Controller failure - low voltage

[Nicktoix]

I have used a very similar circuit for 30+ years but using a TIP121 as the only "transistor ". It works well and can be crammed into a handheld unit. Can get warm when operating O gauge!

One useful addition is to have an led with suitable resistor across the collector and emitter of the output device. That will give you an indication that a loco is in  circuit. A sad omission from almost all commercially available controllers.

Nick

[ikcdab]

2 hours ago, Nicktoix said:

I have used a very similar circuit for 30+ years but using a TIP121 as the only "transistor ". It works well and can be crammed into a handheld unit. Can get warm when operating O gauge!

One useful addition is to have an led with suitable resistor across the collector and emitter of the output device. That will give you an indication that a loco is in  circuit. A sad omission from almost all commercially available controllers.

Nick

Is that an additional LED to the one I have already shown in the original post?

[Nicktoix]

No I don't think it is. That only shows you are sending power to the loco it will light up without any loco in circuit.

My version shows that a loco is detected even if the controller is set to zero.

However I could be wrong.  I work by experimentation as much as theory.

Nick

[AndyID]

8 hours ago, Nicktoix said:

I have used a very similar circuit for 30+ years but using a TIP121 as the only "transistor ". It works well and can be crammed into a handheld unit. Can get warm when operating O gauge!

One useful addition is to have an led with suitable resistor across the collector and emitter of the output device. That will give you an indication that a loco is in  circuit. A sad omission from almost all commercially available controllers.

Nick

 

Using TIP121 will have a greater minimum voltage drop, something like 1.8 volts when delivering a decent amount of current at maximum output. The arrangement in this circuit will have a smaller minimum drop - around 1.1 volts.

[ikcdab]

On 12/12/2022 at 18:53, Michael Hodgson said:

I would comment that it's usual practice to have an electrolytic capacator across the output of the the bridge rectifier to smooth the supply.

The advice I was given is as follows:

"Because it is a closed loop controller using the back EMF of the motor as feedback the slow start and smooth running beats a lot of ready to use designs.

 

"Note that there is NO smoothing capacitor, this is because the voltage needs to fall though the 0v point for the back EMF voltage to be measured. If a smoothing capacitor was fitted the controller would still work but, slow starts and smooth slow running would be impossible."

[AndyID]

4 hours ago, ikcdab said:

The advice I was given is as follows:

"Because it is a closed loop controller using the back EMF of the motor as feedback the slow start and smooth running beats a lot of ready to use designs.

 

"Note that there is NO smoothing capacitor, this is because the voltage needs to fall though the 0v point for the back EMF voltage to be measured. If a smoothing capacitor was fitted the controller would still work but, slow starts and smooth slow running would be impossible."

 

Yes, the capacitor is not a good idea. It will increase the DC voltage well above 12 volts and because the output is smooth DC (or at least a lot smoother) it will affect slow starts and slow speed operation with some types of motor.

 

But, the paragraph about "closed loop" and "back EMF" makes no sense at all for the circuit posted above. There is no "closed loop" anything about it. I doubt if it was written by the original designer.

[ikcdab]

5 hours ago, AndyID said:

 

Yes, the capacitor is not a good idea. It will increase the DC voltage well above 12 volts and because the output is smooth DC (or at least a lot smoother) it will affect slow starts and slow speed operation with some types of motor.

 

But, the paragraph about "closed loop" and "back EMF" makes no sense at all for the circuit posted above. There is no "closed loop" anything about it. I doubt if it was written by the original designer.

Ok.  I got the circuit and comments from here:

unfortunately the diagram was lost in the great crash and they op appears to be no longer active.  I believe this is what I built.  

Ian

 

[ikcdab]

But interestingly going back to the original problem.  I have still not solved it.  The problem is intermittent.  Sometimes the controller is ok, other times its on half voltage.  I can't see any logic to this.

My solution is to totally rebuild it and, in particular, substitute a 5a bridge rectifier in place of the diodes.

Ian

[AndyID]

15 hours ago, ikcdab said:

Ok.  I got the circuit and comments from here:

unfortunately the diagram was lost in the great crash and they op appears to be no longer active.  I believe this is what I built.  

Ian

 

 

Yes, it does sound like the same circuit but the idea that it senses the back EMF is not correct. One way to confirm that would be to monitor the track voltage while it's running a train then increase the load on the train (a little force applied from a finger would do.) If the controller is sensing the back EMF the output voltage will increase to try to maintain motor speed as the load increases. If it doesn't increase the voltage there is no feedback.

[AndyID]

15 hours ago, ikcdab said:

But interestingly going back to the original problem.  I have still not solved it.  The problem is intermittent.  Sometimes the controller is ok, other times its on half voltage.  I can't see any logic to this.

My solution is to totally rebuild it and, in particular, substitute a 5a bridge rectifier in place of the diodes.

Ian

 

Possibly just a dry solder joint. Try retouching them with your soldering iron. If that doesn't work go for the 5A bridge.

[ikcdab]

On 01/01/2023 at 00:20, AndyID said:

 

Possibly just a dry solder joint. Try retouching them with your soldering iron. If that doesn't work go for the 5A bridge.

Just to finish one off.  I dismantled the controller and resoldered the joints, but no improvement.  So to save anymore fuss, I made up a new circuit board using new components and a 5amp bridge rectifier in lieu of the diodes which I think were underpowered.

Everything now works again and, if anything, better than before, but that may be my imagination. 

In due course I will renew the other controllers in the same way to prevent further issues.

Thank you all for you help.

[Platy787]

I have a couple of  the Roger Amos books. I hope the following is helpful.

 

This is the Roger Amos High-performance closed-loop controller from Complete Book of Model Railway Electronics 2nd Edition. Quoting from the text "Transistor T1 is used as a comparator - it compares the output voltage (on its emitter) with the control voltage (on its base)."

Further on, from the "Performance" paragraph,  because the combination of the BC107 and the TIP147 has a very high gain -

"Consequently the speed regulation is excellent, extraordinarily so for a closed-loop controller"

 

Many other simple controllers don't have the intermediate stage BC107 or the connection from output to its emitter.

 

Nothing is mentioned about falling through 0v, but the diode from BC107 emitter to 0v is mentioned as being required so that the B-E junction of BC107 is protected against inductive overshoot. The omission of the diode making it difficult to stop some types of locomotives.

 

I've made this one (High-performance closed-loop controller), another that voltage divides the feedback (Project 7: Sensitive closed-loop controller), and a centre off version (Project 8: Bi-directional controller - this one doesn't work for some locos).

[ikcdab]

1 hour ago, Platy787 said:

I have a couple of  the Roger Amos books. I hope the following is helpful.

 

This is the Roger Amos High-performance closed-loop controller from Complete Book of Model Railway Electronics 2nd Edition. Quoting from the text "Transistor T1 is used as a comparator - it compares the output voltage (on its emitter) with the control voltage (on its base)."

Further on, from the "Performance" paragraph,  because the combination of the BC107 and the TIP147 has a very high gain -

"Consequently the speed regulation is excellent, extraordinarily so for a closed-loop controller"

 

Many other simple controllers don't have the intermediate stage BC107 or the connection from output to its emitter.

 

Nothing is mentioned about falling through 0v, but the diode from BC107 emitter to 0v is mentioned as being required so that the B-E junction of BC107 is protected against inductive overshoot. The omission of the diode making it difficult to stop some types of locomotives.

 

I've made this one (High-performance closed-loop controller), another that voltage divides the feedback (Project 7: Sensitive closed-loop controller), and a centre off version (Project 8: Bi-directional controller - this one doesn't work for some locos).

Thanks for the reply.  I find this circuit excellent and I would recommend it to anyone. It's simple to build and effective.

Ian

[AndyID]

13 hours ago, Platy787 said:

I have a couple of  the Roger Amos books. I hope the following is helpful.

 

This is the Roger Amos High-performance closed-loop controller from Complete Book of Model Railway Electronics 2nd Edition. Quoting from the text "Transistor T1 is used as a comparator - it compares the output voltage (on its emitter) with the control voltage (on its base)."

Further on, from the "Performance" paragraph,  because the combination of the BC107 and the TIP147 has a very high gain -

"Consequently the speed regulation is excellent, extraordinarily so for a closed-loop controller"

 

 

 

Not to detract from the utility of Amos's design in the slightest but describing it as "closed-loop" is completely misleading 😃

 

It's really just an emitter-follower design where the voltage at the power transistor's emitter "follows" the voltage applied to the transistor's base. That means that the output voltage is effectively independent of the current drawn by the motor. That is very different and a huge improvement over the old rheostat, or variable resistance, controller where the voltage applied to the motor actually decreases substantially as the current drawn by the motor increases. That's what happens when the load on the motor increases due to gradients, curves etc.

 

But it is not "closed-loop". A closed-loop controller must use some method to determine the actual speed of the motor (there are several different ways to do that) and it continually adjusts the voltage applied to the motor to maintain the motor speed determined by the controller setting.

 

The Amos design maintains an output voltage that is always proportional to the controller's position setting independent of the current drawn by the motor which is great but the snag is the motor windings have resistance (just like a rheostat) and even if the voltage applied to the motor is constant its speed will still vary with the load on the motor. A true "closed-loop" controller continually adjusts the output voltage to maintain the desired motor speed.

 

Here's a link that might help to explain the difference between open and closed-loop control systems.

 

https://engineering.electrical-equipment.org/panel-building/difference-between-open-loop-closed-loop-systems.html

 

 

 

 

 

 

[Frank Sharp]

You are getting a reading of 7 volts, almost certainly one of the rectifier diodes as gone and you are effectivley getting half wave rectification. I've fixed 3 various Gaugemaster controllers at shows with the same problem. None of them belonging to our layout! It may be that something else has gone in the circuit and is overloading the diodes, but a new set of 4 diodes, try 3amp versions, even at show prices will only be about 60p

[ikcdab]

1 hour ago, Frank Sharp said:

You are getting a reading of 7 volts, almost certainly one of the rectifier diodes as gone and you are effectivley getting half wave rectification. I've fixed 3 various Gaugemaster controllers at shows with the same problem. None of them belonging to our layout! It may be that something else has gone in the circuit and is overloading the diodes, but a new set of 4 diodes, try 3amp versions, even at show prices will only be about 60p

Thanks.  I replaced the diodes with a bridge rectifier and all is now fine.

Ian

[ikcdab]

On 21/12/2022 at 15:31, Nicktoix said:

I have used a very similar circuit for 30+ years but using a TIP121 as the only "transistor ". It works well and can be crammed into a handheld unit. Can get warm when operating O gauge!

One useful addition is to have an led with suitable resistor across the collector and emitter of the output device. That will give you an indication that a loco is in  circuit. A sad omission from almost all commercially available controllers.

Nick

Can you say a bit more about how this works? So I just connect one leg of LED to the emitter and the other to the collector with a resistor and correct polarity.  If I turn the controller up with no loco on the track then the led stays off.  If I add loco to track  then LED lights. 

Bearing in mind I can measure voltage across the track when there is no loco on circuit, how does this work?

[AndyID]

8 hours ago, ikcdab said:

Can you say a bit more about how this works? So I just connect one leg of LED to the emitter and the other to the collector with a resistor and correct polarity.  If I turn the controller up with no loco on the track then the led stays off.  If I add loco to track  then LED lights. 

Bearing in mind I can measure voltage across the track when there is no loco on circuit, how does this work?

 

Current can only flow through the LED when there is a loco on the rails and it is connected to the controller. No current means no LED light.

[Nicktoix]

Sorry for not responding sooner but I have a very flaky connection at the moment. 

Andy is correct. Voltage only shows you power available not possible connection to it. The led will fade as you increase the output to the loco.

Nick