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1366  Forum 2005-2010 (read only) / Bar Sport / Re: Recommend me a DC Motor :) on: July 30, 2010, 01:46:15 pm
So you're at uni and you have a design project to complete

Isn't what you are asking not "cribbing"

When I was at uni we had to do our own reading and research to ensure we understood what we were working with

I think they called it "learning"

Much as we like to help, I think we would not do you any favours if we provide solutions that, with effort, you can solve yourself.

jack
1367  Forum 2005-2010 (read only) / Bar Sport / Re: Hello from the... thread on: June 01, 2010, 09:57:08 am
And hello from Jack in southern Scotland
1368  Forum 2005-2010 (read only) / Bar Sport / Re: Anyone here with a model bridge building hobby? on: May 09, 2010, 05:02:17 am
Praccas,
Considering where you're from, there might be an opportunity for one of your local enterprises to market a near "copy"
jack
1369  Forum 2005-2010 (read only) / Bar Sport / Re: Anyone here with a model bridge building hobby? on: May 09, 2010, 04:59:58 am
Have you thought about using Meccano.  Now that's real engineering!
Still available in France, I believe, and lots advertised on flee-bay
jack
1370  Forum 2005-2010 (read only) / Bar Sport / Re: Capacitance Education Display on: May 15, 2010, 02:11:05 pm
Without seeing the circuit it's impossible to be specific but here goes.

By installing a capacitor across the secondary winding, which is inductive, there is an LC combination which has a natural resonant frequency dependent upon the values of C and L.  In this case that is 220kHz (which seems rather high - it being in the radio frequency band)

The oscillator drive component (transistor) requires feedback in order to maintain a stable oscillation.  Unlike an amplifier, which uses negative feedback to maintain stability free from oscillation, an oscillator requires positive feedback to initiate oscillation.

It is normal to take the output signal and feed it back, in the correct phase, to the active device.  However, because the output is several hundred volts, it would be much too high for the transistor base drive.   An attenuator is required to reduce this signal down to a few hundred millivolts.    Normally, one might use a resistor divider but this is inefficient in an oscillator circuit so in this case they have made a voltage divider by arranging the C capacitor from a combination of 2 capacitors in series and tapped the feedback from the smaller one.  

Capacitors in series are calculated using a formula similar to that used for resistors in parallel  viz  C= (c1xc2)/(c1+c2)

Trust this helps
jack
1371  Forum 2005-2010 (read only) / Bar Sport / Re: Trimpot size question on: May 03, 2010, 10:30:47 am
Some designs of these miniature open pots also have flat tab connectors rather than round pins.  The circuit board holes for these are actually small slots.

It is my inderstanding that the use of "trimpot" to describe this component is confusing, since I have always understood trimpots to refer to the multi-turn miniature devices that is invariably built as a blue plastic package

jack
1372  Forum 2005-2010 (read only) / Bar Sport / Re: Teaching Capacitor to Kids - Suggestions? on: April 19, 2010, 06:41:52 am
Pracas,
The capacitor will only charge to the voltage you use as a souce.  The energy it stores is dependant upon capacity and voltage.  A 64volt unit has better dielectric strength (insulation) than a 25 volt one - that's all.  a 100volt capacitor of 8 microfarads charged from a 9 volt battery will store exactly the same energy (and voltage) as a 12 volt capacitor of 8 microfarads.
jack
1373  Forum 2005-2010 (read only) / Bar Sport / Re: Teaching Capacitor to Kids - Suggestions? on: April 17, 2010, 07:43:05 am
Forget the 25v capacitor, go for one with a higher voltage rating, say 64 or 100.  With a 9 volt battery you can do what you like regarding polarity, the capacitor will survive and more importantly, will take a charge.  If you introduce a diode to prevent reverse polarity charging, you add an element of "doubt" to those poor grey cells that you are trying to excite.

If you really want to play ultra safe, get an ac rated one from a washing machine or an old fluorescent strip lamp.  These are generally around 4 to 8 microfarad and rated for 400volts.  An added benefit is that they are of reasonable size, have decent metal lugs and are virtually kid proof.  (Give them a small wire-ended one and someones bound to pull its "legs" off !!)
jack
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