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1366  Forum 2005-2010 (read only) / Bar Sport / Re: PC keep turning its self off when playing Games? on: September 26, 2010, 01:30:54 pm
Maybe it's just bored with wasting it's brain power trying to operate as slow as its human operator
1367  Forum 2005-2010 (read only) / Bar Sport / Re: Sodding Blue LEDs... on: August 27, 2010, 08:15:55 am
Who remembers the "blue-whitener" they put in soap powder
1368  Forum 2005-2010 (read only) / Bar Sport / Re: Sodding Blue LEDs... on: August 24, 2010, 05:34:39 pm
have you tried coating it with a blue magic marker
1369  Forum 2005-2010 (read only) / Bar Sport / Re: Sodding Blue LEDs... on: August 24, 2010, 04:35:01 pm
It may not be the "brilliance" of the LED that's the problem but the sensitivity of your eyes.   We have a cooker with a blue LED display and the colour appears to cause excessive flourescence which I feel is possibly a spectral sensitivity peak of the human eye.  Yes I know it's supposed by somewhere in the orange range.  But have you noticed how a flashing blue light on top of certain motor vehicles also draws your eye.

Apart from that, dimming of LEDs is best achieved by PWM rather than simple current limiting resistors.

jack
1370  Forum 2005-2010 (read only) / Bar Sport / Re: Your dream electronics lab? on: August 25, 2010, 05:48:32 am
You forgot the lathe, milling machine and arc welder   smiley
1371  Forum 2005-2010 (read only) / Bar Sport / Re: Train tickets on: August 20, 2010, 01:08:03 pm
Have you tried wrapping it in baking foil ?  Like brass and lead it won't stop static magnetic fields but it will minimise induced currents from variable fields (body scanners, electric motors etc)

Have you ever tried watching a compass whilst on the tube.  You'd be amazed at the varying field as the motors are switched.  Yes, it takes a sad person to "discover" that effect.  Just happened to wonder what direction we were heading in and my compass was swinging all over the place, even when the train wasn't changing direction.

jack
1372  Forum 2005-2010 (read only) / Bar Sport / Re: Want to know how to build a wind turbine? on: August 19, 2010, 02:32:30 pm
I might be wrong, but I think you'll find brushless DC motors use a 3-pahse invertor within their structure to make the motor motor.  The process is not reversible ie turning the motor shaft will not make volts on the DC terminals
jack :-/
1373  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
1374  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
1375  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
1376  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
1377  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
1378  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
1379  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
1380  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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