DC Amps Questions Please

Discussion in 'The Projects Forum' started by danl, Dec 16, 2015.

  1. danl

    Thread Starter Member

    Sep 24, 2013
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    I am building a small hobby circuit - see attached.
    Requires high wattage from a sequence of 9 small nichrome wire heaters. Current will be on each heater for only about 1-2 seconds - just to get a very quick burst of heat.
    28g nichrome, ranging about 100 to 120mm length used to make small coils. At these amps, the nichrome current exceeds its MP, but these coils are bathed in an aqeous (non-flammable!) fluid and only powered for a second or two. Manually tested and confirmed to be OK.
    Question is about safest way to get to the high wattage with the simple circuit. 6VDC is preferred to match existing SLA battery, but 12VDC runs at lower amps. For wiring, connections, etc, what is preferred, 6 or 12V? Are the higher vs lower amps a factor, safety or otherwise? Planning to use 16g silicone wire for the circuit. Max wire length will be about 3ft.
    If the rotary switch sticks on a contact, the nichrome in that heater burns out (tested). Still will use a fuse - is that enough protection?
    Thank you! Fog Carronade Circuit.jpg
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I would use the 12V battery, but it's not a critical decision. You'll get less heating loss in the hookup wires and you could use a car battery if you needed to.

    What is "R" in your diagram?

    I'm not really sure what else you are asking. You could make a circuit to replace the mechanical rotary switch, but you didn't ask about that. There are lots of different types of fuses to consider and you may want to read the data sheets about how long they take to blow under various conditions. (I was surprise how slow they are.) But maybe the fuses cost more than the heating elements?
     
  3. danl

    Thread Starter Member

    Sep 24, 2013
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    wa
    Thanks for the quick answer.
    R of the nichrome coils will depend on the voltage chosen in the final setup.
    This is to go in an R/C sailing ship - biggest battery that can be used is about a 6 or 12V SLA at a few Ah size.
    Switching circuit at these many amps? YES I am interested. Could you get me started please?
    Actual best circuit needs 3 different "ON"-times at each position. At each position, the heater is turned on first, then a fraction of a second later a solenoid air valve is activated for about a second, and at the same time the solenoid goes on, an LED is flashed for a very short duration.
    The rotary switch is built with 3 contacts at each position, and the rotary arm has 3 contacts of slightly varying position and length (to get the desired contact time). The whole thing rotates at 14sec/rotation (just over 4RPM), but only one rotation started every few minutes.
    So a timer circuit would be absolutely GREAT instead of this funky switch I built.
    Thanks again!!!!
     
  4. wayneh

    Expert

    Sep 9, 2010
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    This resembles the type of sequencing they use to launch fireworks.
    So the cycle advances roughly every 1.5 seconds. A 5-10A resistive load fires (how long?) followed a fraction of a second later by an inductive load (how much current does it need?) and an LED pulse. Then all is quiet until the next advance. Sound right?

    I think you can assume this is fairly simple. Just a few details need to be resolved such as the questions above and the required precision of the timing.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    A rotary switch capable of switching 20 Adc (always go for twice the requirement) is not your normal rotary switch. How comfortable are you with the idea of a much smaller switch and some power transistors?

    ak
     
  6. danl

    Thread Starter Member

    Sep 24, 2013
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    Wayne....Great!

    Here are details...

    As you say, advance every 1.5 (approx) seconds. 9 total events, evenly timed over about 10-15 seconds. Adjustable total time would be great, but not necessary. No high precision needed for the total time....+/- 1 or 2 seconds is fine.

    Each event: Time 0: 5-10A resistive load starts. Total ON-time max. about 2 seconds. +/- 0.5sec. probably OK. Adj range of 0.5 to 2.5sec would be great.
    Time 0.5sec: Solenoid valve (12VDC, 250ma) opens. Off when resistor load goes off.
    Time 0.5sec: LED flashes. 5mm (580ohm resistor used at 12VDC). Flash duration to mimic a small scale model canon barrel
    flash - just a fraction of a second. When manually testing, a quick 2 - flicker flashes makes a nice effect,
    See the video of the prototype test of this device to see the effect I'm trying to achieve. Test done manually.
    Heater needs to start to generate "fog". Then air flow started to "puff" the "smoke". Flash should trigger just as air flow starts.
    Video shows fan as air source - now replaced with valved compressed air.
     
  7. danl

    Thread Starter Member

    Sep 24, 2013
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    Ha, ak, you will really laugh at this. I made the switch from chewing gum, baling wire, etc etc.....2" ABS pipe cap is switch body.
    Here's a picture. Rotation is from a small DC gearmotor. 2 rows of contacts (3 would be better) to control heat, air and flash initiation timing and duration over 9 separate positions. Any smaller and I would have to be brain surgeon to make it. Contact arms ar bronze spring stock. Contacts are small rivets. This is why wayne's electronic switching suggestion may be ideal.
    Can a Smoothie board or some such do this???
    ImageRFS.jpg
    Mod note: reduced image filesize
     
    Last edited by a moderator: Dec 16, 2015
  8. wayneh

    Expert

    Sep 9, 2010
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    Ah, all is much clearer now, thanks. So the firing cycle is maybe 2.5 seconds (adjustable) while the overall timer has already moved on to the next cannon after just 1.5 seconds (adjustable). One cannon is still firing while the next is getting started.

    This unfortunately means you cannot run everything off a single timer, because some things are happening in parallel instead of step by step.

    I think at this point many here would suggest a micro-controller. The ever-popular Arduino the cool kids play with could almost certainly handle this. There are old-school ways to do it, though. The only one that comes to my coffee-craving brain this morning is to use one 555 timer as the master and then another 555 for each of the 9 cannons, to time out the delay of the solenoid and LED. The master 555 timer would feed an adjustable ~1.5 second clock signal to a 4017 counter. That IC would then direct a signal to each cannon in serial fashion. That would start the cannon's 555 timer to run through its ~2.5 second "one shot". The output of the timer would trigger two MOSFETs, a type of transistor capable of switching large loads like your's. One MOSFET such as IRF540 for the heater and one for the solenoid (could be just about any MOSFET or transistor for the smaller load of the solenoid.

    I'd love to hear if anyone has a better idea how to build in a ~2 second delay for the solenoid after the heater has been triggered. Hmmm. Now that I say it, I think it could be a lot simpler than a 555 timer. Maybe just a comparator watching an RC tank for each solenoid. With 4 comparators on a single LM393, this would lower the parts count substantially.

    Sorry, thinking out loud.

    The decision you need to make is whether to use a micro or not. If you've never used one and don't see much use for the knowledge in future projects, it's probably not worth the learning curve. But if you would enjoy the process, it could be a good choice. Either way, you'll need to use those transistors for switching the loads. It's just a question of where the smart signal comes from.
     
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  9. danl

    Thread Starter Member

    Sep 24, 2013
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    Wayne - thank you. My info slightly mislead... The time ranges I gave would always be sequential. For instance, if Gun 1 took 2.5 seconds for it's total cycle, the Gun 2 cycle could not start until second 2.5. But if Gun 1 took only 1.5 seconds total, the I would still like the option of Gun 2 cycle not starting until a later time eg 2 seconds.
    But Arduino does seem the way to go, since I could tweak the timings to get the desired effect.
    The hardware side of the Arduino learning curve would be easy for me, but writing the code for the effects would be beyond me in the timeframe I have.
    Could someone here write the code and spec out the Arduino hardware needed if I provide the sequencing and device details?
    I could order and assemble the hardware and load and run the software. I would of course pay for the effort if affordable.

    Thanks again!
     
  10. wayneh

    Expert

    Sep 9, 2010
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    I can't help you there! But thanks for clarifying the timing - that simplifies things.

    The old-school solution is still a 555 timer circuit, which is easily controlled, set to run at your desired gun-to-gun timing. The clock signal then gets distributed by the 4017 counter. When the signal arrives, it turns on the heater and delays the triggering of the solenoid and LED by a controllable delay.

    This would give the opportunity to vary the delay at each cannon, which I think might enhance the reality of the effect. On the down side, you would need to change 9 pots if you want to change them all, and the time delay would not be as precise as you could achieve with the Arduino.
     
  11. Picbuster

    Member

    Dec 2, 2013
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    If you want to do it cheap and good.
    buy pic16F690 10 mosfets and diodes, lm7805 cap's en a few resistors plus a proto pcb board. (total 30$ approx).
    compiler mlab + xc8 is free
    The program is simple and we can help you with it.
    You build a state machine running 0-9 each step interval step_seconds. (interval could be dynamic when needed)
    in each state puls(es) are generated. Fet goes on wait(n) off or a multiply thereoff.

    O, forgot start/stop knob.(2x3$)
     
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  12. danl

    Thread Starter Member

    Sep 24, 2013
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    Picbuster,
    Great forum, great people.
    If you have the patience to work with an old guy, I'm willing to give Arduino a shot. I do a lot of animation on R/C ship models, all mechanically controlled, so I really should jump into Arduino.
    I'll order the following.... clarifications? anything else? Where to order???
    1 - pic16F690
    10 - mosfets - spec?
    ? - diodes - spec? - have a lot of em on hand up to 1A
    ? - 7805's
    ? - specs - have many on hand
    proto board - have, also 30mmx30mm PC boards
    compiler - mlab ? (will check this)
    xc8 - ?
    2 - knobs
    Thank you!
     
    Last edited: Dec 17, 2015
  13. danl

    Thread Starter Member

    Sep 24, 2013
    32
    0
    Picbuster,
    Thank you.
    Would love to give it a shot.
    Where do I order the parts please?
    I'll order ASAP
    More detail needed on the mosfets, diodes, caps? Just one 7805? Have R's, proto board and misc.
    Best,
    Dan
     
  14. wayneh

    Expert

    Sep 9, 2010
    12,100
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    Mouser, Digikey, Newark are a few good suppliers. I wouldn't order anything until you can see a full schematic, unless you don't care about those annoying shipping charges.

    I believe you'll need just one 7805 to supply 5V power to the microcontroller. Read the data sheet and prepare to build what it recommends. (You typically need an input and output capacitor.) Order more than one if you plan for future projects as well. For one thing, it's a lot easier to diagnose a problem on your built project if you leave your prototype on your breadboard.

    Not sure about 10 MOSFETs, I still think it's one per cannon (heater). The diodes are likely intended to place on the solenoids to absorb the inductive spikes when they shut off. So 9 of those as well. Something like 1N400x, maybe 1N400x would be fine.

    I'm not sure what the thinking is for switching the solenoids. I think 250mA is far too much for any microcontroller, so I think some transistor will be needed for each one. Personally, I'd probably just order double (18) MOSFETs but there would be the opportunity to use a cheaper option on the solenoids.

    Not sure where to get a breadboard these days. I got mine years ago at Radio Shack. Here's an example of what I'm talking about (not an endorsement).

    For protoboard, see this thread. The hard part is simply figuring out what size you need. Here are some I happen to be looking at right now.
     
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  15. Sensacell

    Well-Known Member

    Jun 19, 2012
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    The video is really cool- looks very realistic!

    Just a thought-

    Murata makes these neat piezoelectric "microblowers" - as a potential replacement for the cost/size/complexity of compressed air and solenoid valves?

    http://www.murata.com/en-global/products/mechatronics/fluid/feature

    They are a little tricky to drive, but they turn on almost instantly and are very small and light- perfect for this application?
     
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  16. Picbuster

    Member

    Dec 2, 2013
    374
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    I furnish you with a basic drawing if you like?
     
    danl likes this.
  17. Picbuster

    Member

    Dec 2, 2013
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    Usefull?
     
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  18. danl

    Thread Starter Member

    Sep 24, 2013
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    Sensacell,
    Thank you. Great find! I looked at the specs, etc. At 10V, the air volume would be too low for this application. The specs indicate it is best used in a clean, dry environment - very sensitive to contaminates and water. The model ships actually sail and are exposed to rough conditions (see pic)
    Then there is the cost, requirement for AC high freq. driver, etc. Seems I'm stuck with a compressed air source. I did look at fans, blowers and air pumps - the ramp-up and slow down time on them causes "wisping" - a slow start of the "smoke" and a trailing stream rather than a more instant "puff" like a cannon discharge.
    forecourse0037.jpg jammin0076.jpg
     
  19. danl

    Thread Starter Member

    Sep 24, 2013
    32
    0
    Picbuster,
    Love it...wow... thank you.
    I can do the circuits (have breadboards, protoboards, solder station etc) Have done fair amount of circuitry - Vregs, ICs, sensors, and understand your drawing about 90%. Just not familiar with PICs. Would really like to do this, but would need more help from you answering questions. Is that OK w/you?
    I can provide more hardware detail. I ordered the solenoid air valves and need to test them. I think the 250ma spec on them is low. My bet is they are going to spike at least double that.
    Seems this approach will require 3 separate pic outputs - heater (mosfet), air solenoid (> 250ma, probably need mosfet?), and LED flash.
    Question - 6mm sq wire in your drawing - that's 10AWG. Sounds big? Have been running 12VDC 10A thru 16AWG (2 sec burst, max L= 1m.) no problem.
    Thanks again. The ability to program specific heater/air/LED flash to optimize appearance is fantastic.
     
    Last edited: Dec 18, 2015
  20. danl

    Thread Starter Member

    Sep 24, 2013
    32
    0
    Wayne. It is a bummer about Radio Shack. They were the only place to get stuff where I live. I have Mouser and Digikey accounts, but like you say, shipping is annoying on small orders. I need to look at Newark - new to me.
    Thanks for getting and keeping replies to this thread going. Much appreciated.
     
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