enough and/or the FETs are too high 'on' resistance (or too low gate drive to attain the rated RDS-On figure).

    At low voltages & high currents, wiring resistance becomes extremely critical; just 0.1 Ohm will drop 2.6V at the 26A you are trying to pull; that's around 20% of your 12V supply.

    Check the voltages across both the supply bus and the solenoids as they switch, I think you will find a significant drop somewhere.

    Some FETs need 15V gate drive to get a decent low on resistance. What are the devices you are using, and what is the gate drive voltage?
     
  9. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Attached is the spec sheet for the MOSFET and also my schematic. The MOSFETS are located in the top right portion. MS1-4 are the symbols I am using for the MOSFET. Pin1 = Gate, Pin2=Drain, Pin3=Source. I am pulling the Gate high to VIN(16v) with a 10K resistor and pulling it low with a FMB2222A.

    The first two runs yesterday, we had my box activate two stages of nitrous. It activated the first stage with a bit of delay. The second stage never activated. So the last run we left stage one on the box and the second stage literally activated with a push button switch for the driver. The first and second stage then activated.

    Thanks in advance for the help.
     
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    The power switches look fine - the FETs are rated at 5 milliohms max with 10V on the gate, there certainly should not be a problem with them!

    On the overall circuit, you appear to be using low value base drive resistors with all the transistors. I'd change to higher values where practical, such as the bipolar transistors feeding the FETs, just to keep the overall current down on the PIC.

    Typically aim for a base current 1/10 the collector, of if a low current load just use 10K to the base.

    I'd also add some extra bulk decoupling to both the 5V and 12V sides unless you have more than shown already, as vehicle environments are extremely noisy electrically.

    The last bit is adding external pullups to the inputs as the chip pullups are very weak and you may get noise problems there.

    Assuming the MPU is not being overloaded, the only other thing I can think of is still a wiring-resistance related voltage drop.
     
  11. Tobias

    Thread Starter Active Member

    May 19, 2008
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    The thing that I really don't understand is when using a mechanical switch with the same wires, the second stage/solenoids open. My mosfets won't open the solenoid. Will using a mechanical switch kind of force the solenoid open. Meaning if the car is right on the bare minimum of current to open the solenoids, the MOSFET won't work correctly and the mechanical switch will do it?
     
  12. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Are you using *exactly* the same wiring with the switches, and do all four solenoids operate with the switches?

    If the FETs are switching right, their resistance is trivial and they should work just the same as mechanical switches..

    Try unplugging the MPU and switching the pins that drive the outputs to +5V, to see if they all still work together?
     
  13. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Yea its the same wiring and the solenoids work fine with switches. I am going to have the team add an additional cable to supply current and see what happens.


     
  14. timrobbins

    Active Member

    Aug 29, 2009
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    I was reviewing the coil driving techniques in http://www.penninecomponents.co.uk/News/PDFS/Kilovac_Data/economizer_presentation.pdf
    and noticed the explanation of the impact of back emf on the current rise waveform for a DC powered solenoid coil. Has anybody else come across a current rise waveform that is not just L/R characteristic for solenoids powered from DC?

    I would have assumed the comment from RJenkins was accurate, but the linked article shows that coil current is influenced by armature position (in that particular solenoid) for DC.

    Likewise, has anybody come across measured current waveforms for AC supply applications?

    Ciao, Tim
     
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