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Driving a coil — Parallax Forums

Driving a coil

CenlasoftCenlasoft Posts: 265
edited 2010-04-05 23:00 in Propeller 1
Hello,
I am in need of a method to drive a small pancake coil (10mm diameter) with the prop. I was thinking of using a npn transistor to drive the coil from a pin. Do I need a diode to protect from back emf? I have used a 555 timer with a transistor and I would like to use the prop and the synth object to get any square wave. Thanks for any suggestions.
Curtis

Comments

  • kwinnkwinn Posts: 8,697
    edited 2010-04-05 04:21
    Yes, you should use a diode for safety.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-04-05 10:18
    When you say "drive a pancake coil" do you mean ...

    Switch on, for a period, and then off again as a single act, or ...

    On and off continuously at a certain frequency ( posibly resonant ) ?

    For the first a quick reverse diode will be ok, but for the second the resonance shouldn't be damped and the transistor will have to be selected to withstand the peak voltages.

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  • CenlasoftCenlasoft Posts: 265
    edited 2010-04-05 14:07
    Hi Toby and thanks for responding.
    I would like to use the prop pin to continuosly excite apancake coil. I would like to be able to use different frequencies with no capacitor (RL circuit). Probably not at resonance. I was thinking of connecting the prop pin to one lead of the coil and then the other coil lead to ground via a resistor (RL circuit). Would the reverse diode be between the prop pin and the coil lead? I will experiment with this today. I just wanted to protect my prop. Any suggestions would be great.
    Curtis
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-04-05 15:10
    With that cct I would put two diodes to prevent any excursions past +3.3V or below 0V. Even then the diodes would let 0.7V ( or less for better ones ) outside of the normal. I dont know what the forward voltage drop of the Prop's protection diodes is, and so they might end up taking the punishment rather than the added external ones.

    If the resistance is large the the tendancie to resonate would be reduced but without knowing the coils parameters or the intended frequencies, I would be protective of the Prop to start with and get measurements of the voltages and currents generated.

    That extra transistor would provide an extra stage of protection.

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    Style and grace : Nil point

    Post Edited (Toby Seckshund) : 4/5/2010 6:38:05 PM GMT
  • KenBashKenBash Posts: 68
    edited 2010-04-05 16:44
    Tip 120 Darlington transistors are a good way to drive many inductive loads with the propeller. They have built in reverse diodes, a saturation point of 2Volts and are rated up to 5 amps.

    The trick is to drive the GROUND leg of your inductive load. Hook the other end of anything from a motor to a magnetic coil up to any voltage up to the rated voltage of the transistor. I've gone as high as 24 volts with no problems and higher shouldn't be a problem either. They aren't as FAST as some newer transistors, but are easy and cheap. You can use resistors of 1-10K between the base and the propeller which gives some further protection.

    Ken B.

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  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2010-04-05 18:20
    I would isolate the pulsing circuit from the prop using a darlington optocoupler.
  • w8anw8an Posts: 176
    edited 2010-04-05 23:00
    What Holly said... That would be the best way to protect your Propeller
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