help with variable resistors!

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.

"

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!

"fatninja" wrote in news:483d6173$ snipped-for-privacy@news.comindico.com.au:

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].

--
bz    	73 de N5BZ k

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

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

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 news:iOednT_bEcz_IaDVnZ2dnUVZ snipped-for-privacy@posted.cpinternet:

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.

bz+ser@ch100-5.chem.lsu.edu   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.

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!

Here's my offering for a simple solution: Get some little hollow standoffs of the kind used for through hole ICs. Solder two of them in place, where the leads of the resistor would go. Take a good selection of resistors, with leads formed and cut, to the field with you. Then you can swap them out quite readily as you experiment. We use this procedure on one board we assemble, to allow the customer to tune each board with the exact value needed.

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.

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

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

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

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!

OK. I would suggest that you do the sums first, and then go to your local Farnell website, if that is who you are going to use (

then select your country flag ) and have a look at the ranges of values available, as each of the links I gave are just examples picked (not quite) randomly from the lists. Can I ask that you bottom post replies on Usenet (unless you are vehemently opposed) as top posting tends to make the thread a bit difficult to follow chronologically, especially if somone comes into it late on. Thanks d;~}

Arfa

Arfa

Can I ask that you bottom post replies on Usenet

And especially for the partially sighted or blind.

Gareth.

And you also might incur the wrath of the top posting police! Thats no joke.

Mike