need circuit design help ?

Discussion in 'General Electronics Chat' started by treedog, Apr 23, 2006.

  1. treedog

    Thread Starter Member

    Apr 23, 2006
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    i'm trying to design a simple transistor switch for rc use that doesn't use relays.
    planes vibrate too much causing many failures. if some 1 can point the way i'd appreciate it i hate seeing my freinds lose 500 dollar airplanes over a faulty slide switch! ty
    john ps i was thinking of a light activated type that was in waiting state til light hit the photo detector.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    What precisely is this transistor supposed to do? Is it just a matter of finding a power transistor that can handle the current? Or are there some other things about the RC environment that are obvious to you but not to the rest of us.

    What is the relationship between transistor switches and "faulty slide switches"?
     
  3. treedog

    Thread Starter Member

    Apr 23, 2006
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    the amps needed in rc models some times exceeds the 7 amps under high stress manuvers so i was thinking of using 4 of them in parallel [ saturated ] to handel any excess current. the vibration is a problem in the wiring connections failing after many runs rpm is some times 16000 rpms and puts strees on the soldered connections so i'm fighting 2 problems actually. another idea i was thinking might work is a shockley or scr diode. what do u think? to turn on the main transistors handeling the current? it's been 20 yrs since i was in tv radio repair and hav't kept up with new products thats y i'm so ignorant of new devices sorry. and ty for the reply hope this clears things up a bit
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    We can find transistor switches to handle the current, and we can pot the resulting circuit to reduce the vibration stresses on the resulting connections. The thing to use to turn on a big transistor is a small transistor.

    I'm still struggling with a picture of what the transistor is doing. You mention "high stress maneuver" and 7 amps in the same sentence so I'm going to guess that you are trying to hold the control surfaces(rudder, elevator, and aileron) in position with servo motors during these times. Is this correct?

    Are your controls proportional or bang-bang. Are there distinctions between partly on, mostly on and all the way on? I'm guessing that since you are currently using relays that it's on or off with no in between, and the ability to reverse directions.

    One more detail. Can you post a diagram of the current configuration with relays?
    It can be hand drawn and scanned if you like.
     
  5. treedog

    Thread Starter Member

    Apr 23, 2006
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    [attachmentid=1398]
    ok lots of ?'s so little space. 1. i'm up to speed on small devices turning on large 1's. u are correct about the servos using more amps for the more stressfull manuvers. we are into 3d flying now and computerized transmitters and recievers. 3d is what patty wagstaff was doing flying in a box in the sky. only on the rc scale we do the same manuvers she does at 9 g's by watching it from below. some of these planes are worth 5000 bucks and are 1/4 scale. each servo cable [3 wire] comes out of a receiver and the battery lead plugs into a seperate place in the receiver[2 lead] ie +/-. 3. rudder , aileron, elevater, etc. all have 3 wires and come out of the receiver to each servo. like this +, -, signal lead. prety simple so far. now as to the power transiters , im trying to get completely away from any moving mechanical parts relays and such for the main power from 2700 amp hr batts. 4. pretty much u are correct to an extent about bang bang except the newer computerized radios have almost an infinite adjuable travel rate from off to fully on and slow in certain areas and faster servo travel in other areas so u see its getting more complicated every day but the power reqirements are going to be more diverse and higher so i was thinking of using power transistors in the fully on or off state as switches then tie them to a block and heat sink them[transistors] this would be right before they go into the servos, from the receiver. o know this is probly an impossible task but im trying my best heres' a beginning idea of the kind of circuit i was thinking of and ty sir for ur reply.
    john reece sorry look up top a pdf file as to what kind of circuit i was thinking of as a starting place
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    do you have the part number for the servo's your using?
     
  7. treedog

    Thread Starter Member

    Apr 23, 2006
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    the servos were on a friends plane at rc universe website and i think they were hitec heavy duty servos but there are so many models of i can't be sure but close to top of the line digital high torque at the very least ty for the reply. i'd be guessing but i'd say some where in the 100ounce inches of torque per servo, and at least 6 servos
     
  8. treedog

    Thread Starter Member

    Apr 23, 2006
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    check ur email joe i could only send u a jpeg of the final demise
     
  9. pebe

    AAC Fanatic!

    Oct 11, 2004
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    I'm trying to make sense of your description of the R/C controls.

    Are you using proportional servos? If so they will already have solid state switching for their motors. Or are you talking about the system on/off switch?
     
  10. treedog

    Thread Starter Member

    Apr 23, 2006
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    as i said in the post above they are proportional servos mainly but can be adjusted for the likes of the operater's likes or needs for certain manuevers. the computerised transmitter can be set for each persons likes as per the exponential rate of travel at certain points along the lenth of travel of the servo arms. and yes its the slide switch that failed [on/off]. it supplies power to the receiver that divides it as needed to the servos being activated. the farther the travel needed up to 180 degrees of rotation. there are no switches inside the servo housing. I've never taken a receiver apart but as far as i know there are no mechanical switches inside the receiver. and it's wrapped in foam so vibration probs are at a minimum there as far as i know. pebe ty for the reply i hope u understand my post. some times i make the simple comlicated thats my fault and i'm sorry. u are correct. everything after the on off switch is probly electronic ie no relays. ty all again for the help
    john
    ps i guess actually when it comes right down to it if u take the switch out of the system then where or what will handle all of the current the reciver needs to send out to each servo? thats what i'm asking about. what kind of switch will handle all the current be it a trasistor or whatever ? then how do i tirn that transistor on off and saturate it so it can transmitt all amps and volts up to 6 volts and the amperage i'd be guessing at because some tomes the tx operator needs full up dow lefy right or whatever combination of servos being used at 1 time shoot clear as mud i bet arrg im tired
     
  11. Papabravo

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    Feb 24, 2006
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    Treedog, the following is for the benefit of the other readers. I know that you know this stuff.

    As near as I can tell you don't need any kind of switches for the servomotors. They have three leads, two of which are connected directly to the battery. This is where the high current path is. The signal lead does not require any appreciable current. For an "analog" servo, at 50 Hz. you vary a pulse from 1 msec. to 2 msec. for the full travel of the servo. A 1.5 msec pulse puts the servo at the center of its travel. The problem here is that power is applied for only a fraction of the 20 msec period.

    There are so called "digital" servos which up the frequency to 300 Hz. but with a corresponding adjustment in pulse width. The claimed advantage of these devices is power applied to the motor for a greater percentage of the time.

    I cannot see how adding a transistor switch of any kind is going to help with the problem of holding the aircraft control surfaces in place. These so called digital servos already have a microprocessor in them to anylyze the incoming pulses and control the motor. The guys who make the motors are already working on the torque problem.

    What is left is the battery and the battery wiring. If you think the motors should have sufficient torque to hold the control surfaces, and the battery has sufficient capacity to supply the required current then the next step is to look at the wiring. If the wiring is 22ga. consider using 18 ga. or lower. There is not much you can do if the wires to the motor are sealed. If they are of a small gauge that should be a hint that the motor is undersized.

    Your battery may not be able to supply the peak current demanded by the high stress maneuvers. You can test this on the ground however. Place an ammeter in series with the battery and the power lead to a motor. Command the deflection of a control surface. When the surface reaches the commanded position, deflect it gently with your hand to simulated the stress of a maneuver. Watch the current as the servo tries to maintain the commanded position. If the motor tries to draw 7 Amps from a 3000 mAh battery then you know a bigger battery is required. If it were me I would try to keep the peak current at 0.7C to 0.8C. The average discharge should be in the 0.2C to 0.3C range.

    Hope this helps
     
  12. pebe

    AAC Fanatic!

    Oct 11, 2004
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    I've got your message, but I wish you would write words out in full so I don't have to read a sentence 3 times before making any sense of it.

    That said, I used to fly 1/6 scale RC scale models some years back, and they ran 4 servos plus a HD servo for the retracts. They used a 450mA NiCd and there was never a problem with switches. What I did find was corrosion on the -ve battery lead and plug.

    I think you should examine the connections to the plugs and switch and prove conclusively that the switch is at fault, before considering a solid state replacement. A good switch will be more reliable and less lossy than a pair of transistors.

    On the subject of expensive crashes, I am surprised that such large and potentially dangerous models are flown without using redundant equipment for emergencies.
     
  13. Papabravo

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    Feb 24, 2006
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    As I have not seen a schematic diagram of any of this stuff would it be too much trouble to show me where the "switch" is and tell me what it's function is?

    By the way, I agree with pebe that you'll make more progress in problem solving and life if you can construct coherent sentences and paragraphs. It is a chore to read your posts.
     
  14. pebe

    AAC Fanatic!

    Oct 11, 2004
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    I think the switch in question must be the system on/off switch.

    The gear that I had, which was typical at that time, had a receiver and decoder mounted in a plastic box. Integral with these was a bank of sockets for output to the servos. Each servo had a 3 pin plug, ie. a +ve and a –ve lead that was common to all, and an individual pulse wire.

    Power was a 4.8V or 6V rechargeable battery (2 wire) via plug/sockets to the on/off switch (mounted on the model), and from there via more plugs/sockets to the receiver box. Earlier models used a 4.8V centre-tapped battery, in which case there was a switch in both +ve and –ve leads.

    The reason for the plug/socket arrangement was that if you had more than one model you could unplug the battery and receiver and transfer them, leaving the switch in position.
     
  15. dbwgwee

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    Mar 30, 2006
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    Celebrate nonsense; it wakes up the brain cells :)
     
  16. Papabravo

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    Feb 24, 2006
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    OK, I see the light, so to speak. You want to be able to kill the power to the servos when the aircraft is not in flight. I'm thinking of a way to make a vibration resistant kill switch that would survive the high stress maneuvers.

    As I see it, there will be too much loss, which you cannot afford, using a semiconductor. I'm thinking of a mechanical fuse holder with a shorting bar in place of the fuse. The shorting bar can be conveniently removed when the aircraft is idle, and placed in position just before engine start and takeoff. I think it should be easy to find such a system which could be adapted to the RC environment. This has the added security bonus that your plane wo't fly without the magic "key".

    How about a standard, spring loaded, inline fuse holder. Machine an aluminum bar the size of a standard fuse. Epoxy one side of the fuse holder to the frame and let the other side float. I think it would take a major event to make it let go.
     
  17. treedog

    Thread Starter Member

    Apr 23, 2006
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    so that we can all be on the same page, please scroll down on this link to "ultimate demise" post, and watch the video all the way thru and u'll see what i'm referring to. i appologize for my bad typing and grammar. my keyboard is old and black with white letters that wore off yrs ago so when i type i'm lucky if i can read it at all. ty for the help
    john reece
    http://www.rcuniverse.com/forum/forumid_96.../smode_1/tt.htm

    at the end of the post is a video link and pics of the crash. the poster is very upset at the switch [on/off] as u can see.
     
  18. pebe

    AAC Fanatic!

    Oct 11, 2004
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    And you diagnosed a faulty switch from THAT mess? From the low altitude, I would agree with one of the posters that it was probably pilot error.
     
  19. Papabravo

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    Feb 24, 2006
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    The text with the video suggested that it was the wire from the switch. What is the normal method of coupling a wire to a switch. Solder? Could this be a cold solder joint?

    From the nature of the maneuvers, the skill with which they were executed, and my own abilities at aerobatics in a full size machine, I think pilot error is extremely unlikely.
     
  20. treedog

    Thread Starter Member

    Apr 23, 2006
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    Its apparent to me that you did not view the video at least in its entirty,as at the very end both the owner and his freind were wiggling the switch left then right and it was intermittent. Its the 5:05 minute win media file just above the crash jpegs called Ultimate demise. and he mentioned the switch failure in his post.
     
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