My turn to ask for advice

Sorry uncle, but your attempt isn't much better, and thus are urban myths born.

No, Back EMF is the voltage generated due to the motor acting as a genarator.

i.e. the armature windings are moving in the field of the permanent magnets and the motor acts as a generator, just like Chris said. You obviously have the basic theory so why do you then have to try and complicate things with totally innapropriate theory?

It's nothing to do with collapsing magnetic fields, when you remove power to the motor the armature keeps moving due to inertia and acts like a generator.

That's due to the inductance of the relay coil, which tries to keep the current flowing when it's switched off. The diode acts as a short circuit for that current.

You may indeed get such a spike from a motor coil, but only one, when the power is removed. From then on we are talking about Back EMF which is a totally different mechanism, I'll say it again, the motor is acting as a generator.

Wrong mechanism. Back EMF occurs because the motor is acting as a generator and the voltage is proportional to the rotational speed of the motor. That's how feedback controllers work, by measuring the

*actual* rotation speed of the motor versus the desired speed. To work properly, the system needs to be tuned to each individual motor. The best DCC decoders give you full control over the tuning of the Back EMF to suit the motor in each loco.

It depends what you mean by "work best". Maximum torque, maximum power, maximum efficiency? Max torque occurs at zero revs. Maximum power at half the no-load speed of the motor.

Back EMF is being generated by a motor all the time, even when it's being driven. At lighter loads, and thus higher speed, the Back EMF is greater. Back EMF has opposite polarity to the drive voltage and opposes it. It's the lower effective *voltage* that causes lower current at higher speed. The armature resistance is constant. It's nothing to do with inductive resistance. At the extreme, if the motor is totally stalled there is no Back EMF and the total supply voltage is applied across the motor.

You measure the Back EMF generated by the motor (acting as a generator) and it gives you a measure of the rotational speed of the motor.

MBQ

Why do you say this method is best?

Noise and coreless motors are only a problem if the frequency is too low, hence we have DCC decoders that drive the motor with higher frequency (usually something > 15KHz).

Your "best" method is actually a form of frequency modulation and more likely to result in too low a frequency.

MBQ

" snipped-for-privacy@hotmail.com" wrote in news: snipped-for-privacy@i39g2000cwa.googlegroups.com:

Only from what I've been told previously, I do have more than what can perhaps be described as a basic knowlage but I'm far from being an expert.

Yes, but you've still got to make up solutions and then get rid of them. It's all so unecessary for something as simple as a controller, unless of course you want to hide some pretty looking setup in a case where it can't be seen but you know it's there. I've made pcbs for complex circuits and it was great fun - wouldn't deny anybody the pleasure if that's what lights their bulbs. And Penfold shows the working layout so there's no excuse for cutting the wrong strip.

Ken.

That would be most kind of you Chris - email is ian (a t) uknn [.] com - much appreciated.

If only I could build something which replicated the intuitive center-off of the old H&M units..... :-)

Rgds, Ian.

MBQ

Very good! Just goes to prove that whatever daft question you ask on UseNet someone has the answer! :-)

It's an interesting little circuit though, and I might be tempted to have a go at it at a later date, thanks.

BTW, I've been reduced to reading/posting via Google for the next few days - Wannado have this evening killed off their usenet server (may they rot in hell for that!!!!) :-(

Rgds, Ian. (email [myname] at uknn dot com) (posting address auto-deletes)

They have ?? So how come you're reading this ??

Cheers, Martyn (posted via Wannado/ex-Freeserve news server)

and this (18:10)

ken

Ian H wrote in news: snipped-for-privacy@4ax.com:

Certainly, I'll try and get it done today once I finish the tiling.