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Help with Variable resistors

Discussion in 'Hobby Electronics' started by fatninja, May 28, 2008.

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  1. fatninja

    fatninja Guest

    Hi Guys!

    Hope I can get a little help here and I'm stumped!

    I've got a pulse going to a coil, Normally I use a 3w Metal film resistor to
    collapse the field at a set rate and adjust the values of the resistor to
    change the rate at which the field is dampened.

    What I am after, is a variable resistor that can do the same job.. Is there
    any type of unit that will act similar to a metal film resistor as the
    resistance is increased/decreased?
    It can get quite warm as the field is charged and collapsed every 30us.

    Thanks for any help!
  2. Phil Allison

    Phil Allison Guest


    ** A potentiometer is the obvious thing, but there are problems.

    A wire wound type could have too much inductance, seeing as your PRR is 30

    A 3 watt cermet type will not dissipate the power without damage unless it
    is set near full resistance.

    Anyhow, this might do:

    ...... Phil
  3. fatninja

    fatninja Guest

    Hi Phil,

    Thankyou for your reply to my question even though it wasn't explained
    adequately. I will give you as much information as I possibly can so you
    understand the application.

    It is for my own basic pulse induction metal detector and works as follows.

    It starts with a squarewave pulse of 24v, 1ah from the the detector box to
    the coil. While the pulse is on the power goes to the coil. When the pulse
    is switched off, the field collapses using resistors (3w metal film) across
    the coil.
    The next part is the critical part. The field created by the coil has to
    collapse at a certain rate. If it collapses too quickly or too slowly, the
    sample is taken at the wrong time, so incorrect.

    Now, unfortunately I don't have any test equipment to know how much
    resistance I need across the coil to see the point at which the sample needs
    to be taken.
    So, I thought I may be able to use trial and error and adjust the
    Potentiometer while using the detector, then use a multi meter to read the
    value from the Pot to determine the value of the resistor I would need
    across the coil.

    But now I think this may not work because the Pot creates its own inductance
    and capacitance.

    Any suggestions as to how I could do something similar with a rheostat or
    pot? Would the one suggested below still work for this?
    Would a cheap wirewound pot work if I used a shielded cable and mounted it
    away from the actual control box?

    Thanks again!
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