Help with Variable resistors

[fatninja]

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!

 

[Phil Allison]

"fatninja"

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.

** 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
kHz.

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

Anyhow, this might do:

http://au.farnell.com/1141308/passives/product.us0?sku=vishay-sfernice-rt12-l-as-100r-10&_requestid=15748




...... Phil

 

[fatninja]

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!




"Phil Allison" <philallison@tpg.com.au> wrote in message
news:6a3vqoF30dvbiU1@mid.individual.net...

"fatninja"
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.



** 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
kHz.

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

Anyhow, this might do:

http://au.farnell.com/1141308/passives/product.us0?sku=vishay-sfernice-rt12-l-as-100r-10&_requestid=15748




..... Phil

 

[Daniel]

fatninja wrote:

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!

A cheap (or expensive for that matter) wirewound pot would still be a
wire wound pot, so would effect the rate of decay of your waveform. Many
rheostats are also wirewound, so be careful with them as well.

Do you know the value of the coil you are testing, or is this what you
are trying to determine?? How long after the pulse is removed is the
sample taken??

(P.S. you are aware that the pulse generator you are using will also
have an output impedance (which is part resistance) so will effect the
rate of decay as well as the resistor you have connected across the coil!!)

HTH

Daniel
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