1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Solenoid driver

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by earckens, Oct 1, 2016.

  1. earckens

    earckens Member

    Joined:
    Aug 25, 2016
    Messages:
    430
    Likes:
    7
    Location:
    Ghent, Belgium
    Some years ago an interesting discussion went on about a small unit to drive solenoids for model railroad turnouts: http://www.electro-tech-online.com/threads/capacitor-circuit-and-current-flow.125521/

    Now I would like to continue/restart/update this discussion although with some extra input.
    Again, given is a solenoid with approx. 10mH inductance that has to be powered during a short (300ms to max 1sec) with a DC pulse of between 16V and 25V in order to get the solenoid to drive a model railroad turnout.
    Requirements:
    1. as fast charge as possible
    2. no charge when load is applied
    3. as high discharge current as possible
    4. protection against back-emf (although this is not such a problem when reviewing the discussion of 2012)
    Here is a simple explanation of the concept: http://www.rebelhosts.com/tt/te/p16.htm

    My questions are:
    1. one circuit (train2.gif) uses a power 2N3055, the other (Cap_Dis_Circuit.gif) a Darlington BD679. Although I would rather use a higher power Darlington TIP120 I wonder what the performance difference is between the 2N3055 and the Darlington. the purchase price of a Darlington is about half that of the 2N3055
    2. circuit "CDblock" uses full bridge rectification: is there any advantage re. one of the requirements over the previous two circuits?
    3. is circuit "CDblocksplit" in an advantage of "CDblock" re. charging time when both simultaneously discharge over 2 solenoids?
    4. Circuit "CDSCR" uses SCR's to do the switching operation of the current but what interests me is the use of an opamp as voltage regulator:
    a. is there an operational advantage in this?
    b. does any of the above requirements get better resolved in this circuit?

    Thank you for your valuable inputs, it is very appreciated!
    Erik
     

    Attached Files:

  2. chemelec

    chemelec Well-Known Member

    Joined:
    Apr 4, 2004
    Messages:
    2,123
    Likes:
    77
    Location:
    Hedley, B.C. Canada
    Using a 555 driving a Mosfet would be much better.
    When triggered, it just give one pulse to the solenoid, than shuts Totally Off .
     
  3. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT
    Hy earckens,

    You asked me to provide a bit more information in response to your opening post of this thread; wow, quite a wide ranging scope.:)

    But right from the start, chemelec in post #2 has given the optimum method for generating a pulse to smartly and safely drive the point switching solenoid on a model railway.

    I will respond to your questions in your opening thread with individual posts.

    spec
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT

    Issue 4 of 2016_10_04
    Question 1: difference between following transistors:

    (1.1) 2N3055: http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF
    Metal can, NPN, high power, bipolar junction transistor (BJT): rugged, cheap, ubiquitous. The original consumer high power transistor. The 2N3055 can handle much more current (15A) and power (115W) than the other two transistors, but being only a single transistor only has a current gain (hFE) of around 50, dropping badly to 15 at 10A.

    versus

    (1.2) BD679: http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00000939.pdf
    Plastic case, NPN, BJT medium power Darlington: cheap. popular. A Darlington transistor is essentially two transistors in one case to give double the hFE of a single transistor. In this case the hFE is 750 at 1.5A. Also included in the case are bias resistors and, normally, as in this case, a snubbing diode to catch voltage over swing with an inductive load.

    versus

    (1.3) TIP120: http://www.onsemi.com/pub_link/Collateral/TIP120-D.PDF
    Plastic case, NPN, BJT medium power Darlington: cheap, ubiquitous. The TIP 120 is simply better than the BD679 in all respects and can be safely used as a substitute for the BD679 in all normal applications. hFE= 1,000 at 3A.

    Darlington transistors, and to a large degree medium power and high power BJTs, have been completely eclipsed by MOSFETs in most circuits these days. High end audio power amplifiers still use BJTs though.

    spec
     
    Last edited: Oct 4, 2016
  6. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT
    Issue 0 of 2016_10_04
    Question 2: half wave versus full wave rectification.

    In general full wave rectification offers the following advantages:
    (2.1) charges the reservoir capacitor twice as fast
    (2.2) halves the ripple voltage
    (2.3) halves the ripple current
    (2.4) doubles the ripple voltage frequency, which means easier ripple voltage filtering
    (2.5) balances the current taken from the mains transformer/mains supply

    spec
     
    Last edited: Oct 4, 2016
  7. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT
    Issue 1 of 2016_10_04
    Question 3: split charging

    I may be missing something specific to model railways, but as far as I can see there is no advantage with this circuit that could not be achieved by simply doubling up on the positive part of the circuit.

    spec
     
    Last edited: Oct 4, 2016
  8. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT
    Issue 1 of 2016_10_04
    Question 4: use of opamp

    The opamp does not provide any real benefit; it does better define certain circuit parameters. But the base drive to the BJT is limited due to the opamp limited current sink capability.

    spec
     
    Last edited: Oct 4, 2016
  9. earckens

    earckens Member

    Joined:
    Aug 25, 2016
    Messages:
    430
    Likes:
    7
    Location:
    Ghent, Belgium
    Hi chemelec, I think I do not understand your response. In which circuit drawing would you use a 555 and MOSFET?
    Thank you,
    Erik
     
  10. earckens

    earckens Member

    Joined:
    Aug 25, 2016
    Messages:
    430
    Likes:
    7
    Location:
    Ghent, Belgium
    spec, my question 3. is not really relevant I realise now because the charge current is dependent on the transformer current supply rating, not on the charging transistor. And I just now see the rest of your answer, I concur. Thank you!
     
  11. earckens

    earckens Member

    Joined:
    Aug 25, 2016
    Messages:
    430
    Likes:
    7
    Location:
    Ghent, Belgium
    That answers about all of it pending explanation of the use of a 555 with a MOSFET (chemelec post #2). And then my further question: MOSFET or SCR?
     
  12. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT
    Yes, you are correct that the charge current is limited by the input power.

    In general there are many ways to implement a particular function: one designer will favor one approach and another designer will favor another approach and, quite often there is little to chose between the two approaches. Also, tradition and common practice seems to have a big influence on the design approach used, as in this area where discharging a capacitor seems to be favored.

    chemelec is proposing a new approach which will required a completely new circuit.

    I will do a schematic showing chemelec's approach (update: no need, chemelec has posted a circuit).

    spec
     
    Last edited: Oct 4, 2016
  13. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT
    MOSFET by a mile for this application.:)

    spec
     
  14. chemelec

    chemelec Well-Known Member

    Joined:
    Apr 4, 2004
    Messages:
    2,123
    Likes:
    77
    Location:
    Hedley, B.C. Canada
    possibly something like this:
    (500Ms on time)
    The 10k And/Or the 47uF cap can be changed to Change the On Time.
    555-BW.PNG
     
    Last edited: Oct 4, 2016
    • Like Like x 1
  15. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,315
    Likes:
    1,230
    Location:
    Cardiff, Wales
    With that circuit you need a pull-up resistor (e.g. 10k) from the trigger switch to the 12V rail.
     
    • Agree Agree x 1
  16. chemelec

    chemelec Well-Known Member

    Joined:
    Apr 4, 2004
    Messages:
    2,123
    Likes:
    77
    Location:
    Hedley, B.C. Canada
    Only Pin 2 Requires a Pull Up, unless you need Quick Repetition.
    The cap and Switch Pull pin 2 Low, for just a Split Second to trigger the 555.
    The cap doesn't hold a charge for very long.
     
    Last edited: Oct 4, 2016
  17. chemelec

    chemelec Well-Known Member

    Joined:
    Apr 4, 2004
    Messages:
    2,123
    Likes:
    77
    Location:
    Hedley, B.C. Canada
    Not With them, But Instead of those Other Circuits.
     
  18. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT
    In addition to your circuit's elegance: accuracy, simplicity, and low cost, there is another advantage: the NMOSFETs intrinsic diode from drain to source will catch negative voltage swings (kickback) from the solenoid coil.

    spec
     
    Last edited: Oct 4, 2016
  19. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    710
    Location:
    Somerset UK GMT
    Hi earckens,

    You would not need one of chemelec's circuits for each solenoid, as is shown with the other circuits. Instead, you can simply have an N way switch to select any one solenoid to operate from from N solenoids.

    spec
     
    Last edited: Oct 4, 2016
  20. earckens

    earckens Member

    Joined:
    Aug 25, 2016
    Messages:
    430
    Likes:
    7
    Location:
    Ghent, Belgium
    Why? Excuse my ignorance, I had my studies finished 30 years ago
     
  21. earckens

    earckens Member

    Joined:
    Aug 25, 2016
    Messages:
    430
    Likes:
    7
    Location:
    Ghent, Belgium
    Wow, great! Thanks a million.

    Just what do you suggest to use for the MOSFET? I happen to have IRL520N and IRLB8721 in stock here..
     

Share This Page