Switching Circuit Help for 20 Resistors & 19 Dc Fans

Discussion in 'The Projects Forum' started by kiranp, Mar 13, 2010.

  1. kiranp

    Thread Starter New Member

    Mar 13, 2010
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    Hi Everyone,

    New to this forum and have a circuit design problem.

    My project- Aroma Generator (smell-o-vision) prototype, will have 20 oil scents which can be mixed by being heated and released to the end user.

    It consists of 20 Heaters (Ceramic Resistors: 33 or 47 Ohm- whichever is best?, 5 Watt) and 19 DC Fans (12v, 0.06Amps). So around 39 channels?

    What I require is a switching circuit that can either turn on a maximum of 3 ceramic resistors and 3-4 fans at the same time.

    I have been recommended to use the following componants:

    ·39 MOSFET Transistors ,N Channel (TIP121 OR TIP141)
    ·8bit TTL Shift Registers , SI-PO (SN74AL164)
    ·20 Base Resistors (2K2 Ohms)
    ·20 Resistors (1K Ohms)
    ·19 Diodes (IN4002)
    ·20 Red LED’s (2v Drop) - to indicate which heaters are on
    ·9 Way D-Type Sockets and connectors with common ground (banks of 6?) to give a nice secure connection

    The circuit board will be controlled by a microcontroller which is connected to the PC. I cant remember which one it is but it has a usb, serial and lan? Connection and is made by microchip.

    I will have to learn how to programme the thing after the PCB is made, which I’m absolutely dreading since i have no prior experience in C!

    The supply voltage will be 12V, and i understand that the FETs switch and VGS need to be <5v so a 5v regulator will need to be in the circuit.

    So basically I’m tying to simulate the circuit in Multisim 10.1 then create the PCB in Ultiboard 10.1.
    I have drawn the circuit diagrams for the individual heaters and fans which is attached below.

    My problem is that i have no idea how to implement the shift registers with the regulator and the other components (the wiring etc) within multisim.

    Can anyone give me any guidance on how to go about doing this? Any help will be massively grateful and stop me from tearing my hair out!



    Circuit:
    [​IMG]
    And heres cad version of what the unit will look like:(sorry abou the size!)
    [​IMG]
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    Are you going to use a negative ion generator to neutralize the aromas between scenes?

    As for the help, you seem to know what you want.
    Without knowing what PIC you are using, it is not very possible to give you real accurate advice.

    From you we need to know, what temperature you want to heat the oils to. Just because you chose some resistors, doesn't equate into temperature. You need current through a resistor to achieve heat, and thats not necessarily the best way to do it. The heat is actually not helping the resistor and if you are using the oil as a heatsink, you can destroy the resistor if the oil ran out.

    Once we have the temps you want, we can move ahead.
     
  3. kiranp

    Thread Starter New Member

    Mar 13, 2010
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    If forgot to mention that the whole unit will be enclosed with a dome shape cover, (mixing bowl, bowl like container) with a covered hole at the top so that the user can smell the scent through the hole when the mini cover is removed.

    As for neutralising the scent, the dome cover can be easily lifted off to air out the device, or use a central fan which is based centrally below the ceramic resistors to vent the dome.

    I am using these ceramic type resistors because they are used in those plug in the wall air fresheners like one of those glade products. After taking one apart the resistor is situated right next to the wicking material.
    Please see:
    http://img534.imageshack.us/img534/5373/glade.jpg

    An example of the heaters: http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=1997725

    The 33 and 47 ohm ceramic heaters are already in my possession as well as the fans, I could not say what temperature I require them to run at as I will have to run tests to see how long they would have to be turned on for a scent to be effectively released, which is part of the testing procedures of my project.

    Oh also the scents are ethanol based, not oil!

    The controller I would be using is:
    http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en021940

    Sorry about the lack/misguided information, hope this helps
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    Oh jeez... You are going to burn the place down.

    SAFETY FREEKING FIRST! got it? good.

    Where are you located? Where in the world? It will help with what you have near by to get parts from.

    Ok. That design is a very odd design to see. No regulation at all. Just a resistor in the mains.

    I wouldn't go that If I were you. You are going to want to use a transformer to isolate your device from the mains power, or someone leaning into that bowl full of moist scented air may get the shock of their life...or death.

    So, you want a uC switching circuit that will turn on up to 3 heaters at a time, correct?
    Are the fans running full time? Or are they on a different circuit all together?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Those are not MOSFETs. They are power Darlington transistors.
    You would be better off to use N-channel logic-level power MOSFETs, as your power would be used much more efficiently.

    Darlingtons will have a Vce of 0.8v to 2v or more depending upon the load current; this causes power to be dissipated in the transistor rather than the load (fans, heaters). Additionally, a fair portion of the current through the Darlington will be used to simply turn the transistor on rather than power the load.

    With a MOSFET, once the gate is turned ON or OFF, virtually no current is required to keep the MOSFET in the desired state.
     
  6. kiranp

    Thread Starter New Member

    Mar 13, 2010
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    Retched: Ok got ya!! Actually I’m going have to email the scent manufacturer, I remember somewhat that the liquid on the scent strips evaporated quiet quickly, so I’m not too sure what the scent is based on.

    The device won’t be connected straight to mains, its running on 12v.

    Yes up to a maximum of 3 or 4 depending on the complexity of the smell, I will be using a whisky set of smells: here they are
    http://www.scotchwhiskynosingkit.com/contents.htm

    the fans won’t be on all the time; hopefully I can programme them to turn on after a certain time after the heaters have done their job. So a different circuit?

    I am in the UK, and can order through RS and Farnell.

    SGTWOOKIE: thanks for pointing that out, it is MOSFETS that I need and if they are better than Darlington transistors then all the better.
    Could you suggest an appropriate one from RS of Farnell?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Are you certain that the fans are 60mA?

    What kind of mounting do you want, thru-hole or surface mount?

    Instead of an SN74AL164, why not a SN74HC164? Less power consumption.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Here's an interesting N-ch MOSFET in surface mount:
    http://uk.farnell.com/rohm/rtl035n03tr/mosfet-n-30v-3-5a/dp/1525551
    Better check the manufacturer's site to make sure it's not in a last-time-buy or obsolete status.

    Here's one in a DFN package:
    http://uk.farnell.com/zetex/zxmn2f34mata/mosfet-n-dfn322/dp/1583666

    Here's another SMT; SOT-23 package:
    http://uk.farnell.com/taiwan-semiconductor/tsm2302cx/mosfet-n-20v-sot-23/dp/1299316

    These would be useful for prototyping, but getting a bit expensive for production:
    http://uk.farnell.com/toshiba/2sk4017-q/mosfet-n-ch-5a-60v-to251aa/dp/1714350
     
  9. MMcLaren

    Well-Known Member

    Feb 14, 2010
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    I wonder if you couldn't do away with the 74HC164 and MOSFET components entirely and simply use five (5) 8-bit MIC5841 serial-to-parallel sinking driver ICs (with 30v 500-ma outputs)?

    Regards, Mike

    [​IMG]
     
    Last edited: Mar 14, 2010
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    Its possible. I think the main thing the OP has to find out, it the evap temps of the scents and the evap rates. You can heat the scents, but if you don't heat them enough, they can separate and you will get odd smells. And if he doesn't heat them long enough, some can be way stronger than others and too much fan can dissipate them too much. You want the oil to be in a semi gasious state that will travel where aimed rather than a full dispersion.
     
  11. kiranp

    Thread Starter New Member

    Mar 13, 2010
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    SGWOOKIE: the fan is rated .06 Amp and I have to use surface mount mosfets. Thanks for those links! Looks like I will be using those instead.

    MMCLAUREN: with the chip and circuit you’re suggesting, can that directly drive the heaters and fans( no need for base resistors)? and can that PIC be programmed just like the PIC i mentioned i will be using, Including the fact that i should be able to turn on and off any heater and fan i specify at the same time or with a delay?

    Which output of the CA42 cable will go into the GP1 and DATpins? from this diagram
    http://www.infernallegion.extra.hu/downloads/wrrs_my_ca-42_cable_ok.jpg

    I must say if this will work it would be fantastic!

    RETCHED: I hadn’t thought about that, pretty good points. Unfortunately I won’t be able to play around with the scents until a few weeks; I have only a week to manufacture the PCB which is a shame.

    So if I were to use my PIC and the recommended shift register and mosfets including the 5v regulator, how do I wire it all up?

    I don’t understand which pins to use etc on multisim, if anyone can show me a couple of the circuits wired up so I can get a basic understanding.

    Thanks for all the input so far guys!
     
    Last edited: Mar 14, 2010
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Well, if you're going to use those MIC#### IC's, that will pretty much force using the 47 Ohm resistors; with those you'll be sinking 250mA and consuming 3.06W of power in the resistors. With the 33 Ohm resistors, you'd have to sink 364mA current, and that's a good bit over the 2x specification rule of thumb.

    Before making a decision, you need to determine if the 47 Ohm resistors will generate sufficient heat when used with a 12v supply.

    There may be another issue; such highly integrated IC's frequently go off the market rather quickly. Unless you wish for a redesign in your near future, better find out what the company's production plans are for that IC.

    The shift registers are standard. The 74HC595 is quite handy; many people make it. The MOSFETs I suggested (in ascending order according to cost) were all rated for at least 1A current, so as long as the traces are able to carry away what little heat will be generated, no problems.

    Oh, I'll suggest the 74HC595 instead of the 74HC164, as the 595 has a latch feature. You clock the bits in from the end, and the outputs don't change until you latch them in. If you use the 164, you'll be turning fans and resistors on and off.

    Another note; the fans are inductive loads. Do they have built-in reverse-EMF protection diodes? If not, you will have to implement them on the board; otherwise you risk destroying the MOSFETs.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    Remember, the heat generated by the resistors have to be hot enough to vaporize the scent while combating the cooling effects of the fans.
     
  14. kiranp

    Thread Starter New Member

    Mar 13, 2010
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    Looks like I’ll be using the MIC circuit due to the lack of time i have. I will have to play around with other scents, either thinning them or use a totally different set if the 47ohm resistors don’t generate enough heat.

    The fans i am using are:
    http://www.pchub.co.uk/uph/laptop/88-13010-1142/Compaq-Armada-7710T-Cooling-Fan-.html

    I don’t think they have EMF protection, but if i am to use the MIC circuit do i still need the diodes?

    I’m trying to create the design in ultiboard now but the MIC is not listed within its directory, is there a way of creating the component or using a generic 18SOIC?

    Primarily I need a PCB design, it does not have to simulate in multisim but just have all the correct footprints!
     
  15. retched

    AAC Fanatic!

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    Without the diodes, you will most likely end up with a dead chip. With relays turning on and off and the fans inductive loads, you will want protection. Diodes are cheap.
     
  16. kiranp

    Thread Starter New Member

    Mar 13, 2010
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    Ok got ya, those are being ordered.

    any ideas on creating the chips in the multisim software?
     
  17. retched

    AAC Fanatic!

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    Sorry, Im not multisim informed.
     
  18. MMcLaren

    Well-Known Member

    Feb 14, 2010
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    The Micrel MIC5841 has the back EMF diodes built-in. Another source for this 8-bit serial-to-parallel 30v 500ma sinking driver IC is Allegro (part # A6841).

    These drivers are roughly equivalent to a 74HC595 plus driver transistors plus back EMF diodes.

    I'm afraid I can't help you with MULTISIM either.

    Good luck with your project.

    Regards, Mike
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    Farnell UK does stock the MIC5841, about £2.2 each. You will require three of these ICs, so £6.6 added to the cost.
    It has integrated flywheel diodes, which eliminates that worry. Make certain that you connect all of the pin 10's to +12v.

    Package power dissipation is still a concern. Absolute maximum is 1.82 Watts.
    The fans take 60mA current at speed; start-up current will be considerably higher. Using PWM to ramp up the speed of the fans and current through the resistors over several seconds would help a great deal to reduce the power dissipation in the MIC5841
    I'll estimate the output saturation voltage to be 1v max with a 60mA load, so 60mW per fan.

    At 200mA, saturation voltage is 1.3v
    At 350mA, saturation voltage is 1.6v.
    With the 47 Ohm resistors, you will get roughly (12v-1.3v)/47 = 228mA current, 2.44W in the resistor and 215mW dissipation in the IC per output. At an absolute maximum, you could have 7 resistors powered simultaneously by one IC, providing good thermal coupling to a sink, or thick/wide copper traces with good air flow.

    A more realistic maximum would be three resistors and three fans for (215mW+60mW) x 3 = 825mW; this will be below 1/2 the maximum rating. Note that the power dissipated in the MIC5841 is wasted, and the available power dissipation in the resistor is reduced by 11%.

    With the 33 Ohm resistors, you will get roughly (12v-1.6v)/33 = 315mA current, 3.28W in the resistor and 504mW dissipation in the IC per output. Two resistors and two fans would place you at well over 1/2 the maximum package power dissipation rating.
     
  20. kiranp

    Thread Starter New Member

    Mar 13, 2010
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    Thanks for your help so far, im just wondering how the usb works,

    http://www.infernallegion.extra.hu/downloads/wrrs_my_ca-42_cable_ok.jpg

    White goes to gnd but which others go into GP1 and other connections?


    Am i right to say that:

    Vee=GND
    Clock=CLK
    Serial in-DAT
    Vss=GND
    Vdd=VCC
    Serial out= OUT
    Strobe=LAT
    Output enabler=!OE
    Vee=GND

    SGTWookie ill give you a reply when i get home
     
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