Help building high speed flasher

Discussion in 'The Projects Forum' started by dkmac, Jul 11, 2013.

  1. dkmac

    Thread Starter Member

    Jul 11, 2013
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    Hello all, I am new here and am learning my way around the forum. So far I love seeing the great comunity the is here and the helpfulness provided. Now more to the subject, I have tore up both the net and the forum trying to find a resolution to my problem. I have been trying to find a schematic or already build high speed flasher that is adjustable from 5 to 200 flashes per second, (yes per second). The problem is that I need a 12vbc 3amp output capability supplied to the load. I did some research on 555 circuits and the way I understand it, they supply a signaled output. The source going into the flasher will be a 12vdc battery. Any help on this would be greatly appreciated. Thank you and take care.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    5-200 Hz is fast for a human but slow in the electronics world. A 555 IC based flasher will work fine at this rate.

    For a load of that size, you'll need an n-channel MOSFET switch under control of the timer output. Time output on the gate pin, source pin to ground, drain pin to the low side of your load. High side of your load connected to +12V. I use the IRF540N for such things but there are many other choices. Just be sure to use one rated for at least about 10A or more.

    What are you flashing? Some loads require special considerations.
     
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  3. dkmac

    Thread Starter Member

    Jul 11, 2013
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    Thank you for the expedited response. The load is a coiled field for a project that I am building. It requires 12vdc and a maximum of 3amp draw. Do you happen to have any schematics for the circuit that you suggested? I have been having issues finding what you described, even already constructed. Thank you again for your help.

    Donny
     
  4. dkmac

    Thread Starter Member

    Jul 11, 2013
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    Wayne, forgot to mention that I was also looking into the possibilities of using this type of circuit to power a transistor or a logic versus the load and having the transistor or logic handle the load. I am thinking that this will give the circuit more longevity. What are your thoughts on this? Thank you again for your help. Take care.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Check out the circuit at the bottom of this page. It's more elaborate than what we just described because it includes the ability to change the duty cycle of the pulses, ie. PWM. You might find that useful so I think it's good to consider it now rather than later.

    Your load would replace all the LEDs show in the example.

    Switching a large inductor at 200 Hz is one of those things that require special attention. The coil's stored energy needs to go somewhere when it is switched off and can destroy things in its path. Do you have any specifications for your coil?
     
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  6. dkmac

    Thread Starter Member

    Jul 11, 2013
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    Wayne, unfortunately I do not have any other specifications than what I gave you. That is all the info I was able to obtain from the manufacturer of the coil I bought. Essentially I am learning as I go on this project. I will check out that link that you posted. Thank you again.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Well, what does it do? You must know more about it than just that it needs 3A. Diameter? Winding count? Wire gauge? Inductance?
     
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  8. dkmac

    Thread Starter Member

    Jul 11, 2013
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    Sorry for the delay, the coil is simular to that of what is used in vehicles for ignition. As for the inductance and such, I have yet to receieve the coil. So I will not be able to take any exact measurements until after I receive it. What I was planning on doing was building this circuit while I am waiting on the part to come in. Thank you again.

    Sorry, just looked at the specs again. The diameter is 2.5" and the hight is 2".
     
    Last edited: Jul 11, 2013
  9. wayneh

    Expert

    Sep 9, 2010
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    At the least you will need a snubber diode in reversed bias position. That means it will not normally conduct when the coil is on, but will help short out the coil and absorb its energy when the magnetic field collapses.

    Other experts will have to tell us the nuances of driving an inductive load. If you're really never above 200Hz, this may not be such a big concern.
     
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  10. dkmac

    Thread Starter Member

    Jul 11, 2013
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    Hi again, I have a quick question pertaining to the schematic at the bottom of this page: http://forum.allaboutcircuits.com/showpost.php?p=117646&postcount=11. Does everything labeled Vcc indeed get Vcc connected to it, more specifically does the Drain of the of the MOSFET receive Vcc in addition to theother Drain connections? I have never used MOSFETs before and want to make sure I build this circuit right the first time. Thank you and take care.

    Donny
     
    Last edited: Jul 18, 2013
  11. wayneh

    Expert

    Sep 9, 2010
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    Are we talking about Fig. 10.4?

    What are you referring to as Hdd?
     
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  12. dkmac

    Thread Starter Member

    Jul 11, 2013
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    Hi Wayne, yes it is Figure 10.4, the bottom schematic you referred for my build. The VDD was an auto correct error, I put Vcc then it my phone changed it. Sorry for that.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    In that circuit, the MOSFET acts as a switch on the path to ground (connected to source) below all the LEDs (connected to drain). The voltage on its gate controls the switch.

    If you connected Vcc directly to the drain, flipping the switch would short Vcc to ground and bypass the load. The LEDs would not light. Something would blow up - either the power supply or the MOSFET or the wiring, whatever is the weakest link.
     
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  14. dkmac

    Thread Starter Member

    Jul 11, 2013
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    That is what I was thinking as well. When I saw Vcc on the drain, I drew a question mark. So to be sure, everything else in the circuit that is labeled Vcc does indeed get connected directly to Vcc? Thank you again for all your help on this. Take care.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    You have to be misreading the schematic. In between Vcc and drain is the load. There should not (ever) be a direct connection of Vcc to drain, and I don't see one in that figure.
    Yes, as long as you are reading it correctly. ;)
     
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  16. dkmac

    Thread Starter Member

    Jul 11, 2013
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    Hi Wayne, when I looked at the schematic it looked like a run connecting the collector of the NPN and R8 only connected to the drain of the MOSFET, but then I noticed the Vcc above the tie point which got me a little confused. So as I understand it, only the run connecting both R8 and the collector of NPN (Q1) connects to the drain and no Vcc? Sorry, but this is my first time using a MOSFET. I was just confused about the Vcc labeled above the Drain on the MOSFET. Thank you and take care.
     
  17. wayneh

    Expert

    Sep 9, 2010
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    I'm still baffled by what you are seeing. Where is there any connection of Vcc to the drain of Q3 in this picture?

    [​IMG]
     
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  18. dkmac

    Thread Starter Member

    Jul 11, 2013
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    So I am finally able to upload the image to better show you what I am looking at. Everything that I have circled in GREEN is connected directly to Vcc, the RED circle does not get connected directly to Vcc, and I labeled what I believe is where the tie point is for the MOSFET Drain. I am sorry again, this is my first time working with a MOSFET. Thank you again and take care.

    Donny
     
  19. dkmac

    Thread Starter Member

    Jul 11, 2013
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    The point above R10 at the tie point. This is my first time looking at a schematic with a MOSFET like this. Again I am sorry for my ignorance, I am learning.
     
  20. wayneh

    Expert

    Sep 9, 2010
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    Nope, the red circle really does go to Vcc as shown, the same Vcc noted everywhere else. The drain is switching the low side - completing the path to ground - for the load.

    An n-channel MOSFET is normally used as a switch just as shown. You can use a p-channel MOSFET to switch the high side, and I believe high-side switching is what is in your head. The p-MOSFET would be placed between Vcc and the load, and the load would connect directly to ground. A low voltage (less than Vcc) causes the p-type to conduct and turns on the load.

    There are subtle reasons why n-channel MOSFETs are better, so that is why you usually see switching done with an N-type unless the P-type is unavoidable.

    Low-side switching confuses every single newbie when it is first encountered, so you are in good company.
     
    Last edited: Jul 19, 2013
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