Re: help with variable resistors!

Now this is the information i was hoping to find!!!!

Thanks Arfa, mate you are a complete legend!!!
I'll have to order one from our local farnell, and now I now how to go about
it the 'easy' way!!

Cheers!


"Arfa Daily" <arfa.daily@xxxxxxxxxxxx> wrote in message
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"Arfa Daily" <arfa.daily@xxxxxxxxxxxx> wrote in message
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"William Sommerwerck" <grizzledgeezer@xxxxxxxxxxx> wrote in message
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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




"fatninja" <noemail@xxxxxxxxxxx> wrote in message
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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!


That being the case, when you determine the correct value, I would feel
inclined to make the 'final' value from two resistors in series, as the
typical voltage rating is only two or three hundred volts.

http://uk.farnell.com/3278232/passives/product.us0?sku=tyco-electronics-rss3-1k5-5&_requestid=41927

Note the voltage rating. I think that what you want to do is easily
achievable using a 3 watt cermet pot e.g.

http://uk.farnell.com/1141634/passives/product.us0?sku=vishay-sfernice-pe30al-10k-10-a

This will be substantially non inductive or capacitive, and although it
might get a bit hot whilst you are carrying out your tests, cermet pot
tracks are pretty stable and reliable, so it is unlikely that you will
damage it. If you know that you need between 1k and 3 k, then use a fixed
resistor of 1 k, in series with a pot. That way, the power dissipation
will be split betwen the two. It will also allow you to get a better
adjustment range, over the mechanical range of the pot. When you have your
optimum setting, just measure the combination with a digital ohm-meter.

Another alternative might be to get close to what the optimum value is for
all cases, then substitute a permanent resistor / trimmer combination.
For instance

http://uk.farnell.com/9352376/passives/product.us0?sku=bourns-3006p-1-501lf

This trimmer is rated at 1.25 watts, so you should be able to work out a
combination of fixed R and trimmer to keep the dissipation below that
figure. You then have the best of all worlds - ie a circuit that's
adjustable over its lifetime as other components drift, a low power
dissipation in the adjustable element, and a sufficient voltage rating
that the damping network won't be damaged or degraded over time, by back
emf.

Arfa



.

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