Electro magnet alternating circuit

Discussion in 'General Electronics Chat' started by customwelds, Aug 6, 2011.

  1. customwelds

    Thread Starter New Member

    Aug 6, 2011
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    Hi everyone,
    Im excited to be part of this site as I'm new to circuits and this forum.
    I have been reading up on some things and I'm trying to make a circuit that will alternate power to two small electro magnets. While I'm testing the circuit, I'm using leds just to make sure everything is working properly. And that's where the problem comes in.... it's not.
    I found a diagram online, http://wild-bohemian.com/electronics/flasher.html [​IMG]
    and have attempted making it multiple times nows to no avail. At first I thought, maybe the transistors were backward, so I reversed the collector and emitter, no fix, same with the capacitors trying each seperatly of course. I even purchased new transistors to make sure they were NPN and not PNP. Still no fix. The problem I'm having is that while the leds light up, they dont alternate, both are on, and thats about it. I'm pretty sure that the problem is "right under my nose" and I simply cant figure it out (I already feel pretty dumb about not being able to put together a simple circuit). Any advice you can give would be extremely helpful as Ive been up since last night trying to get it to work.
    [​IMG]
    [​IMG]

    More info on the project,
    Its a mock hand for a non time keep clock, (the hand will tick back and forth instead of going in a circle, like an upside down pendulum) being made for my fiance's birthday. The hand will tilt left and right from the forces of the two electro magnets on either side of it, the magnets work well at moving the hand with 5 volts 500ma as its pretty well balanced.
     
  2. customwelds

    Thread Starter New Member

    Aug 6, 2011
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    Note: ive also tried lower voltage led's, which just burn brighter. Thanks again in advance.
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Well, you said you inverted them but the way the transistors are mounted right now they are inverted. collector is connected to minus. http://www.fairchildsemi.com/ds/2N/2N3904.pdf

    Also, it cannot be seen on the photos but make sure you don't connect any leads accidentally.
     
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  4. #12

    Expert

    Nov 30, 2010
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    The frequency of this circuit is so high that a human will think both leds are on all the time when it is working correctly. Either look at it with a 'scope or increase the capacitor size to 100uf. PS, be sure those crossed capacitor wires are not touching.

    edit. Oops. I think I'm wrong. Somebody double check me, please.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Yep, your transistors definitely have the emitters and collectors swapped as they're shown.

    If you reversed the polarity across your electrolytic caps, you probably ruined them. They are connected properly now, but if you had them reversed before, try some fresh caps.

    However, this circuit won't be able to put out anything close to 500mA; as shown, the LEDs get about 15mA current.

    You will need some sort of driver arrangement to be able to source/sink 500mA, but let's just concentrate on getting this part working for now.
     
  6. SgtWookie

    Expert

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    I beg to differ with you; the frequency should be less than 1Hz. In simulation with ideal components, I get 766mHz/0.766Hz.
     
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  7. customwelds

    Thread Starter New Member

    Aug 6, 2011
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    Thanks for all the fast replys everyone.
    Regarding the transistors being reversed, Is the image from fairchild standard on all transistors? the reason I ask is that the package they came in shows them as being the opposite. see pic [​IMG]

    Also, im confused why they would be flashing so fast, I thought this might be the case origionally, though I dont have a scope to prove it, I swung the bord around with the power leads as fast as I could and the steak of light was not solid. What makes me confused is that based on the site where I got the diagram, it shows the leds blinking pretty slow, so is the site just misinformed or am I using the wrong caps? the caps I got are :[​IMG]
     
  8. #12

    Expert

    Nov 30, 2010
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    Transistor pinouts are horribly non-standard. You must look up every single one to be sure you're hooking them up correctly.
     
  9. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Yeah this picture is confusing. No ,the pinout is not the same for all transistors in this package. Refer always to the manufacturers datasheet (online: www.alldatasheets.com) and if you are still not sure try to look up the datasheets of the same transistor by other manufacturers.
     
  10. customwelds

    Thread Starter New Member

    Aug 6, 2011
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    they arnt (made sure of that ;))
     
  11. customwelds

    Thread Starter New Member

    Aug 6, 2011
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    I noticed that right when power is supplied, the lights jump bright for a split second, then go to a dimmed state, they are still on, just not as bright as when it first recieves power. This "simple circuit" is giving me headaches. ill try switching the transistors again? any other trouble shooting ideas for someone with out a scope?
     
  12. customwelds

    Thread Starter New Member

    Aug 6, 2011
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    You were RIGHT! the last transistors I tried, (the first ones, i think were bad, then I followed the diagram on the package the new ones came in, WRONG because i just switched them, and it works! Thanks everyone! I really like this forum and am excited to get more involved!:D:D
     
  13. SgtWookie

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    Actually, the package has the correct pinout. Where you got confused is that on the package, the pins are shown facing up towards you; as if you're staring down at the bottom of the transistor with the flat front facing to the left. However, when you install the transistor, the pins are going down - you simply forgot to correct for that.
     
  14. customwelds

    Thread Starter New Member

    Aug 6, 2011
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    Ok, so now that Ive got the circuit alternating between LEDs, I'm attempting to hook up the magnets. Is the resistance between the cathode and Vdd relevant to the oscilating? the reason I'm asking is because when I diconnect one of the LEDs and replace with the magnet, I no longer recieve a oscilation, only a steady low current to the LED and magnet. would ensuring the restiance between both sides was equal? Im testing this with 4.5v 1.5a, hopefully not enought to blow anything, and once I get it working, I plan to replace the 2n3904 transistors with TIP31A's and the power to 12v 1.5~3amps.
    If anyone could help me with the theory behing all this, such as explaining if I need to change the caps for the larger load of the magents, or are the caps simply for triggering the transistor. Im excited to learn more so any help and clarification is greatly apprecciated. Thanks for everyone's help already!
     
  15. #12

    Expert

    Nov 30, 2010
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    Your post #14 seems very odd.

    Using the word, "magnet" when you mean "coil" is confusing.
    2n3904 transistors can't survive 1.5 amps. They are rated at 2/10ths of an amp, absolute maximum.
    The resistance in the LED parts of the circuit is very important. It is likely that your coils have very little resistance and the transistors can't handle the resulting current. Remember, the 100k resistors limit the drive current to the transistors. That drive current times the gain of the transistor sets a limit on how much current can happen.

    The 100k resistors and the capacitors set the oscillation speed. It is probably not very important to change the capacitors to accomodate the coils.
    Equalness in each leg is good.
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    He means electromagnets, of course. ;)

    Like I mentioned before, your little circuit won't be able to drive an electromagnet directly, even if you replaced the transistors with more powerful versions.

    Perhaps your cheapest option is to pick up a few L2722 power opamps, and use your flasher circuit as an input to the opamps.
    Digikey stocks them: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-1384-5-ND
    They're rated for up to 1A current. You'd use the transistor collectors of your flasher circuit as one input, and a voltage divider as the other input.
    Look at the datasheet:
    http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000055.pdf
    5th page, top schematic, figure 8.
    E1 and E2 would be connected to your two transistors' collectors.
    You will probably have to use a resistor in series with your electromagnet to keep the current within the opamps' limits.
     
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