Ohm's Law?

[Rob Cope]

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

[Rob Cope]

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

[Peter Eve]

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

[Rob Cope]

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

[Myron Beaumont]

Not quite Rob  --They go in one direction & stay there ! Thats life

[Pete Rieden]

Myron Beaumont wrote (see)

Not quite Rob  --They go in one direction & stay there ! Thats life

I'm not sure what you're saying - care to exapnd a bit?

PDR

[Myron Beaumont]

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

[Peter Beeney]

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.

[Myron Beaumont]

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

[Former Member]

[This posting has been removed]

[Peter Beeney]

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.   

[Tim Mackey]

Well it has completely drifted off the point of the OP

[Pete Rieden]

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

[Peter Beeney]

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. 

[Pete Rieden]

...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

[Guest]

sounds like a memory aid

the only one I can remember is "Bad Boys .............. Without "

[Pete Rieden]

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

[Guest]

thank goodness for the old days when common sense prevailed

[Martin McIntosh]

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!

[Pete Rieden]

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 = admittance

1 / resistance = conductance

1 / reactance = susceptance

So we get another equation:

Admittance = Conductance + Susceptance

QALOPKT(BTRDGAS)

PDR

[John Cole]

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.

[Pete Rieden]

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

[Rob Cope]

Hi Peter,

(-:   Would you be able to set up your profile to accept private message please?  

Regards, Rob

[Rob Cope]

Hi Peter,

Impedance = resistance plus reactance

1 / impedance = admittance; 1 / resistance = conductance; 1 / reactance = susceptance

However, 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 get

Impedance = resistance + reactance

1 / admittance = 1 / conductance + 1 / susceptance

multiplying 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 get

admittance = (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

[Brian Parker]

Hi Rob,

I think you will find it's Parallel inductors.