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!

fatninja wrote:
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!



30 us is a tall order.

Have you looked into using a Mosfet ? it can act as a
variable resistor via a bias voltage source on the gate.

I don't know how clean you need the pulse to me but I'm
guessing that will do it.

P.S.
You may still want to use the highest Fixed value of R along
with the MosFet. this will help to keep transient HV peaks reaching
the Mosfet from the coil.
And also, Apply the lowest value R you would use in series with the
Drain of the Mosfet.
this way when the mosfet is biased full on/off, it'll fit in the range.
seeing that you are paralleling the circuit, you don't want to
short the mosfet if it happens to be in zero ohm state. and if it's
in infinite state, you don't want Back EMF to kill it.

A power MosFet should handle that. Make sure you get one that is
spec for the linear operation that fits your needs.
Many of them are very high amps at switching (Full On) but no
where near that spec in the linear region.


http://webpages.charter.net/jamie_5"

"fatninja" wrote in message
...

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.

What, in the name of heaven, are you talking about? Your grammar is poor,
and your ability to express yourself clearly even worse. You don't seem to
understand what you're talking about. I certainly don't.

Electrically, resistance is resistance, and (in principle) it doesn't matter
what type of resistor you're using, whether fixed or variable, metal-film or
bulk carbon.

In practice, resistors aren't purely resistive -- they also have stray
capacitance and inductance. A variable resistor might have sufficient
inductance to disrupt the normal operation of an inductive circuit.

Come back when you can explain just what your problem is.

Hi guys,

Thankyou for your replies 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 of the inductance and capacitance
of the pot.

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

Thanks again!

"William Sommerwerck" wrote in message
. ..
"fatninja" wrote in message
...

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.

What, in the name of heaven, are you talking about? Your grammar is poor,
and your ability to express yourself clearly even worse. You don't seem to
understand what you're talking about. I certainly don't.

Electrically, resistance is resistance, and (in principle) it doesn't
matter
what type of resistor you're using, whether fixed or variable, metal-film
or
bulk carbon.

In practice, resistors aren't purely resistive -- they also have stray
capacitance and inductance. A variable resistor might have sufficient
inductance to disrupt the normal operation of an inductive circuit.

Come back when you can explain just what your problem is.

"fatninja" wrote in
:

Hi guys,

Thankyou for your replies 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.


Might it be simpler to collapse the field at an arbitrary fixed rate and
use some adjustable timing circuits to adjust both the time the sample is
taken and the length of time spent sampling?
Those should be much easier to control than the field collapse rate.

It might help if you had a good scope with variable delayed sweep. You
should be able to pick up a surplus tube type cheap.
If you live near Baton Rouge, I will GIVE you one if you come pick it up.
[If I remember correctly, it is a tektronix 545 with cart and plug ins.
Needs a bit of work].


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 of the inductance and
capacitance of the pot.

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

Thanks again!









--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

bz wrote:

snip
It might help if you had a good scope with variable delayed sweep. You
should be able to pick up a surplus tube type cheap.
If you live near Baton Rouge, I will GIVE you one if you come pick it up.
[If I remember correctly, it is a tektronix 545 with cart and plug ins.
Needs a bit of work].

A working 545x is a joy to use, but repairing and aligning one, without
silver solder, source for matched pairs in the delay line vert section,
etc. is perhaps a non-starter for a newbie. Wish I were nearer to your
QTH however.

Michael

msg wrote in
ernet:

bz wrote:

snip
It might help if you had a good scope with variable delayed sweep. You
should be able to pick up a surplus tube type cheap.
If you live near Baton Rouge, I will GIVE you one if you come pick it
up. [If I remember correctly, it is a tektronix 545 with cart and plug
ins. Needs a bit of work].

A working 545x is a joy to use, but repairing and aligning one, without
silver solder, source for matched pairs in the delay line vert section,
etc. is perhaps a non-starter for a newbie. Wish I were nearer to your
QTH however.

Yeah. The trace is too bright, if I remember correctly.
I think one of the power supply voltages was wrong but I don't remember
which one it was.

I have a couple of other scopes so I would be glad to give this to someone
that would give it a good home.






--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

I'm not trying to be funny when I suggest that you get a box of resistors
and experiment. You can wire them in series, parallel, and series-parallel
to get a range of values.

And you might not need a 3W resistor. The rating is for continuous power. A
resistor with lower dissipation might be perfectly "happy" with pulses.

"William Sommerwerck" wrote in message
...
I'm not trying to be funny when I suggest that you get a box of resistors
and experiment. You can wire them in series, parallel, and series-parallel
to get a range of values.

And you might not need a 3W resistor. The rating is for continuous power.
A
resistor with lower dissipation might be perfectly "happy" with pulses.



However, it should be noted that resistors have an often forgotten about
voltage rating, and the peak level of back emf pulses from an inductor, may
well exceed that voltage, which can be quite low for some lower power
resistor types ...

Arfa

Afra,
It's funny that you mention this, because measuring it on a friend's scope,
the back EMF was in the vicinity of 700 Volts!!

But William, as you suggest this is something I HAVE thought about. I know
the resistance I need is somewhere between about 1k5 and 2k5.
The way you suggest is how I originally went about it, though it took me
hours and hours. Hence why I was (and still am!) looking for a simple
solution :-)

Thanks!


"Arfa Daily" wrote in message
...

"William Sommerwerck" wrote in message
...
I'm not trying to be funny when I suggest that you get a box of resistors
and experiment. You can wire them in series, parallel, and
series-parallel
to get a range of values.

And you might not need a 3W resistor. The rating is for continuous power.
A
resistor with lower dissipation might be perfectly "happy" with pulses.



However, it should be noted that resistors have an often forgotten about
voltage rating, and the peak level of back emf pulses from an inductor,
may well exceed that voltage, which can be quite low for some lower power
resistor types ...

Arfa