Rob Cope Posted September 5, 2008 Share Posted September 5, 2008 Hi All,Sorry to butt in PDR. I think that would lead to a perpetual motion machine... in reductuo ad absurdum...The motor will not run at 71% power... The effective voltage supplied is reduced to 71% of the full power value. However, the current drawn by the motor is reduced in the same way.. Thus the POWER is reduced to 71% of 71% (voltage times current) which is, of course a half.Regards, Rob Quote Link to comment Share on other sites More sharing options...
Rob Cope Posted September 5, 2008 Share Posted September 5, 2008 No chagrin... It's an easy slip to make. Done it lots myself in the (recent) past until someone put me right. Power is a tricky subject - especially in audio with decibels when it gets even more confusing...Conservation of enery (or power) is always a good check. Half power in... so somehow I must get half power out! No criticism implied even though reading back, my post "sounds" a little abrupt.Regards, Rob Quote Link to comment Share on other sites More sharing options...
Peter Eve Posted September 5, 2008 Share Posted September 5, 2008 Early 80's?! You should be so lucky - for some of us it was very early 60's, when they hadn't quite worked out how to teach transistors and i/c only meant internal combustion!I have a theory that the lousy weather outside my window might just be perpetuating this fascinating thread! Maybe the formula is really W (weather) = (gives rise to) V (vintage) X I (intelligence)Well recalled chaps, it's great to know that someone still remembers what they learned! Pete Quote Link to comment Share on other sites More sharing options...
Rob Cope Posted September 5, 2008 Share Posted September 5, 2008 Hi again, Don't you just love the fact that speaker cables must not only be oxygen-free, but fitted in the drawn direction from amplifier to speakers... Now I am a cynnic, but the ignorance of the fact that it makes no difference anyway is exceeded in absurdity by the ignorance of the fact that the conductors not only carry the current in different directions (one in, one out) but that it's an alternating current travelling to the speakers anyway. Supply DC to them and they fry!Regards, Rob Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted September 5, 2008 Share Posted September 5, 2008 Not quite Rob --They go in one direction & stay there ! Thats life Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted September 5, 2008 Share Posted September 5, 2008 Myron Beaumont wrote (see)Not quite Rob --They go in one direction & stay there ! Thats lifeI'm not sure what you're saying - care to exapnd a bit?PDR Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted September 6, 2008 Share Posted September 6, 2008 Peter If you put DC through a speaker coil it will move the cone in one direction and stay there .Its called Flemings L hand rule ( or is it R hand I've forgotten) Anyway its the electro motive force exerted by current flow in a conductor in the prescence of a magnetic field ! Hope that is a bit clearer -End of lecture Of course with AC applied it vibrates the cone back & forth moving air at the same frquency causing sound waves Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted September 6, 2008 Share Posted September 6, 2008 Yet another 6d worth I seem to remember little diagrams of amplifier circuits with a capacitor in series with the speaker? To block the DC component? And the ohmic value was derived from the impedance at a 1kHz test frequency? This was a long time ago, plus it was only toy stuff! Jonnine Ray was Just Walkin’ in the Rain then. Perhaps things are much different now, I shall very pleased to find out how much.Pete. Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted September 6, 2008 Share Posted September 6, 2008 Peter I think you are "remembering"part of a passive filter circuit blocking low frequencies to tweeters (consequently blowing them ) & not wasting high frequencies sent to woofers.that could not physically reproduce high frequencies .With just two speakers ( depending on many factors of course ) not withstanding their own resonant frequencies the "cross over" target was any where between 2 & 8 Kcs Re ;Helmholtze's formulae for resonance / speaker cabinet design for optimum Hi-Fi sound Just how far off thread do I go ? Grumpy .'cos its raining .Thats my excuse today Good news is I now have my Lipos for my Tx & Rx so I can now set about more problems ( over heating / fires / burnt out components /etc etc ) I expect Quote Link to comment Share on other sites More sharing options...
Former Member Posted September 6, 2008 Share Posted September 6, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted September 6, 2008 Share Posted September 6, 2008 Thank you gentlemen, I can now sleep easy.I’ve never been into speakers, r/c has been my downfall.The way I imagine it, the DC resistance should be as low as possible. The swings as high as possible, 24v in today’s mega-watt cacophony? If so, and 24 volts DC was applied without any current limit, then I would tend to think that Rob Cope would be on the button.That should have Jonnie Ray, not Jonnine. (I think!) Sorry. But it was the 1950’s and that’s my excuse. And it’s not stopped Raining since.Hopefully this post is still obeying Ohm’s Law?Pete. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted September 6, 2008 Share Posted September 6, 2008 Well it has completely drifted off the point of the OP Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted September 6, 2008 Share Posted September 6, 2008 Just to complete the drift - decoupling the output stage was the cheaper alternative to transformer output stages. It was cheap, but it had the basic limitation that it couldn't exceed the voltage rails, so it limited the output power of a car stereo driving a standard speaker (8 ohm in those days) to ((12*0.7071)^2)/8 = 9Watts RMS. Transformer coupling allowed the voltage to be stepped up for more power. This is how the classical car power amplifiers work - they drive a higher current into a transformer-coupled output.QAFPKT(BVFGAS)PDR Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted September 7, 2008 Share Posted September 7, 2008 I’m beginning to think that I’m understanding some of this, nearly.Re: the drift, I think that one of my lad-about-town friends summed it up very succinctly in a recent conversation at the field. He remarked how he occasionally enjoyed a pleasant evening with some sparkling wine, some scintillating conversation in convivial company and then a nice early night to bed. ‘Cos he had to get up early in the morning. “Oh’ said I “ You get up early and go to work?”“Oh no,” he replied “ I gets up early an’ goes Ohm!”Apologies for the very old joke and I think I’ve drifted so far I’m right off tune.It’s stopped raining so I’m off to the field.Pete. Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted September 7, 2008 Share Posted September 7, 2008 ...or for the electronic engineers out there: "Resistance is useless! But reluctance can be aluring, and susceptance is always welcome..."That's unlikely to mean much to the non-engineers unfortunatey.PDR Quote Link to comment Share on other sites More sharing options...
Guest Posted September 7, 2008 Share Posted September 7, 2008 sounds like a memory aidthe only one I can remember is "Bad Boys .............. Without " Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted September 7, 2008 Share Posted September 7, 2008 No, it's not that one - although the version of that one I was taught (translated into politically correct speak) would be:"Gentlemen of colour (but dubious parentage) indulge in non-consnesual interaction with our non-age-specific people of gender but the non-interacted remain non-intellectually challenged".PDR Quote Link to comment Share on other sites More sharing options...
Guest Posted September 7, 2008 Share Posted September 7, 2008 thank goodness for the old days when common sense prevailed Quote Link to comment Share on other sites More sharing options...
Martin McIntosh Posted September 7, 2008 Share Posted September 7, 2008 Peter R, I vaguely remember tuning resonators and susceptances many years ago on 1.9gHz. Glad we do not have to do this to get our gear to work! Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted September 7, 2008 Share Posted September 7, 2008 For the ininitiated - in electronic engineering there is the general equation:Impedance = resistance plus reactance (reactance is the resistance of something whose resistance varies with frequency)It is often useful when doing the sums to deal with the "inverse" of something, so we have:1 / impedance = admittance1 / resistance = conductance1 / reactance = susceptanceSo we get another equation:Admittance = Conductance + SusceptanceQALOPKT(BTRDGAS)PDR Quote Link to comment Share on other sites More sharing options...
John Cole Posted September 13, 2008 Share Posted September 13, 2008 Peter: going back to some earlier posts of yours on brushless motors and the pulsed DC used to drive them: I seem to remember reading somewhere that the potential for radio interference was greater from the two battery leads to the ESC than from the three power leads from the ESC to the B/L motor, and it was therefore desirable to site the Rx (and aerial) further from the battery lead than the motor lead. The reason given for this was that "the AC current from the ESC to the motor is smoother than the DC feed to the ESC".Your explanation and diagrams make me feel this reasoing is specious. Do you agree? Mind you, specious reasoning does not mean the conclusion is wrong! So comment on that too please. Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted September 13, 2008 Share Posted September 13, 2008 The potential for interference is identical from both - that is to say that it would depend on the length and position of each wire rather than whether the wire was on the battery or motor side of the ESC. The current on the motor side most certainly isn't "smoother" - I can only assume that whoever made this assertion was under the impression that the ESC outputs a 3-phase, sinusoidal current (which it most certainly doesn't).PDR Quote Link to comment Share on other sites More sharing options...
Rob Cope Posted September 15, 2008 Share Posted September 15, 2008 Hi Peter,(-: Would you be able to set up your profile to accept private message please? Regards, Rob Quote Link to comment Share on other sites More sharing options...
Rob Cope Posted September 18, 2008 Share Posted September 18, 2008 Hi Peter,Impedance = resistance plus reactance 1 / impedance = admittance; 1 / resistance = conductance; 1 / reactance = susceptanceHowever, the bottom equation does not seem right to me. Here's my reasoning. If we do the algebra starting at the well known equation, and substitute the reciprocal names above, we getImpedance = resistance + reactance 1 / admittance = 1 / conductance + 1 / susceptancemultiplying by admittance and dividing by (1 / conductance + 1 / susceptance), [so long as it's not zero] we get admittance = 1 / (1 / conductance + 1 / susceptance))multiplying top and bottom on the right by (conductance x susceptance), [so long as that's not zero] we getadmittance = (conductance x susceptance) / (conductance + susceptance)This is the product over sum form that is also applied to parallel resistors and series capacitors (and I suspect series inductors.) Regards, Rob Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted September 18, 2008 Share Posted September 18, 2008 Hi Rob,I think you will find it's Parallel inductors. Quote Link to comment Share on other sites More sharing options...
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