Small Heating Element Circuit

Discussion in 'General Electronics Chat' started by B.Weaver, Nov 24, 2010.

  1. B.Weaver

    Thread Starter New Member

    Nov 24, 2010
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    Hello,

    I'm looking for a bit of guidance with building a circuit to quickly switch on/off a small heating element that will be used to repeatedly light fuses.

    The heating element will be a small glow plug typically used for nitro RC engines and I'd like to keep the overall package as small as possible. I'm able to get the element red hot almost instantly with a single CR123 battery and this is satisfactory.

    For the switch I would like to use a small detect switch like Radio Shack 275-0008 1mA 5VDC. The reason for the small switch size is that I want to position the lever so that it protrudes into a small tube (slightly larger than the fuse) so that as the fuse passes down the tube, the fuse will trigger the switch and activate the heating element. The element would then switch off when the fuse is removed from the tube. The switch is not robust enough to carry the current on its own and I believe I need a FET of some sort that can be triggered by the switch to carry the load.

    My overall knowledge of electronics and circuit design is very shallow so if someone could provide a schematic and/or parts list it would be greatly appreciated. I'm open to varying the power source from the CR123's I mentioned earlier so long as the package can stay compact.

    Thanks in advance!

    - Brian
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Welcome to AAC

    Shows us or tell us with a drawing how u wud want it. Pic's will help.
    This way u will get more response.

    Further more it will help in giving u a working schematic
     
  3. tom66

    Senior Member

    May 9, 2009
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    Use a small transistor as a switch.

    How much current do you require?
     
  4. whale

    Active Member

    Dec 21, 2008
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    Using SCR will be the right choice for your application ........
    It will be better if you provide us the block diagram of your device.
     
  5. tom66

    Senior Member

    May 9, 2009
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    An SCR is not the right choice. It will switch on and then become uncontrollable due to the latching behavior. It might be okay for an AC source, but the OP doesn't have this - he mentions a battery.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    And, a 1ma switch is also not going to be enough current to get that hot instantly...if ever.

    You will want to switch a transistor...probably.

    Well, you could use a momentary switch if the current spec of the switch is up to the draw of the glow-plug.
     
  7. B.Weaver

    Thread Starter New Member

    Nov 24, 2010
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    Alright, I've attached the best MS Paint description that I can give of the circuit as it currently exists, however the switch I need to use cannot handle the current being drawn to heat the element (small glow plug). So, I need the switch to be able to activate some other device that can supply the higher current to the element.

    I imagine it would go something like this:

    Battery --> Switch (5v 1ma) --> Transistor/FET --> Glow Plug

    The battery can be a single CR123 since it is able to make the glow plug heat up as desired.

    The switch I mentioned in my first post is necessary because of its small size. It is 5v 1ma.

    The transistor/FET needs to be able to handle the draw of the glow plug (small coil).



    I'm not certain I can be more descriptive of my requirements.

    Thanks for the help!

    - Brian
     
  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    With a 3V battery, I don't think even logic-level power mosfets are going to saturate and pass high current without a significant Vds drop...loss. You never spec'd the glow plug current draw @ 3v. Have you measured the CR123's voltage while heating the glow plug?

    Ken
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    What is the voltage and current rating of the glow plug, or voltage and wattage?

    3v is a bit low to control the gate of a MOSFET with; even for a logic-level MOSFET.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Without more information, this should work OK for up to around 500mA (1/2 Ampere)

    [​IMG]
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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    Last edited: Nov 26, 2010
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Wikipedia reports:
    "To start a glow engine, a direct current (around 3 amps and 1.25 to 2 volts, often provided by a single, high current capacity rechargeable NiCd, NiMH or lead-acid battery cell, or a purpose-built "power panel" running on a 12VDC source) is applied to the glow plug, initially heating the filament"
    Link: http://en.wikipedia.org/wiki/Glow_plug_(model_engine)

    So, a lower voltage and a different transistor configuration is needed.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    ....I just spent half an hour looking for info, and I completely forgot wikipedia.

    That 1ma switch would be burning a hole into your finger mighty quick at 3000x the rated current.

    I cant believe it didnt weld the contacts.

    A power transistor is in order. ;)

    There is a schematic on the forum I linked to. Thet simulate the glow plug as a .25ohm resistor.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Here's a revised schematic; Q2 is a very high-gain transistor that is rated for the current needed.

    It would be difficult to get it working with a single 1.2v NiMH cell, so a "button" cell was added. The increase in weight will be negligible.

    [​IMG]
     
  15. B.Weaver

    Thread Starter New Member

    Nov 24, 2010
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    Sorry for not replying sooner but I've been busy over the holiday weekend.

    I'd like to express how thankful I am for all of the time/effort you all have put into this for me. I'm not as knowledgeable in the subject as I would like to be and as simple as my original question seemed to me, it was clearly more complex than I expected. I too spent about a half hour looking for glow plug specifications before I posted this; I suppose I should have looked here first for follow up's. :)

    SgtWookie,
    Would there be any reason not to use two AA NiMH batteries? Is the button cell shown in your schematic providing extra voltage to get the transistors working? What is the wattage rating for the resistors? Would you care to explain to me how the circuit works?

    SgtWookie, retched and others,
    Thank you for taking your time to research, provide feedback and solutions to my question, I appreciate it very much.


    I'll order up the parts and give the circuit a try.

    - Brian

    P.S. retched, the contacts of the little switch did briefly weld themselves together but a few taps on the table freed them. :D
     
  16. B.Weaver

    Thread Starter New Member

    Nov 24, 2010
    5
    0
    SgtWookie,

    What is the wattage of the resistors in your schematic? Also, what is .tran 10m?

    Thanks!

    - Brian
     
  17. B.Weaver

    Thread Starter New Member

    Nov 24, 2010
    5
    0
    If I wanted to be able to vary the amount of current to the glow plug, could Q2 be replaced with a trim pot and a triac?

    - Brian
     
  18. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Don't mean to step on Sarge's toes...

    .tran 10m is for simulation purposes to test the circuit virtually in LTSpice.

    Resistors would be 1/4 watt standard size, nearly all of the power is actually going through the power transistor, ZTX849 and the glow plug when turned on.

    The circuit is pushing the transistors into saturation or "Full On", so minimal heat is dissipated by the transistors, and is dissipated by the glow plug instead. Attempting to lower the current to the glow plug with the transistor would cause the transistor to be in "linear mode", where it would heat up quickly.

    The reason for the 1.5V is that is the designed voltage of the glow plug. The glow plug may act like an electrical fuse at 3V, failing due to high current.

    If the glow plugs are only expected to last for only a few cycles, this wouldn't be as large of a concern, the current could be changed by switching between 1.5V and 3V supplying the glow plug.

    Sarge will be along sometime to clarify. :)
     
  19. BillO

    Well-Known Member

    Nov 24, 2008
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    Any reason why this would not work? the 2N3055 could be an MJ2955 or something similar. Just trying to keep it simple.

    (sorry for the hand drawn diagram)

    [​IMG]
     
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  20. retched

    AAC Fanatic!

    Dec 5, 2009
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    Pbbbbtttt! That guy uses hand-drawn diagrams...what a maroon! ;)

    That looks alot better than many that come through here!
     
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