3 transistor feedback controller - Lost circuit

I am trying to track down a circuit for a model railway controller that used 3 transistors and was a pulsed/feedback design.
I used the controller for many years on my exhibition layouts, which I sold to Solihull MRC many years ago. I have decided to make another small O gauge layout, cannot find the circuit anywhere and haven't even got an old controller that I could work out the circuit from.
I know the person who designed it was a member of the 3mm society and that he worked with fork lift trucks.
I would be very grateful if anyone could help me.
Kind regards - Jim Read
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Jem Raid wrote:

Pop down your library for a copy of Electronics for Railway modellers. Its a basic design using a 2n3055 as the main power output and two smaller ones to control it I cut it down to 2 transistors by using a Tip 35 I think which is 2 transistors in one and does not produce as much heat
--
Trev
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"Trev" <trevbowdenAT.dsl.pipex.COM> wrote in message

Dear Trev,
I have a copy of that book alas the controllers are just variable volatge ones and do not have feedback or pulsing. I made small locos that are not very heavy and the controller would bring them down to a crawl without stalling.
Rgds - Jim
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Jem Raid wrote:

A transistor has three connections the output is dependent on the voltage across the input one of which is + as One Neg. the neg is connected to the output.The back emf will very depending on how the motor is performing. under heavy load its low but running downhill kind of its high when its high it send voltage back to the neg. contact that changes the balance between the pos and neg so doing lowering the output. Thats feedback.
--
Trev
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"Trev" <trevbowdenAT.dsl.pipex.COM> wrote in message

Dear Trev,
A transistor does have three connections, one to the supply voltage this is called the Collector, one to the control voltage this is called the Base, and one to the output this is called the Emitter. Depending on the configuration of the transistor the Collectors can be connected to either the negative or the positive supply rails.
In order to effect feedback the Base voltage must be altered, ergo the feedback must use a path other than the one you suggest, in order to work.
The circuit I am looking for used three transistors but not in a conventional cascade, as I recall two were NPN and one was PNP, as well as creating the feedback these two acted as an oscillator to generate the pulse.
If and when I get the circuit I will gladly send you a copy, please let me know your email address. It is very easy to build just a bit of veroboard and a box and it works superbly.
There is a much more complicated circuit here;
http://www.webtech.co.uk/controller/fig2.gif
You can see that the negative feedback is fed to the op-amp IC1a (via R14, D9 etc) which is configured as a comparitor with the input voltage from the speed control potentiometer. this output is then passed to IC1b and from there to the Base of the 2n3055.
Kind regards - Jim
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Ignoring the fact that bipolar transistors are current operated devices rather than voltage, what the OP was trying to convey is that the current through the transistor is dependant in the current flowing into the base, that current is usually dependant on the base to emitter voltage (Vbe) up to the point where the transistor becomes saturated and Vbe stops rising (about 0.7V).
Now if the emitter is connected to the output, any back emf from the motor will raise the voltage on the emitter, so reducing the Vbe and so reduce the output. ie feedback.
Regards Jeff
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Dear Jeff,
You are of course quite correct, my apologies.
In practice though it doesn't work very well :-)
Rgds - Jim
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That's all very well but in that case every transistorised controller has "feedback".
In the context of train controllers "feedback" is usually taken to mean some form of active monitoring of the BEMF during a short period when the track supply is cut off (hence the pulsing requirement) and adjustment of the drive to the transistor. On more sophisticated controllers the amount of correction is adjustable for motors with differing characteristics.
MBQ
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No, see my reply to Trev. BEMF does not change the voltage on the terminals of the motor.
MBQ
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<No, see my reply to Trev. BEMF does not change the voltage on the <terminals of the motor.
The voltage on the motor terminals will be affected by back emf. The back-emf is seen as another voltage source is series with the supply, but of opposite polarity. Without losses, in a 'ideal' world, the back-emf would equal the applied voltage, giving perpetual motion!!
The difference between the applied voltage and the back-EMF is the net voltage that the motor actually sees. As the mechanical load on the motor is increased, and the motor slows down, the back-EMF is reduced and the net voltage that the motor sees increases. It turns out that the actual motor current is determined by the difference between the applied voltage and the back-EMF divided by the parasitic resistance. When the motor speed is zero and therefore the back-EMF is zero (motor stalled) the only thing that controls the current is the parasitic resistance. Since the parasitic resistance is small, the resultant current is much higher than it would be under normal running conditions.
Regards Jeff
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The BEMF is entirely internal to the motor and is always less than the voltage applied to the motor. The voltage at the motor terimnals (ie the applied voltage) does not change when the BEMF changes unless the voltage source cannot regulate for the change in motor current.

The BEMF cannot be greater than the applied voltage so you could only get out what you put in. That's not perpetual motion.

Thanks for basically repeating what I said in my other post.
MBQ
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For those with 5 mins spare.....
www.ee.ucl.uk/~ktong/E3002LT10.pdf
<old git in cap mode>
takes me back a bit.....
</old git in cap mode>
Cheers Richard
--
www.beamends-lrspares.co.uk snipped-for-privacy@beamends-lrspares.co.uk
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It's a lot less error prone to copy and paste a link than type it all in again:
http://www.ee.ucl.ac.uk/~ktong/E3002LT10.pdf
MBQ
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On Mar 29, 10:08pm, "Trev" <trevbowdenAT.dsl.pipex.COM> wrote:

No. You cannot measure BEMF this way. You need to cut off the drive voltage (hence the pulsing requirement) to measure the BEMF.
The BEMF is always opposite polarity and less than the drive voltage. In simple terms, under load the BEMF reduces so more of the drive voltage is avaliable and more current is drawn by the motor. The voltage at the motor terminals will not change unless the source (the controller) has a high impedance and cannot supply the higher current.
MBQ
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Jem Raid wrote:

This site may be useful:
http://home.cogeco.ca/~rpaisley4/Throttles.html
--
wolf k.

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On Sat, 29 Mar 2008 20:37:38 -0000, "Jem Raid"
I am trying to track down a circuit for a model railway controller that used 3 transistors and was a pulsed/feedback design. I used the controller for many years on my exhibition layouts, which I sold to Solihull MRC many years ago. I have decided to make another small O gauge layout, cannot find the circuit anywhere and haven't even got an old controller that I could work out the circuit from. I know the person who designed it was a member of the 3mm society and that he worked with fork lift trucks. I would be very grateful if anyone could help me. Kind regards - Jim Read
I have a copy of Roger Amos's book "Practical Electronics for Railway Modellers" and it contains both a simple 3tran PWM and a more sophis job with fB.
You are welcome to a copy of the ccts. my email is real -- I hide from no one.
Regards
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Is there anything here?
http://www.merg.org.uk /
I've not looked, admittedly!
Cheers Richard
--
www.beamends-lrspares.co.uk snipped-for-privacy@beamends-lrspares.co.uk
I have become... comfortably numb
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Thanks for all the interesting replies.
I have the circuit now.
I'm extemely pleased to say :-)
Jim
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